WO1992011901A1

WO1992011901A1 - Rate responsive pacer

Info

Publication number: WO1992011901A1
Application number: PCT/US1991/009528
Authority: WO
Inventors: Gianni Plicchi
Original assignee: Medtronic, Inc.
Priority date: 1991-01-09
Filing date: 1992-01-02
Publication date: 1992-07-23
Also published as: AU1206792A; IT1244639B; ITRM910010A1; ITRM910010A0

Abstract

An implantable rate responsive pacer (30) for pacing a patient's organs or living tissues. The pacer (30) comprises a pacer control circuit (70) for generating pulses to control the cardiac activity at a rate which varies between a predetermined upper limit and a predetermined lower limit as a function of the metabolic demand of the patient. The pacer (30) further includes a single electrode (A) for sensing the patient's physiologic parameters, and for generating signals indicative thereof. A pacer control device (70) responds to the electrode's signals, for generating control pulses to control the cardiac activity at the rate variable between the upper rate and the lower rate. The pacer control device (70) is located within a housing (30), and the electrode is physically coupled or connected to the housing (33). The electrode (A) has a surface area which is sufficiently large to optimize the sensitivity of the pacer (30).

Description

I RATE RESPONSIVE PACER

BACKGROUND OF THE INVENTION

FIELD OF THE INVENTION

The present invention generally relates to medical cardiac pacers, and more particularly, it relates to a pacemaker, and to an electrode for use in the pacemaker for responding to changes in activity level and pulmonary physiologic parameters.

DESCRIPTION OF THE BACKGROUND ART

Pacemaker rate control has been conventionally derived from control signals obtained from a plurality of measuring elements such as cardiac catheters and special breathing and temperature sensors. Functional parameters used for the control of the pacing rate are dependent upon the patient's physical conditions and dynamically changing physiologic variables. It is therefore desirable to have the pacing rate controlled by information derived from parameters closely representative of such physiologic conditions.

Several pacing methods have addressed this objective in an attempt to adjust the stimulation rate to the patient's metabolic demand by means of a suitable physiologic variable sensor circuit. These pacing methods have targeted various physiologic parameters, such as: atrial activity, blood Ph, body temperature, oxygen saturation in the venous blood, QT interval obtained from the endocavitary E.C.G. , respiratory rate, minute ventilation, physical activity, cardiac output and i electromyogram of the diaphragm.

A discussion of these pacing methods can be found in the following publications: "Technical Aspects of Sensors", I. Bourgeois and F. Lindemans, New Perspectives in Cardiac Pacing, 229-254.

Clin. Prog. Pacing and Electrophysiol. Vol. 1 n. 1-1983 "Rate Responsive Pacing" by Anthony F. Rickards M.D. and Robert M. Donaldson M.D., National Heart Hospital-London W 1 England.

" A Physiologically Controlled Cardiac Pacemaker", Krasner, Voukydis and Nardella, J. A. A.M.I. , Vol. 1 n. 3-1966; 14-20.

"A Pacemaker Which Automatically Increases Its Rate With Physical Activity", Kenneth Anderson, Dennis Brumwell, Steve Huntley of Medtronic Inc., Minneapolis, Minnesota, U.S.A.; and "Variations of Cardiac Pacemaker Rate Relative to Respiration", Getzel W.,

Orlowski J., Berner B., Cunningham B., Esser M., Jacob M., Jenter D., IEEE/Engineering in Medicine and Biology Society First Annual Conference-p. 50, 1979.

Several patents have also described the pacing rate control of a pacemaker by measuring signals based on the detection of a physiologic parameter to provide pacing rate control dependent upon pulmonary activity. Thus, in United States Patent No. 4,567,892 issued to Plicchi et al., the respiratory rate is determined from an implanted electrode by an impedance measurement.

In the Lekholm et al. United States Patent No. 4,697,591, the respiratory rate is determined from the impedance across the chest cavity by using the pacemaker can and the heart implanted electrodes. In United States Patent No. 4,596,251 issued to Plicchi et al., the respiratory minute ventilation is measured by detecting variations of the geometry of a part of a patient's chest as a part of the pulmonary ventilation.

