Continuous Cardiac Output by Impedance Measurements in the Heart

This invention generally relates to medical apparatus for measuring characteristics of a heart. More particularly, the invention relates to a balloon flotation electrode catheter which can be used with appropriate equipment to monitor cardiac outputs on a beat-by-beat basis over a prolonged period of time.

While the invention is particularly applicable to the measurement of cardiac output in the right ventricular chamber of a human heart, it should be appreciated that the measurement of cardiac output in another chamber of a heart, such as the left ventricular chamber and of a nonhuman heart such as a suitable mammalian heart can also be performed by the present invention.

Several parameters are routinely monitored in patients having heart problems or those undergoing cardiovascular surgery. These include the electrocardiogram (ECG), the arterial blood pressure (ART), the central venuous pressure (CVP), the pulmonary artery pressure (PAP), and the cardiac output (CO). With the exception of cardiac output, technology now exists which permits these time varying parameters to be monitored continuously. However, all present techniques for clinically obtaining cardiac output involve indirect methods with sample intervals of several minutes. In addition, these techniques require either the injection of an indicator substance or the gathering of significant respiratory and blood gas patient data.

Cardiac output is generally measured in terms of liters per minute which corresponds to the heart's stroke volume multiplied by heart rate. Cardiac output values change depending on the activity level of the body, the level of metabolic demand, the condition of the heart and many other factors. During major operations, cardiac output is clinically significant because it is an indicator of how well the heart itself is performing and it demonstrates whether a sufficient supply of blood is being circulated to maintain metabolic demands.

One of the indirect methods of measuring cardiac output is the Fick method which determines such output by examining both the oxygen consumption of the lungs and the difference between arterial and venuous oxygen concentrations. A second method involves indicator dilution. Early indicator techniques used injectates such as cardio green dye which was injected as a bolus into the vascular system and allowed to mix with the venuous blood. An arterial sampling through a densitometer was then used to measure the time varying concentration levels of dye. The concentrations recorded were directly related to the flow rate of the dye mixed blood through the circulatory system.

The currently accepted clinical indicator method is a technique known as thermodilution. This method relies on thermal changes as a flow indicator. A bolus of cold fluid, at least 10.degree. C. less than the patient's core temperature, is injected into a venuous site. After mixing in the right ventricle, the adjacent cooled blood and fluid pass a small thermistor temperature sensor which has been placed via a catheter in the patient's pulmonary artery. The time varying temperature changes are directly related to the flow rate of the mixed fluid through the right side of the heart. Since the circulatory system is a series circuit, the right side value is also reflective of the left side ejections. Thus, a cardiac output can be calculated from the indicator dilution curve using a known equation.

Non-invasive techniques for obtaining cardiac output have been recently developed. Echocardiographic instruments can be used to measure aortic sizes and ventricular volumes at specific times during the cardiac cycle. Stroke volumes can then be derived from this information. In this connection, flow doppler instruments have been developed to measure blood velocity via external probes which are placed on the skin of the patient and aimed at a major arterial vessel such as the ascending or descending aorta. Cardiac output is then derived by estimating the vessel diameter in determining blood flow. Further calculations can convert the flow determinations to cardiac output by multiplying the heart rate and the flow per beat. Also, instruments which attempt to measure transthoracic impedance have also been developed in an attempt to determine non-invasive cardiac output. Finally, a non-invasive technique known as the pulse wave contour technique has been developed which makes use of the concept that the area under the arterial waveform curve is related to the stroke volume and the aortic compliance.

Each of the above recited methods has deficiencies which greatly limit either its use and/or functionality for clinical applications, especially during surgery. The Fick principle requires special equipment and careful attention in collecting the required samples and present technology does not allow all of the required patient data to be continuously monitored and analyzed. Non-invasive methods have also demonstrated severe limitations with regard to the size and expense of equipment, the requirement for highly trained personnel and may lead to inaccurate information in patients with cardiac diseases. Finally, the thermodilution technique is not capable of providing real time data on a beat-by-beat basis.

It would be very desirable to provide the clinician with the ability to evaluate cardiac function in certain circumstances, such as with critically ill patients or during surgery, on a continual basis since all other hemodynamic information except cardiac output is currently gathered on a beat-to-beat basis. By obtaining beat-to-beat cardiac output, a hemodynamic assessment of the patient could be performed continuously by the attending staff.

Accordingly, it has been considered desirable to develop a new and improved catheter for measuring cardiac output together with a method for determining the instantaneous volume of blood in a chamber of a heart and a cardiac output monitoring system with which the catheter can be used which would overcome the foregoing difficulties and others while providing better and more advantageous overall results.

In accordance with the present invention, a new and improved diagnostic catheter is provided for measuring cardiac output in the right ventricular chamber.

