CARDIAC STIMULATOR FOR VERIFYING CAPTURE OF THE ATRIUM

Abstract: A cardiac pacemaker capable of verifying whether an atrium of the heart has been captured by an atrial cardiac stimulation pulse. An atrial signal is sensed via an electrode located in the atrium during a period of time from before delivery of the stimulation pulse to after delivery of the stimulation pulse. The sensed signal is analyzed by detecting whether a P-wave occurred prior to delivery of the cardiac stimulation pulse such that stimulation pulse would have been delivered within a refractory period. If a P-wave did not occur, the slope of the sensed waveform signal is determined within a selected interval of time beginning at a selected time after the time of delivery of the stimulation pulse. A capture status signal is generated that indicates non-capture, if a P-wave was detected, or capture, if a P-wave component was not detected and the determined slope of the sensed waveform signal surpasses a pre-selected slope threshold demarcating capture from non-capture.

Description: Description Cardiac Stimulator for Verifying Capture of the Atrium Technical Field The present invention relates generally to cardiac pacing using an i plantable cardiac stimulator, and more particularly to verification of capture of the atnum of the heart following application of an electrical stimulating pulse by the cardiac stimulator Background Art An implantable cardiac stimulator, specifically a pacemaker, "captures" the heart by delivering an electrical pulse via an electrode to the myocardium of a selected chamber durmg an interval in the cardiac cycle m which the cardiac tissue is excitable The electrical pulse causes depolarization of cardiac cells and a consequent contraction of the chamber, provided that the energy of the pacing pulse as delivered to the myocardium exceeds a threshold value It is desirable to adjust the cardiac stimulator so that the pulse energy delivered to the myocardium is at the lowest level that will reliably capture the chamber Such a level assures therapeutic efficacy while maximizing the life of the implanted battery Because the threshold for capture varies from one implantation to another, and can change over time, it is also desirable that the pulse energy delivered by the cardiac stimulator to the myocardium be adjustable during and subsequent to implantation Adjustment can be effected manually from time to time through use of an external programmer that communicates with the implanted pacemaker It would be desirable, however, to provide a cardiac stimulator that adjusts the pulse energy automatically and dynamically in response to changes in the capture threshold Changes in capture threshold can be detected by monitoring the efficacy of stimulating pulses at a given energy level If capture does not occur at a particular stimulation energy level which previously was adequate to effect capture, then it can be surmised that the capture threshold has increased and that the stimulation energy level should be increased On the other hand, if capture occurs consistently at a particular stimulation level over a relatively large number of successive stimulation cycles, it is possible that the stimulation threshold has decreased and that stimulation energy is being delivered at a level higher than necessary This can be verified by lowering the stimulation energy level and monitoring for loss of capture at the new energy level For automatic and dynamic adjustment of the stimulation energy level to be successful, it is necessary for the implantable cardiac stimulator to be able to verify that capture has occurred Verification of capture m the ventricle has been accomplished by detecting a characteristic electrical potential in the heart evoked by the stimulating pulse If capture has not occurred, there will be no characteristic evoked potential to detect It follows that each time a stimulating pulse is delivered to the ventricle, the heart can be monitored during an appropriate period of time thereafter to detect the presence of the evoked potential, and thereby verify capture In practice, however, reliable detection of the evoked potential of the ventricle is not a simple matter This is because the evoked potential is small in amplitude relative to the residual polarization charge on the electrode resulting from the stimulation pulse The residual charge decays exponentially but tends to dominate the evoked potential for several hundreds of milliseconds thereafter Detection of the evoked potential of the atnum is even more difficult, as the amplitude of the atnal evoked potential is quite small and difficult to discriminate from noise, other cardiac signals, and the residual polarization charge on the electrode It would be desirable to provide a method for analyzing cardiac signals, for use in an unplantable cardiac stimulator, that permits verification of capture of the atnum in the presence of a residual charge from a preceding stimulation pulse and m the presence of other signals This and other desirable goals are met by the present invention Disclosure of Invention According to one aspect of the invention, a method is provided for discriminating between capture and non-capture electrogram signal morphologies that are sensed in the atrium followmg delivery of a stimulation pulse to the atnum by a