Method and System for Non-Invasive Ultrasound Doppler Cardiac Output Measurement

The present invention relates, generally, to methods and means for measuring the cardiac output of a mammalian patient, more especially to noninvasive methods and means for measuring the cardiac output of a human patient, and most particularly to a dual modality insonification technique for ascertaining the cross-sectional area of the patient's ascending aorta and the systolic velocity profile of blood flow therethrough allowing for a determination of cardiac output, cardiac index, stroke volume and stroke index, amongst other physiological parameters.

The present invention advantageously provides a simple yet highly efficient method and means for carrying out the measurement of cardiac output of a mammalian patient, and particularly a human patient on a real time, beat-by-beat basis. The method and means of conducting such measurement in accordance with the present invention are especially desirable for facilitating direct operator interaction with the apparatus over the course of the measurement protocol via an interactive visual display which instructs the operator at each step of the sequence and responds to an election of operator options with certain failsafe features guarding against the entry of invalid data and prompting the entry of maximized physiological characteristics of the patient under examination. Still a further advantage of the present invention is the selective, specific use of dual modality insonification with an eye toward employing an ultrasound mode best suited for the measurement task at hand. A related benefit is the facility with which the operator may interact with the system to achieve reliable, repetitive results without requiring extensive training. As respects the system itself, the same provides the benefits of microprocessor control for fast data computation without the need to resort to elaborate hardware or software.

The foregoing, and other, advantages and benefits are realized in a method wherein the diameter of a patient's ascending aorta is provided as an input to the system, either as a known parameter or as measured via A-mode (pulsed) insonification techniques, and the cross-sectional area computed based on a model of the aorta as a right circular cylinder (hence, area as that of a circle); followed by a continuous wave insonification procedure to measure systolic velocity of blood flow through the ascending aorta using Doppler techniques. The variable parameters of area and flow rate provide the basis for determining cardiac output; rationalized to cardiac index as the ratio of cardiac output to the patient's body surface area. The system for implementing that method is comprised of circuitry for generating the pulsed ultrasonic energy, detecting reflected returns and processing same, cooperating with circuitry for producing continuous wave ultrasonic energy and detecting Doppler shifted returns for processing via spectrum analysis; both integrated through a visual or graphic display having a touch-sensitive overlay for direct operator interaction with the system.

A preferred method for the noninvasive measurement of cardiac output is comprised of the steps of pulsedly insonifying the ascending aorta of the patient with repetitive, intermittent ultrasonic energy propagating along a line generally transverse with respect to the axis of the ascending aorta from a position through the cardiac window of the patient to define a first insonification zone enveloping the aortic region; receiving pulses of ultrasonic energy reflected from the anatomical structure within the first insonification zone, including energy reflected from the anterior and posterior walls of the ascending aorta characteristic of the separation thereof along the transverse line of propagation therethrough; discriminating the pulses of received ultrasonic energy to detect the transverse dimension of the ascending aorta between the anterior and posterior walls; developing an aortic diameter signal proportional to and indicative of the transverse separation between walls; computing the cross-sectional area of the ascending aorta in the plane of the transverse line of propagation of pulsed energy, most preferably modeling the aorta throughout the procedure as a right circular cylinder to facilitate the ease of computation--here the area of a circle; thence continuously insonifying the ascending aorta with uninterrupted ultrasonic energy propagating along a line generally axial with respect thereto from a position within the patient's suprasternal notch to define a second insonification zone within the aortic region; receiving Doppler-shifted ultrasonic energy reflected from pulsatile blood flow through the ascending aorta, and principally the red blood cells thereof, frequency shifted from the frequency of the transmitted ultrasonic energy by values characteristic of systolic flow velocity; developing a systolic velocity energy signal as a time domain function proportional to and indicative of the systolic velocity of blood flow; subjecting that systolic velocity energy signal to frequency spectrum analysis, and most preferably to fast Fourier transform analysis, at a predetermined signal sampling rate to develop a velocity component profile signal as a frequency domain function characteristic of the composite of peak frequency components in the sampled signal; integrating the velocity component profile signal over time for each period of pulsatile flow to calculate a systolic velocity integral; computing stroke volume as a combined function of the cross-sectional area of the aorta and the systolic velocity integral for each of n periods; and computing cardiac output as the time-averaged sum of stroke volumes for the n periods. Each stage of the overall protocol is accompanied by a sequence of instructional steps to be followed by the operator, appearing on a cathode ray tube display (CRT) having associated therewith a touch-sensitive overlay facilitating direct operator interaction with the system. The combination of the CRT display and the touch-sensitive overlay provides the operator with a graphic indication of specific optional steps throughout the measurement sequence accompanied by active field areas allowing a given one or more options to be elected simply upon a manual touch of the display. Data is introduced to the system via the display upon a controlled presentation of active areas in the form of an alphanumeric keyboard to facilitate the use of the system by a wide range of individuals having equally diverse backgrounds. Known physiological characteristics of the patient under examination, for example aortic diameter and/or body surface area, may simply be introduced to the system. Otherwise, the system will accept raw data, for example body height and weight to compute the desired characteristic, i.e., body surface area; or the system may be keyed to present a visual display facilitating the measurement of the desired characteristic, e.g., aortic diameter. Within operational limits, the system will insist upon the entry of required data, will limit the entry of certain data to values within statistically anticipated ranges, and will prompt the operator to maximize the accuracy of technique in measuring unknown parameters.

Further along these summary lines of system control for maximized efficiency and reliability, the process of the present invention most preferably includes a self-contained procedure for enhanced data processing, particularly in respect of the frequency spectrum analysis of Doppler-shifted data. This procedure is comprised of the steps of establishing a plurality of signal sampling rates based upon corresponding ranges for statistically anticipated systolic velocities for the patient under examination; selecting high and low threshold values for these separate velocity ranges; monitoring systolic velocity to determine its value within a given one of the ranges as measured with reference to the selected threshold values; and, adjusting the signal sampling rate for data analysis to a predetermined one of the plurality of sampling rates corresponding to the systolic velocity within the threshold values for that range. For example, in the most preferred embodiment of the present invention, three anticipated velocity ranges are established--viz., 0-82 cm/s, 0-165 cm/s, and 0-330 cm/s--with correlative sampling rates for the spectrum analysis. The system initially processes data at the first sampling rate while monitoring the velocity signal. Upon the occurrence of systolic velocities in excess of the high threshold value for the first range, the system automatically adjusts the sampling rate to the second rate for further processing. Monitoring the velocities continues, now with reference to a high and low threshold value within the second range. Systolic velocities lower than the low threshold causes a downward adjustment to the first sampling rate, while velocities in excess of the high threshold for the second range adjusts the sampling rate to the third range. Should the third rate be selected, and subsequent monitoring reveals systolic velocities lower than a low threshold for that range, an adjustment in the sampling rate to the second rate is made. In this manner, processing of data is correlated with the appropriate velocity range, enhancing processing capabilities while simplifying system hardware and firmware.

In those situations where the operator has elected to measure aortic diameter, the visual display presents a graphic image showing returns detected upon reflection of pulsed ultrasonic energy from anatomical structure within the insonification zone (including returns from the anterior and posterior walls thereof), scaled to the propagation distance of the transmitted energy. The graphic display assists in proper positioning and manipulation of the pulse transducer to insure proper placement of the probe to direct the pulsed energy through the patient's cardiac window at an orientation generally transverse with respect to the ascending aorta. This is achieved by employing the highly characteristic returns from the aortic region, and particularly the leaflets of the aortic valve which provide a distinctive echo pattern observable by the operator on the visual display. Manipulation of the probe results in a corresponding alteration in the display to optimize the appearance of the signals and allow improved operator confidence in the proper position of the pulse transducer probe. Once that probe has properly been positioned, the operator may freeze the graphic image and, by way of moveable cursors operated through the touch-sensitive overlay, make a direct, scaled measurement of the aortic diameter. In this mode, the pulse-echo portion of the system serves principally to detect and present echo returns through generally conventional, digital raster scan circuitry for the CRT display while the overlay and coordinated control means serve to develop the aortic diameter signal as the scaled separation between moveable cursors manipulated by the operator.

In a generally like vein, the cardiac output data processed as aforesaid is presented on the visual display during the corresponding continuous wave measurement of systolic velocity. The display thereby provides the operator with direct visual indication of cardiac output and its changing rate from cardiac cycle to cycle; further providing a message to the operator when the signal level is too low, prompting manipulation for more accurate positioning of the transducer probe. In this continuous wave mode, the operator is also supplied with audio information indicative of systolic velocity; developed by mixing the Doppler-shift signal downward, preferably in two mixing stages, to yield an audio signal where frequency is directly proportional to velocity. Thus the operator may rely upon aural and/or visual means of perception during the diagnostic procedure.

The foregoing and other advantages of the present invention will become more apparent, and a fuller appreciation of its construction and methods of operation will be gained, upon an examination of the following detailed description of preferred embodiments, taken in conjunction with the figures of drawing, wherein:

FIG. 1 is a perspective view illustrating an exemplary cardiac output monitoring system embodying features of the present invention, here mounted on casters so as to enhance the mobility of the system;

FIG. 2 is a perspective view here illustrating the cardiac output monitoring system shown in FIG. 1 in use, and wherein a suitable medical practitioner (for example, a doctor, nurse, technician or the like) has positioned a pulse-type ultrasonic transducer at the cardiac window between the second and fourth intercostal spaces of a patient's chest and with the medical practitioner's right index finger positioned to "freeze" the graphic image appearing on the screen of the cardiac output monitoring system, which image is here representative of the internal diameter of the patient's ascending aorta;

FIG. 3 is a perspective view similar to FIG. 2, but here illustrating the patient, medical practitioner and apparatus at that stage in the process wherein the medical practitioner has positioned a continuous wave ultrasonic transducer within the patient's suprasternal notch with the focused energy beam being directed substantially axially along the patient's ascending aorta, and with the medical practitioner's right index finger being positioned to initiate a cardiac output measuring cycle;

FIGS. 4a and 4b, when disposed in adjacent top-to-bottom relation, comprise a block-and-line drawing here illustrating in time sequence the various graphic images that are presented sequentially on the screen of the cathode ray tube used with the present cardiac output monitoring system and, illustrating further, the process that would be employed by the medical practitioner to progressively step through a cardiac output monitoring operation that might vary from patient to patient dependent upon what data is known for each patient and what data must be calculated;