More particularly in accordance with this aspect of the invention, the catheter comprises an elongated multilumen flexible member having a distal end and a proximal end. A first lumen extends the entire length of the member and terminates in a distal port. A second port extends through the side wall of the member at a location immediately proximate of the distal end of the flexible member and a second lumen extends from the proximal end of the member to the second port. An expandable sleeve surrounds the member and spans the second port. The sleeve is inflatable by a fluid introduced into the proximal end of the second lumen. A plurality of ring electrodes are secured to the outer surface of the member at a predetermined axial spacing. The electrodes include a distal ring electrode located at a first predetermined distance proximal of the distal end of the flexible member and a proximal ring electrode located a second predetermined distance greater than the first predetermined distance from the distal end of the flexible member. The electrodes further include a plurality of intermediate ring electrodes disposed between the distal ring electrode and the proximal ring electrode. A plurality of electrical conductors extend longitudinally through a third lumen in the flexible member from the proximal end of the flexible member and are individually connected to separate ones of a plurality of ring electrodes. A first stiffening member is disposed in a fourth lumen in the flexible member and extends from a third predetermined distance to a fourth predetermined distance.

According to another aspect of the invention, a catheter is provided for measuring cardiac output.

More particularly in accordance with this aspect of the invention, the catheter comprises a catheter body having an outer periphery and a distal section terminating in a distal end and a proximal section terminating in a proximal end. A plurality of spaced electrodes are secured to the body outer periphery along the body distal section. A plurality of electrical leads are provided each one of which extends in the catheter body from a respective one of the electrodes to the proximal end of the catheter body. An elongated rigid means is provided for stiffening a portion of the catheter body. One end of the rigid means is located adjacent a proximal most one of the plurality of electrodes. The rigid means so locates the plurality of electrodes as to space them away from endocardial tissue.

In accordance with still another aspect of the invention, a catheter is provided for measuring cardiac output.

More particularly in accordance with this aspect of the invention, the catheter comprises an elongated flexible multi-lumen catheter body having an outer periphery and a distal section terminating in a distal end and a proximal section terminating in a proximal end. A balloon is attached to the distal end of the body. A first lumen extends the entire length of the catheter body and terminates in a first port which communicates with an interior surface of the balloon. A plurality of spaced electrodes are secured to the body outer periphery along the body distal section proximal of the balloon. A second lumen extends from a distal most one of the plurality of spaced electrodes to the proximal end of the body. A plurality of electrical leads are provided each one of which extends through the second lumen from a respective one of the electrodes to the proximal end of the catheter body. A third lumen is provided which extends longitudinally in the catheter body from the proximal end to a port which is intermediate to the plurality of spaced electrodes.

According to a further aspect of the invention, a method is provided for determining the instantaneous volume of blood in a chamber of an animal heart.

More particularly, the method comprises the steps of inserting an elongated tubular catheter percutaneously into the heart chamber. The catheter has a plurality of longitudinally spaced electrodes on the surface thereof which electrodes are individually connected to a corresponding plurality of terminals at the proximal end of the catheter by conductors passing through the tubular catheter. The longitudinal spacing of the electrodes are such that a distal electrode and a proximal electrode are located at the pulmonic valve and the tricuspid valve of the heart, respectively. The distal electrode and the proximal electrode are driven with a constant current source. The potential signal developed between pairs of sensing electrodes located intermediate the pair of driving electrodes and attributable to the application of the driving constant current source to the pair of driving electrodes is selectively and sequentially detected. The potentials are proportional to the instantaneous impedance of the medium existing between the selected pairs of intermediate sensing electrodes. The detected potential signals are then converted to digital quantities. The digital quantities are applied to a programmed digital computing device. A single corrected instantaneous impedance value is generated for each of the intermediate sensing electrodes determined to lie within the ventricle. The impedance value detected is due to the application of the constant current source to the pair of driving electrodes. A single corrected instantaneous impedance value is calculated for a ventricular segment volume for each pair of the sensing electrodes. The segment volumes for each pair of sensing electrodes are summed to produce the total instantaneous ventricular volume.

According to a further aspect of the invention, an apparatus is provided for measuring the instantaneous volume of blood in a chamber of a heart.

More particularly in accordance with this aspect of the invention, the apparatus comprises an elongated tubular intravascular catheter having a proximal end and a distal end with a pair of drive electrodes attached to the exterior surface thereof and spaced apart from one another by a predetermined distance D1 which is less than the length dimension of a catheter section that is held in the chamber. A plurality of pairs of sense electrodes are attached to the surface of the catheter and longitudinally spaced therealong between the drive electrodes. The pair of drive electrodes and the plurality of sense electrodes are electrically coupled individually to a terminal at the proximal end of the catheter. A constant current source of a frequency F.sub.1 is provided together with a switching means which is joined to the terminals for coupling the constant current source to a selected pair of drive electrodes. A signal detector means is connectable through the switching means to predetermined pairs of the plurality of pairs of sense electrodes for producing signal waves corresponding to the impedance of the medium present between a sense electrode pair selected by the switching means attributable to the constant current source. A means is operatively coupled to the signal detector means for sampling the signal waves at a predetermined rate and converting the signal waves to digital values representative of impedance values. A computing means is coupled to receive the digital values. The computing means is programmed to compute the volume of the segments between selected pairs of sense electrodes using the formula Volume =(i.sub. c .times.p x L.sup.2)/V.sub.EE where i.sub.c is a known constant current source, p is the resistivity of the medium, L is the distance between electrodes and V.sub.EE is the measured end to end voltage.

