Cardiac and Pulmonary Physiological Analysis via Intracardiac Measurements With a Single Sensor

This invention relates to the analysis of instantaneous physiological parameters affecting the heart and lungs and more particularly it relates to means and methods of measuring with a single sensor both cardiac and pulmonary physiological parameters of a patient to derive and analyze data useful in treating the patient for cardiac and pulmonary conditions such as the control of a cardiac pacemaker in accordance with dynamic changes incurred in physiological activity of the patient.

In modern intensive medicine a successful therapeutic intervention is only possible on the basis of extensive diagnostic information. Knowledge of cardiac and respiratory activity as physiological functional parameters is essential. They are usually monitored using a plurality of measuring elements such as cardiac catheters, special breathing sensors, etc. For this monitoring, not only is the display and evaluation of the functional parameters difficult to coordinate, but also the placement of the detection instruments within the heart and lungs of a patient is difficult.

Many such functional parameters are also dependent upon a patient's exercise, so that they can also be used to control dynamic variations in the pacing rate of a cardiac pacemaker.

Some available publications describe pacing rate control of a pacemaker by measured signals based on the detection of one physiological functional parameter. Thus, in U.S. Pat. No. 4,566,456, G. Koning et al., Jan. 28, 1986, the systolic pressure and change in time of the right ventricular pressure is used as the functional parameter. In German Offenlegungsschrift No. 27 17 659, A Wirtzfeld, et al., published Oct. 26, 1978, the central venous oxygen saturation parameter is used. In U.S. Pat. No. 4,535,774, W. H. Olson, Aug. 20, 1985 and U.S. Pat. No. 4,674,518, R. W. Salo, June 23, 1987, the ventricular stroke volume of the heart is determined by means of an impedance measurement. In U.S. Pat. No. 4,567,892, G. Plicchi, et al., Feb. 4, 1986, the respiratory rate is determined from an implanted secondary electrode by an impedance measurement. In U.S. Pat. No. 4,697,591, A. Lekholm, et al., Oct. 6, 1987, the respiratory rate is determined from impedance across the chest cavity by using the can and heart implant electrodes. In U.S. Pat. No. 4,596,251, G. Plicchi, et al., June 24, 1986, the respiratory minute volume is measured by impedance changes from at least one electrode located in the chest cavity. Other related respiratory rate controls are effected in U.S. Pat. Nos.: 3,593,718, J. L. Krasner et al., July 20, 1971; 4,721,110, M. S. Lampadius, Jan. 26, 1988 and 4,702,253, T. A. Nappholz et al., Oct. 27, 1987. In U.S. Pat. No. 4,291,699, L. A. Geddes, et al. Sept. 29, 1981 the change of impedance between two electrodes in one ventricle is used to indicate and control fibrillation of the heart. In U.S. Pat. No. 4,576,183 G. Plicchi, et al., Mar. 18, 1986 subcutaneous electrodes in a patient's chest are used to measure impedance for obtaining a respiratory parameter.

Recently there have also been proposals to control the pacing rate of a cardiac pacemaker from two or more physiological functional parameters. In German Patent No. P 36 31 155.C2, published Mar. 24, 1988, pacing rate is controlled for stable long-term control from the temperature of the venous blood within the heart and from an activity sensor for short-term exercise related activity. The temperature signals can be modulated by the activity signals for an optimal adaptation of the pacing rate to the particular exercise of the patient. Different sensors may be used to check the two functional parameters. The pacemaker control is based on the finding that essentially only absolute parameters such as the blood temperature and activity should be used as absolute values for determining a relationship between these parameters and the pacing rate, whereas other physiological functional parameters are merely relative parameters, which at least impede stable long-term control of the pacemaker.

U.S. Pat. No. 4,722,342, D. Amundson, Feb. 2, 1988 provides a plurality of different body activity sensors to derive variable pacer controls for body activity.

In this and related prior art instrumentation for analyzing a patient's physiological condition as related to desired and actual cardiac activity there are unresolved problems corrected by this invention. Thus, the prior art does not provide simple easy to install in the patient detectors, nor do the detectors produce adequate physiological functional parameters providing little possibility of error in derived signals for introducing dynamic changes in heartbeat stimulus produced by pacemakers.

For example, when multiple sensors are used at different body locations, such as chest and heart cavities, not only is there a chance for erroneous control signals, but there is the corresponding necessity to implant special detectors for this purpose. The chances for control error may be typified by the measurement of impedance across a chest cavity to derive ventilatory response signals under different conditions of activity. Thus, the measured impedance can vary with the position of a patient's body or arm, and is not solely restricted to the period or magnitude of inspiration or expiration. Thus, false control signals could adjust the pacing rate in response to sensed respiratory physiological parameters. Such false signals can also come from interference between multiple sensors and complex processing systems for analyzing and merging various detected signals.

