Source: http://www.google.com/patents/US6473649?dq=6031454
Timestamp: 2014-07-14 15:32:49
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Patent US6473649 - Rate management during automatic capture verification - Google PatentsSearch Images Maps Play YouTube News Gmail Drive More »Sign in<nobr>Advanced Patent Search</nobr>PatentsAn implantable cardiac rhythm management device capable of automatically detecting intrinsic and evoked response of a patient's heart and suitable for use during capture verification. The device of the present invention may operate in an automatic capture verification mode, wherein an electrocardiogram...http://www.google.com/patents/US6473649?utm_source=gb-gplus-sharePatent US6473649 - Rate management during automatic capture verificationAdvanced Patent SearchPublication numberUS6473649 B1Publication typeGrantApplication numberUS 09/470,269Publication dateOct 29, 2002Filing dateDec 22, 1999Priority dateDec 22, 1999Fee statusPaidAlso published asDE60044978D1, EP1239919A1, EP1239919B1, WO2001045792A1Publication number09470269, 470269, US 6473649 B1, US 6473649B1, US-B1-6473649, US6473649 B1, US6473649B1InventorsMark Gryzwa, Qingsheng ZhuOriginal AssigneeCardiac Pacemakers, Inc.Export CitationBiBTeX, EndNote, RefManPatent Citations (54), Non-Patent Citations (1), Referenced by (41), Classifications (8), Legal Events (4) External Links: USPTO, USPTO Assignment, EspacenetRate management during automatic capture verificationUS 6473649 B1Abstract An implantable cardiac rhythm management device capable of automatically detecting intrinsic and evoked response of a patient's heart and suitable for use during capture verification. The device of the present invention may operate in an automatic capture verification mode, wherein an electrocardiogram signal of a patient's heart is received and used by the device to determine whether a stimulation pulse evokes a response by the patient's heart. The device suspends the automatic capture verification mode and/or adjust the detection threshold dependent upon detected and/or measured noise, a determined amplitude of evoked response, a determined modulation in the evoked response, or detected and/or measured artifact. Further, the sensing circuit of the rhythm management device of the present invention reduces afterpotentials that result due to delivery of the stimulation pulses.
What is claimed is: 1. An implantable cardiac rhythm management device capable of automatically detecting intrinsic and evoked response of a patient's heart, said rhythm management device including:
(a) a pulse generator that generates stimulation pulses; (b) a controller having a timing circuit, sensing circuit and stimulation circuit for controlling activation of the pulse generator and delivery of the stimulation pulses; (c) electrodes for positioning within a patient's heart and electrically coupled to said controller, wherein electrocardiogram signals are electrically conducted to said controller, said electrocardiogram signal including electrical impulses corresponding to a cardiac depolarization and noise; (d) said sensing circuit coupled to said timing circuit and including an adjustable detection threshold; (e) wherein said controller detects an evoked response of the patient's heart from the electrocardiogram signal, determines an amplitude associated with the evoked response, and adjusts the detection threshold dependent upon the determined amplitude; and (f) wherein said controller determines a value associated with modulation of the evoked response, wherein said value is determined from the amplitude of a detected evoked response. 2. A rhythm management device as recited in claim 1, wherein the controller adjusts the detection threshold dependant upon the value associated with modulation.
(a) a pulse generator that generates stimulation pulses; (b) a controller having a timing circuit, sensing circuit and stimulation circuit for controlling activation of the pulse generator and delivery of the stimulation pulses; (c) electrodes for positioning within a patient's heart and electrically coupled to said controller, wherein electrocardiogram signals are electrically conducted to said controller, said electrocardiogram signal including electrical impulses corresponding to a cardiac depolarization and noise; (d) said sensing circuit coupled to said timing circuit and including an adjustable detection threshold; and (e) wherein said sensing circuit includes a sense amplifier electrically connected to the electrodes and controller in a manner wherein a polarity of an amplitude of the electrocardiogram signal corresponding to an evoked response is opposite a polarity of an amplitude of the electrocardiogram signal corresponding to afterpotential. 11. A rhythm management device as recited in claim 10, wherein a positive pole of the sense amplifier is coupled to an indifferent contact and a negative pole of the sense amplifier is coupled to said electrodes.