According to another aspect of the invention, a continuous cardiac output monitoring system is provided.

In accordance with this aspect of the invention, an elongated tubular intravascular catheter is provided which is adapted for insertion into a patient's heart. The catheter includes a plurality of spaced electrodes positioned on a periphery of the catheter. A distal most one and a proximal most one of the electrodes are configured as drive electrodes and the remaining electrodes are configured as sense electrodes. Each of the electrodes is connected to a terminal located at a proximal end of the catheter. A signal conditioning and control unit is provided which is in electrical contact with the catheter through the catheter terminal. The unit comprises a constant current source, a selector means for coupling the constant current source to drive electrodes and a signal processing means for processing a signal received by the unit. A computing means is electrically connected to the unit for converting signal waves from the unit to digital values and then computing a stroke volume of the heart.

According to still another aspect of the invention, a cardiac output monitoring system is provided.

More particularly in accordance with this aspect of the invention, the system comprises a first signal means for sending analog data related to a stroke volume in a right ventricle of a patient's heart. A signal processing means is provided for processing the analog data from the first signal means into processed analog data. A computing means is provided for converting the processed analog data from the signal processing means to digital values and thereafter computing the stroke volume of the patients heart.

One advantage of the present invention is the provision of a new and improved catheter for use in monitoring stroke volume.

Another advantage of the present invention is the provision of a method and apparatus for measuring stroke volume and cardiac output with an accuracy greater than has heretofore been possible using known prior art techniques.

Still another advantage of the present invention is the provision of a method and apparatus for measuring stroke volume and cardiac output on a beat-to-beat basis in a continuous manner.

Yet another advantage of the pr.RTM.sent invention is the provision of a catheter which, together with apparatus for measuring stroke volume facilitates the measurement of cardiac output on a beat-to-beat basis. The catheter can also be used in ventricular pacing and the diagnosis of complex arrhythmias.

Still yet another advantage of the present invention is the provision of a balloon catheter having a series of axially aligned electrodes extending over a predetermined length proximally of the balloon such that when the balloon is guided into the pulmonary outflow tract of the heart, the portion of the catheter bearing the electrodes extends between the tricuspid valve and the pulmonary valve of a right ventricle of the heart.

A further advantage of the present invention is the provision of a flow directed catheter having a stiffening member contained in a lumen thereof for causing the catheter to assume the correct orientation in a right ventricle of the heart.

A still further advantage of the present invention is the provision of a method and apparatus for measuring ventricular volume of a heart Wherein the catheter is capable of conducting stroke volume measurements using two different techniques so that a comparison or a calibration can be performed.

A yet further advantage of the present invention is the provision of a ventricular volume measuring system including a catheter having electrodes, a signal conditioning and catheter control unit and a microcomputer. The system allows any electrode pair to be selected for use as either sensing electrodes or drive electrodes as desired and the electrodes can be scanned to determine catheter position.

Still other benefits and advantages of the invention will become apparent to those skilled in the art upon reading and understanding of the following detailed specification.

The invention may take physical form in certain parts and arrangements of parts, a preferred embodiment of which will be described in detail in this specification and illustrated in the accompanying drawings which form a part hereof and wherein:

FIG. I is a plan view of a catheter according to the preferred embodiment of the present invention;

FIG. 2 is an enlarged cross-sectional view along line 2--2 of the catheter of FIG. 1;

FIG. 3 is a block diagram of a continuous cardiac output measuring system according to the present invention;

FIG. 4 is a front elevational view of a signal conditioning and control unit housing of the system of FIG. 3 according to the present invention;

FIG. 5 is a block diagram of the electronic modules within the signal conditioning and control unit of FIG. 4;

FIG. 6 is a block diagram at the input isolation unit of FIG. 5.

FIG. 7 is a block diagram of the signal processing unit of FIG. 5.

FIG. 8 is a block diagram of the interface/ oscillator unit of FIG. 5.

FIG. 9 is a block diagram of the major sections of the signal conditioning and catheter control unit of FIG. 4;

FIG. 10 is a block diagram of a microcomputer of the system of FIG. 3;

FIG. 11 is a block diagram of the three primary software modules utilized in the computer of FIG. 10;

FIG. 12 is a flow diagram of the software routines in module 1 of the modules illustrated in FIG. 11;

FIG. 13 is a flow diagram of the software routines in module 2 of the modules illustrated in FIG. 11;

FIG. 14 is a flow diagram of the software routines in module 3 of the modules illustrated in FIG. 11;

FIG. 15 is a sectional view of a heart showing the catheter of FIGURE 1 inserted in the right ventricle;

FIG. 16 is a perspective view of a removed section of the right ventricle of the heart of FIG. 15;

FIGS. 17A-l7I are schematics of the actual circuitry in which an embodiment of the subject system is presented; and

FIGS. 18A-18II is a listing of the software modules with which the subject system operates.