cardiac stimulator The method involves detecting whether an intrinsic P-wave has occurred prior to the tune of delivery of the atnal stimulating pulse, and determining the slope of the sensed atnal electrogram within a selected time interval beginning at a time following delivery of the atnal stimulating pulse If no P-wave has been detected pπor to the atnal stimulating pulse and the slope of the sensed electrogram surpasses a pre-selected slope threshold, as measured between two time-spaced sampled amplitudes of the sensed electrogram, then capture of the atnum is deemed to have occurred Otherwise, a non-capture is presumed It is an object of the present invention to provide an improved method for distinguishing non- capture and capture waveform morphologies as sensed by an intracardiac atnal electrode following delivery of a cardiac stimulating pulse to the atnum Other objects and advantages of the present invention will be apparent from the following description of a preferred embodiment made with reference to the drawings Brief Description of the Drawings FIG 1 is a block diagram of the preferred embodiment of a cardiac stimulator for practicing the method of the present invention FIG 2 is a block diagram of the capture sense block of FIG 1 , showing in greater particularity a capture sense signal processing unit FIG 3 is an intracardiac atnal electrogram showing an atnal stimulation pulse delivered by unipolar pacing, preceded by an intrinsic atnal depolaπzation, and showing intrinsic ventricular depolarization as sensed in the atnum FIG 4 is an intracardiac ventricular electrogram showing the atnal stimulation pulse of FIG 3, as sensed m the ventricle, and showing intrinsic ventricular depolarization FIG 5 is an intracardiac atnal electrogram showing a non-capturing atnal stimulation pulse preceded by an intrinsic atnal depolarization FIG 6 is another intracardiac atnal electrogram showing a non-capturing atnal stimulation pulse preceded by an intrinsic atnal depolarization FIG 7 is an intracardiac atnal electrogram showing a non-capturing atnal stimulation pulse that is not preceded by an intrinsic atnal depolarization, but that is followed by an intrinsic atnal depolarization FIG 8 is an intracardiac atnal electrogram showing a capturing atnal stimulation pulse that is not preceded by an intrinsic atnal depolarization FIG 9 is an overplot of intracardiac atnal electrograms duπng a period of time following delivery of an atnal stimulation pulse, some of which electrograms depict capture morphologies and some of which depict non-capture moφhologies Best Mode for Carrying Out the Invention Referring in particular to FIG 1, there is illustrated a block diagram of a cardiac stimulator, or pacemaker, 10 with which the method of the present invention can be practiced A microprocessor and control circuit 20 preferably provides pacemaker control and means for processing digital signals Microprocessor 20 has input/output ports connected in a conventional manner via bi-directional bus 22 to memory 24 Memory 24 preferably includes both ROM and RAM, and stores the pacemaker operating routine and various programmable parameters and variables Microprocessor 20 preferably also has an input/output port connected to a telemetry interface 26 by line 28 The pacemaker when implanted is thus able to receive pacing control parameters and variables from a transmitter of an external programmer and send data to a receiver of the external programmer if desired Telemetry communication is preferably effected by transmission and reception, via antenna 30, of electromagnetic radiation modulated in accordance with the data to be communicated Microprocessor 20 also has output ports connected to inputs of an atnal stimulus pulse generator 32 and a ventricular stimulus pulse generator 34 by control lines 36 and 38, respectively Microprocessor 20 sends pulse parameter data, such as amplitude and width, as well as enable/disable and pulse initiation codes to the generators 32 and 34 on the respective control lines 36 and 38 Microprocessor 20 also has input ports connected to outputs of an atnal sense amplifier 40 and a ventricular sense amplifier 42 by lines 44 and 46, respectively The atnal and ventricular sense amplifiers 40 and 42 detect P-waves and R-waves respectively The input of the atnal sense amplifier 40 and the output of the atnal stimulus pulse generator 32 are connected to a first conductor 48 which is connected via a conventional atnal lead to a pacing/sensing electrode 50 preferably lodged within the right atnal chamber of the heart 52 The input of the ventricular sense amplifier 42 and the output of the ventricular stimulus pulse generator 34 are connected to a second conductor 54 which is connected via a conventional ventricular lead to a pacing/sensing electrode 56 preferably lodged within the right ventricular chamber of the heart 52 The conductors 48 and 54 conduct the stimulus pulses generated by the atnal and ventricular stimulus pulse generators 32 and 34, respectively, to the pacing/sensing electrodes 50 and 56 The pacing/sensing electrodes 50 and 56 and corresponding conductors 48 and 54 also conduct sensed cardiac electrical signals in the right atnum and right ventricle to the atnal and ventricular sense amplifiers 40 and 42, respectively An atnal capture sense signal processor 