Another problem with prior art sensors is the ability to dynamically respond closely enough to real time to those physiological parameters of a patient that produce proper control signals for heartbeat stimulus provided by a pacemaker. Thus, for example, temperature measurements slowly respond and pressure measurements are subject to outside noise influences such as coughing or sneezing that should not affect the heartbeat rate.

The simplicity of prior art detectors was primarily due to the use of a single variable such as respiratory or blood pressure fluctuations. However, this does not give enough data for successful diagnosis, therapy or pacing rate control. Thus, the problem of simplicity in system and installation of detectors has not been resolved in such a way to produce the desired physiological parameters for analysis and control. The present invention thus affords a single sensor within the heart operable to measure a single intracardiac functional parameter, and means to derive from that measurement both pulmonary activity and cardiac activity. This detector in the case of a patient having a pacemaker is the already implanted stimulus electrode, preferably on the right side of the heart and to use that electrode both as an intracardiac detector and as a heart pacer.

Accordingly, both intracardiac pressure fluctuations correlating with the patient's breathing and physiological signals coming from the heart itself can be detected using only one measuring element located within the heart and detecting only one integral intracardiac functional parameter.

Investigations by applicant correlate breathing and intrathoracic pressure fluctuations with physiological parameters of blood measured in the heart. Thus, for example, a single sensor in the right ventricle can determine from changes of blood parameters, preferably impedance, the necessary functional parameters for proper heartbeat rate control of a pacemaker, namely the rate and depth of respiration, contractility of the myocardium, stroke volume, etc.

Great advantages are obtained in control of a cardiac pacemaker in response to these detected signals. Significant is the simple construction with the measuring element constituting the simultaneously used stimulation electrode, thus necessitating no further implants.

An essential advantage of the invention is that the sole intracardiac measurement, preferably impedance, allows for respiratory and cardiac functional parameters to be clearly distinguished from each other by appropriate filtering with respective high pass and low pass filters. This is not possible with prior art measurements such as the impedance measurement of breathing with a thoracic secondary electrode, in part because the respiratory effects on the signals detected are such that the overall information signal has little indicative value and is superimposed with high interference. Such mutual interference of respiratory or pulmonary and cardiac functional parameters is reliably eliminated by this invention.

The invention shall be explained in more detail in an exemplary embodiment in connection with a cardiac pacemaker with reference to the accompanying drawings, in which:

FIG. 1 shows a schematic view of an inventive cardiac pacemaker electrode system implanted in a patient's heart,

FIG. 2 shows in block diagram form the electronic diagnosis system afforded by this invention,

FIG. 3 is a waveform diagram showing the bandwidth characteristics of the respiratory and cardiac signal components for forming low-pass and high-pass filters,

FIGS. 4, 5 and 6 are signal waveform diagrams illustrating the impedance curves for intracardiac measurements in animals, with various parameters, and

FIG. 7 shows inter-related human impedance waveforms with the lower frequency respiratory and higher frequency cardiac signal components.

FIG. 1 shows a cardiac pacemaker 2 implanted in a patient 1, comprising a can 3 from which a probe 4 leads via a vein into heart 5. Probe 4 is designed at its front end located in the ventricle as a single sensor 6, which in this embodiment has a first electrode pole 7 located at the tip of the probe 4 and a second electrode pole 8 located more proximally. As seen from FIG. 2, electrical lines 9 and 10 within probe 4 connected the two spaced poles 7 and 8 located within the right ventricle with a control circuit 11 disposed in can 3.

The block diagram control circuit 11 has functionally defined control circuit elements which can be embodied into an integrated circuit along with associated microprocessing means and appropriate software. The control circuit comprises generally a measurement section 12, a logic section 13 and a stimulation section 14. Energy is supplied by battery 15.

One line 10 from detector electrode pole 8 is connected to ground potential. The other detector signal line 9 is coupled to the signal measurement section 12 by high pass filter 16. An oscillator 17 provides alternating current of low amplitude unable to pace the heart for impedance measurement with a frequency between approximately 1 kHz and 100 kHz that does not interfere with pacer stimulation. A low current amplitude in the range of a few microamperes, or alternatively single pulses of 0.01 msec duration and amplitude of less than a milliampere, reduces battery load. The current signal of oscillator 17 is also applied in addition to the stimulus pacer signal 25 to the measuring-stimulus electrode 6 by way of the dotted lead. This is an interrogating electric signal which responds at the single sensor 6 to variations of cardiac activity and of pulmonary activity to produce a single variable signal representative of heart and pulmonary activity, and has such low energy that it does not pace or stimulate the heart, nor interfere with the pacer signals 25.