Referring now to the drawings, wherein the showings are for purposes of illustrating a preferred embodiment of this invention only and not for purposes of limiting same, FIG. 1 shows the subject new diagnostic catheter A which is adapted to be positioned in a heart B as is illustrated in FIG. 15 and is adapted to convey information to the continuous cardiac output measuring system C illustrated in FIG. 3. While the catheter will be described for use in monitoring cardiac output in the right ventricle of a human heart, it should be appreciated that the catheter can be used for monitoring cardiac output elsewhere in the heart, such as in the left ventricle, and can also be used to monitor cardiac output in hearts other than human hearts, such as suitable mammalian hearts and others.

More specifically, the catheter A is a balloon flotation catheter of the type known as a Swan-Ganz catheter. The catheter A comprises an elongated tubular member 10 which can be approximately 110 cm long if desired and which can be made from a plasticized PVC extrusion, if desired. The member 10 is extruded so as to have a predetermined outer diameter which, for purposes of illustration only, may be about a French 7.5 diameter (2.475 mm) and which is preferably formed from silicone rubber, polyurethane or some other suitable plastic that tends to be non-thrombogenic. It should be appreciated, however, that the tubular member could have a diameter between about French 4 (1.32 mm), for pediatric applications, and French 8 (2.64 mm). The tubular member 10 includes a distal section 12 having a distal end 14 and a proximal section 16 having a proximal end 18 which terminates in a pigtail sheath 20.

Extending from the pigtail sheath are a first inlet tube 22, a second inlet tube 24, a third inlet tube 26, and a fourth inlet tube 28. Also extending from the sheath is a first electrical conduit 30 and a second electrical conduit 32. Secured on a free end of the first inlet tube 22 is a connector terminal 34. Similarly secured on the free ends of the second and third inlet tubes 24 and 26 are suitable second and third connector terminals 36 and 38. To a free end of the fourth inlet tube 28 is secured a fluid connector terminal 40 known as a Luer valve. A first electrical terminal 42, which is for the thermistor and can be a three pin Edwards type connector if desired, is connected to a free end of the first electrical conduit 30. Similarly, secured to the free end of the second electrical conduit 32 is a suitable second electrical terminal 44, which is for the electrodes and can include ten pins, if desired.

The distal end 14 of the catheter is provided with a first outlet port 50 which is in fluid communication with the first inlet tube 22 through a first or distal lumen 52 as shown in the cross-sectional view of FIG. 2. Similarly, second and third outlet ports 54, 56 are in fluid communication with a respective one of the second and third inlet tubes 24, 26, through suitable lumens only one of which, 58, is illustrated in FIG. 2 since the port 56 can terminate the other lumen before the cross-sectional view of FIG. 2. A balloon section 60 is in fluid communication with the fourth inlet tube 28 through a third lumen 62 as is illustrated in FIG. 2.

Formed through the side wall of the tubular member 10 in the zone spanned by the balloon 60, is a port, not visible in FIG. I, which communicates with the third lumen 62. Thus, when fluid under pressure is introduced through the open fluid terminal 40, it flows through the lumen 62 and out the mentioned port so as to inflate the balloon. By then closing the valve 40, the balloon can be retained in its inflated state.

Secured on an outer periphery of the tubular member 10 are a plurality of spaced ring type surface electrodes 70, which can be made from Elgiloy. The electrodes are spaced apart by approximately .8 to 1.0 cm and can be approximately 2 mm wide. The most proximal electrode is identified by the numeral 70P and the most distal electrode is identified by the numeral 70D. Preferably, ten electrodes are provided with each of the electrodes being connected to a separate insulated conductor 72 which is positioned in a fourth or electrical lumen 74 as is illustrated in FIG. 2. If desired, the distal-most electrode 70D can be located approximately 9 cm from the distal end of the catheter with the proximal-most electrode being located approximately 16.4 cm from the catheter distal end, when the electrodes are spaced apart by 0.8 cm. Such an electrode spacing may be advantageous for patients with small ventricles. The conductors 72 extend in the fourth lumen proximally to the second electrical terminal 44 and terminate in individual connector pins 76 contained in the terminal or housing 44. The terminal is adapted to be connected to a control unit as described hereinbelow.

Located on the tubular member 10 is a port 80 adjacent the balloon section 60 for holding a conventional thermistor element 82 which is normalized for blood temperature measurement and is disposed within the tubular member. As is well known in the art, a suitable plastic such as polyurethane having good heat conducting properties covers the thermistor in the port 80 in order to prevent the ingress of blood and other body fluids. The thermistor 82 is in electrical contact with the thermistor terminal 42 through a suitable insulated conductor 84 (FIG. 2) which for the sake of convenience, can also extend through the fourth lumen 74 if desired.