58 has an input connected to conductor 48, and an output connected via line 60 to an input port of microprocessor 20 A signal sensed in the atnum by electrode 50 is conducted via conductor 48 to capture sense signal processor 58, where the sensed signal is processed in a manner described further below The processed signal from capture sense signal processor 58 is conducted via line 60 to microprocessor 20 where the signal undergoes further processing and analysis in accordance with a method described below. The present invention contemplates verifying capture of the atnum by sensing via an electrode placed in the atnum a waveform signal that is, among other thmgs, indicative of the effect of a stimulating pulse delivered in the atnum A significant advantage of the present invention is that the same electrode that is used to deliver the stimulating pulse can also be used for verifying capture This allows use of unipolar pacing between the lead tip and the pacemaker housing without requiring a separate πng electrode for capture verification Alternatively, bipolar sensing between the lead tip and a ring electrode can be used together with unipolar pacing This has the advantage of minimizing the sensing of far-field R-waves In addition, bipolar pacing can be used together with bipolar sensing The method is suitable for single chamber pacing/sensing, but may also be used with a dual-chamber device An advantage of the latter arrangement is that detection of the R-wave in the ventricle can be used to identify far-field R-waves sensed in the atnum Referring to FIG 2, the capture sense signal processor 58 of FIG 1 is illustrated in greater detail In a preferred embodiment, signal processor 58 includes a pre-amphfier 62 having an input to which sensed electrical activity signals from the atnum are applied The input of pre-amphfier 62 is electrically connected via conductor 48 ot an endocardial lead to the tip electrode 50 located in the right atnum of the heart The signal from tip electrode 50 is sensed relative to a second electrode, preferably an external conductive surface of the cardiac stimulator housing, or "can, " in a unipolar pacing/sensing configuration Nevertheless, it should also be understood that the input to pre-amphfier 62 can also be connected to a ring electrode Alternatively, the input to pre¬ amplifier 62 can be connected to the tip electrode 50 with the signal being sensed relative to a πng electrode in a bipolar sensing configuration, together with either unipolar or bipolar pacing The amplified output signal of pre-amphfier 62 is applied to the input of a following filter stage 64 having filter characteristics calculated to discriminate against frequencies that are characteristic of R-waves originating in the ventricle Filtering techniques for discriminating against R-waves are well known in the art The filtered output of filter stage 64 is applied to the input of a following analog to digital converter stage 66 in which the amplified and filtered analog signal is digitized for further processing and analysis by microprocessor 20 in accordance with the method for verifying capture described below Referring to FIG 3, there is illustrated an atnal intracardiac electrogram 70, showing in particular an atnal unipolar stimulus pulse 72, as detected at a tip electrode on a unipolar lead implanted in the atnum Moφhological artifact 74 is an intrinsic P-wave occurring prior to atnal stimulation pulse 72 Moφhological artifact 76 is a far-field sensed R-wave originating in the ventricle Because stimulus pulse 72 occurs within the refractory period following intrinsic P-wave 74, stimulus pulse 72 does not effect atnal capture Referring to FIG 4, there is illustrated a corresponding ventricular intracardiac electrogram 78 as detected at a tip electrode on a unipolar lead implanted in a right ventricle Moφhological artifact 80 is the atnal stimulus pulse of FIG 3, as sensed by the ventricular electrode No ventricular stimulus is present Moφhological artifact 82 is a near-field sensed intrinsic ventricular R-wave to which artifact 76 of FIG 3 corresponds Moφhological feature 84 is the T-wave that follows R wave 82 It is evident trom FIGS 3 and 4 that intrinsic ventricular depolarizations may be sensed in the atnum Such ventricular depolarizations can occur before, during, or after the atnal stimulation pulse and, if not recognized as ventricular R-waves, may be mistaken for P-waves Hence, it is desirable to discriminate against far-field R-waves as part of any method for analyzing atπal-sensed waveforms to verify atnal capture One method for discriminating against far-field R-waves is to employ a filter in the atnal capture sense amplifier having characteristics that reject frequencies known to be correlated with R-waves Such filtering methods are well known in the art Another method involves detecting the R-wave in the ventricle and blanking out the time interval in which it occurs in the atnal capture sense amplifier This latter method would require a ventricular electrode, and would be most readily implemented in a dual-chamber pacemaker In accordance with the method of the present invention, it is necessary to detect intrinsic atnal depolarizations (P-waves) that precede the atnal stimulus pulse within an interval of time such that the atnal stimulus pulse occurs during