FIGS. 5 through and including 24 are representative illustrations depicting the successive images that might be presented on the face of the cathode ray tube as the medical practitioner progresses through a cardiac output monitoring operation; and, more specifically:

FIG. 5 is a view of the master image, herein referred to as the "Menu", which appears on the screen of the cardiac output monitoring system when the system is first powered up;

FIG. 6 is a view illustrative of the image that appears on the face of the CRT screen when the operator touches the appropriate active area of the screen shown in FIG. 5 so as to enable resetting of date and/or real time information and, illustrating also, the "keyboard" which appears on the face of the CRT screen for enabling resetting of date and/or time;

FIG. 7 is a view of the face of the CRT screen illustrating the Menu which reappears with corrected date and/or time information once the medical practitioner has entered corrected date and/or real time data by touching appropriate areas of the CRT screen shown in FIG. 6 and has touched the active screen area in FIG. 6 labelled "Menu";

FIG. 8 is a drawing illustrating the graphic image that appears on the face of the CRT screen when the operator activates the Menu shown in FIG. 7 for the purpose of entering patient data with the first item of patient data to be entered being an appropriate identification number (hrein "ID number") and with the drawing here illustrating the CRT screen image following entry of the first four digits of the patient's ID number;

FIG. 9 is a drawing illustrating the next successive graphic image appearing on the face of the CRT screen following entry of the patient's ID number, with such graphic image presenting the medical practitioner with an option to enter either a known value for the patient's body surface area or to calculate such value in the event that it is unknown;

FIG. 10 is a view of the graphic image appearing on the CRT screen when the medical practitioner selects the option presented in FIG. 9 for entry of a known value of body surface area;

FIG. 11 is a drawing illustrating the graphic image that appears on the face of the CRT screen and which enables the medical practitioner to calculate the patient's body surface area in the event that the value thereof is initially unknown and that the medical practitioner had selected that option by touching the appropriate active area of the CRT screen shown in FIG. 9;

FIG. 12 is a view illustrating the face of the CRT screen and the image presented thereon following entry of the patient's body surface area either from inputting known data (FIG. 10) or from calculating such value by inputting data representative of the patient's height and weight (FIG. 11), and with FIG. 12 showing the two optional paths that can be selected by the medical practitioner for entering the patient's aortic diameter--i.e., either from known data in the patient's records or through an ultrasonic computational mode;

FIG. 13 is the graphic image which is next presented on the face of the CRT screen in those instances where the medical practitioner touches the active screen area in FIG. 12 by which the option is selected for entry of known values of the patient's aortic diameter;

FIG. 14 is illustrative of the graphic image that appears on the face of the CRT screen in those instances where the medical practioner does not know the patient's aortic diameter and, consequently, activitates the ultrasonic computational mode by touching the appropriate area of the screen depicted in FIG. 12;

FIG. 15 is a graphic image that might appear on the face of the CRT screen had the medical practitioner elected to shift from the "Normal Scale" shown in FIG. 14 to the "Twice Scale" shown in FIG. 15 by touching the face of the screen in FIG. 14 in the rectangular area labeled "Normal Scale" and, with the image shown in FIG. 15 having been frozen by the medical practitioner and with the left and right cursors having been moved so as to provide an accurate measurement of the inside diameter of the patient's aorta;

FIG. 16 is a view of the graphic image next appearing on the face of the CRT screen following entry of either known data (FIG. 13) or computed data as measured in FIG. 15 (or in FIG. 14 had the cursors been readjusted and the "Enter" data area touched), and illustrating particularly the option granted to the medical practitioner either to retain the existing data or to correct such data;

FIG. 17 is a view of the graphic image which would next appear on the face of the CRT screen in those cases where the medical practitioner wished to change all or certain of the data previously entered for a given patient and which would result by touching the area labelled "No" on the screen depicted in FIG. 16;

FIG. 18 is a view of the real time Menu that reappears on the face of the CRT screen in those cases where the medical practitioner touches the area labelled "Yes" on the screen shown in FIG. 16,

FIG. 19 is a view of the graphic image that first appears on the face of the CRT screen following selection of the "Measure CO (cardiac output)" function by touching the appropriate area of the screen depicted in FIG. 18;

FIG. 20 is a view similar to FIG. 19, but here illustrating the image that appears on the face of the CRT screen when the ultrasonic transducer is properly aimed but before accumulation of data representative of at least twelve consecutive beats containing acceptable data values, it being understood that the horizontal lines representative of successive heartbeats are here moving across the face of the screen from right to left with the latest heartbeat detected being at the right hand side of the image;

FIG. 21 is representative of the graphic image that appears on the face of the CRT screen at that point in time when at least twelve consecutive heartbeats have been detected having data within the acceptable range;

FIG. 22 is a portrayal of the graphic image that appears on the face of the CRT screen when the cardiac output monitoring apparatus of the present invention has detected twenty-four consecutive heartbeats each presenting data within acceptable limits;

FIG. 23 is a view similar to FIG. 22, but here illustrating the image appearing on the face of the CRT screen when the medical practitioner determines that optimum data values are being presented and enters those values by touching the active screen area labelled "Measure C0";

FIG. 24 is a view similar to FIG. 23, but here illustrating the CRT screen image that appears whenever the medical practitioner is not properly aiming the ultrasonic transducer axially along the ascending aorta--for example, after the cardiac output measurement has been completed and the transducer has been removed from the patient's suprasternal notch;

FIGS. 25a and 25b, when disposed in adjacent side-by-side relation, comprise a block diagram illustrating the various electronic components that might be employed in a cardiac output monitoring system made in accordance with the present invention;

FIG. 26 is a simplified block diagram here illustrative of the echo-ranging signal path associated with a pulse-type ultrasonic transducer utilized to measure a patient's aortic diameter;

FIG. 27 is a diagrammatic frontal elevational view illustrating generally a human heart and certain of the major arterial vessels, and particularly illustrating the relative positions of the patient's suprasternal notch and ascending aorta with a continuous wave ultrasonic transducer positioned in the suprasternal notch and directing a focused ultrasonic beam substantially axially along the ascending aorta;

FIG. 28 is a highly diagrammatic illustration of the various heart structures as detected by a pulse-type ultrasonic transducer which is directed through the cardiac window on the patient's chest and looking at the aortic root;

FIG. 29 is a highly diagrammatic side view illustrating both proper and improper positioning of a pulse-type transducer with respect to the patient's aorta;

FIG. 30A is a graphic view of the image which might appear on the face of the CRT screen in those instances where a pulse-type ultrasonic transducer is improperly positioned as shown in FIG. 29 so as to intercept the valve leaflets of the heart in the Sinus of Valsalva;

FIG. 30B is a view similar to that shown in FIG. 30A, but here illustrating the image that appears on the face of the CRT screen when the pulse-type transducer is properly positioned as shown in FIG. 29;

FIG. 31 is a highly diagrammatic top or plan view illustrating a patient's aorta looking axially downwardly along the aorta with a pulse-type transducer positioned properly in the solid line position and improperly in the two phantom line positions;

FIG. 32A is a view illustrating the graphic presentation that might appear on the face of a CRT screen in those instances where the pulse-type transducer is improperly positioned in FIG. 31 and the ultrasonic beam is too medial with respect to the axis of the ascending aorta;

FIG. 32B is a view similar to that shown in FIG. 32A, but here illustrating the image that might appear on the CRT screen when the pulse-type transducer is properly positioned in the solid line position shown in FIG. 31 with the beam cutting substantially diametrically through the ascending aorta;

FIG. 32C is a view similar to those shown in FIG. 32A and 32B, but here illustrating the image that might appear on the face of a CRT screen when the pulse-type transducer is improperly positioned as shown in FIG. 31 with the energy beam being directed too lateral with respect to the diameter of the aorta;

FIG. 33 is a simplified block diagram herein illustrating the continuous wave Doppler signal path resulting from echo signals reflected from the red blood cells moving vertically upward through the patient's ascending aorta; and,

FIG. 34 is a highly diagrammatic side elevational view of the upper chest and lower neck portion of a patient in a supine position and illustrating particularly proper placement of the continuous wave ultrasonic transducer within the patient's suprasternal notch with a focused ultrasonic beam being directed substantially axially along the patient's ascending aorta which is here illustrated diagrammatically.

The present invention relates, generally, to methods and means for measuring the cardiac output of a mammalian patient, more especially to noninvasive methods and means for measuring the cardiac output of a human patient, and most particularly to a dual modality insonification technique for ascertaining the cross-sectional area of the patient's ascending aorta and the systolic velocity profile of blood flow therethrough allowing for a calculation of cardiac output, cardiac index, stroke volume and stroke index, amongst other physiological parameters, on a real time, beat-by-beat basis. Accordingly, the present invention will now be described with reference to certain preferred embodiments within the aforementioned contexts; albeit, those skilled in the art will appreciate that such a description is meant to be exemplary only and should not be deemed limitative.

The present invention employs two different ultrasonic measurement modes to gather data used for determining stroke volume and cardiac output of the patient. The first is an echo-ranging mode to measure the diameter of the aorta; which involves placing an echo-transducer over the heart, obtaining a so-called "A-mode" image of the aorta on a visual display, and measuring the aortic diameter from the image. That measurement is then employed by an adaptive algorithm to compute aortic cross-sectional area, modeling the aortic structure as a right circular cylinder throughout the procedure for ease of computation with minimal sacrifice in accuracy. The second mode is a continuous wave mode to insonify the ascending aorta and detect Doppler-shift caused by moving red blood cells within it. This measurement is made by placing a Doppler ultrasound transducer in the suprasternal notch of the patient and aiming the transducer toward the ascending aorta. The returning signals are processed into Doppler-shift signals that are analyzed and converted into discrete frequency components by digital fast Fourier transform ("FFT"). Through Doppler computation, the Doppler-shift frequencies are converted to velocities, in turn employed to calculate a systolic velocity integral ("SVI"). Combining the SVI with aortic cross-sectional area yields the cardiac stroke volume on a beat-by-beat basis; summing stroke volumes over a predetermined number of consecutive beats and then dividing by the time duration thereof yields cardiac output. Cardiac output may then be normalized to cardiac index simply by ratioing cardiac output to the patient's body surface area. The foregoing protocol is carried out by and implemented in the system of the present invention as illustrated more particularly in the figures of drawing.