Other related respiratory rate controls are effected in United States Patent No. 3,593,718 to Krasner et al.; United States Patent No. 4,576,183 to Plicchi et al.; United States Patent No. 4,702,253 to Napphholz et al.; and United States Patent No. 4,721,110 to Lampadius. However, the concept of determining respiratory activity by determining the changes of transthoracic impedance is susceptible to recurrent disturbances that are not respiratory in nature. These disturbances can be caused, for instance, by coughing, speaking, laughing, sudden changes of position, and the movement of the pacemaker can in the subcutaneous pocket where it is implanted. Such motion of the pacemaker in the pocket can be induced by physical activity or normal respiration. With the miniaturization of the can, the macrodisplacement of the pacemaker is also induced by the patient twiddling the can, thus accentuating the signal disturbances. The signals generated by these disturbances generally compete with the respiration induced signals. It is therefore desirable to minimize the effect of these interference signals and to provide a more reliable determination of the patient's activity level and pulmonary ventilation. Alt et al. United States Patent No. 4,919,136 describes a ventilation controlled rate responsive cardiac pacemaker for minimizing the signals caused by non- respiratory events. In this patent, the intracardiac impedance is measured with a sensor located within the right side of the heart. The resulting signal is then fed through a low-pass filter circuit and a high-pass filter circuit for splitting the signals into lower and higher frequency portions. The low-pass filter circuit passes the signals associated with the patient's lower respiratory rate, and the high-pass filter circuit passes the signals associated with the patient's higher-rate cardiac activity.

Wherefore, it is desirable to have a new electrode for use in cardiac pacemakers and in particular with activity and/or ventilation dependent rate responsive pacers. The electrode should substantially reduce non-pulmonary and non-activity induced signals caused by extrinsic disturbances, such as the relative displacement of the can and the electrodes caused by the heart contraction.

BRIEF SUMMARY OF THE INVENTION

It is therefore one object of the present invention to disclose an improved impedance sensing electrode for use in cardiac pacers and more particularly in activity and/or minute ventilation rate responsive pacers using a standard unipolar or bipolar lead. Briefly, the above and further objects and features of the present invention are realized by providing an implantable rate responsive pacer for pacing a patient's organs or living tissues. The pacer comprises a pacer control circuit for generating pulses to control the cardiac activity at a rate which varies between a predetermined upper limit and a predetermined lower limit as a function of the metabolic demand of the patient. The pacer further includes a single electrode for sensing the patient's physiologic parameters, and for generating signals indicative thereof. A pacer control device responds to the electrode's signals, for generating control pulses to control the cardiac activity at the rate variable between the upper rate and the lower rate. The pacer control device is located within a housing, and the electrode is physically coupled or connected to the housing. The electrode has a surface area which is sufficiently large to optimize the sensitivity of the pacer.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects and features of the present invention and the manner of attaining them, will become apparent, and the invention itself will be best understood, by reference to the following description and the accompanying drawings, wherein: Figure 1 illustrates a conventional unipolar pacemaker;

Figure 2 illustrates in block diagram form various impedances measured between the sensors in the conventional pacemaker;

Figure 3 is a front face view of a pacemaker according to the present, showing a novel electrode mounted on the pacemaker can; Figure 4 is an enlarged side view of the electrode of Figure 3;

Figure 5 is an enlarged front view of the electrode of Figures 3 and 4; Figure 6 illustrates the pacemaker and electrode of Figures 3, 4 and 5 implanted in a patient;

Figure 7 illustrates a simplified block diagram of a single chamber demand pacemaker for use with the electrode of Figures 3 - 6;

Figure 8 is an alternative embodiment of a pacemaker using a bipolar electrode arrangement in accordance with the present invention; and

Figure 9 illustrates another alternative embodiment of a pacemaker using a multi-sensor arrangement in accordance with the present invention. DESCRIPTION OF THE PREFERRED EMBODIMENT

Figure 1 illustrates a conventional unipolar pacemaker 10 having a pulse generator enclosed inside a can 12, and a lead 14 which extends from a connector 16 for implantation within the heart. The lead 14 includes an electrode which terminates at its distal end 18 in a tip B. This unipolar pacemaker 10 is conventionally used to measure the impedance Z_BC or the impedance variation ΔZ_BC between the tip B and the can 12.