the refractory period when the atnum cannot be captured The importance of sensing the P-wave is described below with reference to FIGS 5-7 Techniques for sensing P-waves are well-developed in cardiac pacing technology, and will not be discussed further herein FIG 5 shows an atnal electrogram 86 m which an atnal stimulus pulse 88 is preceded by a P-wave 90 Thus, the atnum is in a refractory period at the time of delivery of atnal stimulus pulse 88 and cannot be captured Electrogram 86 exhibits a post-stimulus moφhology having a negative slope during the time interval between times Tl and T2 FIG 6 shows an atnal electrogram 92 in which an atnal stimulus pulse 94 is preceded by a P-wave 96 As in FIG 5, the atnum is not captured Unlike FIG 5, however, electrogram 92 of FIG 6 exhibits a post-stimulus moφhology having a positive slope during the time interval Tl to T2 Analysis of human electrograms has revealed that the electrograms illustrated in FIGS 5 and 6 are very typical and that the moφhology of the sensed electrograms will conform to those general shapes, differing only in relative amplitudes at Tl and T2 Referring now to FIG 7, there is shown an atnal electrogram 98 in which an atnal stimulus pulse 100 is not preceded by a P-wave, and yet there is no capture, as indicated by the presence ol intrinsic P-wave 102 following stimulus pulse 100 Electrogram 98 exhibits a post-stimulus moφhology having a negative slope during the time interval between times Tl and T2 Analysis of human electrograms has revealed that the post-stimulus moφhology illustrated in FIG 7 is virtually always associated with a non-capture condition, provided that the stimulus pulse is not in the refractory period The amplitudes at Tl and T2 may be different from that depicted Referring now to FIG 8, there is shown an atnal electrogram 104 m which an atnal stimulus pulse 106. which is not preceded by an intrinsic P-wave, captures the atnum Electrogram 104 exhibits a positive slope during the post-stimulus time interval between times Tl and T2 The post-stimulus moφhology illustrated in FIG 8 has been observed through analysis of human electrograms to be virtually always associated with a capture condition, provided that the stimulus pulse is not in the refractory period The amplitudes at Tl and T2 may differ from those shown It also has been observed that there is a general correlation between the energy level of the stimulation pulse and the steepness of the slope of the electrogram following the stimulus pulse The polarity of the slope, 1 e , negative versus positive, has been found to be well correlated with capture versus non-capture in those cases where there is no preceding intrinsic P-wave, l e, where the stimulus pulse is not delivered within the refractory period Comparing FIGS 6 and 8, it can be observed that the electrogram moφhologies 92 and 104 are similar and exhibit positive post-stimulus slopes Yet, FIG 6 shows a non-capture condition while FIG 8 shows a capture condition These two cases can be distinguished in that the non- capture shown in FIG 6 involves an intrinsic P-wave that precedes the stimulation pulse It has been observed that there is no reliable correlation between polarity of the slope of the post-stimulus electrogram and capture where an intrinsic P-wave precedes the stimulation pulse such that the stimulation pulse is delivered during the refractory period Thus, detection of the post-stimulus electrogram slope is not sufficient to verify capture, as it is indeterminate where there is a preceding P-wave In the absence of a preceding P-wave, however, the post-stimulus electrogram slope is highly correlated with capture Since the presence of a preceding P-wave virtually assures that the stimulating pulse will not capture, due to being delivered withm the refractory period, detecting whether a preceding P-wave occurred, and detecting the post-stimulus electrogram slope (in at least those cases where there was not a preceding P-wave), provides sufficient information to determine capture status To avoid misunderstanding, the definition of "slope" as used above should be made clear The preferred method of calculating the slope during the interval between times Tl and T2 is to subtract the amplitude at Tl from the amplitude at T2, thus representing the slope of a straight line drawn between the amplitudes at times Tl and T2 Examination of human electrograms has revealed that a preferred value for Tl is a time 40 msec following the leading edge of the atnal pacing pulse For T2, values of 70 msec following that leading edge have been found to be adequate in most cases, but the most preferred value is 100 msec To account for variability among patients, it would be desirable to provide for the values of Tl and T2 to be programmable in a cardiac stimulator in which the present method is implemented FIG 9 is an oveφlot of human atnal electrograms for a chronic implanted atnal lead, resulting from stimulus pulses having various amplitudes and durations The period of time from 10 msec before delivery of the atnal stimulation pulse to 110 msec after delivery of the atnal stimulation pulse is shown Each of the electrograms that are labeled NCP represent non-captures that were preceded by an intrinsic P-wave It can be seen that the class of NCP electrograms includes post-stimulus moφhologies having