Abstract:
An implantable tachyarrhythmia control system includes a patch electrode having a sensor integrated therein, such as a piezoelectric sensor, which is capable of monitoring mechanical heart activity. When the patch electrode is sutured to the cardiac tissue, the piezoelectric sensor will be deformed due to the mechanical activity of the heart muscle, and will generate a corresponding electrical signal. The electrical signal from the sensor will exhibit relatively low-frequency periodicity and relatively low amplitude during normal heart activity. In the event of tachycardia or fibrillation, the signal will exhibit excursions beyond those occurring for normal heart activity, and will consequently have a higher energy content. The signal is thus an indicator for the onset of these cardiac events. The signal can be supplied to an implantable defibrillator and can be used as a primary or secondary trigger for initiating defibrillation therapy, such as one or more defibrillating pulses. The signal may alternatively be supplied to an implantable combination pacemaker/defibrillator, which also undertakes standard R-wave detection via a conventional pacing lead. The signal from the patch electrode sensor can be used as before as a primary or secondary trigger to initiate defibrillation therapy alone or in combination with the detected R-wave signal. The patch sensor signal may also be used, alone or in combination with the R-wave signal, to initiate a change in the pacing mode or to initiate a particular pacing therapy, such as an antitachycardia sequence.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to cardiac defibrillating and pacing systems, and in particular to implantable systems of this type employing a patch electrode. 
     2. Description of the Prior Art 
     Many types of implantable systems are known for tachyarrhythmia control. Such systems have gained greater acceptance in recent years as an alternative therapy to chronic pharmacologic treatment. Such tachyarrhythmia control systems typically include an implantable device capable of tachyarrhythmia detection and delivery of an automatic therapeutic response to the arrhythmia, including bradycardia pacing support, anti-tachyarrhythmia pacing, low energy synchronized cardioversion or high energy defibrillation shock, an electrode system for sensing and pacing, and a high energy electrode system for delivery of defibrillation shock. Typically the pacing and sensing electrode system will consist of a bipolar endocardial lead or two unipolar myocardial leads. The high energy electrode system generally consists of two myocardial patches or a transvenous shocking electrode and a myocardial or subcutaneous patch. 
     Any device which is intended to provide automatic treatment of ventricular tachyarrhythmias must be capable of first detecting the presence of such arrhythmias prior to the onset of therapy. Several methods are known for detecting ventricular tachyarrhythmias. These include monitoring an absolute heart rate interval, and initiating therapy when the interval becomes less than a programmable interval threshold. It is also known to try to differentiate pathologic rhythms from normal physiologic rhythms by analyzing the rate of onset (sudden change, as opposed to gradual change in the heart rate interval) and/or heart rate stability. It is also known to determine the probability density function of a signal corresponding to heart activity, which involves the evaluation of the time which the cardiac electrical signal spends at an isoelectric base line, and to initiate therapy when deviations beyond a predetermined threshold occur. 
     The known detection techniques have several limitations and disadvantages. The two major disadvantages are (1) no accurate method of differentiating between a pathologic (i.e. hemodynamically compromising) rhythm versus a physiologic (i.e., sinus) rhythm, and (2) total reliance on a processed electrogram for detection of cardiac depolarization. As a result of the second disadvantage, certain rhythms, particularly low amplitude ventricular fibrillation, may not be detected. These limitations in the known detection techniques may result either in a false positive detection response (inappropriate shock delivery) or a false negative detection response (failure to respond to a pathologic rhythm). 
     SUMMARY OF THE INVENTION 
     In accordance with the principles of the present invention, a tachyarrhythmia control system is provided with a patch electrode having a motion sensor, such as a piezoelectric sensor, integrated in the patch electrode. The patch electrode is otherwise of the type conventionally used for delivery of high energy defibrillating shock. The patch electrode is attached to cardiac tissue, such as by placement directly on the myocardial surface, or the pericardial sac, so that wall motion of the cardiac muscle can be evaluated. Activity of the cardiac muscle deforms the piezoelectric sensor, thereby causing an electrical signal to be generated corresponding to the cardiac motion. The output of the sensor is processed and is monitored by the implantable device to determine the extent of the wall motion, the cardiac energy, and/or the frequency of myocardial motion to determine and differentiate the specific myocardial rhythm of an individual patient. The sensor can be used as a primary or secondary indicator of the onset of ventricular tachyarrhythmias, including ventricular fibrillation in a conventional implantable defibrillating system, or can be used in combination with a conventional R-wave detection system in a combination defibrillating and pacing system. In the latter system, the signal from the sensor can be used, alone or in combination with the R-wave signal, as a primary or secondary indicator of tachycardia, and can thus trigger the initiation of an antitachycardia pacing mode, low energy cardioversion or defibrillation, or such other pacing modes as may be responsive to the pathology indicated by the sensor signal. 