(a) a pulse generator that generates stimulation pulses; (b) a controller having a timing circuit, sensing circuit and stimulation circuit for controlling activation of the pulse generator and delivery of the stimulation pulses; (c) electrodes for positioning within a patient's heart and electrically coupled to said controller, wherein electrocardiogram signals are electrically conducted to said controller, said electrocardiogram signal including electrical impulses corresponding to a cardiac depolarization and noise; (d) said sensing circuit coupled to said timing circuit and including an adjustable detection threshold; (e) wherein said stimulation circuit includes a coupling capacitor arrangement that reduces afterpotentials; and (f) wherein said coupling capacitor arrangement includes a capacitor having a capacitance less than 5 microfarads. 13. An implantable cardiac rhythm management device capable of automatically detecting intrinsic and evoked response of a patient's heart, said rhythm management device including:
(a) a pulse generator that generates stimulation pulses; (b) a controller having a timing circuit, sensing circuit and stimulation circuit for controlling activation of the pulse generator and delivery of the stimulation pulses; (c) electrodes for positioning within a patient's heart and electrically coupled to said controller, wherein electrocardiogram signals are electrically conducted to said controller, said electrocardiogram signal including electrical impulses corresponding to a cardiac depolarization and noise; (d) said sensing circuit coupled to said timing circuit and including an adjustable detection threshold; and (e) wherein the sensing circuit includes a pre-amplifier electrically coupled to the electrodes, a first high pass coupling capacitor electrically coupled between the electrodes and said pre-amplifier, and a blanking switch electrically coupled between said high pass coupling capacitor and said pre-amplifier, said sensing circuit further including a dedicated evoked response amplifier. 14. An implantable cardiac rhythm management device capable of automatically detecting intrinsic and evoked response of a patient's heart, said rhythm management device including:
(a) a pulse generator that generates stimulation pulses; (b) a controller having a timing circuit, sensing circuit and stimulation circuit for controlling activation of the pulse generator and delivery of the stimulation pulses; (c) electrodes for positioning within a patient's heart and electrically coupled to said controller, wherein electrocardiogram signals are electrically conducted to said controller, said electrocardiogram signal including electrical impulses corresponding to a cardiac depolarization and noise; (d) said sensing circuit coupled to said timing circuit and including an adjustable detection threshold; (e) wherein the sensing circuit includes an afterpotential attenuation system for attenuating afterpotentials which result due to delivery of the stimulation pulses; and (f) wherein said afterpotential attenuation means includes a first coupling capacitor operatively coupled to a second coupling capacitor, and a switching system for selectively coupling said second coupling capacitor in series with said first coupling capacitor so as to reduce the effective capacitance of said first and second coupling capacitor. 15. An implantable cardiac rhythm management device capable of automatically detecting intrinsic and evoked response of a patient's heart, said rhythm management device including:
(a) a pulse generator that generates stimulation pulses; (b) a controller having a timing circuit, sensing circuit and stimulation circuit for controlling activation of the pulse generator and delivery of the stimulation pulses; (c) electrodes for positioning within a patient's heart and electrically coupled to said controller, wherein electrocardiogram signals are electrically conducted to said controller, said electrocardiogram signal including electrical impulses corresponding to a cardiac depolarization and noise; (d) said sensing circuit coupled to said timing circuit and including an adjustable detection threshold, said sensing circuit including a sense amplifier electrically connected to the electrodes and controller in a manner wherein a polarity of an amplitude of the electrocardiogram signal corresponding to an evoked response is opposite a polarity of an amplitude of the electrocardiogram signal corresponding to afterpotential; and (e) wherein said controller detects an evoked response of the patient's heart from the electrocardiogram signal, determines an amplitude associated with the evoked response, and adjusts the detection threshold dependent upon the determined amplitude. 16. A rhythm management device as recited in claim 15, wherein said controller determines a value associated with modulation of the evoked response, wherein said value is determined from the amplitude of a detected evoked response.
(a) a pulse generator that generates stimulation pulses; (b) a controller having a timing circuit, sensing circuit and stimulation circuit for controlling activation of the pulse generator and delivery of the stimulation pulses; (c) electrodes for positioning within a patient's heart and electrically coupled to said controller, wherein electrocardiogram signals are electrically conducted to said controller, said electrocardiogram signal including electrical impulses corresponding to a cardiac depolarization and noise; (d) said sensing circuit coupled to said timing circuit and including an adjustable detection threshold, said sensing circuit including a sense amplifier electrically connected to the electrodes and controller in a manner wherein a polarity of an amplitude of the electrocardiogram signal corresponding to an evoked response is opposite a polarity of an amplitude of the electrocardiogram signal corresponding to afterpotential; and (e) wherein said stimulation circuit includes a coupling capacitor arrangement that reduces afterpotentials; and (f) wherein said coupling capacitor arrangement includes a capacitor having a capacitance less than 5 microfarads. 22. A rhythm management device as recited in claim 21, wherein the sensing circuit includes a pre-amplifier electrically coupled to the electrodes, a first high pass coupling capacitor electrically coupled between the electrodes and said pre-amplifier, and a blanking switch electrically coupled between said high pass coupling capacitor and said pre-amplifier, said sensing circuit further including a dedicated evoked response amplifier.
(a) a pulse generator that generates stimulation pulses; (b) a controller having a timing circuit, sensing circuit and stimulation circuit for controlling activation of the pulse generator and delivery of the stimulation pulses; (c) electrodes for positioning within a patient's heart and electrically coupled to said controller, wherein electrocardiogram signals are electrically conducted to said controller, said electrocardiogram signal including electrical impulses corresponding to a cardiac depolarization and noise; (d) said sensing circuit coupled to said timing circuit and including an adjustable detection threshold, said sensing circuit including a sense amplifier electrically connected to the electrodes and controller in a manner wherein a polarity of an amplitude of the electrocardiogram signal corresponding to an evoked response is opposite a polarity of an amplitude of the electrocardiogram signal corresponding to afterpotential; and (e) wherein the sensing circuit includes an afterpotential attenuation system for attenuating afterpotentials which result due to delivery of the stimulation pulses; and (f) wherein said afterpotential attenuation means includes a first coupling capacitor operatively coupled to a second coupling capacitor, and a switching device for selectively coupling said second coupling capacitor in series with said first coupling capacitor so as to reduce the effective capacitance of said first and second coupling capacitor. 24. An implantable cardiac rhythm management device capable of automatically detecting intrinsic and evoked response of a patient's her, said rhythm management device including:
(a) a pulse generator that generates stimulation pulses; (b) a controller having a timing circuit, sensing circuit and stimulation circuit for controlling activation of the pulse generator and delivery of the stimulation pulses; (c) electrodes positioned within a patient's heart and electrically coupled to said controller, wherein electrocardiogram signals are electrically conducted to said controller, said electrocardiogram signal including electrical impulses corresponding to a cardiac depolarization and noise; (d) said sensing circuit coupled to said timing circuit and including an adjustable detection threshold, said sensing circuit including a sense amplifier electrically connected to the electrodes and controller in a manner wherein a polarity of an amplitude of the electrocardiogram signal corresponding to an evoked response is opposite a polarity of an amplitude of the electrocardiogram signal corresponding to afterpotential; (e) wherein said controller detects the presence of noise in the electrocardiogram signal; and (f) wherein said controller determines a value associated with an amplitude of the detected noise. 25. A rhythm management device as recited in claim 24, wherein the controller adjusts the detection threshold dependant upon the value associated with the amplitude of the detected noise.