either positive or negative slopes in the time interval between times Tl and T2 Each of the electrograms that are labeled NC represent non-captures that were not preceded bλ an intrinsic P-wave It can be seen that the class of NC electrograms includes post-stimulus moφhologies having only negative slopes in the time interval between times Tl and T2 Each of the electrograms that are labeled C represent captures that were not preceded by an intrinsic P-wave It can be seen that the class of C electrograms includes post-stimulus moφhologies having only positive slopes in the time interval between times Tl and T2 The preferred method for determining the slope of the evoked response is illustrated in FIG 9 for one of the capture electrograms labeled C Line 108 connects the amplitudes of the example electrogram at times Tl and T2 Since the amplitude at T2 is greater than the amplitude at Tl , the slope is deemed to be positive It should be recognized that the correlation between positive slope and capture, and between negative slope and non-capture, is dependent upon the relative polarities of the system, and it is to be expected that the correlation would be reversed in a pacing and sensing system having differently arranged polarities Nevertheless, regardless of the polarity convention adopted, the correlation between post-stimulus moφhology and capture status is expected to remain consistent It should also be recognized that in this description of the invention, a slope of zero, 1 e horizontal, has been defined as the demarcation between a slope indicative of capture and a slope indicative of non-capture Within any particular pacing and sensing system, however, it may be necessary to subtract a slope-offset from the measured slope to obtain consistent and accurate verification of capture In other words, the method employs a comparison of a measured slope with a slope threshold that demarcates capture from non capture Thus, where the measured slope surpasses the pre-selected demarcation slope, whatever the value and polarity of the demarcation slope may be, then capture is deemed to have occurred and a signal to that effect is generated The present invention has been described with particularity in terms of a preferred method, for implementation in a preferred embodiment of an apparatus, by way of illustration and not limitation The scope of the invention is defined by the claims appended hereto Variations of the particular embodiment described herein that incoφorate the principles of the present invention may still fall within the scope of the appended claims

Claims: WHAT IS CLAIMED IS 1 A cardiac pacemaker capable of verifying whether an atnum of a heart has been captured by an electrical cardiac stimulation pulse delivered to the atnum, comprising a pulse generator, a lead connected to said pulse generator, at least one electrode attached to said lead and electrically connected to said pulse generator, and means adapted to sense a waveform signal at said electrode during a period of time extending from a time preceding the time of delivery of said cardiac stimulation pulse to a time following the time of delivery of said cardiac stimulation pulse, characterized by means for analyzing said sensed waveform signal by detecting whether said sensed waveform signal includes a P-wave component occurring at a time prior to the time of delivery of said cardiac stimulation pulse such that said stimulation pulse would have been delivered within a refractory period of said atnum, determining the slope of said sensed waveform signal withm a selected interval of time beginning at a selected time after the time of delivery of said cardiac stimulation pulse, and means adapted to generate a capture signal if and only if a P-wave component is not detected and the determined slope of said sensed waveform signal suφasses a pre-selected slope threshold demarcating capture from non-capture, or a non-capture signal, if either a P-wave component is detected, or a P-wave component is not detected and the determined slope of said sensed waveform signal does not suφass a pre-selected slope threshold demarcating capture from non-capture 2 The cardiac pacemaker of claim 1, further comprising means for timing said selected interval of time within which the slope of said sensed waveform signal can determined beginning at about 40 msec following the time at which said cardiac stimulation pulse was delivered 3 The cardiac pacemaker of claim 2, in which the means for timing the selected interval of time within which the slope of said sensed waveform signal is determined is adapted to end said interval at least about 70 msec following the time at which said cardiac stimulation pulse was delivered 4 The cardiac pacemaker of claim 3, in which means for timing the selected interval of time within which the slope of said sensed waveform signal is determined is adapted to end said interval at most about 100 msec following the time at which said cardiac stimulation pulse was delivered 5 The cardiac pacemaker of any of the preceding claims, further comprising means for discriminating against far-field R-wave components of said sensed waveform signal

Inventors: SCHROEPPEL EDWARD A, SCHROEPPEL, EDWARD, A.
Applicants: INTERMEDICS INC, INTERMEDICS, INC.
Application year: 1996
Main classification: A61N1
IPC codes: A61N1