Turning to those figures, in each of which like parts are identified with like reference characters, FIGS. 1-3 diagrammatically illustrate one representation of a system in accordance with the present invention, designated generally as 100, as might be implemented for use in a hospital or like medical facility by an operator "O" in the measurement of the cardiac output of a patient "P". Viewing initially the system as perceived by the operator O, it includes a visual display designated generally as 102, here shown to be a cathode ray tube ("CRT") display having a screen 104 provided with a touch-sensitive overlay 106, greatly facilitating operator interaction with the system as described more fully hereinbelow. From the operator's point of view, the system 100 is further comprised of a pulse-echo transducer designated generally as 108 for A-mode measurement and a continuous wave transducer designated generally as 110 for continuous wave insonification in the examination of systolic velocity. An audio headset designated generally as 112 is included as a part of the system for aural perception of Doppler-shifted signals during that latter, continuous wave examination. A foot switch designated generally as 114 is furnished as an auxiliary means of operator interaction in association with the touch-sensitive overlay 106, allowing the operator to perform certain steps either by touching an active field area on the overlay 106 or depressing the foot switch 114, as may be more convenient depending upon the task at hand. A printer 115, such as a thermal printer, is provided to obtain "hard copy" of the test results for posting in the patient's file. The system 100 is shown to be borne upon a cart 116, for portable mobility about, e.g., a hospital.

A particularly significant advantage of the present invention resides in its use of the operator-interactive display 102 along with the presentation of step-by-step instructional messages thereon to lead the operator through the sequence necessary for determining cardiac output. In this way, operators having no specific background as respects medical instrumentation, or indeed even anatomical structure, may perform the cardiac output measurement in accordance with the instant method without sacrificing either the accuracy of the test or the reliability of its results. This aspect of the present invention is best considered with reference to FIGS. 4-24 which illustrate representative sequences of steps the operator O might perform in conducting the measurement of cardiac output of patient P. FIG. 4 shows the sequence in a flow-chart manner while FIGS. 5-24 show, seriatim, a typical series of graphic images appearing on the display 102 leading operator O through the protocol.

When the system 100 is energized, microprocessor control for the CRT display initially presents a system function menu as shown in FIG. 5. This "MENU" image is one which is displayed periodically throughout the sequence shown in FIG. 4 (as amplified by the individual display illustrations in FIGS. 5-24) or which can be selected at virtually any step throughout that sequence. In this exemplary showing, the menu display appearing on CRT screen 104 includes a first instructional step "SELECT FUNCTION" within a field 118, followed by three options which may be elected by the operator O each bearing an appropriate legend indicative of the choice; viz., "ENTER PATIENT DATA" in a field 120, "MEASURE CARDIAC OUTPUT" in a field 122 and "SET DATA & TIME" in a field 124, prompting the operator to select one or another function. Appearing proximate each legend is a touch-sensitive active area, identified respectively as 126, 128, and 130. As is generally conventional with such touch-sensitive overlays as the overlay 106, a manual touch at one of the sensitive or active areas will be position discriminated and a signal indicative of the area touched will be generated by the overlay and/or its associated interface with the system and raster scan circuitry for the device. As the overlay 106, per se, forms no independent part of the present invention, suffice it to say that the signal may be either an analog or digital signal recognized by, e.g., microprocessor control means for the CRT; in any of which events the important point being the ability for the system to identify the particular option selected by the operator as a function of the position on the screen the operator touches. Irrespective of such considerations, the menu of display of FIG. 5 further includes a fixed field 132 bearing the legend "ID" and a field region 134 for accommodating a patient ID number as described below. Another field 136 shown adjacent the region 134 is provided for the introduction of real time and data information; date information being accommodated within a field region 138 and time information within a field region 140. These provisions for patient identification and real time and date information are preferably included for correlation of the diagnostic measurement to be performed with the patient's chart maintained by the medical facility; as well, in instances where the system provides hard copy output (e.g., printer 115) to give an indication of the timing of each step within the sequence as an aid to diagnostic evaluation by the medical practitioner. Under most circumstances, therefore, the operator O will first enter appropriate patient identification and real time and date data before proceeding further; which option is initiated by the operator's manual touch at touch sensitive area 130.

Electing the option to "SET DATA & TIME" presents the visual display shown in FIG. 6. As noted generally above, the system preferably affords the operator O with the ability to gain access to the menu display of FIG. 5 at any intermediate stage of the sequence outlined in FIG. 4. Accordingly, as shown in FIG. 6, the display includes a touch-sensitive area 142 bearing the legend "MENU"; the operator's touch thereat causing the display to resume the form shown in FIG. 5. Having elected the option of setting data and time, the display of FIG. 6 continues with a first field 144 for the data, including a field region 146 bearing the legend "DATE" followed by three field regions 148, 150 and 152 corresponding to two-digit regions for entry of the appropriate month ("MO"), day ("DA") and year ("YR"). A moveable cursor 154 is shown in FIG. 6 to be assuming a position intermediate the data legend in field 146 and the field region 148 for entry of the month, the operation of which is considered below. Beneath the date field is a corresponding time field designated generally 156 which includes a field region 158 bearing the legend "TIME" followed by two two-digit field regions 160 and 162 for entry of the appropriate hour and minute information, respectively. As indicated in a field region 164 subjacent the time region 156, the time to be entered in the fields 160 and 162 is based on a 24 hour clock. As is also evident from FIG. 6, whatever time and date information which initially appears respectively within the fields 138 and 140 also appears in the corresponding field 144 and 156; which information may or may not be accurate when the system is first energized, therefore requiring in most cases the entry of proper date and time information. This is facilitated by a touch-sensitive active region designated generally as 166, shown here to be in a type of "keyboard" form including the digits "1" through "0" in separate touch-sensitive field regions, each identified as 168. The keyboard region further includes a touch-sensitive area 170 bearing an arrow legend for backspacing the cursor 154; and the display might further include a similar touch-sensitive area for advancing the cursor 154 should that be necessary or desirable. The display provides another touch-sensitive region 172 bearing the legend "ENTER" for introduction of appropriate date and time information to the system.

As is now easily envisioned with reference to FIG. 6, the system when first energized has presented within the date region 138 the numerical designation "10-31-82" to indicate Oct. 31, 1982 and in the time region 140 the numerical designation "09:25" to indicate 9:25 a.m. For purposes of the present discussion, let it be assumed that the data entry is correct but that the correct time is 8:26 a.m. vice 9:25 a.m., requiring correction before proceeding further. Where the display includes a cursor moving area for advancement of cursor 154, corresponding to the backspace provision of touch-sensitive area 170, the cursor 154 might simply be cycled through the date field 144 to assume a position for entry of the correct time within time field 156. Otherwise, as shown in FIG. 6, the operator will simply reintroduce the same date information by sequentially depressing the touch-sensitive areas 168 in the sequence 1-0-3-1-8-2 as the cursor 154 sequences automatically among the field regions 148, 150 and 152 corresponding to the month, day and year of the diagnostic test. Once the operator has sequenced through the date, the cursor 154 will then appear in the time field 156 for similar entry of the correct time. In this instance, the operator will touch the active areas 168 in the sequence 0-8-2-6 to enter the correct time of 8:26 a.m. Where the information has been added correctly, touching the active area 172 will enter the data to the system, whereupon the correct date and time will appear in the field regions 138 and 140 respectively. In the event the operator inadvertently touches a wrong one of the active regions 168, the active region 170 may be used to backspace the cursor 154 so the correct information can be entered to the system.

Once the operator has introduced the correct date and time information, and touches the active enter region 172, the system automatically reverts to the menu display, now shown in FIG. 7. FIG. 7 is identical to the menu display of FIG. 5, save the fact that the correct date and time now appear within the respective regions 138 and 140. At this juncture, the operator will normally enter patient identification data and, for this purpose, apply a touch at sensitive area 126 proximate the legend "ENTER PATIENT DATA" of field 120; whereupon the display of FIG. 8 will appear under control of the microprocessor governing displays on the CRT 104. Again, an active region 142 allowing the operator to revert to the menu display is provided and the real time and date information in fields 138 and 140 appears in this display. The display additionally presents a field 174 bearing the legend "PATIENT ID NUMBER" followed by a field region 176 for introduction of this identification upon sequential movement of the cursor 154 as the operator applies a touch to the active keyboard region 166. In this instance, the keyboard field 166 further includes an active area for a hyphen ("-") and another for a decimal point (".") in addition to the digits "1" through "0". For the sake of illustration, let it be assumed that the appropriate patient identification number is "789-01-2345"; that identification number being entered in the same manner as noted above with reference to the introduction of time and date information; viz., sequentially touching the active field regions 168 in the order 7-8-9---0-1---2-3-4-5, as the cursor 154 advances from place to place within the field region 176. Should the operator inadvertently touch an inappropriate one of the active field areas 168, the cursor may be backspaced by touching the area 170 and correct information then introduced. Once that correct information has been provided within the field region 176, the operator may then touch the active field region 172 to enter the patient identification number within the field region 134. Doing so then causes the display to present the image shown in FIG. 9.

FIG. 9 prompts the operator to the next step in the protocol for conducting the cardiac output measurement in accordance with the present invention. The display, as with the foregoing, includes an active area 142 in order to permit the operator to revert to the menu display. A new region 178 is now included on the display showing the patient's ID number as entered in accordance with the immediately foregoing description. The display likewise includes the ID information within the fields 132 and 134, and also the real time and date data, shown here to be two minutes later than the initiation of the sequence (a relatively realistic time difference). The display of FIG. 9 calls upon the operator to enter the body surface area of the patient, as indicated by the legend "BODY SURFACE AREA?" within a field 180. In this case, the operator is presented with two options for introduction of this surface area data, the first indicated by the legend "CALCULATE FROM HT/WT" appearing in region 182 and the option to "ENTER KNOWN VALUE" appearing as a legend in region 184. Proximate each of the legends within fields 182 and 184 is a touch-sensitive area 186 and 188, respectively, for electing one or the other of these choices. The entry of body surface area at this stage of the protocol is preferred since certain subsequent computations will be made to provide indices based upon the normalizing factor of surface area and, without this information having previously been entered, a "division by zero" will result, obviously yielding inaccurate data.