(a) means for generating stimulation pulses; (b) control means for controlling activation of said means for generating stimulation pulses and for controlling delivery of stimulation pulses, said control means having a timing circuit, sensing circuit and stimulation circuit; (c) electrodes for positioning within a patient's heart and electrically coupled to said control means such that electrocardiogram signals are electrically conducted to said control means, said electrocardiogram signal including electrical impulses corresponding to a cardiac depolarization and noise, said control means determines a value associated with an amplitude of the detected noise; (d) said sensing circuit coupled to said timing circuit and including an adjustable detection threshold, wherein said control means adjusts the detection threshold dependant upon the value associated with the amplitude of the detected noise; and (e) wherein said control means detects an evoked response of the patient's heart from the electrocardiogram signal, determines an amplitude associated with the evoked response, and further adjusts the detection threshold dependent upon the determined amplitude. 28. A rhythm management device as recited in claim 27, wherein said control means determines a value associated with modulation of the evoked response, wherein said value is determined from the amplitude of a detected evoked response.
(a) means for generating stimulation pulses; (b) control means for controlling activation of said means for generating stimulation pulses and for controlling delivery of stimulation pulses, said control means having a timing circuit, sensing circuit and stimulation circuit; (c) electrodes for positioning within a patient's heart and electrically coupled to said control means such that electrocardiogram signals are electrically conducted to said control means, said electrocardiogram signal including electrical impulses corresponding to a cardiac depolarization and noise, said control means determines a value associated with an amplitude of the detected noise; (d) said sensing circuit coupled to said timing circuit and including an adjustable detection threshold, wherein said control means adjusts the detection threshold dependant upon the value associated with the amplitude of the detected noise; and (e) memory means for storing the determined value associated with an amplitude of noise over a plurality of detected cardiac depolarization, wherein the control means adjusts the sensing threshold dependant upon the determined value associated with an amplitude of noise corresponding to prior detected cardiac depolarization. Description
The success of a stimulation pulse in depolarizing or �capturing� the selected chamber of the heart hinges on whether the output of the stimulation pulse as delivered to the myocardium exceeds a threshold value. This threshold value, referred to as the capture threshold, is related to the electrical field intensity required to alter the permeability of the myocardial cells to thereby initiate cell depolarization. If the local electrical field associated with the stimulation pulse does not exceed the capture threshold, then the permeability of the myocardial cells will not be altered enough and thus no depolarization will result. If, on the other hand, the local electrical field associated with the stimulation pulse exceeds the capture threshold, then the permeability of the myocardial cells will be altered sufficiently such that depolarization will result.
While the conventional pacing circuit is generally effective in delivering stimulus pulses to a selected chamber of the heart, it has been found that the detection of evoked depolarization or capture verification is rendered very difficult due to polarization voltages or �afterpotential� which develop at the heart tissue/electrode interface following the application of the stimulation pulses. The ability to verify capture is further affected by other variables including patient activity, body position, drugs being used, lead movement, noise etc.
In the past, the large capacitance of the coupling capacitor was believed necessary in order to sufficiently block any DC components from the heart and to minimize pace pulse voltage droop. However, the large capacitance of the coupling capacitor causes a charge dissipation or �afterpotential� which is relatively large (100 mV or greater) and which decays exponentially over a relatively long period of time (100 milliseconds). This is particularly troublesome due to the fact that the evoked potential of the heart tissue is small in amplitude relative to the polarization voltage or �afterpotential� (100 mV). The amplitude of the evoked potential corresponding to a P-wave typically ranges between 1-5 mV and the amplitude of the evoked potential corresponding to an R-wave typically ranges between 5-2 mV.
Further, the long decay period of the polarization voltage or �afterpotential� effectively masks the evoked potential, which typically begins within approximately (10-40) milliseconds after the stimulation pulse to a selected chamber of the heart. It will be appreciated that this creates difficulty in detecting the evoked response of the heart following the delivery of stimulus pulses. In that evoked response is indicative of capture, the undesirable masking of the evoked response by �afterpotential� thus hampers the ability of the pacemaker to conduct automatic capture verification. Hence, there is a need for a rhythm management device that decreases and/or shortens the pacing afterpotential with minimal increase of the leading edge voltage pacing threshold. It is also desirable to reduce the number or complexity of the implanted components and, thus, there is a need for a system having a stimulation/sensing circuit that minimizes the number of required electrodes positioned within the heart for sensing a response evoked by a stimulation pulse directed to a pre-selected chamber of the heart.
U.S. Pat. No. 4,686,988 to Sholder teaches the use of a separate sensing electrode connected to a detector for detecting P-waves in the presence of atrial stimulation pulses, wherein the P-wave detector has an input bandpass characteristic selected to pass frequencies that are associated with P-waves. U.S. Pat. No. 4,858,610 to Callaghan et al. teaches the use of charge dumping following delivery of the stimulation pulse to decrease lead polarization and also the use of separate pacing and sensing electrodes to eliminate the polarization problem on the sensing electrode. The techniques of the '610 patent and '988 patent, which involve using a separate electrode located at some distance from the stimulating electrode for the purpose of isolating the polarization voltages or �afterpotential� are not completely desirable in that they require the additional cost and complexity of the additional sensing electrode.
U.S. Pat. No. 5,324,310 to Greeninger et al. teaches the use of the �ring-to-ring� sensing with corresponding atrial and ventricular EGM amplifiers whose outputs are multiplied and compared to a predetermined threshold to determine capture. U.S. Pat. No. 5,486,201 to Canfield discloses an active discharge circuit having a switching device which sequentially and repeatedly couples a charge transfer capacitor to the coupling capacitor to transfer charge therebetween and thereby actively discharge the coupling capacitor. None of these devices reduce or shorten the pacing afterpotentials through the use of a simplified pacing output. The present invention addresses these and other needs that will become apparent to those skilled in the art.