Figure 2 illustrates, in a simplified block diagram form, the various impedances measured between the tip B and the can 12, in the pacemaker 10. Z_M, V_M and I_M represent the impedance, voltage, and current, respectively, between the tip B and the can 12, such that:

V_M = Z_M x I_M. (1)

In the conventional pacemaker 10, the total impedance Z_M comprises several impedance components, as shown in the following equation: Z_M = Z_P + Z_L + Z_B, (2) where Z_P is the internal impedance of the pulse generator circuitry. Z_L represents the impedance of the lead 14, and Z_B represents the body impedance.

The body impedance variation ΔZ_B, in turn, includes several components, as indicated by the following equation: ΔZ_B = ΔZ_R + ΔZ_H + ΔZ_C, (3) where ΔZ_R is the impedance variation introduced by the change in the conduction path and distance between the tip B and the can 12, due to respiration and physical activity, ΔZ_H is the impedance variation introduced by the change in the conduction path and distance between the tip B and the can 12, due to contraction of the heart, and ΔZ_C is the contact impedance variations introduced by the interface of the can and the tip B with the body tissues and muscles during heart contraction, respiration and activity.

By substituting the components of the body impedance variation ΔZp (Equation

3) in Equation (2), the total impedance variation Δ2__M will be expressed as follows: ΔZ_M = ΔZ_P + ΔZ_L + (ΔZ_R + ΔZ_H + ΔZ_C) (4) Traditional pacing methods generally attempt to minimize the values of ΔZ_H and ΔZ_C. The contact impedance variation ΔZ_C has been treated as an undesirable parameter which reduces the impedance measurement efficiency. The intrinsic impedances Z_P and Z_L remain unchanged, and hence, the variation of this impedance is approximated to zero.

The inventive pacemaker 30 is illustrated in Figure 3, and purports to measure the variation in the contact impedance Z_c, as an indication of activity and respiratory levels, for controlling the responsive pacing rate.

For this purpose, a specially designed electrode A is affixed to the top of connector 32 of the pacemaker 30. In this manner, the distance and impedance path (ΔZ_K) between the electrode A and the can 33 is fixed and does not vary with pulmonary and non-pulmonary and activity and non-activity induced signals caused by extrinsic disturbances, such as: (1) the movement of the pacemaker can in the subcutaneous pocket; (2) the relative position variation of the can and the electrodes vis-a-vis each other caused by respiration and physical activities; and (3) the relative displacement of the can and the electrodes caused by the heart contraction.

The pacemaker 30 includes a lead 34 which is similar to the lead 14, and which terminates at its distal end 38 in a tip electrode C. In the preferred embodiment, the electrode C is used to stimulate the heart, and is not used to measure the impedance or impedance change between the heart and the can 33. Rather, the variation in the contact impedance Z_c is measured between the new electrode A and the can 33.

Since the electrode A and the can 33 are placed remotely from the heart, the signals caused by the heart contraction do not substantially affect the impedance variation between the electrode A and the can 33. Furthermore, since the electrode A is rigidly connected to the can 33, micro and macro displacements are substantially minimized, if not completely eliminated, when compared to the displacement of the conventional lead 14 which is implanted within the heart.

As a result, there is no need to use a separate sensor for activity and another sensor for respiration. A single electrode A will respond to the patient's activity and respiration levels as well as to change in posture. The use of a single electrode to replace multiple electrodes realizes a substantial cost reduction in the pacemaker manufacture, and enhances its reliability. In addition to the simplification of the hardware implementation, the software programming is now also simplified and the implantation of the pacemaker is rendered less complicated.

The measurement of the variation in the contact impedance Z_c is carried out by the circuit 70 illustrated in Figure 7, which represents a simplified block diagram of a single chamber demand pacemaker for use with the electrode A of Figures 3, 4 and 5. The function and components of the circuit 70 is explained are conjunction with the pacemaker described in the U.S. Patent No. 4,596,251 issued to Plicchi. The Plicchi patent is incorporated herein by reference.