Where the patient's body surface area is known, for example from previous computations as might appear in the patient's file, the operator will then touch the active area 188 to present the image shown in FIG. 10 on the CRT screen 104. Under these circumstances, in addition to the active and non-active field regions shown in the figure and previously described above, the display includes a legend "BODY SURFACE AREA" in a field designated generally as 190 having a field region 192 for the introduction of the patient's surface area, here shown to be in square meters. The data is introduced within the field region 192 by appropriate touching of the keyboard areas 168 as aforesaid with the moveable cursor 154 indicating the appropriate digit position during the entry procedure. The touch-sensitive overlay 106 further provides a signal to a microprocessor wherein the body surface area is stored for subsequent indexing calculations as described below.

Should the patient's body surface area be unavailable, the operator will apply a touch to the active field region 186 to indicate the need for calculation of that data from the patient's height and weight. In that event, the display to be presented is the one shown in FIG. 11. This display now includes a legend "PATIENT HEIGHT" within a field designated generally as 194 which includes a field region 196 for the introduction of the patient's height in centimeters upon touching the active field regions 168 of the keyboard 166 in the same manner as aforesaid. In this instance, the field 194 further includes a touch-sensitive field area 198 bearing the legend "IN" should the operator desire to introduce the patient's height in inches as opposed to centimeters. If that option is elected, the operator will first touch the active region 198 prior to the keyboard region 166 and the system will recognize the data to be inputted in these English (vice metric) units. The display continues with the legend "PATIENT WEIGHT" within a field designated generally as 200. Like the field 194, the field 200 includes a region 202 within which the operator may enter the patient's weight in kilograms by touching the active areas 168 in the appropriate sequence. And, once again the field 200 includes a touch-sensitive area 204 bearing the legend "LB" to allow the operator a choice for entering the patient's weight in pounds as opposed to the metric units. Accordingly, where the operator desires that ability, the region 204 will be touched prior to introduction of the numerical data via keyboard region 166 and the system will recognize the data to be entered in English as opposed to metric units. Preferably, the system 100 will accept mixed units; e.g., height in metric and weight in English units, or vice versa. The adaptive algorithms required for this conversion are simple and straightforward demanding very little in terms of cost or size while the benefits of providing such an ability are significant in terms of broadening the range of individuals who may operate the system 100 efficiently. Irrespective of that consideration, the height and weight data entered in the fields 196 and 202, respectively, are developed as corresponding signals via the touch-sensitive overlay 106 inputted to, e.g., a microprocessor having stored therein an adaptive algorithm for the surface area computation, a random access memory (RAM) having stored therein appropriate numerical values for generating a signal indicative of body surface area (e.g., via a nomograph), or an equivalent device.

With the entry of body surface area to the system 100, the same is now keyed to begin a sequence for determining the aortic diameter of the patent P. Accordingly, upon the operator's touch at active field 172 in the display shown in FIG. 11, the display of FIG. 12 now appears calling for aortic diameter data. The display of FIG. 12 provides (in addition to the menu field, patient identification fields and real time and date fields as noted above) a first field 206 bearing the legend "BODY SURFACE AREA" followed by a field region 208 wherein the patient's body surface area (either as calculated or as entered from records) appears. The display continues with a legend "AORTIC DIAMETER" in a field 210 indicating to the operator that this is the next stage in the sequence. The display calls upon the operator to elect the mode for entering this aortic diameter data, presenting a general instructional legend "DIAMETER DATA SOURCES?" within a field 212 followed by a first touch-sensitive area 214 bearing the legend "PATIENT RECORDS" and a second touch-sensitive area 216 bearing the legend "A-MODE". In the event the patient's aortic diameter has previously been measured and appears as information within the patient's chart, the operator will touch the active area 214 to present the display of FIG. 13. The image displayed in FIG. 13 includes a region 218 adjacent the field 210 ("AORTIC DIAMETER") for introduction of the appropriate diameter in millimeters via the keyboard array of field 166 in the same manner described above. Should the patient's aortic diameter require measurement, the operator would then touch the active area 216 to convert the system 100 into a measurement mode and present the image shown in FIG. 14. The system 100 also now activates the A-mode circuitry destined for a pulse-echo measurement of the patient's aortic region with an eye toward making the diameter measurement; an operation best envisioned with further reference to FIGS. 2 and 28-32.

The A-mode measurement of aortic diameter is one based on a pulse-echo amplitude study where ultrasonic energy is intermittently applied to the cardiac region and the interfacial, reflected energy from the anatomical structure therein is utilized to determine aortic dimensions. A number of threshold principles warrant some brief discussion at this stage. Ultrasound does not travel well through air or tissue containing any significant quantity of air. Consequently, the lung tissue which almost completely surrounds the heart intermediate it and the chest wall prevents unlimited ultrasonic access to the cardiac region. However, the left lung margin beginning at about the second intercostal space and continuing to about the fourth or fifth intercostal space, follows a lateral course away from the midline placing the heart very close to the chest wall in a region commonly termed the cardiac window. The heart, and specifically the aortic anatomy thereof, may be examined using ultrasonic imaging by transmission and reception of ultrasound through the cardiac window; as envisioned generally with reference to the illustration of FIG. 2.

With the pulse-echo transducer probe 108 positioned as shown in FIG. 2 in order to transmit pulsed ultrasonic energy through the cardiac window, and preferably from about the third or fourth intercostal space, the structure giving rise to echo returns is shown generally in FIG. 28. The aortic wall signals reflected from the root region will appear as a pair of parallel moving signals; the structural sequence being the right ventricle outflow tract 220, the anterior aortic wall 222, the aortic valves 224 (leaflets), the posterior aortic wall 226, the left atrium 228, and the posterior left atrial wall 230. Because the leaflets of the aortic valves are moderately reflective, they provide (along with the characteristic signal returns of the contracting heart) a good reference point for the initiation of aortic diameter measurement across the ascending aortic region. As the heart contracts, blood moving past the aortic valves during ejection forces the leaflets toward the aortic walls, leaving the leaflets generally perpendicular to the beam axis 232 and quite visible on the A-mode trace a good portion of the time. When the aortic valve closes the leaflets touch in the center of the aorta to form a centrally positioned, single echo signal. Consequently, the leaflets may serve as a landmark between the aortic root region and the ascending aorta as best considered with reference to FIGS. 29 and 30.

FIG. 29 shows the pulse-echo transducer probe 108 in two different positions vis-a-vis the aorta; through the root region in the lower position and through the ascending aortic region in the upper position. FIG. 30 shows two pulse-echo signal traces which might appear on the CRT screen 104 corresponding respectively to the lower position of probe 108 in FIG. 29 (FIG. 30A) and the upper position (FIG. 30B). With the objective of obtaining an accurate measurement of the transverse diameter dimension of the ascending aorta, proper positioning of the probe 108 to present the beam axis 232 generally transverse to the ascending aorta is of considerable importance. Initial location of the ascending aorta is facilitated by the characteristic pulse-echo returns from the leaflets 224, allowing the operator O first to locate the probe to find the aorta and thence move it to the region of the ascending aorta. More specifically, the probe is positioned as shown generally in FIG. 2 to present the beam axis 232 through the cardiac window and the probe is manipulated until a signal trace like that of FIG. 30A appears. Focusing simply upon the aortic region, signal returns 234 and 236 will appear corresponding to the reflected returns from the anterior and posterior aortic walls, respectively. With contractions of the heart and concomitant opening and closing of the aortic valve, an intermediate signal trace 238 will likewise appear during ejection periods. Thus, the trace of FIG. 30A is a highly characteristic one, with the parallel moving traces corresponding to the aortic walls and the intermediate trace of varying amplitude corresponding to the leaflets of the aortic valve. Having thus located the aortic root, the operator may simply then move the probe slightly upward to the upper position shown in FIG. 29 until the intermediate echo 238 corresponding to the leaflets no longer appears, indicating a beam axis position now in the ascending aortic region having moved from the root region. With the beam now traversing the ascending aorta, it is next important to take into account proper lateral and/or medial placement of the transducer probe 108 in order to achieve accurate measurement of the aortic diameter. Proper probe positioning will be evident by observation of the geometry of echo returns appearing on the CRT screen 104, as best viewed in FIGS. 31 and 32. FIG. 31 shows the beam axis 232 in three orientations vis-a-vis the ascending aorta, identified A, B and C, where B is the optimal orientation while A is too medial and C is too lateral to obtain an accurate diameter measurement. Each of those three orientations correlates specifically with the signal traces illustrated in FIGS. 32A, 32B and 32C, respectively. With the beam axis 232 shifted toward a medial disposition relative to the aorta, the signal trace on the CRT display will commence with the echo 234 for the anterior wall, followed by an echo 240 corresponding to the right cusp, and a signal 242 corresponding to the echo returned by the posterior cusp. The echo returns shown are low in amplitude and rather broad in extent since the ultrasound beam now includes these additional groups of echo sources. Contrariwise, the signal trace of FIG. 32B shows the echo returns 234 and 236 corresponding respectively to the anterior and posterior aortic walls. The signals are relatively crisp and, when the distance separating the two is minimized on the CRT image, the operator may regard the positioning to be optimized. Proceeding to FIG. 32C, the trace again is underdeveloped, reflecting beam positioning too lateral as respects the aorta. The trace is initiated by a low-level, nearly imperceptible echo 234 for the anterior wall, a signal return 240 from the right cusp and a broadening of the signal return 236 for the posterior wall due to a contribution from the posterior cusp as indicated by signal 242.

With the foregoing as background, it is now evident that proper positioning of the pulse-echo transducer probe 108 is greatly facilitated by the image displayed on CRT 104, whereby operator familiarization with the system occurs easily and rapidly. Once the operator identifies the cardiac window and applies the transducer probe as illustrated diagrammatically in FIG. 2, a brief search will present a signal trace like that shown in FIG. 30A indicative of the aortic root region. The probe is then positioned slightly upwardly to eliminate the signal trace 238 representative of the valve leaflets, whereby the ascending aortic region is properly located. The probe is then manipulated slightly from medial to lateral orientation (or vice versa) and the varying signal patterns shown in FIG. 32 observed with an eye toward obtaining that of FIG. 32B. When the intermediate low level returns from, e.g., the cusp structures have been depressed while the amplitude of the signal returns from the anterior and posterior walls have been maximized with the intervening dimension between returns minimized, the operator is then assured of proper placement for an accurate measurement of the aortic diameter.