As illustrated in FIG. 2, a metallic stiffening member or stylet 90 is suitably disposed in a lumen 92 proximally of the proximal most electrode 70P. If desired, the lumen 92 can be a continuation of the lumen which leads also to the third port or proximal port 56. In order to prevent fluid from flowing further down this lumen, a suitable adhesive plug (not visible) is suitably injected into the lumen at a location distal of the port 56, as is well known in the art.

As is evident from FIG. 2, the tubular member can be a five lumen catheter. However, it should be recognized that the member could also be provided with six or more lumens if that was considered desirable or necessary.

The stiffening stylet 90 can comprise a suitable stainless steel wire which is encapsulated in an insulating material such as nylon. In order to give the wire considerable stiffness, it can be made out of a suitable conventional spring wire if desired. The stylet 90 can be positioned immediately proximally of the proximal most electrode 70P and can extend approximately 10 cm proximally therefrom as is illustrated in FIGURE 1. During insertion, the stiffening stylet 90 aids in the proper positioning of the catheter to locate the electrodes away from the heart chamber walls thereby allowing the catheter to be placed in a position which permits impedance measurements.

While the stylet 90 is shown in FIG. 1 as being substantially straight, it should be appreciated that curved, bent, or looped stylets might prove advantageous for certain catheter uses as well. The stylet could be fixed or adjustable as may be required. While the stylet has been illustrated as being made of stainless steel, other types of material, such as for example fiber-reinforced composites may be used instead.

The first lumen 52 which terminates in the first port 50 at the tip of the catheter is useful for monitoring blood pressures during insertion of the catheter. Additionally, blood samples can also be drawn from the first port 50. The third port or proximal port 56 with which the lumen 92 can communicate as explained above, can terminate approximately 30 cm from the distal end of the catheter. When the catheter is correctly inserted in the heart, the port 56 will be located in the right atrium. This port can be used to monitor central venuous pressures and can also be employed as an injection site for fluids and medications. Blood samples can also be obtained through this port.

As mentioned previously, it is advantageous to provide a second port 54 which is located between the series of spaced electrodes 70. The lumen 58 communicating with port 54 can terminate at approximately the 15 cm mark as measured from the distal end locating the port between the eighth and ninth electrodes 70. The port 54 can be used for measuring right ventricular pressures as well as determining catheter location by examining the changes in the pressure wave-form as the port passes through the tricuspid valve and into the right ventricle.

In another embodiment of the invention, ten electrodes can be spaced apart at 1 cm intervals beginning 9 cm from the distal tip of the catheter and terminating 20 cm from the distal tip. A calibrated thermistor bead can be located approximately 4 cm from the distal tip. The catheter can have a balloon of approximately 1.5 cc volume located between the thermistor and the distal tip. A stiffening or stabilizing stylet 10 cm in length can be provided in the catheter between 20 cm and 30 cm from the distal tip of the catheter, that is proximally from the proximal-most electrode. The stylet can be made of stainless steel which is encapsulated in nylon.

This catheter can include four lumens, namely, a proximal lumen which terminates 30 cm from the distal end of the catheter for monitoring central venuous pressures, injecting fluids and medications and drawing blood samples; an electrical lumen which contains the leads for the thermistor and each of the ten electrodes; a balloon lumen which is used to control the inflation and deflation of the balloon; and a distal lumen which terminates at the tip of the catheter, for monitoring blood pressures and drawing blood samples.

With reference now to FIG. 15, the catheter A can, if desired, be inserted via the superior vena cava. The site of entry can be an internal jugular, subclavian or antecubital vein. Insertion and final catheter positioning are guided by pressure waveforms and EKG signals obtained from the catheter. The methods employed for introducing the catheter are identical to those used for the insertion of a conventional Swan-Ganz catheter, and, accordingly, no further description of them is considered necessary. Once the distal tip of the catheter has been routed through a right atrium 100 of the heart B, and a tricuspid valve 102 thereof and into the right ventricle 104, an inflating fluid is applied under pressure to the balloon lumen 62 to inflate the balloon 60. As blood is pumped from the right ventricle, the balloon 60 tends to be carried by blood flow through the pulmonary valve 106 and into the pulmonary outflow tract. Once the tip of the catheter has been located in the pulmonary artery, it is advanced until a wedge condition exists, i.e., the inflated balloon lodges in a branch of the pulmonary artery 108.

When correctly located, the proximal electrode 70P is located adjacent the tricuspid valve 102 and the distal electrode 70D is located at the entrance to the pulmonary outflow tract and preferably adjacent the pulmonic valve 106. Once the catheter is installed, stroke volume measurements can be taken using the techniques set out hereinbelow.

One advantage of the pentamerous lumen embodiment of the invention illustrated in FIG. 1, is that the port 54 can be used to inject medications directly into the cardiovascular system even when blood pressure measurements are being taken through the ports 50 and 56. Also, the port 54 will be positioned in the right ventricle (as shown in FIG. 15) which is advantageous for obtaining a good mixing of the medication with the blood.

On the other hand, the port 56 can also be used to inject medication. This port, since it will be positioned in the right atrium (see FIG. 15) will also assure a good mixing of medication with the blood.