Hence, there is a need for a cardiac rhythm management device that attenuates polarization voltages or �afterpotentials� which develop at the heart tissue/electrode interface following the delivery of a stimulus to the heart tissue, and which minimizes the number of required components of the cardiac pacing system. There is a further need for a device that automatically adjust the detection threshold during a normal mode or an automatic capture verification mode. There is a still further need for a device capable of suspending the automatic capture verification mode and/or capable of adjusting the detection threshold dependent upon detected and/or measured noise, a determined amplitude of evoked response, a determined modulation in the evoked response, and detected and/or measured artifact.
SUMMARY OF THE INVENTION The present invention provides for a cardiac rhythm management device capable of automatically detecting intrinsic and evoked response of a patient's heart during a normal mode or capture verification mode. The implantable cardiac rhythm management device of the present invention generally includes a pulse generator that generates stimulation pulses, a controller having a timing circuit, sensing circuit and stimulation circuit for controlling activation of the pulse generator and delivery of the stimulation pulses, and an electrode lead arrangement electrically coupled to the controller. The rhythm management device further includes an adjustable detection threshold operable in a normal or autocapture verification mode, that may be adjusted or suspended dependant upon one or more of the following: detected and/or measured noise, a determined amplitude of evoked response, a determined modulation in the evoked response, and detected and/or measured artifact. The electrode lead arrangement of known suitable construction is positioned within the patient's heart and is electrically coupled to the controller, wherein electrocardiogram signals are electrically conducted to the controller from the electrodes. The electrocardiogram signal includes electrical impulses corresponding to a cardiac depolarization and noise.
OBJECTS It is accordingly a principal object of the present invention to provide a rhythm management device that may automatically adjust the detection threshold on a beat by beat basis.
DESCRIPTION OF THE DRAWINGS FIG. 1 is a partial sectional fragmentary block diagram depicting a cardiac rhythm management device incorporating a controller having a sensing circuit for automatic capture detection in accordance with the present invention;
FIG. 39 shows a strip chart tracing showing �pseudo-fusions� detected as capture and non-capture and the resulting stimulation sequences, together with a strip chart tracing of a surface ECG for reference;
FIG. 40 shows a strip chart tracing showing the stimulation sequence including two �pre-looks�, together with a strip chart tracing of a surface ECG for reference;
DESCRIPTION OF THE PREFERRED EMBODIMENT The present invention represents broadly applicable improvements to cardiac rhythm management devices. Those skilled in the art will appreciate that the present invention may find application in a variety of implantable or external cardiac rhythm management devices. For purposes of illustration and ease of discussion, the present invention may be described in connection with an implantable rate adaptive cardiac pacer Thus, the embodiments detailed herein are intended to be taken as representative or exemplary of those in which the improvements of the invention may be incorporated and are not intended to be limiting.
Referring now to FIG. 2, a portion of the embodiment of the stimulation circuit 24 and sensing circuit 26 shown in FIG. 1 is illustrated in greater detail. Those skilled in the art will appreciate that the stimulation and sensing circuits 24 and 26 respectively may be modified slightly to sense for atrial evoked response or to sense for ventricular evoked response. Thus, the description of the pacing/sensing circuit as shown in FIG. 2 should not be construed as limiting. As will be explained below, the improved circuit 26 is capable of quickly attenuating any polarization voltages or �afterpotential� which result due to the application of stimulus pulses to the heart 30. By attenuating the polarization voltages or �afterpotential� in this fashion, the improved circuit 26 facilitates the task of capture verification in that the presence or absence of evoked responses may be readily determined without the masking caused by afterpotential. Capture verification advantageously allows the rhythm management device 10 to automatically adjust the pacing output parameters and/or the detection threshold so as to minimize power consumption while assuring therapeutic efficacy.
In the embodiment shown in FIG. 2, the circuit 26 of the present invention includes a power supply or battery 14, a first switch (S1) 58, a second switch (S2) 60, a third switch (S3) 62, a pacing charge storage capacitor (C1) 64, and an afterpotential reduction capacitor/coupling capacitor (C2) 66, all of which are cooperatively operable under the direction of a controller 16 of known suitable construction. The power supply or battery 14 is preferably the battery provided to power the rhythm management device 10 and may comprise any number of commercially available batteries suitable for pacing applications. The switches 58-62 are preferably carried out via any number of conventionally available microprocessor-directed semiconductor integrated circuit switching means. The pacing charge storage capacitor 64 may also comprise any number of conventional storage capacitors, but is preferably provided with a capacitance in the range of 10-30 microfarads so as to develop a sufficient pacing charge for stimulating the heart 30. The primary function of the coupling capacitor 66 is to quickly attenuate the A polarization voltage or �afterpotential� which result from pacing and additionally block any DC signals from reaching the heart 30 during pacing. The coupling capacitor 66 has a capacitance in the range less than 5 microfarads, with a 2.2 microfarad capacitor being preferred.
The recharge cycle involves keeping open the first switch 58 and opening the second switch 60 while closing the third switch 62. This allows the circuit 24 to passively recharge, such that the charge within the heart 30 is allowed to flow back into the pacing output circuit to balance out. During this passive recharge period, the charge on the coupling capacitor 66 is such that the signal decays over a short period of time and less than the required blanking period preceding detection of any evoked response from the heart 30. This is because the evoked responses from the heart 30 typically begins within 12 milliseconds from the delivery of a stimulus pulse to the atrium and within 20 milliseconds from the delivery of a stimulus pulse to the ventricle, which is substantially longer than the required recharge time. Advantageously, it has been found that reducing the overall capacitance of the coupling capacitor 66 quickly attenuates the polarization voltages or �afterpotentials� which result immediately following the application of a stimulus pulse such that the evoked responses within the heart 30 will not be masked or buried within the �afterpotential.� By eliminating the adverse affects of �afterpotential� in this fashion, the rhythm management device 10 can easily sense an evoked response and track the capture threshold of the heart 30 over time. Those skilled in the art will appreciate that with the continuous evaluation of an evoked response, the rhythm management device 10 may be automatically adjusted to maintain an optimal pacing stimulus level which ensures safe pacing while minimizing power consumption.