     The system disclosed herein permits a nonelectrogram-dependent means of either primary detection and differentiation of a pathologic versus physiologic cardiac rhythm, or as a secondary decision point of confirmation in the treatment algorithm for the selection of a particular therapy. The system embodying the motion sensor can be used in combination with conventional electrogram detection methods described above, and can be programmable by a physician to allow selection by the physician of use of the sensor signal alone as a therapy-determining indicator, or use of the sensor signal in combination with other detected signals. 
     The system operates based on the knowledge that in a normal physiologic cardiac rhythm, electrical depolarization and subsequent myocardial wall motion follows a specific pattern and sequence, and that an effective myocardial systole will result in a recognizable, identifiable myocardial excursion and energy. A pathologic (hemodynamically compromising) cardiac rhythm, by contrast, will result in an ineffective and/or chaotic cardiac wall motion. Monitoring of the excursion of the cardiac wall and the energy associated with the sensor signal permits discrimination between pathologic and physiologic rhythms. 
     During testing of the system at the time of implantation, various cardiac rhythms can be deliberately induced and the output of the sensor can be recorded and stored during these rhythms, for use as a template for comparison purposes during actual clinical use following implantation. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     The advantages of the present invention are best understood with reference to the drawings, in which: 
     FIG. 1 is a plan view of a first embodiment of a patch electrode having a two-terminal lead for use in an implantable tachyarrhythmia control system constructed in accordance with the principles of the present invention; 
     FIG. 2 is a plan view of a second embodiment of a patch electrode having a single terminal lead for use in an implantable tachyarrhythmia control system constructed in accordance with the principles of the present invention; 
     FIGS. 3 is a plan view of a further embodiment of a &#34;butterfly&#34; shaped patch electrode for use in an implantable tachyarrhythmia control system constructed in accordance with the principles of the present invention; 
     FIGS. 4 is a plan view of a further embodiment of a &#34;rabbit ear&#34; shaped patch electrode for use in an implantable tachyarrhythmia control system constructed in accordance with the principles of the present invention; 
     FIGS. 5 is a plan view of a further embodiment of a triangular shaped patch electrode for use in an implantable tachyarrhythmia control system constructed in accordance with the principles of the present invention; 
     FIGS. 6 is a plan view of a further embodiment of a &#34;figure-eight&#34; shaped patch electrode for use in an implantable tachyarrhythmia control system constructed in accordance with the principles of the present invention; 
     FIG. 7 is a schematic block diagram of an implantable defibrillator system constructed in accordance with the principles of the present invention using a single patch electrode; 
     FIG. 8 is a schematic block diagram of an implantable defibrillator constructed in accordance with the principles of the present invention using two patch electrodes; 
     FIG. 9 is a schematic block diagram of an implantable pacing and defibrillating system constructed in accordance with the principles of the present invention; 
     FIG. 10 is a graph showing the piezoelectric sensor signal, the aortic pressure signal, and a surface EKG signal for explaining the operation of the tachyarrhythmia control system disclosed herein; 
     FIG. 11 is a side view of a patch electrode constructed in accordance with the principles of the present invention showing a first location for placement of the motion sensor; and 
     FIG. 12 is a side view of a patch electrode constructed in accordance with the principles of the present invention showing a second location for placement of the motion sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A patch electrode 1 constructed in accordance with the principles of the present invention is shown in FIG. 1. The electrode consists of a carrier 2 consisting of Dacron®--reinforced silicone sheeting. The surface of the patch electrode 1 which is intended for contact with the myocardium is visible in FIG. 1. The opposite side of the patch electrode 1 is completely covered by the silicone sheeting. Embedded within the carrier 2 is a wire mesh 3, such as a titanium mesh, which is electrically conductive. The carrier 2 has a number of windows or openings 4 on the side thereof intended for contact with the myocardium. The mesh 3 is directly exposed to the myocardium through these openings. 