Turning now to FIG. 3, a block diagram of a continuous cardiac output measuring system C of the present invention will be described. The entire monitoring system C is contained in the portable cart D. The monitoring system C receives electrical power from a power source connection 110. Power entering through connection 110 passes through an isolation transformer 112, and then to a power distribution network 114 which functions to condition power to appropriate levels and distribute it throughout the system. The power isolation transformer 112 functions to provide a level of patient safety for the equipment when operating in a critical environment.

Signals received from the multi-electrode catheter A into the continuous monitoring system C are acquired by a signal conditioning and catheter control unit 118 ("SCCCU"), the user interface of which is illustrated more fully by FIG. 4. The SCCCU 118 provides a user interface to control operation parameters of the system. Included is user selected auto position control; pacer balancing controls; input channel gain select; electronic filtration; position control; signal gain; and a master power control.

It will be recalled that analog signals are received by the continuous monitoring system C. Signals received by the unit 118 are passed through a gain select 120 which functions to isolate a desired signal level. Analog outputs from the gain select 120 are fed to a four channel analog recorder 122, which in turn interfaces a patient monitor through an interface adapter 124. Analog signals from the gain select 120 are also fed to a microcomputer 130 via an analog to digital ("A to D") interface 132. In this fashion, a digital signal representative of the analog values obtained from the multi-catheter electrode A is obtained for use in the microcomputer 130 which, in the preferred embodiment, is digital. The microcomputer 130 will be described more fully in conjunction with FIG. 10, below.

The microcomputer 130 is also in data communication with a hard-copy data recorder illustrated by printer 134. The microcomputer 130 is also similarly in data communication with an external display such as that illustrated by display screen 136 which is suitably comprised of a conventional cathode ray tube ("CRT") display. The microcomputer 130 is also shown as including a contiguous CRT monitor 138, a data entry device such as key board 140, and a mass storage medium 142 which is illustrated as a pair of disk drives 142a and 142b. The mass storage medium 142 is suitably comprised of a hard disk, a floppy disk, a CD-MEMORY (compact disk memory), or the like, or any combination thereof. A data port 146, which is suitably comprised of a parallel port or a serial port, provides a means for communicating data to an exterior of the microcomputer 130. As illustrated, the data port 146 communicates data back to the signal conditioning and catheter control unit 118 in a feed-back manner.

Turning now to FIG. 5, a block diagram of the signal conditioning and catheter control unit 118 is presented. Power is received into the SCCCU unit 118 via interconnect 150 which is in turn connected to the power distribution network 114 (FIG. 3). The power network interfaces circuit breakers 152 and a power transformer 154, which steps down the voltage therethrough to suitable levels for operation of the remaining circuitry. The control unit 118 includes primary circuit modules comprising an input isolation unit 158, a signal processing unit 160, and interface/oscillator/filter unit 162, a power supply filter unit 164, and a power supply unit 166. All devices are interfaced via a common signal bus 170. The signal bus 170 also interfaces the control panel of FIG. 4.

The power supply unit 166 receives power from the power transformer 154, stepping it down to appropriate values for use throughout the control unit. The voltage levels obtained from the power supply unit 166 are filtered, prior to distribution to the remaining circuitry of the unit, by power supply filter unit 164.

Use of the common connector bus 170 provides a means by which any or all of the above units may be implemented by "plug-in" modules which facilitates selective replacement, enhancement, or modification. Implementation of this bus structure also provides for minimization of noise problems.

The signal bus 170 also interconnects the multielectrode catheter A via an input protection circuit 174. The input protection circuit 174 isolates the signal bus 170, and accordingly the remaining components interfaced thereto, from voltage levels which may otherwise damage circuitry within the control unit 118.

Turning to FIG. 6, the input isolation unit 158 will be described in detail. The input isolation unit 158 contains circuitry which provides five channels of input isolation, a constant current source, electrode select control, and ground isolation. All signals, as well as power entering or leaving this module, are isolated via opto-isolators or transformer coupling. Blocks within the dash line of FIG. 6 are isolated. Blocks through which the dash line passes are providing the isolation. Block 180 illustrates a series of six high impedance voltage follower amplifiers which function as buffers between the multi-electrode catheter and the remaining circuitry. Outputs from the input buffers 180 are in turn fed to a series of five channels of differential input amplifiers illustrated by block 1822. Signals resultant from the amplifiers 182 are in turn fed outward, again through the signal bus 170 (FIG. 5) via a group of opto-isolators 184. The opto-isolators isolate the signals passed therethrough from the next stage via optic coupling. This circuitry is powered via a transformer coupled with nonearth ground references. This forms the iso-power input 190. The iso--power used is referenced to a potential known as isolated ground. Isolated ground is not tied to earth ground. This feature provides a level of patient isolation preventing currents which are flowing due to ground reference potentials from passing into this circuitry. Each optical coupler provides a signal isolation well above several thousand volts.