Returning to a specific consideration of the precise visual display presented for the operator O, touching the active area 216 of the display image shown in FIG. 12 creates the image display shown in FIG. 14. In addition to the active area 142 allowing a return to the menu display, the patient identification and real time and date data appear as indicated above. The display now further includes a legend "AORTA MEASUREMENT" in a field 244 to indicate to the operator that this is the next step in the protocol. Beneath that legend is a graph field region designated generally 246 which includes a signal trace like those discussed above with reference, e.g., to FIG. 32, including therefore pulse echo returns 234 and 236 created by the anterior and posterior walls of the aorta when the probe 108 is properly positioned. The trace 246 further includes a distance scale 248 placed on the screen 104 by the control circuitry driving the CRT raster scan. The scale bears numerical designations, in this case corresponding to the distance from the head of the transducer probe 108 scaled in centimeters. First and second moveable cursors 250 and 252 are also created on the image display of FIG. 14 to assist in the diameter measurement. The first or left cursor 250 is controlled by active field regions 254 and 256 bearing arrow legends indicating the direction of movement upon a manual touch by the operator, while the second or right cursor 252 is positioned via touch-sensitive areas 258 and 260 in like manner. These active areas on the touch-sensitive overlay are indicated by the legends "LEFT" and "RIGHT" "CURSORS" appearing in a field 262 in order to provide convenient operator interaction with the system.

The objective at this stage of the protocol is to obtain the optimized signal trace 246 noted in detail above and thence position the left and right cursors over the signal traces 234 and 236 corresponding to the anterior and posterior aortic walls. With the cursors thus positioned, the measurement of aortic diameter may be made directly from the visual display. However, as also noted above, the signal trace is a changing one as the heart goes through its rhythmic cycling. The measurement of diameter is, therefore, facilitated by freezing the image appearing on CRT screen 104 when the same has been optimized and the operator is confident of its accuracy. The image may be frozen by the operator's manual touch at an active area 264 bearing the legend "FREEZE" or by depressing the foot switch 114 should that be more convenient. In either event, that will stabilize the image allowing accurate positioning of the cursors 250 and 254. The ease of that measurement is further improved by an upward scaling capability, elected by the operator's manual touch at active field region 266 bearing the legend "NORMAL SCALE" which will convert the display from that shown in FIG. 14 to the one illustrated in FIG. 15. In the latter figure, the scale is now doubled in size allowing greater discrimination in the placement of the cursors. As best viewed in FIG. 15, the left cursor 250 is positioned toward the trailing edge of the signal trace 234 while the right cursor 252 is positioned near the leading edge of the trace 236 in order to obtain a measurement of the internal dimension of the aorta. Empirically, it has been determined that the system 100 provides the most accurate results where the left cursor is positioned near the peak of the echo return 234, but on the trailing edge side thereof while the right cursor 252 is positioned midway along the leading edge of the echo 236; albeit this precise positioning of cursors to achieve accurate aortic diameter measurement may vary with variations in system design. In any event, the display includes a field 268 which shows the scaled, dimensional separation between left and right cursors in millimeters and, accordingly, aortic diameter when the cursors are properly located. As shown in FIG. 14, the cursors 250 and 252 are outside the echo returns corresponding to the aorta, the field region 268 indicating a scaled distance of 55.9 millimeters. However, with the cursors then properly positioned as shown in FIG. 15, the aortic diameter may be read directly, indicated there to be 35.0 millimeters. With the display in that configuration, the operator then touches the active area 172 bearing the legend "ENTER" to introduce the measured diameter to a storage register in the system.

The system 100 preferably calculates cardiac output as a function of the cross-sectional area of the ascending aorta, thus requiring an area determination from either entered, known data or the measured diameter; although it is equally possible to employ the diameter data directly (e.g., by appropriate scaling in subsequent computation of certain output parameters) should that approach prove more desirable. Following the more preferred approach of computing area, the same is greatly facilitated by modeling the region of the ascending aorta as circular, an assumption which introduces only very slight deviation and one which is tolerable at the savings realized thereby. With that assumption, area can simply be computed by a microprocessor algorithm using standard mensuration formula or, optionally, a read only memory ("ROM") having pre-calculated area measurements corresponding to statistically anticipated diameters might be employed. Regardless, the area data is then stored in a register for subsequent use in the computation of cardiac output.

Once the aortic diameter measurement has been completed, indicated to the system upon the operator's touch at region 172, the system then presents the display shown in FIG. 16. That display recapitulates the extant data respecting the patient P, including the patient's ID number in the field 178, the patient's body surface area in the field 206 and now the patient's aortic diameter in a field 270. The image then continues with the instructional question to the operator "RETAIN EXISTING DATA?" in a field 272 prompting the operator to reconsider the potential validity of the information previously entered. Touch-sensitive areas 274 and 276 bearing, respectively, the legends "YES" and "NO" allow the operator to elect the retention of existing data or reject it upon a touch at the appropriate field. Should the operator decline to retain the entered data, touching the sensitive area 276 will present the image shown in FIG. 17.

The image of FIG. 17 begins with the instructional legend "TOUCH TO REVISE" in a field 278. The display then presents four touch-sensitive areas, an area 280 bearing the legend "PATIENT ID NUMBER" followed by the number previously entered, an area 282 bearing the legend "BODY SURFACE AREA" followed by the surface area either formerly entered or calculated, a touch-sensitive area 284 bearing the legend "AORTIC DIAMETER" followed by the diameter previously entered or measured, and a last touch-sensitive area 286 bearing the legend "ALL PATIENT DATA". Thus, the operator is presented with the option to make a selective choice in deleting certain patient data and either re-entering or re-computing it as might be required or, indeed, revising all patient data upon the application of a manual touch to area 286 returning the system to the configuration described above beginning with FIG. 8.

On the other hand, the operator may have complete confidence in the data theretofore entered within the system and, accordingly, touch the active area 274 in the display appearing in FIG. 16. That will return the system to a mode presenting the menu, as shown in FIG. 18. That display is generally similar to the menu displays described above with reference to FIGS. 5 and 7 and warrants no further detailed discussion; save to say that the next procedure at this stage of the sequence is the measurement of cardiac output prompted by the operator's manual touch at active area 128. The signal developed through the touch-sensitive overlay causes the image of FIG. 19 to appear on the CRT 104 and now keys the system to the continuous wave mode for the measurement of blood flow parameters through the patient's ascending aorta. This aspect of the invention is best understood with initial, brief digression to FIGS. 3, 27 and 34.

While aortic diameter is measured via the A-mode or pulse-echo insonification technique, blood flow through the aorta is measured in accordance with the instant invention by continuous wave insonification. The approach transmits an uninterrupted wave of ultrasound to insonify the region of the ascending aorta along a line generally axial with respect thereto from a position within the patient's suprasternal notch. Reflections from static structure will be returned and detected at the same frequency as the transmitted wave, while moving structure will return energy shifted away from the transmitter frequency by a Doppler-shift frequency proportional to velocity and its directional sense. Accordingly, during each ejection period from the heart, pulsatile blood flow will cause echoes returning therefrom to be frequency-shifted as an indication of velocity; the red blood cells being principally responsible for the reflected return. Detecting the frequency shift and normalizing same yields a velocity signal following the systolic velocity of blood through the ascending aorta during active ejection periods. Subjecting that frequency signal to a spectrum analysis to account for the various frequency contributions and thence integrating the signal over time for the systolic contraction period yields a systolic velocity integral. The product of the systolic velocity integral and the cross-sectional area of the ascending aorta then gives a measure of stroke volume per cycle which, when summed over a predetermined number of cardiac cycles and time averaged with respect thereto, yields cardiac output. While both stroke volume and cardiac output are important individual physiological parameters for the patent under examination, the same are usually normalized based on the patient's body surface to yield, respectively, cardiac index and stroke index. The system 100 leads the operator through these various steps and provides a visual display during the sequence assisting in the efficiency of the measurement technique.

The first step in the measurement of cardiac output is the proper positioning of the continuous wave transducer probe 110 within the patient's suprasternal notch as shown diagrammatically in FIG. 3. The transducer probe 110 includes discrete transmitting and receiving crystals 282 and 284, respectively, positioned within the probe in a manner providing a focal point 286 within a focal zone 288. The configuration is made with an eye toward providing a focal point within the statistically anticipated distance of the ascending aorta from the patient's suprasternal notch, with a focal zone accounting for the physical vagaries among patients to be examined by the system. The ascending aorta lies almost completely behind the sternum, identified generally as 290 in FIG. 34, with a portion of the ascending aorta pointed toward the suprasternal notch identified generally as 292. Placing the transducer probe 110 within the suprasternal notch and aiming slightly anterior within the saggital plane, will generally place the beam in the ascending aorta as shown in FIG. 34. The probe is then manipulated in order to maximize the signal returns appearing on the CRT display, to which attention is now directed, beginning with FIG. 19.

The display image of FIG. 19 provides a legend "CARDIAC OUTPUT" within a field 294 to provide an indication to the operator that this is the stage within the protocol next to be performed. A grid 296 appears in the form of an X-Y coordinate system where the X axis represents time from right to left and the Y axis signal amplitude. A field 298 bears the legend "SIGNAL LEVEL" and includes a field region 300 immediately below wherein a numerical value for the signal level will appear during later stages of the sequence. Likewise, a field 302 bearing the legend "HEART RATE" appears on the display shown in FIG. 19 and includes a field region 304 immediately below where a numerical indication of heart rate will later appear as the sequence progresses. With the probe 110 positioned in the suprasternal notch 292 as aforesaid, the display will begin to present a series of discrete signals 306 (in bar graph form) corresponding to discrete cardiac cycles, the amplitude of which is an indication of stroke volume. Following a predetermined number of cardiac cycles allowing time for positioning of the probe 110, a numerical value appears within the field 300 providing the operator with a direct indication of the existing signal level detected upon systolic ejection of blood. In the preferred embodiment shown, the graphical representation shown in FIG. 20 corresponds to the magnitude of the systolic velocity integral calculations noted briefly above while the signal strength is shown as a simple numeral. Optionally, but preferably, the signals applied to the display are converted first to signals within the audio frequency range, thereby allowing the operator to use either the headset 112 or a speaker to provide both aural and visual observation of the results as the sequence progresses. By combining the use of these Doppler sounds, the SVI values, and visual transducer positioning, the Doppler ultrasound beam can be consistently and correctly located for the cardiac output calculation.