     Also embedded within the carrier 2 is a motion sensor 5, such as a piezoelectric sensor. The carrier 2 is sufficiently flexible so that when the carrier 2 is sutured to the myocardium, or to the pericardial sac, the motion sensor 5 will mechanically interact with the cardiac tissue and will generate an electrical signal corresponding to the mechanical activity of the cardiac tissue. Specifically, the wall motion of the cardiac muscle will deform the carrier 2, and the motion sensor 5 embedded therein. It will be understood that the outline indicating the motion sensor 5 is for schematic representation only, and the actual extent and size of the motion sensor may be varied as needed. In general, however, the motion sensor 5 will preferably be disposed in the neck of the patch electrode 1, where the carrier 2 begins to taper for the cable which leads to the remainder of the implantable system. Alternately, the motion sensor 5 could be located on the main portion of the patch electrode 1. 
     As also shown in FIG. 1, the motion sensor 5 has leads 6 and 7 associated therewith, which are conducted via an insulated cable 9 to a distal end of the patch electrode. The wire mesh 3 also has a lead 8 extending therefrom, which is also conducted via the cable 9 to the distal end of the electrode. In the embodiment of FIG. 1, the distal end of the electrode is divided into a branch 10, with a terminal 12, for the wire mesh lead 8, and a branch 11 with a terminal assembly 13 for the motion sensor leads. The cable 9 may be a Teflon®coated cable with the leads being drawn-brazed-stranded (DBS) wire. The motion sensor 5 may be formed by a piezoelectric crystal 5a mounted on a metallic backing 5b, such as a titanium strip, as schematically shown in FIGS. 11 and 12. Standard connectors may be used as the terminals -2 and 13. 
     A second embodiment of a rectangular patch electrode I is shown in FIG. 2, wherein components identical to those shown in FIG. 1 have been provided with the same reference numerals. In the embodiment of FIG. 2, the cable 9 is not branched, and terminates in a single terminal assembly 13. In order to avoid the branching and the dual terminals of the embodiment of FIG. 1, the embodiment of FIG. 2 uses the lead 8 from the wire mesh 3 as one of the cable leads for the motion sensor 5. To this end, one of the leads from the motion sensor 5, such as the lead 7, is connected to the lead 8 within the carrier 2. 
     As shown in FIG. 11, the motion sensor 5 may be placed adjacent to a surface 46 of the carrier 2 which is intended for contact with the myocardium, or as shown in FIG. 12 may be placed adjacent a surface 47 of the carrier 2 which is intended to face away from the myocardium when the patch electrode 1 is in place. As can also be seen in FIGS. 11 and 12, the neck portion of the patch electrode 1 which forms the transition to the cable is slightly thicker than the remainder of the patch electrode 1. The carrier 2 is still sufficiently flexible at this location to permit the motion sensor 5 to flex in response to cardiac wall movement, however, placement of the motion sensor 5 at this thicker portion of the electrode affords protection against severe bending of the motion sensor 5, which could crack or otherwise damage the sensor. 
     The patch electrode is not limited to the rectangular configuration shown in the embodiments of FIGS. 1 and 2, but may be configured in any number of other shapes, examples of which are respectively shown in FIGS. 3 through 6. In each of these embodiments, only the location of the motion sensor 5 is schematically shown, with the other details being omitted. It will be understood that in each of the embodiments of FIGS. 3 through 6, either type of electrical connection as shown in FIGS. 1 and 2 can be used, and the motion sensor 5 can be placed at either of the locations shown in FIGS. 11 and 12. 
     A patch electrode 14 is shown in FIG. 3 having a general &#34;butterfly&#34; or &#34;figure-eight&#34; shape with two generally elliptical regions 15 being joined by a narrow region 16. The connection to the cable lead, and the location of the motion sensor 5, are at a side of one of the regions 15 closest to the narrow region -6. 
     The embodiment of FIG. 4 shows a patch electrode 17 having a general &#34;rabbit ear&#34; shape consisting of two elongated lobes 18 joined by a narrow region 19. The attachment to the cable and the location of the motion sensor 5 are at a side of the electrode 17 close to the narrow region 19. 
     A triangular patch electrode 20 is shown in FIG. 5 consisting of a single triangular electrode region 2-. The cable connection and the motion sensor 5 are located at an apex of the triangle. In the embodiment of FIG. 5, the triangle is an isosceles triangle, and the apex at which the motion sensor 5 is located is the apex at which the two equal-length sides meet. 