Turning to FIG. 7, fabrication of the signal processing unit 160 of FIG. 5 will be described. The signal processing unit 160 contains circuitry which provides AC buffering, bandpass filtering, amplitude demodulation, signal smoothing, signal inversion, and waveform isolation. FIG. 7 depicts a signal flow and control related to this module. The board is connected, via the signal bus 170, to two external devices. These external devices include a circulating fan (not shown) via an interconnect 192, and an offset control (not shown) via an offset control interconnect 194. Unlike the circuitry of FIG. 5A the signal processing unit 160 includes no circuitry which uses isolated power. Outputs of the optoisolators 184 from FIG. 6 form an input to AC buffering circuitry 200. This circuitry buffers analogously to the buffers 180 of FIG. 6. Waveforms from the buffer circuitry 200 are passed to a series of five 2 Khz bandpass filters 202. Use of multi-feedback active bandpass filters permits modulated signals of up to 40 Hertz to pass with minimal effect. The filters are implemented given that the modulated impedance signals contain frequencies of up to 40 Hertz. The bandpass filters effectively block undesirable physiologic signals such as electrical signals generated by the contractions of the heart.

After the bandpass filters 202, the signals are passed into five channels of amplitude demodulation present in demodulation circuitry 204. In this circuitry, each channel of impedance waveforms is amplitude demodulated by an absolute value and wave smoothing circuit comprised of a pair of operational amplifiers. The output waveforms from each of these stages is a demodulated signal with some carrier frequency noise. The signals are next passed to a low pass filter 206 to further reduce carrier frequency noise. The low pass filters 206 are suitably comprised of Butterworth-type filters with a maximally flat frequency response. Attenuation is suitably 12 dB at twice the cut-off frequency of 100 Hertz. Signals after the 100 Hertz low-pass filters represent real time dynamic impedance waveforms for each of five selected pairs of electrodes from the catheter A (FIG. ).

Signals are next fed to a signal inversion circuit 210. In this stage, each impedance value is inverted to form an admittance value. The admittance signal level is desired as it forms a signal from which volume to be measured is directly proportional. The signal inversion circuit functions by implementation of a analog divider chip. Impedance signals are used as denominator values, while an adjustable but constant voltage is used for a numerator. With these two analog voltages, a real time quotient (admittance) is developed for each channel which is inversely proportional measuring impedance values. One of two resistive voltage dividers can be selected, via the front panel control of FIG. 4, to determine the constant numerator voltage.

Output from channels are selected from the signals from the signal inversion circuit 210 by channel selector circuit 212. Such selection is accomplished by means of a signal on waveform select control lines which are obtained as described further below. These control lines determine whether a particular channel is selected. In the event a channel is not selected, no connection is made to further stages, thereby leaving the circuitry open or at a high impedance state. Accordingly, the channel selector circuitry 212 functions analogously to a tri-state device. Outputs from the channel selector circuit 212 form inputs to a waveform summing amplifier 216. The summing amplifier 26 generates a composite waveform, which in turn forms an input to an offset adjust buffer 218. In the offset adding circuit 218, an externally selected DC level adds a value to the composite admittance waveform resultant from the summing amplifier 216. This functions to "window" the waveform within the range of an analog to digital convertor which is implemented in the computer 132 (FIG. 3).

Turning now to FIG. 8, fabrication of the interface/isolator/filter unit 162 will be described. The interface/isolator/filter unit 162 provides interface buffering, latching control, constant current source development, control level shifting, waveform filtering, pacemaker rejection, and auto positioning. The composite admittance waveform developed by the signal processing unit 160 of FIG. 7 is routed via interconnect 224 to circuitry which accomplish the above-stated operations. The interconnect 224 receives its signal from an output 220 of the waveform summing amplifier 216 to FIG. 7. The composite waveform input obtained from interconnect 224 enters a signal inverting stage 226 which is comprised of an inverting buffer with unity gain. The output from the inverter 226 forms an input for a filter selector switch (single pole, double throw) 228 on the panel of FIG. 4. The selector switch permits operator selection of an in-line 40 Hertz filter or a straight-through non-filtered waveform.

The output from the signal inverting stage 226 also forms an input to a low-pass filter 230. An output of the 40 Hertz low-pass filter is selectively fed to the filter selector 228 through a pacer reject module 232 prior to being additionally fed to the filter selector 228.

The pacer reject module is implemented in conjunction with the signal placed on pacer reject line 234 which controls a selector switch 236. A signal on the pacer reject line 234 is user selected by the control panel of FIG. 4. The pacer reject circuitry is comprised of a slew rate limiter fabricated from a diode bridge circuit. The circuitry functions by saturating if a feed-back path coupled to an RC network is not at the same voltage as its input. A state of the feed-back path is determined by the slew rate of the input signals. For dynamic values greater than 50 Hertz, the output of the amplifier saturates. Saturation may be positive or negative depending on a direction of a detected signal spike. Upon activation, the circuit deselects the actual wave-form and connects the output to a high impedance source. This essentially forces the circuit to behave as a sample and hold circuit, thus locking out spike potentials from the pacer and forcing the circuit to remain at its last detected value.