After the initial period of positioning, during which the operator maximizes the signal returns shown on the display of FIG. 20, the system will recognize the rhythmic contraction of the heart and thereby monitor heart rate as well as signal level. In the preferred embodiment shown, the system requires at least twelve contraction periods as indicated generally by the vertical phantom line appearing in FIG. 21. After whatever number of cycles required by the system for statistical significance has been reached (here twelve) a legend "MEASURE CO" appears in an active field region 308 indicating to the operator that a cardiac measurement may be taken. In this exemplary embodiment, the cardiac output is calculated on the basis of the twelve most recent contraction periods and, accordingly, those signals appearing to the right of the phantom line in these figures of drawing. Thus, while the operator may initiate a measurement calculation with the display in the configuration shown in FIG. 21, a greater number of cycles may be allowed to accumulate in order for the operator to observe any erratic data either visually by observing the display or aurally through the headset or speaker. Thus, let it be assumed that the operator allows the display to achieve the configuration of FIG. 22 before applying a touch at the sensitive area 308. Once that is done, the computation of cardiac parameters is made by the system firmware and the values displayed as shown in FIG. 23 within a field designated generally as 310. The field includes the legends "CO" for cardiac output, "CI" for cardiac index, "SV" for stroke volume, "SI" for stroke index, and "HR" for heart rate, beneath each of which specific fields appears the calculated or measured parameter. Optionally, but preferably, this image is transferred to hard copy via, e.g., the printer 115 associated with the system in order to provide a permanent record for the patient's chart.

In the event the operator moves the transducer probe 110 out of proper position with respect to the region of the ascending aorta, and thereby loses signal returns from systolic flow, the image of FIG. 24 is one which will appear. As can be seen with reference thereto, approximately eight cardiac cycles have passed without a return, whereupon the system presents the legend "LOW SIGNAL . . . IMPROVE AIM" in a field 312; prompting the operator to reposition the probe in order to obtain valid, maximized data.

As is now evident from the foregoing description of the methodological sequence comprising the process of the present invention, the same offers simple yet highly efficient and reliable cardiac output measurement. The system is operable by a wide range of operators having perhaps widely varying technical and/or medical skills; but, due to the step-wise instructions provided on the visual display in association with the operator-interactive touch-sensitive overlay therefor, the operator is instructed explicitly at each separate step in the process and offered the ability to make elections of various options presented simply upon a manual touch of the overlay or, depending upon the stage, depression of a foot switch. The display, furthermore, assists the operator in proper, accurate positioning of the transducer probes employed, enhancing ease of operation and reliability of result. The system circuitry responsible for the implementation of this process is likewise designed with the same objectives in mind.

Turning to FIG. 25, a schematic block diagram of the principal components of the system is shown. Briefly summarizing the circuitry, the same is constituted of an A-mode section for generating and processing the pulsed ultrasonic energy used in an aortic measurement, a continuous wave section responsible for generating and processing the uninterrupted insonification of the ascending aorta for the measurement of systolic blood flow therethrough, an interface section for associating the various peripheral devices such as the CRT display, touch screen, switch and thermal printer, and processors both for controlling the operative interaction among or between the various sections and for managing the calculations required to obtain the ultimate cardiac output parameters.

Looking first to the central, digital section of the system circuitry, the same is comprised of a system or first central processor unit ("CPU") 320, a continuous wave or second central processor unit ("CW-CPU") 322, an interprocessor bus control circuit 324 and an input/output control circuit ("I/O") 326. The digital section of the system provides the necessary timing and control functions associated with its operation, including data management and transfer, along with necessary interface functions allowing for operator interaction therewith. The two central processor units are essentially identical in structure, with the CW-CPU 322 dedicated to processing the data in respect of the continuous wave measurement of systolic velocity in order to compute the systolic velocity integral for each heart beat; the system CPU 320 being employed for all remaining timing and control functions along with control and display interface with the operator. Each of these functions is under control of firmware program within the CPU devices with EPROMS. Briefly stated, the central processors are conventional devices including a microprocessor, 32K of dynamic random access memory ("RAM") a dynamic RAM control element, read only memories ("ROM") with programming capacity to 64K, and standard miscellaneous timing and control logic. Suitable devices are available in the marketplace meeting these criteria and may be selected by the system designer to meet the demands of the system; the most preferred being a microprocessor known in the trade as a "Z8000" marketed by, e.g., Zilog or Advanced Micro Devices, or its equivalent. The interprocessor bus control 324 provides means for interfacing between the CPU's themselves and the other principal sections of the system.

The interprocessor bus control 324 is likewise of standard commercial design, having timing and control logic, address latches, latches for control discretes and a clock circuit used as a real time clock reference. Although data transfer may be intiated by external interrupts to the central processing units directly, all data transfer activities are more preferably managed through the interprocessor bus control under appropriate firmware control. The timing and control central logic of the interprocessor bus control provides means for converting the data transfer protocol requirements of the, e.g., microprocessors to the protocol requirements of each of the data processing sections within the system.

The input/output control circuitry 326, as is conventional, provides interfacing for the peripheral elements of the system, mainly the printer, foot switch, touch-sensitive overlay, and the video monitor. Again a conventional device, the I/O assembly contains an array of circuitry to provide for address decoding, dynamic RAM data storage, video display drivers, data latches, signal conditioners, analog to digital converters ("ADC") and interrupt logic for the keyboard and video interfaces. A monolithic display chip is preferably included to develop and present the CRT display under firmware control. Through the means of video address decoding and video interrupt logic, the dynamic RAM of the I/O control circuitry can be filled under control from the central processing unit as necessary to present each of the various display images discussed in detail above. Operator interaction with these images is thereby implemented through the touch-sensitive overlay which, in the preferred embodiment shown herein, provides analog signals from a transparent switch memory multiplexed in the ADC for conversion and transfer to the system CPU for data processing. A local microprocessor is included, programmed to convert the keyboard actuation into appropriate X-Y coordinates for the CPU. The line printer interfacing incorporates latches and receivers in order to convert the data transfer rate requirements of the line printer into a compatible interface with the system CPU as well.

Under the control of the digital section, the A-mode section, designated generally as 330, provides the means for implementing aortic diameter measurement. The A-mode circuitry is comprised of a transmitter 332, a receiver 334, pulse control circuitry 336, a video memory 338 and a video generator 340.

The transmitter 332, shown in this embodiment to be shared for joint use in the continuous wave mode as described below, serves to power the pulse transducer 108. In this pulse-echo mode of operation, the transmitter delivers a series of timed pulses to the transducer crystal, exciting the same into bursts of repetitive, intermittent ultrasound applied to the patient's ascending aorta, following which the transmitter is silent allowing returns to be processed in the remaining circuitry. The transmitter 332 is preferably comprised of a power transformer, power rectifiers and filters, FET power switches, an output coupling transformer and appropriate control logic to provide the necessary ringing pulses to the transducer. An enable signal provided by the pulse control circuitry 336 establishes the proper timing and pulse repetition rate for the transmitter. For example, in the embodiment shown in FIG. 25, the application of an enable signal from the pulse control circuit 336 gates the FET's to provide a pulse of, for example, 200 nanoseconds at an amplitude of about 150 volts. This pulse, in turn, causes the transducer crystal to ring at a frequency of about 2.25 MHz. A preferred pulse repetition rate for implementing the instant system is about 60 Hz, allowing a 260 microsecond period following pulse excitation within which to receive echoes returning from the anatomical structure within the insonification zone of the ascending aorta.

The reflected energy incident upon the transducer probe 108 is converted into oscillating electric signals delivered to the pulse receiver 334. The receiver 334 provides means for amplifying the return signals from the transducer 108 and converting the same into a digital format for display processing on the CRT screen 104. Pulse receiver 334 is most preferably comprised of a transmit/receive switch to protect its circuitry against the relatively high power ringing pulse from the transmitter, band pass filters for signal conditioning, video gain amplifiers, a full wave peak detector, ADC means, time compensated gain control circuitry and an automatic gain control loop along with appropriate timing and control logic. These elements of the invention are best considered with reference to FIG. 26.

The input to receiver 334 is made via a transmit receive switch 342 which isolates the transmitter circuitry 332 during times when it applies the excitation pulse to the transducer 108. Echoes received by the transducer are converted to oscillating electrical signals passing through the transmit/receive switch to an RF amplifier 344. The ultrasonic echoes reaching the transducer are very close to the operating frequency of 2.25 MHz, transduced to an oscillating electrical signal with the same frequency as the echo, but one of fairly low amplitude. The amplifier 344 serves to increase the level to one permitting signal processing. Due to tissue attenuation, echo signals from interfaces close to the transducer are larger than those from more distant anatomical structures. For ease of operator interaction with the system, it is preferable to bring these signal amplitudes into conformity by attenuating signal amplitudes from structure closer to the transducer and amplifying signals more distant to achieve a type of spatial normalization in the nature of time compensated gain ("TCG"). In the exemplified embodiment, this time compensated gain is provided by a TCG circuit 345 interposed between the central timer 336 and RF amplifier 344, controlling the signal conditioning achieved in the amplifier in coordination with the timing of insonification pulses to realize spatial normalization and enhance echoes representative of the aortic structure of the patient. The thus-conditioned signals are thence applied to a controlled attenuator stage 346, cooperating with an automatic gain control element 348 for signal amplitude conditioning and a detector 350 for signal processing. The detector is designed to remove the radio frequency components from the received echo signals, leaving but the signal envelope for display. Accordingly, the detector includes rectifiers and filters for converting the oscillating radio frequency signal to pulsating direct current, pulsating at a radio frequency rate. Subsequently, the RF variations are removed to leave merely the envelope representing each echo, the shape of which follows the shape of the echoes received at the transducer 108.

In a highly preferred embodiment, the received echo signals are routed to a 2.25 MHz band-pass filter and then to two video amplifier stages with variable gain control. The gain levels of these two amplifiers are controlled as a function of time in order to increase gain as a function of distance or depth, as indicated above. The output of these amplifiers is routed through another 2.25 MHz band-pass filter and a third variable gain amplifier which provides the automatic gain control. The output of that amplifier is full wave peak detected to provide the video and automatic gain control signals. The video portion is subjected to further processing in a low pass filter.