     The patch electrode 22 shown in FIG. 6 is a &#34;butterfly&#34; or &#34;figure-eight&#34; electrode similar to that shown in FIG. 3, consisting of two generally oval regions 23 separated by a narrower region 24. Differing from the embodiment of FIG. 3, the motion sensor 5 and the cable connection are located at a side of one of the lobes 23 farthest from the narrow region 24. 
     An implantable defibrillator system 25 constructed in accordance with the principles of the present invention is schematically shown in FIG. 7. The system 25 consists of a patch electrode having a motion sensor 5 embedded therein, as described above, electrically connected to components within an implantable housing 26. The leads (schematically indicated as a single line) from the motion sensor 5 are electrically connected to an input of an amplifier 27. The amplified sensor signal from the output of the amplifier 27 is supplied to a sensor signal analyzer 28. As explained in more detail below, the sensor signal analyzer will examine one or more characteristics of the sensor signal, such as amplitude, frequency, energy content, etc., and will generate an output signal if one or a selected combination of these characteristics deviates from a standard or normal value by more than a predetermined threshold. 
     The chaotic cardiac wall motion, for example, which occurs during ventricular fibrillation will cause amplitude excursions of the motion sensor signal which dramatically exceed the amplitude which will be present during normal cardiac activity. This type of deviation will be identified within the sensor signal analyzer 28, and upon the detection of a characteristic, or set of characteristics, indicating abnormal cardiac activity, the sensor signal analyzer 28 will supply a signal to control logic 29. The control logic 29 may use the signal from the sensor signal analyzer 28 as a primary indicator of the onset of fibrillation or ventricular tachycardia, or may use the signal from the sensor signal analyzer 28 as a confirmation of the occurrence of fibrillation or ventricular tachycardia which has been detected by one of the aforementioned known techniques. In the event of an identification or confirmation of the onset of fibrillation or ventricular tachycardia based on the output of the sensor signal analyzer 28, the control logic 29 will enable a pulse generator 30 for generation of a high energy defibrillation pulse. This is delivered via the wire mesh electrode in the patch electrode 1. 
     In the embodiment of FIG. 7, a single patch electrode is shown, with the electrode preferably being sutured in the region of the ventricle of the heart, primarily for detecting ventricular defibrillation (but also capable of detecting tachycardia). A second patch electrode (not shown) could be placed on the opposite side of the heart. The second patch in this embodiment would not have a sensor mounted thereon. Alternately, a second electrode (not shown) could be transvenously placed within the heart, or close thereto. 
     In the embodiment shown in FIG. 8, however, two patch electrodes 1, each having a motion sensor 5 embedded therein, are employed. These patch electrodes 1 will be respectively sutured to the front and back (or left and right) of the cardiac myocardium. This permits the location of abnormal cardiac wall activity to be more precisely isolated, so that different types of defibrillation or antitachycardia pacing therapy may be instituted. In the dual patch/piezo approach, one patch may be placed in the region of the right atrium to allow differentiation of atrial or ventricular arrhythmias, and also to allow determination of the relationship between atrial and ventricular activity to further discriminate a pathologic from physiologic disarrhythmia. 
     The defibrillation system 25a of FIG. 8 includes an implantable housing 26 having components therein to which the patch electrodes 1 are electrically connected. The respective leads (again schematically shown as a single line) from the motion sensors 5 are supplied to the respective inputs of amplifiers 27. In the embodiment of FIG. 8, a sensor signal analyzer 28a, having channel A for analyzing signals from one of the motion sensors 5, and a channel B for analyzing signals from the other motion sensor 5, is provided. The sensor signal analyzer 28a provides an output signal to the control logic 29 which not only identifies the occurrence of abnormalities as described above in connection with FIG. 7, but also identifies which channel, and thus from which patch electrode the abnormalities have been sensed. The sensor signal analyzer 28a may generate a signal on a single line leading to the control logic 29 which includes encoded information identifying the abnormalities in both channels, however, it will be understood that alternatively a separate line may be provided from each channel to the control logic 29. 