The sense amp/filter circuit 162 conditions the electrogram signal and then applies the conditioned signal to an analog-to-digital converter 164 which converts the conditioned signal to corresponding digital values compatible with a peak detector 166. From the analog-to-digital converter 164, the signal is transmitted to both peak detector 166 and a comparator 168. Without any limitation intended, the peak detector may include a digital comparator and register, wherein the signal transmitted from the A/D converter 164 is continuously compared with an initial value stored in the peak detector register. If the current signal is greater than the value stored in the peak detector, the current value is loaded into the register value and is then stored in the peak detector register as a �maximum� amplitude. The peak detector 166 includes a clearing mechanism controlled by the controller 16. Those skilled in the art will recognize that the timing circuit 28 utilized to activate and deactivate the peak detector, may be either external or internal to the controller 16. Once the peak detector 166 times out, the final peak detector register value is transmitted to the controller 16. In this manner the peak detector 166 may be utilized to determine the amplitudes of the cardiac depolarization events.
While the foregoing conventional pacing circuit is generally effective in delivering stimulus pulses to the heart 30, it has been found that the detection of evoked depolarization or capture verification is rendered very difficult due to polarization voltages or �afterpotentials� which develop at the heart tissue/electrode interface following the application of the stimulation pulses. The inventors of the present invention have discovered that these polarization voltages are due, in large part, to the relatively large capacitance (e.g. 33 microfarads) of the coupling capacitor 246. The large capacitance of coupling capacitor 246 was believed necessary to deliver sufficient energy to the heart. However, the large capacitance of the coupling capacitor 246 also causes a charge dissipation or �afterpotential� which is relatively large (100 millivolts or greater) and which decays exponentially over a relatively long period of time (100 milliseconds). This is particularly troublesome due to the fact that the evoked potential or R-wave of the heart tissue is small in amplitude (5-20 millivolts) relative to the polarization voltage or �afterpotential� (100 millivolts). Moreover, the long decay period of the polarization voltage or �afterpotential� effectively masks the evoked response, which typically begins within approximately 10-20 milliseconds after the stimulation pulse. It will be appreciated that this creates difficulty in detecting the evoked response of the heart following the delivery of stimulus pulses. In that evoked response is indicative of capture, the undesirable masking of the evoked response by �afterpotentials� thus hampers the ability of the pacemaker to conduct automatic capture verification.
With reference to FIG. 12, another embodiment of the present invention includes an improved pacing output circuit 250 for delivery of stimulation pulses with reduced affects of afterpotential. As will be explained below, the improved pacing output circuit 250 is capable of quickly attenuating polarization voltages or �afterpotentials� which result due to the application of stimulus pulses to the heart 30. By attenuating the polarization voltages or �afterpotentials� in this fashion, the improved pacing circuit 250 of the present invention facilitates the task of capture verification in that the presence or absence of evoked responses may be readily determined without the masking caused by afterpotentials. Capture verification may advantageously allow the rhythm management device 10 to automatically adjust the capture threshold so as to minimize power consumption while assuring therapeutic efficacy.
One function of the second coupling capacitor 254 is to block DC signals from reaching the heart 30 during pacing. In order to minimize the pacing pulse droop the second coupling capacitor 254 should have a sufficiently large capacitance, for example, greater than 10 microfarads. In an important aspect of the present invention, the first coupling capacitor 66 is advantageously provided having a capacitance preferably less than 5 microfarads and substantially smaller than that of the second coupling capacitor 254. The first coupling capacitor 66 may be selectively operable, via the fourth switch 252, so as to selectively reduce the effective capacitance of the second coupling capacitor 254, thereby quickly attenuating the polarization voltage or �afterpotentials� which result from pacing.