Physiological activity detection or measuring means 18 then processes the single variable signal or subsignal derived from it such as the lower and higher frequency components of the dynamic signals sensed at electrode 6 and modulated on the high frequency oscillations from oscillator 17 supplied via filter 16. This single variable signal responds to the changes in physiological parameters such as changes in volume, flow or pressure in the patient's vascular system preferably within the right heart. The preferred embodiment of a detection and measurement system for this purpose responds to the variations of blood impedance indirectly indicating the changes in volume.

Accordingly the resistance (or impedance) within the heart between the single sensor 6 electrode poles 7 and 8, as defined by Ohm's law, is determined in the impedance measuring means 18 following high-pass filter 16 which transmits the carrier signals from the single signal reproduced by sensor 6 varying with cardiac and pulmonary activity modulated upon the oscillator 17 signal frequency, and rejects the pacing signals 25 passed through low-pass filter 26. The resulting raw signal varying dynamically in impedance is fed on one hand to a low-pass filter circuit 19 and on the other hand to a high-pass filter circuit 20, which splits the impedance modulated signal into lower and higher frequency subsignal portions. Thus, low-pass circuit 19 passes physiological activity signals associated with the patient's lower rate respiratory activity, whereas high-pass circuit 20 passes physiological activity signals associated with the patient's cardiac activity. Since the heart rate is generally four to five times greater than the respiratory rate, these respiratory or pulmonary and cardiac signals can be separated by filters within the known state of the art.

The output signals of low-pass and high-pass circuits 19, 20 are each fed to a respective signal shaping circuits 21, 22 for pre-evaluation, e.g. averaging, determination of the derivative in time, evaluation of amplitude and frequency and subsequent integration, or the like. An output line 23-1 of low-pass circuit 21 then provides a signal associated with the respiratory rate and corresponding to the periodic frequency of the low-frequency signal. The further output line 23-2 provides a signal associated with the depth of respiration and corresponding to the amplitude of the low-frequency signal. An output line 24-1 of high-pass circuit 22 provides a signal associated with the contractility of the heart and corresponding to the derivative in time (dV/dt) of the high-pass filtered impedance signal, i.e. the rate of change in time of the systolic stroke volume. A further output line 24-2 provides a signal associated with the stroke volume of the heart and corresponding to the amplitude of the high-passed impedance signal.

All output lines 23, 24 are connected to logic circuits 13, which calculate on the basis of available signals, an optimum pacing rate related to the exercise of the pacemaker wearer. As represented in the above mentioned prior art, it is known in the art how to use physiological signals to control the pacing rate signals 25 for a heart pacer. This pacing rate is fed to a pulse generator 25 in stimulation signal section 14, which provides corresponding stimulation pulses to the single sensor electrode 6 via a filter 26. The frequencies of measuring channel 12 and stimulation channel 14 are separated by filters 16 and 26 so that the signals in one channel do not interfere and impair the functioning of the other channel. In this manner the single sensor electrode 6 can be used both as a measuring electrode and as a stimulation electrode.

FIG. 3 in the lower frequency curve denoted by the square coordinate markers the filter characteristic of low-pass filter 19 are typified. It is seen that the degree of transmission stated in % has dropped virtually to zero at a frequency of one hertz. Signals correlating with cardiac activity have a frequency higher than this value, so that there is no interference with the low-passed signals representative of breathing rate. The low-passed breathing signals are processed in signal shaping circuit 21, and if necessary can be amplitude corrected to allow for filter characteristics, to provide signals for evaluation of the amplitude of respiration. The filter characteristics of the high-pass circuit 20, as shown by the curve with the circular coordinate points, permits the signals based on cardiac activity to pass without interference with or distortion by the respiratory signals.

As shown in FIG. 4 in numerous measurements with animals, and with corresponding tests on more than twenty healthy persons and pacemaker patients, applicant has confirmed the features on which the invention is based. An initial series of tests on dogs shows that changes in intracardiac physiological parameters, i.e., pressure, volume and the corresponding flow through the heart, correspond to cardiac activity, upon which changes due to the respiratory activity are superimposed. More on these changes in flow following respiration has been disclosed in an article by Gerhard A. Brecher, published in Circulation Research, Volume III, Mar. 1955, pp. 210 to 214. Appropriate alternative means to measure flow within the vascular system are known from the publication, "Subselective Measurement of Coronary Blood Flow Velocity Using a Steerable Doppler Catheter," by David H. Sibley, et al. in JACC, Vol. 8, No. 6, Dec. 1986, pp. 1332-1340.