Due to the fixed conduction path and distance between the electrode A and the can 33, the impedance Z_R resulting from respiration and physical activity, and the impedance Z_h resulting from the contraction of the heart are relatively negligible. Consequently, the variation in the total impedance measured by the pacemaker

30 is closely approximated, as follows:

ΔZ_M = ΔZ_C. (4)

Investigations have shown that by sizing and shaping the electrode to particular dimensions, an optimal stability of the impedance measurement can be achieved. It was experimentally found that a surface area of about 400 square millimeters (mm²) provides an optimal long term stability for impedance measurement.

Figures 3, 4 and 5 illustrate the preferred shape of the novel electrode A, and its position on the can 33. It will however be understood that other shapes and positions of the electrode A can also achieve substantially similar results. A similarly acceptable result can be achieved by selecting the surface area of the electrode A from a range between 100 mm² and 500 mm². The surface area of the electrode A is an important feature of the inventive electrode A. It was experimentally found that if the surface area of the electrode A is too small to accurately measure the variation in the contact impedance Z_c, the reliability of the electrode A is adversely compromised. The electrode A includes two generally symmetrical and identical front and rear face plates 40 and 42, respectively, and which are bridged by an intermediate plate 44. The plates 40, 42 and 44 are integrally connected to one another to form the electrode A.

The front and rear face plates 40 and 42 being generally symmetrical, only the front face plate 40 will now be described in greater detail. The front face plate 40 is generally rectangular in shape, and extends along one of its lengths 45 into a curved portion 46 for interconnecting to the intermediate plate 44. The preferred radius of curvature R of the portion 46 is about 1.6 mm. The preferred thickness T of the front face plate 40 is about 0.40 mm. The height H of the front face plate 40 is about 5 mm, while its length L is about 20 mm. The width W of the electrode A, that is the distance between the front and rear face plates 40 and 42 is about 10 mm.

Considering now the intermediate plate 44, it is generally formed of a flat rectangular plate, with substantially the same thickness as the front face plate 40. The intermediate plate 44 extends along the lengths of the front and rear face plates 40 and 42. Therefore, in the preferred embodiment, the length of the intermediate plate 44 is about 20 mm, while its width is about 10 mm. The width of the intermediate plate 44 can vary with the thickness of the top portion of the can 33, since the electrode A is straddled across the top portion of the can 33, and is dimensioned to fit snugly and fixedly thereto. The electrode A can therefore be formed by using a generally flat rectangular plate having the following dimensions: Length 20 mm, Width 20 mm, and Thickness 0.40 mm. The plate is then bent into the shape illustrated in Figure 4. The electrode A can be composed of generally known conventional bio-compatible conductive materials. The surface of the electrode A can be made porous material, or, it can be treated by conventional surface treatment methods to increase its effective surface. The relative design simplicity of the new electrode A makes it universally adaptable for use with existing pacemakers with minimal modifications.

Considering now the function of the electrode A, the patient's activity and respiration levels cause a movement of the pacemaker in the pocket where it is implanted. The impedance variation measured between the electrodes A and B is mainly caused by changes induced by respiration and activity (inertial forces), which effect muscle pressure on the can 33. It should also become apparent to those skilled in the art after reviewing the present disclosure that the impedance variation measured in the absence or substantial reduction of one physiologic parameter is indicative of the remaining parameters. For instance, during sleep, when activity is at a generally minimal level, the impedance variation between the electrodes A and B is indicative of the pulmonary minute ventilation.

While the electrode A is shown connected to the connector 32, it should be understood that the electrode A can be placed in any suitable and non critical position on the can B to permit the measurement of the variation in the contact impedance Zc. Figur" 6 illustrates the subcutaneous placement of the pacemaker 33 and the electrode A. The electrode C is shown implanted within the right ventricle of the patient. However, since the electrode C is not used to measure the body impedance, the electrode C could be located outside the heart, as indicated by the electrode C. In the alternative, if is desired to simultaneously stimulate the heart and other tissues of the body, both the electrodes C and C could be deliver stimulation pulses in response to the signals from the electrode A.