     In the event of the detection of an abnormality indicating the onset of fibrillation or ventricular tachycardia, the control logic 29 supplies an enabling signal to an input stage of a pulse generator 30a. If defibrillating therapy is to be applied, the signal from the control logic 29 may be encoded to identify the polarity of the high energy pulse. In systems using more than two electrodes, such as a transvenous lead with two patch electrodes, the polarity configuration of the shock delivery may be modified to deliver the pulse to the primary site of the disarrhythmia. 
     A tachyarrhythmia control system 42 is shown in FIG. 9 which provides both defibrillation therapy and cardiac pacing assistance in a single implantable system. The system includes a patch electrode 1 having a motion sensor 5 embedded therein electrically connected to components in an implantable housing 41, and a conventional pacing lead 38, also electrically connected to components in the housing 41. The lead 38 may be a standard bipolar pacing/sensing lead. The second high energy shock lead could be a second patch electrode (not shown), or a transvenous electrode (not shown which could be mounted on the pacing lead 38). The signal from the motion sensor 5 is again supplied to an amplifier 27 and is analyzed in a sensor signal analyzer 28 as described above. The cardiac activity sensed by the pacing lead 38, such as R-waves, is supplied in a known manner via an amplifier 31 to conventional detection logic 32. 
     The respective outputs of the sensor signal analyzer 28 and the detection logic 32 are directly supplied to control logic 29a as separate signals, but are also logically combined in a gate 33, such as an AND gate, the output of which is also supplied to the control logic 29a. The control logic may assume any one of a number of modes or states by virtue of being telemetrically programmed by an external programmer 40 via a telemetry stage 39 contained within the implantable housing 41 and electrically connected to the control logic 29a. The telemetry system may also be used, via the control logic 29a, to program any of the other components within the implantable housing 41, and for this purpose the logic 29a may be provided with storage capacity, or a separate memory may be provided. 
     The control logic 29a may be programmed to make use of only the signal from the detection logic 32, only the signal from the sensor signal analyzer 28, the combined signal from the logic gate 33, or combinations thereof. The signal from the sensor signal analyzer 28 may thus not only be used as a primary or secondary indicator of the onset of fibrillation, but may also be used as a primary or secondary indicator of less severe arrhythmia, such as the onset of tachycardia. 
     The detection logic 32 may be used in a standard manner as providing data for standard bradycardia pacing support, in which case the control logic 29a will enable a bradycardia pacing control unit 36 for generating standard pacing pulses via a pacing output stage 37, delivered the lead 38. This may be accomplished in any of the known pacing modes. When the onset of tachycardia is identified either by the detection logic 32 alone, the sensor signal analyzer 28 alone, or a combination of the respective signals therefrom, the control logic 29a enables an antitachycardia pacing control stage 35, which causes pacing pulses to be generated by the pacing output stage 37 in a sequence calculated to terminate the tachycardia using any of the known pulse sequencing techniques for this purpose. Alternately, control logic 29a could enable a cardiovert/defibrillator control stage 34 which would cause a low energy cardioversion. When the onset of fibrillation is detected, the control logic 29a enables the cardiovert/defibrillate control stage 34 which causes a high energy defibrillation pulse to generated by the pulse generator 30 via the patch electrode 1. 
     A standard or base line signal 43 from the motion sensor 5 as occurs during &#34;normal&#34; cardiac activity is shown at the top of FIG. 10. For reference purposes, this signal is shown on the same time scale with an arterial pressure signal 44 and a surface EKG signal 45. Note that the arterial pressure signal 44 is delayed somewhat due to the location of the pressure sensor; the large pressure spike in fact closely follows the QRS complex. 
     As can be seen in FIG. 10, during normal cardiac activity the motion sensor signal 43 exhibits a regular periodicity, and a relatively low frequency and amplitude. Upon the onset of tachycardia, it has been observed that the frequency and amplitude of the signal 43 significantly increase, the amplitude in particular doubling or tripling. Upon the onset of ventricular fibrillation, the increase in amplitude becomes even more dramatic, approximately four to five times the amplitude of the normal signal shown in FIG. 10. Since the amplitude and frequency of the signal 43 in combination determine the &#34;energy content&#34; (proportional to the area under the curve) of the signal 43, any one or a combination of the amplitude, frequency or energy content of the signal 43 can be used as a reliable indicator for the onset of these types of cardiac activity. 
     Although modifications and changes may be suggested by those skilled in the art, it is the intention of the inventor to embody within the patent warranted hereon all changes and modifications as reasonably and properly come within the scope of his contribution to the art.