In the same series of tests it was also shown that myocardial contractility exerts an influence on the rate of change in time of the pressure and volume change due to heartbeat within one heartbeat in the right ventricle. The reference waveform is the electrocardiogram ECG, with the indirect volume measurement taken by impedance measurement shown as oscillation signals occurring during the ventilation period and superimposed on the cardiac activity waveform. The pressure (P.sub.RA) within the right atrium during a series of heartbeats is prominently shown to be influenced by intervening pulmonary activity. The pressure waveform in the right ventricle (P.sub.RV) with both higher and lower frequencies is shown to correlate with the corresponding impedance (Imp) waveform taken from the intracardiac impedance measurements. Therefore, the intracardiac impedance measurements and the intracardiac pressure measurements within the right ventricle provide periodic and amplitude signal data from both the cardiac and the pulmonary activity of the patient.

Thus, it is possible to determine the relative contractility of the heart via the systolic rate of change in time of the impedance signal during one heartbeat as seen from FIGS. 5a, b, c, d. The signal waveform RV dP/dt, representing contractility, taken from the right ventricle is thus compared with the pressure P.sub.RV and impedance signal Imp RV for various patient conditions, i.e., at rest and with different medications simulating different hemodynamic conditions as noted in FIG. 5 sections a, b, c and d. The various comparison waveforms show the correlation between pressure and impedance in the right ventricle and relates myocardial contractility to the rate of change of the intracardiac pressure as derived in impedance measurements as well. Note that variations in rate and amplitude are available for denoting the corresponding cardiac activity.

Furthermore, as evident from FIGS. 6a, b and c, not only the influence of the respiratory rate is determined from the intracardiac impedance signal, but also the depth of respiration in a dog externally ventilated. In these comparative waveform sections a, b and c, the respiratory minute volume was kept constant and the respiratory rate was increased from 10 to 20 to 30 breaths per minute. It is evident that the intracardiac impedance measurement within the right ventricle accordingly carries the periodic respiration rate information together with the depth or tidal volume information. Note that the basic parameter is a signal related to the periodic cardiac activity upon which is superimposed and modulated with signals relating to respiratory activity. Further experiments confirmed this theory in humans.

As related in FIG. 7, in numerous more advanced investigations of patients and test persons we were able to prove the dependence of intracardiac impedance changes also on spontaneous breathing in humans.

FIG. 7 shows the impedance in the upper line, the directly measured exhalation (EX) in terms of time and volume in the middle line and the ECG in the lower line. The inhalation phases are clearly recognizable in the upper line by a decrease of impedance due to inhalation. The intrathoracic pressure drop during inhalation has a strong blood suction effect and thus leads to a greater filling of the right ventricle. This increase in right ventricular volume is expressed in a corresponding drop in impedance, since more blood of lower impedance is found in the vicinity of the electrode. The increase in pulmonary impedance following an increased amount of air in the lungs due to inhalation plays a negligible part in the inventive intraventricular measurement. In the phase of breath holding after inhalation it can further be seen that after initial increased filling due to inhalation the ventricle returns to its normal size again. Subsequent exhalation then leads to a further, but quite small, increase in impedance due to a further decrease of volume of the ventricle. The fine changes of impedance which correlate with the ECG are due to the right side of the heart volume changes caused by cardiac activity.

The principle of simultaneous detection of cardiac and pulmonary parameters in the heart by means of the stated principle of measurement not only opens up possibilities for rate adaptive pacemaker therapy, but also offers a good posibility of detecting and monitoring essential physiological parameters for other diagnostic or therapeutic purposes. An example is care of patients in an intensive unit, where therapeutic consequences depend on knowledge of vital data such as cardiorespiratory activity. Since many of these patients are supplied for a short time with pacemaker electrodes anyway, the data can be obtained at the same time without any additional intervention.

The preferred object of our investigations was the change in intracardiac impedance due to the influence of cardiac and respiratory activity. The use of impedance measurements is appropriate because the bipolar electrode already used routinely to stimulate the heart with both poles being located in the heart also can be used for impedance measurement. In this case no additional sensor is required.

Nevertheless, the statements made on the basis of our results also can also be applied fundamentally to the measurement of intracardiac and intravascular pressure changes with cardiac and respiratory activity. Corresponding measured data are apparent in FIGS. 5 to 7. The change of pressure behaves as a complement to volume. As a further feature of the invention, the measurement of blood flow in the heart or the surrounding vessels can also be used in the same way to determine cardiac and pulmonary changes under varying conditions, since pressure, volume and flow variations exhibit a defined dependency between each other and react predictably to pulmonary activity and to cardiac activity.