Without any limitation intended, when an electrocardiogram excursion picked up on lead 32 is signal processed by the sense amplifier/filter circuit and converted to a digital quantity by A/D converter, a digital quantity proportional to the excursion is applied to one input of the digital comparator and to the controller 16. If the electrocardiogram excursion exceeds the sensing threshold, the controller processes the signal as a cardiac depolarization, measuring the amplitude of the depolarization wave, initiating the refractory period 358 and predetermined period, and measuring the amplitude of noise deflections detected in the noise measurement window 360. Once the refractory period 358 times out, the controller 16 initiates a sequence to determine and adjust the sensing threshold 356. The sequence that the controller 16 follows will now be discussed. First, the detected cardiac depolarization or r-wave amplitude is �smoothed� or �averaged�
Ravg(t)=1/4R(t)+3/4Ravg(t−1), according to the following equations:
Ravg(t)=Ravg(t−1)−rm, wherein the first equation is applied if the detected cardiac depolarization is intrinsic (see FIG. 30, block 370) and the second equation is applied if the detected cardiac depolarization results from pacing stimulus (see block 372). R(t) is the current amplitude of the cardiac depolarization, Ravg(t−1) is the previous �smoothed� r-wave amplitude, and rm is a preselected constant that, without limitation, may range between 0.001-2.5 mV. The preselected constant, rm, will vary depending upon whether sensing in the atrial autosense or ventricular autosense mode, with 0.14 mV being preferred for ventricular autosense and 0.03 mV being preferred for atrial autosense. Those skilled in the art will recognize and appreciate that the rm may, for convenience, be set equal to the resolution of the A/D converter 26. Once a current �smoothed� r-wave amplitude is determined, then the noise level is determined (see blocks 374 and 376) from the following equation:
N(t)=Max[Min(5 mV; NW Amp);0.375 mV: N(t−1)−rm] wherein NW Amp is the maximum amplitude of noise measured in the noise measurement window 360, N(t−1) is the previously determined noise level, and rm is a preselected constant as described previously. After the noise level and current �smoothed� r-wave amplitude are determined, then a value for the sensing threshold may be determined according to the following equation: Stdnext  ( t ) = Max  [ Ravg  ( t ) - N  ( t ) x + N  ( t ) ; ymV ; zN  ( t ) ] wherein Stdnext(t) is defined as the next sensing threshold, x is a constant ranging between 1-5 with 2 being preferred for atrial autosense and 3 being preferred for ventricular autosense (see blocks 378 and 380). In the alternative, x may be set as a x = Ravg  ( t ) N  ( t ) function of noise. For example, the following equation may apply:
Rate_NEI =
Std_next(t) =
Std_next(t−1) =
Nm(t) =
Pavg(t) =
Pavg(t−1) =
N(t−1) =
RNW =
At the end of the noise measurement interval the controller 16 implements a subroutine that first determines whether the sensed cardiac depolarization is a result of a pacing stimulus or is an intrinsic event (see FIG. 32 decision block 400). If the cardiac depolarization is a result of a pacing stimulus, the controller 16 follows the sequence shown in FIG. 33 which is interconnected with the flowchart in FIG. 32 by connector �A� . If the cardiac depolarization is a result of an intrinsic event, the controller 16 then determines the rate, in beats per minute (bpm), of the number of deflections during the noise measurement interval having an amplitude that exceeds the preceding sensing threshold level (see decision block 402). If the rate of the number of deflections is greater than 180 bpm but less than 500 bpm the p-wave detection is ignored (see block 404) and the sensing threshold value is set equal to the previous sensing threshold value (see block 406). When the rate of the number of deflections is greater than 180 bpm but less than 500 bpm, it is considered that the detected deflections are the result of atrial flutter or fibrillation. Without any limitation intended, in accordance with the above description, the predetermined lower limit may be set equal to the URL, which may preferably be set at 250 bpm.
If the rate of the number of deflections is not between 180-500 bpm then the Signal to Noise Ratio (SNR) is determined and compared to a predetermined constant A (see decision block 408). The SNR is determined by taking the measured amplitude of the p-wave cardiac depolarization and dividing by the measured noise amplitude, wherein the measured noise amplitude may be either the maximum amplitude of noise detected during the noise measurement interval or the average of all noise deflections detected during the noise measurement interval. The predetermined constant A is preferably set at 2 but may range between 1.5-5. If the SNR does not exceed the preset constant A, the p-wave detection is ignored, (see block 410) and the controller determines whether the previous noise level minus a constant �sm� exceeds the measured noise level (see decision block 412). If the SNR exceeds the preset constant A, then the current �smoothed� p-wave (Pavg(t)) is determined (see block 414) in accordance with the following:
Pavg(t)=1/4P(t)+3/4Pavg(t−1) where P(t) is measured amplitude of the p-wave and Pavg(t−1) is the value for the previous �smoothed� p-wave. Once the Pavg(t) is determined, then the controller determines whether the previous noise level minus a constant �sm� exceeds the measured noise level (see decision block 412), where constant sm, without limitation, may range between 0.01-0.5 mV, with 0.05 mV being preferred. If the previous noise level minus constant sm exceeds the current measured noise amplitude, the noise level is set equal to the previous noise level minus the constant sm (see block 416), otherwise, the noise level is set equal to the measured noise amplitude within the current noise measurement interval (see block 418). Once a noise level value and �smoothed� p-wave value have been determined, the next sensing threshold is determined in accordance with the following: Stdnext  ( t ) = Max  [ Pavg  ( t ) - N  ( t ) x + N  ( t ) ; ymV ; zN  ( t ) ] where x, y, and z are constant values having a range as previously described. The controller 16 then sets the ATH 170, for example, equal to the calculated value and sensing continues until the next cardiac depolarization is sensed.
Referring again to connector �A� and FIG. 33, if the detected cardiac depolarization is the result of a pacing stimulus, following the end of the noise measurement interval the controller 16 determines the �smoothed� p-wave value (see block 422 from the following equation:
Pavg(t)=Pavg(t−1)−sm Once a value for the �smoothed� p-wave is determined, the controller 16 then determines the rate, in beats per minute (bpm), of the number of deflections during the noise measurement interval having an amplitude that exceeds the preceding sensing threshold level (see decision block 424). If the rate of the number of deflections is greater than 180 bpm but less than 500 bpm the next sensing threshold is set equal to the previous sensing threshold value (see block 426).
If the rate of the number of deflections is not between 180-500 bpm then the controller determines whether the previous noise level minus a constant �sm� exceeds the measured noise level (see decision block 428), where constant sm, without limitation, may range between 0.01-0.5 mV, with 0.05 mV being preferred. If the previous noise level minus constant sm exceeds the current measured noise amplitude, the noise level is set equal to the previous noise level minus the constant sm (see block 430), otherwise, the noise level is set equal to the measured noise amplitude within the current noise measurement interval (see block 432). Once a noise level value and �smoothed� p-wave value have been determined, the next sensing threshold is determined in accordance with the following: Stdnext  ( t ) = Max  [ Pavg  ( t ) - N  ( t ) x + N  ( t ) ; ymV ; zN  ( t ) ] where x, y, and z are constant values having a range as previously described. The controller 16 then sets the ATH 170, for example, equal to the calculated value and sensing continues until the next cardiac depolarization is sensed.