The video signal being developed through the receiver circuitry is an analog one following the RF envelope as aforesaid. The analog signals are then converted to digital form in an analog-to-digital converter 349 ("ADC"). Preferably, the video signal processed through the low pass filter as noted above is scaled to drive the ADC. The digital video signals are stored in the digital memory of video memory circuit 338 and are read out of that memory under the influence of the video generator 340 in a standard video (CRT) format controlling the raster scan.

The video memory circuit 338 preferably includes a memory address counter and line generation circuit. When enabled with an A-mode command, the video memory alternately reads digitized data from the pulse receiver and stores same in memory, which is thence read out of memory and applied to the video generator 340 for display purposes. The data update rate for the memory is preferably 60 Hz. in concert with the pulse rate of the A-mode transducer. The output of the video memory is routed to the vertical line generator circuitry where digital comparators and a RAM are used to define and format the data suitable for a raster scan display. Offset and scan inputs are also provided from the interprocessor bus control 324 to provide for the display magnification noted above with reference to FIGS. 14 and 15 and truncation of the data when desired by the operator.

The video generator 340 provides the necessary means of converting the video data stored in memory into the appropriate raster display. Within the context of the most preferred embodiment of the present invention, the video generator includes a 3.5 MHz counter and suitable decoding logic and latches for defining bit times, CRT lines and CRT frames to implement a 30 frame per second interlaced display. Video output components include pedestal, blanking, black and white video levels, color burst, enable and synchronization pulses. Suitable clock signals are also provided by the pulse control circuit 336 to correlate these operations.

That control circuit provides the necessary timing and control signals for coordinating the A-mode operational sequence. Included in the most preferred design for the pulse control circuit 336 is a multiphased clock providing 21.48, 10.74, 7.16, 5.37, and 3.57 MHz clocking frequencies, additional counting and bit controlling functions for the video generator circuit, memory clocking and control functions for the video memory circuit, A to D and TGC control functions for the pulse receiver circuit and a composite video summing amplifier for the CRT video display. All functions, including transmitter pulsing, pulse receiver A to D conversion, video memory writing and reading, and video generator component generation, are synchronized to the multiphased clock of the pulse control circuit to minimize display interference which might occur as a result of asynchronous operation. The display may be frozen at any time by disabling the write function in the video memory control in order to facilitate the aortic diameter measurement. All control functions are disabled when the A-mode section is not in use.

Correlating the system circuitry design for the A-mode section 330 with the system operation described above, the operator's manual touch at active area 216 (FIG. 12) is sensed by the overlay 106 which, in turn, provides an input to the input/output control circuit 326 indicative of the operator's need to measure aortic diameter via the A-mode insonification technique. The command is relayed through the system CPU 320 and interprocessor bus control 324, activating the A-mode circuitry 330.

Pulse control circuit 336 provides the enabling pulse train to transmitter 332 which, in turn, develops the excitation voltage pulses causing intermittent, repetitive insonification of the ascending aorta via the transducer crystal. Following a burst of insonification energy, the interpulse period is dedicated to the detection of returns from the anatomical structure within that insonification zone. Pulse echoes are received at the transducer, converted to oscillating, RF signals routed to the pulse receiver 334. Those RF signals are conditioned, processed to yield the envelope replicating the echo patterns, and digitized to yield binary data supplied to video memory 338. In concert with the video generator 340 and pulse control circuit 336, the data is displayed as the trace 246 of, e.g., FIG. 14.

Once the operator has achieved a level of confidence that the image appearing on the display of, e.g., FIG. 14 is an accurate representation of the patient's aortic structure, a manual touch at active area 264 of the display or depression of the foot switch 114 interrupts the write capability of the video circuitry and thereby freezes the image pattern. Manipulation of the left and right cursors 250 and 252, respectively, via the overlay 106, thereby yields a direct, scaled measurement of aortic diameter. Touching the active area 172 on the display at that time enters the measured value (as determined by the scaled distance between cursors), and a signal developed through the overlay 106 is applied to the data bus for ultimate determination of cross-sectional aortic area.

Once the A-mode aortic measurement is concluded, measurement of the systolic blood flow through the aorta is next in order. That aspect of the procedure is governed by the continuous wave ("CW") section of the system circuitry, designated generally as 350. In capsule summary, the CW circuitry is comprised of a CW transmitter 352 (shown to be in physicaly sharing relationship with the pulse transmitter 332), a CW receiver 354, a CW intermediate frequency circuit 356 ("CW-IF"), a fast Fourier transform ("FFT") processor 358 and an FFT control circuit 360; all in concert with and under the guiding control of the digital circuitry described above.

The CW transmitter 352 provides output power to the continuous wave transducer 110, and specifically the transmitting crystal thereof. It is preferably comprised of a line receiver, shaping filter, step-up transformer and an output current buffer. When the CW mode is enabled, a clock signal (here shown to be a 2.5 MHz signal) is applied differentially to the line receiver; the signal is then shaped and filtered to eliminate harmonics greater than 2.5 MHz, and thence amplified through the step-up transformer to, e.g., 10 volt peak to peak. The signal is next preferably routed through the output buffer, for high current output capability, to the transmitter transducing crystal.

The CW receiver 354 serves to amplify incoming reflected CW signals and condition the same to an acceptable level for further signal processing. In the most preferred embodiment of the invention, the CW receiver circuitry includes a band stop crystal filter, several video amplifier gain stages, plural automatic gain control stages, mixers and a frequency locked loop ("FLL") oscillator. Collectively, the components of the CW receiver 354 serve to amplify and condition the signal representative of detected energy within the continuously insonified zone within the ascending aorta and, most preferably, convert the radio frequency signal to an audio one. This aspect of the invention is best considered with reference to FIG. 33.

A master oscillator circuit 362 within the FFT control element 360 provides clocked pulses to a transmitting amplifier 364 of the transmitter circuitry 352. As noted above, the transmitter then creates and delivers a 2.5 MHz signal to the transmitting crystal 282 of the transducer probe 110. The continuous wave of ultrasonic energy is applied to the region of the ascending aorta as described above with reference to FIGS. 27 and 34. Red blood cells comprising the systolic flow of blood through the aorta reflect a portion of that transmitted energy, shifted in frequency from the transmitter frequency by a value proportional to and indicative of the velocity of blood flow in accordance with the Doppler principle. Doppler-shifted, reflected energy is detected at the receiving crystal 284 of the transducer probe, which serves to convert the mechanical oscillations to electrical ones preserving the same frequency excursions and hence Doppler deviation. In the most preferred embodiment, the transmitter frequency is about 2.5 MHz and, accordingly, the signal emanating from receiver crystal 284 will vary above and below that value; the overall signal being a radio frequency signal. It is that radio frequency signal which is applied to the continuous wave receiver 354. The signal is first amplified in an RF amplifier 370 and is next applied to a first stage variable attenuator 372 under the influence of automatic gain control provided by AGC circuit 374. The thus-conditioned, RF Doppler signal is applied as a first input to a mixer 376 receiving as its second input a reference frequency signal from an in-phase divider 378. The two signals (RF Doppler and reference) are beat in the mixer 376 to yield an intermediate frequency Doppler signal. The intermediate frequency Doppler signal is then applied as an input to a second conditioning stage comprised of a variable attenuator 380 likewise controlled by AGC circuit 374 to improve or condition the Doppler signal, accounting for variations in signal amplitude due to differences in tissue characteristics in the insonofied region. The conditioned IF Doppler signal is then applied as a first input to a mixer 382 receiving a second reference frequency signal from the in-phase divider 378 to achieve frequency translation while preserving frequency variations indicative of systolic velocity. Preferably, the in-phase divider 378 provides reference frequencies suitable for translating the initial RF signal to an audio one whereby the operator may "listen" to variations in velocity flow during each cardiac cycle and over successive cycles as related above. Most preferably, the audio signal created is one where the frequency is a direct indication of velocity--for example 0 Hz corresponding to no flow, 2 KHz corresponding to 60 cm/s, etc. in a linear relationship. This is achieved by providing the in-phase divider 378 with the same clock signals from master oscillator 362 controlling the transmitter frequency. The in-phase divider then yields two reference signals, a first having a frequency f and a second having a frequency equal to the difference between the transmitter frequency and f. The higher frequency signal is employed in the first mixer stage 376 and the lower frequency signal in the second mixer stage 382. As a specific exemplification, for the most preferred transmitter frequency of 2.5 MHz, the in-phase divider 378 will provide a 12.5 KHz intermediate frequency signal and a 2.4875 MHz reference frequency signal as the difference between the transmitter frequency and the intermediate frequency. RF signals applied to the first mixer stage will vary about the 2.5 MHz transmitter frequency by values indicative of the velocity of systolic flow in accordance with Doppler-shift principles, the deviation representing velocity. Beating that signal against the 2.4875 reference signal will yield an IF Doppler signal with frequency variations about the 12.5 KHz beat frequency difference. The subsequent mixing with the 12.5 KHz signal from the in-phase divider in the second mixer 382 will effectively strip the signal of all frequency contributions save those directly related to velocity, leaving the desired audio frequency Doppler signal for further processing as well as for aural perception by the system operator. In some instances, the reference signal frequency f may be added to the base transmitter frequency and in that way achieve an indication of direction as well as the magnitude of systolic velocity.

The CW-IF circuit 356, operating cooperatively with the CW receiver 354, preferably also includes clocked filter means for eliminating spurious or otherwise undesirable frequency contributions; i.e., those other than frequencies directly correlated to systolic flow. A high pass clocked filter ("clocked wall filter") 384 provides variable corner frequencies to insure that Doppler frequencies associated with tissue movement or wall motion is effectively rejected during subsequent signal processing, as described below. That signal is then further processed through a clocked low pass filter 386 which also provides selectible corner frequencies to guard against "aliasing" of the higher frequencies into the signal ("clocked anti-aliasing filter"), also considered in somewhat greater detail below.

The audio Doppler signal is applied as an input to a detector 388 which develops the AGC control voltages for circuitry 374. That signal is also applied to the headset 112 for aural perception by the operator and/or audio speaker associated with the system 100. The audio signal is further directed to a spectrum analyzer in order to process the frequency excursions into signals indicative of cardiac output. In the preferred embodiment illustrated herein, that spectrum analysis is provided by the FFT processor 358 in combination with its FFT control circuit 360 in order to perform fast Fourier transform processing.