It will be also understood to those skilled in the art, after reviewing the description of the present invention that, while a unipolar demand pacemaker 30 is illustrated as the preferred embodiment, a bi-polar, or a multi-polar pacemaker could be used instead.

Furthermore, due to location and surface area of the inventive electrode A, it is no longer necessary to use sophisticated measuring devices, such as tripolar or quadripolar instruments. Figures 8 and 9 illustrate alternative embodiments of the pacemakers 80 and

90, respectively, in multi-electrode configurations, where the references Al, A2, and A3 refer to electrodes which are disposed on the can 33 and which are electrically insulated from the electrode A. An electrode D can be disposed at the lower portion 40 of the can 33 in a substantially opposite relationship to the electrode A. The electrode D can be sized to substantially the same measurements as the electrode A and can be used in conjunction with the electrode A to the exclusion of the other electrodes Al and B2 (Figure 8). Thus, with the continuous need for ways to miniaturize the pacemakers, each of the electrodes A and D can have a surface of about 200 square millimeters. In this way, the reduction in the size of the pacemaker does not affect the overall measurement by the pacemaker 80.

This concept of splitting the total electrode surface can be expanded to a multi- electrode pacemaker, with more than just two electrodes. For instance, in certain applications, where it is desirable to have a generally circular pacemaker shape, as illustrated in Figure 9, an electrode F can be centrally located and surrounded by the electrodes Al, A2, and A3, such that the combined surfaces of these electrodes total about 400 square millimeters.

The operation of the electrode A and the pacemaker P will now be described in a simplified way in relation to the block diagram of a single chamber demand pacemaker, as illustrated in Figure 7. Block 71 refers to a strobed Impedance/Voltage converter which uses sampling frequencies of approximately 10 Hz for sending narrow pulses of proper amplitude to the electrode A and B. Block 72 refers to a signal change responsive circuit for measuring the absolute variations of the signal corresponding to the impedance between two consecutive pulses. The output signal is V'(ΔZ_AB) which is proportional to the chest volume variation is therefore proportional to the instantaneous respiratory flow in absolute value, with the flow being a variation of the chest volume.

The signal V'(ΔZ_AB) at the output of block 73 has peaks corresponding to the phases of the respiratory cycle in which the expiratory and inspiratory speeds reach their maximum values and will have a zero value when respiratory dynamics are absent.

Another function of the signal change responsive circuit of block 72 is the offsetting of the slow variations of the impedance between electrodes A and B resulting, for instance, from the histologic changes in the tissues surrounding the same electrodes or from the change in the relative position of the same electrodes or to the slow variation of the bodily mass of the patient, or from the posture variation of the patient or, from the variation of the lung residual functional capacity which shows a positive increase under strain conditions.

Block 73 refers to a low-pass filter with a time constant of a few tens of seconds, e.g., approximately 30 seconds. This low-pass filter determines the mean value or the average of the absolute values of the impedance variations with a time constant, in order to minimize the ripple in the output signal and to be sufficiently fast to physiologically adjust the heart stimulation rate. Signal Vm (ΛZ_AB) corresponds to the mean value of the input signal. Experimental data has shown that the signal Vm (ΔZ_AB) is proportional to the minute ventilation. The signal Vm (ΔZ_AB) is the physiologic variable driving the pacemaker stimulation or inhibition rate.

Block 74 refers to a programmable correlator which correlates two values Vml(ΔZ_AB) and Vm2(ΔZ_AB) of the output signal from block 73, which are "a priori" programmable or obtained in two different physical activity situations of the patient, to two stimulation/inhibition rates (fl and f2) of the pacemaker 33. These rates define a possible operational mode of the pacemaker on the basis of which fl and f2 may, not necessarily but possibly, coincide with the minimum and maximum working rates of the pacer. The stimulation/inhibition rate (f) at the input of block 75 is proportional to the signals Vm (ΔZ_AB) at the input of block 74.