Referring next to FIGS. 34 and 35, alternate preferred algorithms are shown that may be implemented by a cardiac rhythm management device incapable of a direct measurement of the amplitude of noise while in an atrial autosense mode. The sequence shown in FIG. 34 is implemented by the controller 16 when the PVARP is set equal to 250 ms or the PVARP exceeds 250 ms. The sequence shown in FIG. 35 is implemented when the PVARP is less than 250 ms. In the case where PVARP exceeds 250 ms, the �smoothed� p-wave amplitude and the number of events exceeding the sensing threshold is determined over a preset period (250 ms) of time or noise measurement interval within the PVARP interval.
The sequence shown in FIG. 34 is implemented by the controller after the noise measurement interval or PVARP times out. The controller 16 then determines whether the current cardiac depolarization is a result of a paced or intrinsic event (see decision block 440). If the cardiac depolarization is the result of a paced stimulus the controller 16 calculates the �smoothed� amplitude (see block 442) for the detected p-wave deflection in accordance with the following equation:
Pavg(t)=Pavg(t−1)−sm where, without limitation, sm is a constant ranging between 0.01-0.5 mV, with 0.05 mV being preferred. If the cardiac depolarization is the result of a sensed stimulus the controller 16 calculates the �smoothed� amplitude (see block 444) for the detected p-wave deflection in accordance with the following equation:
Pavg(t)=1/4P(t)+3/4Pavg(t−1) as previously described. The controller then determines the number of deflections counted exceeding the sensing threshold during the predetermined period. If there were no deflections detected during the noise measurement interval and the retriggerable noise window of 40 ms, for example, is set (see decision block 446), then the noise amplitude value is set equal to the previous sensing threshold value (see block 448). If deflections are detected during the noise measurement interval, and the number of detected deflections exceed 3 (see decision block 450), then the noise amplitude value is set equal to the previous sensing threshold value (see block 448). If the number of detected deflections equals 2 (see decision block 452), then the value for the amplitude of noise is set equal to the previous amplitude of noise value (see decision block 454), otherwise, the amplitude of noise value is set equal to the previous value for the amplitude of noise minus a constant �sm� as previously described (see block 456). In this manner the measured amplitude of noise is estimated for the current noise measurement interval. Once the �smoothed� amplitude of the p-wave deflection and the amplitude of noise are calculated, then the value for the next sensing threshold is determined in accordance with the following: Stdnext  ( t ) = Max  [ Pavg  ( t ) - N  ( t ) x + N  ( t ) ; ymV ; zN  ( t ) ] where x, y, and z are constant values having a range as previously described (see block 458). The controller 16 then sets the ATH register 170, for example, equal to the calculated value and sensing continues until the next cardiac depolarization is sensed.
As previously indicated, the sequence shown in FIG. 35 is implemented by the controller 16 after the noise measurement interval or predetermined period times out and when the PVARP is less than 250 ms. When this is the case, the controller 16 determines whether the current cardiac depolarization is a result of a paced or intrinsic event (see decision block 460). If the cardiac depolarization is the result of a paced stimulus the controller 16 calculates the �smoothed� amplitude (see block 462) for the detected p-wave deflection in accordance with the following equation:
Pavg(t)=Pavg(t−1)−sm where, without limitation, sm is a constant ranging between 0.01-0.5 mV, with 0.05 mV being preferred. If the cardiac depolarization is the result of a sensed stimulus the controller 16 calculates the �smoothed� amplitude (see block 464) for the detected p-wave deflection in accordance with the following equation:
Pavg(t)=1/4P(t)+3/4Pavg(t−1) as previously described. The controller then determines the number of deflections counted exceeding the sensing threshold during the PVARP interval. If there were no deflections detected during the noise measurement interval and the retriggerable noise window of 40 ms, for example, is set (see decision block 466), then the noise amplitude value is set equal to the previous sensing threshold value (see block 468). If deflections are detected during the noise measurement interval, and the number of detected deflections exceed 2 (see decision block 470), then the noise amplitude value is set equal to the previous sensing threshold value (see block 468). If the number of detected deflections equals 1 (see decision block 472), then the value for the amplitude of noise is set equal to the previous amplitude of noise value (see decision block 474), otherwise, the amplitude of noise value is set equal to the previous value for the amplitude of noise minus a constant �sm� as previously described (see block 476). In this manner the measured amplitude of noise is estimated for the current noise measurement interval. Once the �smoothed� 25 amplitude of the p-wave deflection and the amplitude of noise are calculated, then the value for the next sensing threshold is determined in accordance with the following: Stdnext  ( t ) = Max  [ Pavg  ( t ) - N  ( t ) x + N  ( t ) ; ymV ; zN  ( t ) ] where x, y, and z are constant values having a range as previously described (see block 478). The controller 16 then sets the ATH register 170, for example, equal to the calculated value and sensing continues until the next cardiac depolarization is sensed.
As seen in FIG. 37, a pacing stimulus 520 may be delivered during a QRS complex, thus requiring a backup pace 522 proximate the T-wave or vulnerable period 524. Delivery of a backup pace 522 proximate the vulnerable period may lead to an undesirable arrhythmia and may be avoided by reducing fusion and pseudo-fusion during autocapture and autothreshold determination. FIG. 38 further illustrates four possible timing sequences resulting from delivery of a pacing stimulus 526. The timing sequence identified as �Capture� shows delivery of a pacing stimulus that evokes a response. The other three timing sequences show delivery of a pacing stimulus proximate with an intrinsic event 528.