The FFT processor 358 provides a means for converting the analog Doppler signal, based upon a time domain function, into its corresponding frequency spectrum through fast Fourier transform processing. The processor circuitry, per se, is generally conventional for this purpose, being comprised of a four quadrant multiplier, a track and hold circuit, an A to D converter, an eight-bit digital multiplexer, a 256.times.12 bit static RAM, a multiplying/accumulator and sequence controller. The four quadrant multiplier allows the FFT processor to multiply the input Doppler signal by a weighting function depending upon the phase of the FFT sequence. This result is then processed through a track and hold element, converted by an A to D converter into eight bits of binary data, and the digital signal is then multiplexed with its Q component. That multiplexed signal is stored in an FFT data memory for subsequent processing in the ALU in accordance with the FFT program sequence stored in the FFT sequence control PROMS producing a 32 point fast Fourier transformation of the frequency spectrum of the Doppler signal.

The processing function is performed under the guidance of the FFT control circuit 360, which provides the necessary timing and control functions for performing the CW data processing function. In the most preferred embodiment of the present invention, the FFT control assembly contains suitable clocks, latches and decoding functions to implement a mulitphased clocking capability and the controls for the filter clocks of the CW-IF circuit, the control functions for the FFT processor, protocol signals for data transfer to the interprocessor bus control 324, clocking signals for the transmitter, CW receiver, CW-IF circuit and FFT processor, and also the multiplexing capability for switching the various audio signals to the CRT monitor. The analog signals multiplexed to the audio monitor include the Doppler signal itself for audio queues for probe positioning as well as a feedback queue indicating keyboard actuation.

Insofar as the FFT circuitry is conventional, the foregoing description is provided merely to exemplify the most preferred functional characteristics to be exhibited, thereby allowing those skilled in the art to select appropriate discrete devices. It will be appreciated, however, that devices meeting the overall functional capability of spectrum analysis of the audio Doppler signal might be employed to equal advantage and, accordingly, could well be implemented within the broader ambit of the present invention by those skilled in this art.

Irrespective of the precise design employed in respect of the FFT processor, per se, the same functions in combination with an analog-to-digital converter used to digitize analog Doppler signals. The FFT circuitry proceeds with the transformation of data at a data sampling rate varying with the systolic velocity of blood flow in order to minimize the hardware and firmware requirements of the system while providing correlation between data frequency and data processing. In brief, the FFT processor receives data at one of a plurality of predetermined data sampling rates for the signal under transformation, the data sampling rate being adjusted to take into account the velocities encountered. The velocity ranges on which sampling rates are based are themselves selected on the basis of those velocities statistically anticipated.

More specifically, and simply by way of exemplification, the present system most preferably provides three data sampling rates for fast Fourier transformation of the analog Doppler signal, correlated with three ranges for anticipated systolic velocities (responsible for the analog Doppler signals). In the most preferred implementation, the three anticipated velocity ranges are overlapping ranges of from 0 to 82 cm/s, 0 to 165 cm/s, and 0 to 330 cm/s. Threshold values within each range determine the need for adjustment or shifting of the correlative data sampling rate. The criteria employed in this highly preferred embodiment causes shifting upon the reception of substantial signals in excess of 78% of the maximum in the specific range, while a lower range will be elected upon reception of substantial signals less than about 26% of the maximum of that range. Thus, for example, shifting from the first data rate to the second data rate will occur upon the reception of substantial signals in excess of 64 cm/s while the data samplaing rate will shift from the second to third upon reception of substantial signals in excess of about 129 cm/s. Down shifting will occur from the third to the second rate where substantial signals less than about 86 cm/s are received, while the same will occur between the second and first rate where substantial signals less than about 43 cm/s are encountered. These shifting points (high and low thresholds) for rate adjustment have been selected to optimize signal processing within the context of the preferred implementation of the system of the present invention; others perhaps being more appropriate depending upon the specific design elected for implementation. Irrespective, and again viewing the most preferred embodiment, a complete FFT spectrum is processed every 2.5, 5 or 10 milliseconds depending upon the sample rate of the FFT processor as dictated by the velocity of blood flow detected. The velocity is monitored and, depending upon the design requirements, the occurrence of one velocity signal above or below a given threshold may cause shifting, a statistical average over several cardiac cycles may determine the substantiality of the signal levels governing shifting, or values determined over plural sequential caridac cycles may dictate this shifting.

Merging the foregoing features within the overall system of the present invention, the audio Doppler signal emanating from mixer 382 (suitably filtered) is applied to an analog-to-digital converter 390 which serves to provide digitized data to an FFT analysis circuit 392 within FFT processor 368 at a predetermined signal sampling rate. The FFT analyzer 392 performs the Fourier transformation in accordance with an algorithm stored therein. The peak component frequency in each sampling period is employed collectively with others in the same cardiac cycle to calculate cardiac output and stroke volume. As the detected velocity of the systolic flow (represented by the processed signal) varies, so too does the sampling rate of the analog-to-digital converter and the processing rate of the FFT analyzer. Also, the clocked wall and anti-aliasing filters of the CW-IF circuitry are adjusted to conform the high and low frequency rejection functions to the frequency of the signal being processed, it being remembered that the audio frequency of the Doppler signal is directly indicative of velocity. The velocity profile signals produced upon Fourier transformation are directly correlated to the actual velocity profile of systolic flow. These velocity profiles are thence integrated over the time of the corresponding cardiac cycle to yield a systolic velocity integral (the area under the curve described by the velocity profile signal) in a microprocessor 394 to yield that value. That value is further used, as described below, the calculate stroke volume from which cardiac output, cardiac index and stroke index may be obtained. It also is responsible for generating the signals appearing on CRT display 104, as described above with reference to FIGS. 19-24.

Microprocessors of conventional design are employed to calculate caridac parameters, based upon the calculated systolic velocity integral and aortic cross-sectional area as aforesaid. Stroke volume on a per cycle basis is achieved as the product of the patient's systolic velocity integral and aortic cross-sectional area. Cardiac output is simply achieved by taking the time-averaged sum of stroke volumes over the collective time period represented by the selected number of cycles. Each of stroke index and cardiac index is achieved as the simple ratio of the respective parameter to the patient's body surface area, a parameter previously computed or otherwise introduced to the system as noted above.

Correlating the system operation from the operator's point of view with the circuitry 350 comprising the CW mode of cardiac measurement, the same is keyed upon the operator's manual touch at active area 128 when the display is in the configuration of FIG. 18. The touch-sensitive overlay provides a signal through the input/output control circuitry 326 prompting the system to the CW mode via the central processors 320 and 322 and the interprocessor bus control 324. The CW transmitter section 352 receives clocked pulses, its output in turn causing the transmitting transducer crystal 282 to generate a 2.5 MHz, uninterrupted ultrasonic wave directed to the patient's ascending aorta along a propagation line generally axial with respect thereto from a point within the patient's suprasternal notch as viewed in FIGS. 3, 27 and 34. During each active ejection period, systolic flow through the ascending aorta begins from a static no-flow condition, with velocity rising to a maximum followed by a period of decreasing velocity and thence reversed diastolic flow. Transmitted ultrasonic energy is refelected during this period of active ejection, and principally by the red blood cells. Reflected energy is detected by receiving crystal 284; energy reflected from blood cells moving toward the transducer having an increased frequency, Doppler-shifted by a value proportional to velocity while stationary structure returns signals equal to the transmitter frequency. Diastolic flow likewise will return Doppler-shifted signals, but here shifted below the transmitter frequency; thereby providing a benchmark for ascertaining the initiation and termination of systole.

The receiving transducer crystal converts mechanical vibrations from reflected ultrasound into oscillating electrical signal having a radio frequency equal to the mechanical vibration frequency. These electrical signals are amplified, conditioned, and beat with the reference signals from the in-phase divider to yield an analog, audio signal having a frequency directly correlated with blood velocity. That audio may be perceived by the operator via either the headset 112 or an audio speaker associated with the apparatus. It is further applied to spectrum analysis in the FFT processor 358, the data sampling rate thereof being governed by the detected velocity itself. The FFT processor performs the transformation at a rate consistent with the sampling rate, dictated by observed velocity, while pre-processing signal filtration (i.e., frequency contributions from tissues in the clocked wall filter and carrier frequencies in the clocked anti-aliasing filter) is likewise keyed to the velocity (hence frequency) experienced. FFT-processed data in the form of a velocity profile signal, replicating the advancing and then declining velocity of blood flow, is integrated over time to yield the systolic velocity integral in microprocessor circuitry, further employed to determine stroke volume, cardiac output, stroke index and cardiac index. The ability to discriminate systole from diastole as aforesaid provides the further capability to recognize heart rate and the temporal excursion of each such cardiac cycle contributing thereto.

The signal value of the SVI calculation is displayed for each heart beat, as is the heart rate itself, on the video display. The operator will manipulate the transducer probe in response to the displayed signal on the CRT (either the graphical representation or the numerical indication) and/or through aural perception via the headset/speaker arrangement.

Once the predetermined number of consecutive cycles has occurred, (e.g., twelve) the operator's manual touch at sensitive area 308 keys the system, through the touch-sensitive overlay and interfacing circuitry, to make the calculations of cardiac output, stroke volume, cardiac index, and stroke index. The computed values are likewise routed to the display as shown, for example, in FIG. 23; the signals being routed along the data bus, through the interprocessor bus control and applied via the input/output control.

The microprocessor/firmware control implemented in the system described above is all quite straightforward and relatively uncomplicated. Those of ordinary skill in the art will have little difficulty in adapting the method and/or system of the present invention to a particular design given the principles set forth herein and the knowledge possessed by such designers. However, merely for the sake of illustrating a most preferred approach to this end, representative binary programs for the CPUs 320 and 322 are annexed hereto as Appendix I for particular use in association with the most preferred embodiment set forth in detail above.

While the invention has not been described with referenced to certain preferred embodiments, those skilled in the art will further appreciate that various substitutions, modifications, changes and omissions may be made without departing from the spirit thereof. Accordingly, it is intended that the foregoing description be viewed as exemplary only and not work a limitation on the scope of the claims granted herein. ##SPC1## ##SPC2## ##SPC3## ##SPC4##