The operator selected signal from filter selector 228 is passed to a low pass filter 242 and a filter selector 246. The filter selector 246 functions to selectively pass the output of the filter selector 228 through the 10 Hertz low-pass filter 242 prior to passing it to a gain select potentiometer 248 which is also found in FIG. 4.

The output of gain potentiometer 248 is selectively placed, via selector switch 250, through an auto-position circuit 252, prior to being fed to a final output driver 254. The auto-position circuit 252 inverts the polarity of the signal.

Turning to the top portion of FIG. 8, parallel data from the microcomputer 130 is input to the interface/oscillator filter unit 162 via a data bus 260. Data enters control buffers 262 and 264. The buffer 262 holds admittance select-channel data with the buffer 264 holding electrode select-line information. The lines 266 form the electrode select control lines which are coupled directly to the catheter A.

Data from the buffer 262 is also fed to a control level translator 270 to provide for control within the interface/oscillator filter unit 162. These signals are latched by control signal latch 272, and translated by control level translator 274 which converts them to voltage levels suitable for a logic control external of the unit 162. The voltages levels are made available on channel select control lines 276.

Latch detect circuitry 280 triggers a control pulse at its output in response to a low to high transition on its input port, which is derived from an output of the control data buffer 262. In response to an output of latch detect 280, data from the select-lines control buffer 264 is made available at lines 282.

A sine wave generator 286 has its frequency and symmetry adjusted by controls 288 and 290, respectively. The sine wave generated by this circuit is routed through an offset adjust 292 and a gain adjust potentiometer 294. These components form a constant current source for use elsewhere in the circuit which also includes an isolator.

Turning to FIG. 9, a summary of the interaction of all hardware components illustrated in conjunction with FIGS. 5-8 as presented.

Turning now to FIG. 10, a more detailed description of the microcomputer 130 as implemented in the preferred embodiment is presented. It will be noted that the microcomputer is illustrated as based on an 8086-2 microprocessor, of Intel Corp. of Santa Clara, Calif. Suitable computers having the characteristics illustrated by FIG. 10 are commonly available in the market.

Turning now to FIGS. 11-13, a software routine for the microcomputer of FIG. 10 will be disclosed. With particular reference to FIG. 11, the three primary software modules and their interactions will be described.

FIG. 11 shows common structural links between modules and how they are initiated. A shared or common memory 300 is provided to hold variables which are used by all modules or units in the subject system. This provides means by which modules which may have been programmed in different languages are able to change a state of a stored variable independently of one another. Each routine is initiated without respect to a status of any other routine. SCCCU operation and signal analysis is operationally separated from other code modules, but by virtue of the shared memory 300, shares common variables with other code sections. Activation is preferably not user controlled, but instead is a function of a hardware timer and clock circuit 302 which activates the module at a frequency of suitably 200 times per second, regardless of a state of the remaining modules. This section of code is essentially running in the background while all other code is running in the foreground. Because of its mode of activation, the SCCCU operation and signal analysis module has priority over other code modules. This module is termed "freerunning" and requires no operator interaction to begin or complete its task. The software routine implemented in modules 1, 2, and 3 (304, 306, and 308 respectively), are illustrated in FIGS. 12, 13, and 14, respectively.

The computation of the volume of the right ventricular segments (one is illustrated in FIG. 16) between selected pairs of sense electrodes is done according to the formula Volume =(I.sub.c .times..rho..times.L.sup.2)/V.sub.EE where I.sub.c is a known constant current source, p is the resistivity of the medium, L is the distance between electrodes and V.sub.EE is the measured end to end voltage. This formula is substantially accurate although it does not take into consideration the loss of current to surrounding tissue or the varying conductivity of blood.

If L is designated to be 1 cm., then each segment 5 volume is directly proportional to .rho./Z.sub.EE, where Z.sub.EE is the impedance of the blood volume in the measured segment and is equal to Ic/V.sub.EE. Using the thermodilution technique, .rho. can be determined as well as any signal losses due to the leakage of drive current through surrounding tissue. Thus, the total ventricular volume, i.e. the sum of the segments, can be determined by the formula V.sub.T =.SIGMA.1/Z.sub.EE where K is a constant which represents the effect of blood resistivity and drive signal losses.

It should be appreciated that the potential from each sensing electrode pair can also be used to detect the position of the catheter within the cardiovascular system. The dynamic potentials from the sensing pairs of electrodes are examined and the pair location is determined from the timing and waveform characteristics.

In the present system, the current source for the electrodes is a sinusoidal waveform of approximately 20 microamperes at a frequency of approximately 2 KHz. Twenty (20) microamperes has been determined to be the maximum RMS safe current at 2 KHz by the Association for the Advancement of Medical Instrumentation.

It should also be appreciated that the construction of the inventive system allows any electrode to be a sense electrode or a drive electrode as desired and selected.

This invention has been described with reference to a preferred embodiment, obviously modifications and alterations will occur to others upon the reading and understanding of this specification. It is intended that all such modifications and alterations be included insofar as they come within the scope of the appended claims or the equivalents thereof.