Referring now to FIG. 43, the delivery of a stimulation pulse with prelook will now be described. The controller may initiate a stimulation with prelook (see block 650) during beat to beat, autocapture or autothreshold determination. The controller first sets an early detection threshold equal to two times a predetermined amount �N� and enables the early detection threshold a predetermined time immediately before delivering the stimulation pulse (see blocks 652 and 654. Those skilled in the art will appreciate that the early detection threshold may be determined in a manner similar to determining the event threshold as described above in greater detail. The controller then determines whether the amplitude of a portion of the electrogram signal received during the predetermined time immediately before delivering exceeds the early detection threshold (see decision block 656). If the amplitude of a portion of the electrogram signal does not exceed the early detection threshold, then the stimulation pulse is delivered (see loop 658 and block 660). If the amplitude of a portion of the electrogram signal exceeds the early detection threshold, then delivery of the stimulation pulse is delayed by a predetermined amount (see block 662) and the portion of the electrogram signal is analyzed to determine whether an intrinsic event has occurred (see decision block 664). If no intrinsic event is detected then a backup pace is delivered having an output sufficient to assume capture (see block 666). If an intrinsic event is detected at decision block 664, then the RR interval is extended as described above (see block 668 and continues to be extended until an intrinsic event is not detected (see loop 670), at which time a backup pace is delivered (see block 666).
Referring now to FIG. 45, the maximum amplitude of several R-waves indicated at point 696 are shown measured over time, wherein the �measurements� of the amplitude occurred during a five-beat step down capture detection pacing protocol. The evoked response detection threshold baseline 698 as determined by the method of the present invention is shown, wherein fluctuation in the baseline coincides with the modulation of the evoked response amplitude. In this manner, during beat by beat autocapture, for example, a decrease in the evoked response amplitude is not likely to generate a false negative in capture decision making, thereby eliminating unnecessary backup pacing.
As discussed above in greater detail, when an electrocardiogram excursion picked up on lead is signal processed by the sense amplifier/filter circuit and converted to a digital quantity by A/D converter, a digital quantity proportional to the excursion is applied to one input of the digital comparator and to the controller. If the electrocardiogram excursion exceeds the sensing threshold, the controller may process the signal as a cardiac depolarization, �measuring� the amplitude of the depolarization wave. Once the amplitude measurements have been made for a predetermined number of beats, the controller initiates a sequence to determine and adjust the evoked response detection threshold. FIG. 46 illustrates another embodiment of determining and modulating the evoked response detection threshold in conjunction with a modulating amplitude of evoked response.
Once the automatic evoked response detection threshold determination is initiated (see FIG. 46, block 700), the patient's heart is paced at the current capture level for a predetermined number of beats. The maximum amplitude of each evoked response over the predetermined number of beats is measured utilizing the peak detector, comparator and controller as described above in greater detail (see block 702). A value associated with each maximum amplitude may be stored in the memory of the controller. The predetermined number of beats should include at least one complete cycle of the respiration modulation. The mean amplitude and minimum amplitude for all of the maximum amplitudes over the predetermined number of beats is then determined and stored (see block 704). A first constant value referred to as the �Respiration Modulation Index� or RMI is determined according to the following equation:
(ER mean −ER min)/ER mean wherein ERmean is the mean amplitude for all of the maximum amplitudes over the predetermined number of beats and ERmin is the minimum amplitude for all of the maximum amplitudes over the predetermined number of beats (see block 706). A second constant value identified as the ERfilter (described below in greater detail) is initially set equal to the determined ERmean and a value corresponding to an amplitude of artifact is initially set equal to zero (see block 708). The evoked response detection threshold (ERDT) is then determined according to the following equation:
ERDT=k(Artifact+ER Min) where k is a predefined constant that may range between 0.1 to 0.9 and ERMin is the minimum evoked response due to modulation. The ERMin takes into account both the modulation due to respiration and the modulation due to other factors as follows:
ER Min=(1−RMI)(ER Filter) (see block 710). This evoked response detection threshold (ERDT) may be utilized and updated during a capture detection step down pacing protocol (see block 712). A pacing stimulus is delivered and then the rhythm management device senses for an evoked response (see blocks 714 and 716). A determination is then made whether a signal is sensed having an amplitude greater than the ERDT (see decision block 718).
ER Filter(n)=α(ER Filter(n−1))+b(ER n) where n=0 for the initial determination of the ERFilter value and increases by an integer number for each subsequent determination of the ERDT, and �a� and �b� are coefficients wherein a+b=1 (see block 720). In the preferred embodiment �a� is set equal to 0.75 and �b� is set equal to 0.25. Those skilled in the art will appreciate that the ERFilter provides a moving average of the evoked response, thereby adapting the evoked response detection threshold to changes in the evoked response amplitude. The ERDT is updated according to the following equation:
ERDT n=0.5(Artifact+((1−RMI)ER Filter(n))) where the RMI is updated at predetermined intervals, with 21 hours being preferred (see block 722). Without limitation, the pacing output may then be updated according to a known suitable pacing protocol (see block 724). The next pace in the pacing step down protocol may be delivered (see loop 726). If after a pacing stimulus is delivered and the maximum amplitude of the electrocardiogram signal does not exceed the ERDT then a backup pace is delivered (see block 728). The artifact amplitude is then measured from the electrocardiogram signal (see block 730) and it is determined whether the autothreshold protocol is complete (see decision block 732). If the protocol is not complete, the pacing output is updated according to known suitable pacing protocol (see loop 734, block 724 and loop 726). If the autothreshold protocol is complete, then the autothreshold is terminated (see block 736). In this manner the detection threshold is defined greater than the amplitude for maximum artifact and less than the amplitude of the minimum evoked response.
Referring to FIG. 49, several points 770 are shown plotted relative to an evoked response detection threshold baseline 772, an artifact baseline 774, and an evoked response filter baseline 776. The resulting modulating waveform 778 is shown in relation to the threshold baseline 772. Each point 770 corresponds to a maximum amplitude for a corresponding evoked response. FIG. 50 shows the modulating waveform 778 in relation to the evoked response �mean� baseline 780 and the evoked response �minimum� baseline 782. The evoked response mean baseline 780 represents the mean of several maximum amplitudes of several R-waves over several beats. Likewise, the evoked response minimum baseline 782 represents the minimum amplitude of several maximum amplitudes of several R-waves over several beats.
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