Block 75 represents a conventional pacemaker circuit well known to persons skilled in the art. The input signal V (Z_AB) includes pulses having an amplitude which is proportional to the impedance periodically measured between electrodes A and B. SHI and SH2 indicate two sample and hold circuits and AMP-1 indicates a differential amplifier. SHI stores the amplitude of the N* D pulse and SH2 stores the amplitude of the N^ + 1 pulse. A CK time signal synchronous with the pulses of block 1 first sends the output signal from SHI and SH2 and then, after a delay caused by the DEL circuit, stores in SHI the new value of the N*-. 1 pulse amplitude. At this stage, an AMP-1 circuit performs the difference between the N⁴ + 1 amplitude stored in SHI . This signal processing is thereafter repeated. The Dl signal at the output of AMP-1 is shown in Figure 15. The functioning Blocks 72, 73 and 74 can be realized with analog and/or digital or microprocessor based circuits. Although the present invention is disclosed within the context of a single chamber demand pacemaker, a substantially similar design can be applied to other pacing modalities, including the double chamber modes which maintain the atrioventricular sequentiality, or even applied to other therapeutic or diagnostic, portable or implantable devices or to artificial organs, such as an artificial heart, operating in response to the minute ventilation of the patient.

While particular embodiments of the present invention have been disclosed, it is to be understood that various different modifications are possible and are contemplated within the scope of the specification, drawings, abstract and appended claims.

What is claimed is:

Claims

1. An implantable rate responsive pacer for pacing a patient's organs or living tissues comprising: a. single electrode means for sensing the patient's physiologic parameters, and for generating signals indicative thereof; b. pacer control means, responsive to said electrode signals, for generating control pulses to control the cardiac activity at a rate variable between a predetermined upper limit and a predetermined lower limit; c. means for housing said pacer control means; d. said electrode means being physically coupled to said housing means; and e. said electrode means having a surface area sufficiently large to optimize the sensitivity of the pacer.

2. The pacer as defined in Claim 1, wherein said electrode means measures the impedance variation of a portion of the patient's chest cavity generally adjacent to said housing means, and wherein the surface area of said electrode ranges between 100 square millimeters and 500 square millimeters.

3. The pacer as defined in Claim 1, wherein the surface area of said electrode means is about 400 square millimeters.

4. The pacer as defined in Claim 1, wherein said electrode means is responsive to the patient's pulmonary minute ventilation.

5. The pacer as defined in Claim 1, wherein said electrode means is responsive to the patient's physical activity levels.

6. The pacer as defined in Claim 4, wherein said electrode means is further responsive tp the patient's physical activity levels.

7. An electrode for use in an implantable rate responsive pacer for pacing the patient's organs or living tissues comprising: a. front face plate means; b. rear face plate means disposed generally in opposite relationship relative to said front face plate means; c. intermediate plate means for coupling said front face plate means and rear face plate means; and d. wherein the electrode is physically coupled to the pacer.

8. The electrode as defined in Claim 7, wherein the combined surface of said front face plate means, said rear face plate means and said intermediate plate means ranges between 100 square millimeters and 500 square millimeters.

9. The electrode as defined in Claim 7, wherein the combined surface of said front face plate means, said rear face plate means and said intermediate plate means is about 400 square millimeters.

10. The pacer as defined in Claim 1, wherein said electrode means includes: a. front face plate means; b. rear face plate means disposed generally in opposite relationship relative to said front face plate means; c. intermediate plate means for coupling said front face plate means and rear face plate means; and d. wherein the electrode is mounted onto the pacer.

11. The pacer as defined in Claim 10, further including sensor means physically coupled to the pacer, and wherein the impedance variations are measured between said electrode means and said sensor means.

12. The pacer as defined in Claim 11, further including a plurality of sensor means disposed on the pacer, and wherein the impedance variations are measured between said electrode means and said sensor means.

13. The pacer as defined in Claim 10, further including a plurality of electrode means disposed on the pacer and electrically insulated from one another, and wherein the impedance variations are measured between said electrode means.

14. The pacer as defined in Claim 13, wherein said electrode means are generally similarly shaped and dimensioned.