CA2228578A1 - Threshold templating for digital agc - Google Patents

Abstract

A system and method automatically adjusts a sensing threshold in a cardioverter/defibrillator which receives electrical activity of the heart and delivers shock pulses in response thereto. An amplifier amplifies the electrical activity. A detection circuit detects depolarizations in the amplified electrical activity and provides a detect signal representing a cardiac event indicative of a depolarization when the amplified electrical activity exceeds a variable sensing threshold. Digital template generation circuitry responds quickly to track amplitudes of the electrical activity and to set the variable sensing threshold to a level proportional to a peak value of the amplitude of the amplified electrical activity and then decreases the variable sensing threshold from the level in discrete steps until the variable sensing threshold is at a low threshold value. The discrete steps are grouped into step groups. Each step group decreases the variable sensing threshold by a defined percentage.

Description

W O 97/06851 PCT~US96/13181 TEI~ ~OI.n T~l~IPl,A.T~l~G FOR DIGITA~, AGC

S Field of the Inventioll The present invention relates generally to imrl~nt~hle medical devices, and more particularly, to systems such as automatic gain control systems for i1.,t~ ically adj~li,lg the sensing threshold in cardiac rhythm management devices, such as p~cem~kers, cardioverter/defibrillators, and 10 cardioverter/defibrillators with pacing capability.
~ ~ro~,apd of the Invention Cardiac rhythm management devices such as p~cPm~kçr~, cardioverter/defibrillators, and cardioverter/defibrillators with pacing capability typically include a system for detecting dangerous cardiac arrhythrnia conditions 15 in the heart, such as bradycardia, Lachycaldia, and fihrill~tion by measuring the time interval between conse.;uli~e cardiac depolarizations. Cardiac rhythm management devices receive a sensed cardiac signal comprising electrical activity of the heart and detect cardiac depolarizations in the electrical activity when an amplitude of the electrical activity exceeds a predetermined amplitude 20 level or "sensing threshold." The sensing threshold may be fixed, or may vary over time.
A fixed sensing threshold is not a~ op,iate for detecting certain alll,ylhlllias, such as polymorphic tachyc~dia and fibrillation, wherein extremev~ri~tion~ occur in the ~mrlihl~le of the electrical activity during the arrhythmia.
25 The problem of tracking variations in the amplitude of the electrical activity is further complicated when the cardiac rhythm management device delivers pace pulses to the heart, which cause invoked responses which are quite high in amplitude as c~ pdl~d to normal cardiac depolarizations.
One approach to compensate for problems associated with a fixed 30 sensing threshold is to program the sensing threshold at a value d~PterminPd by the ~ttPn-ling physician after careful study of the variety of ~mrlitudes in cardiac signal activity experienced by a patient. In other words, a sensing threshold is CA 02228~78 1998-02-04 W O 97/06851 PCT~US96/13181 programmed into the cardiac rhythm management device, and any cardiac signal amplitude larger than the pro~nl,lcd sensing threshold is considered a cardiac depol~ri7~tion. If, however, the pro~ d sensing threshold is set too high r and the cardiac signal amplitude decreases ~i~nific~ntly, as is often the case in fibrillation, the cardiac rhythm management device may not sense the a~l,ylh,~lia. If the progr~mmlod sensing threshold is set too low, the device may over-sense. For example, a system designed to detect ventricular depolarizations(R-waves) may erroneously detect atrial depolari_ations (P-waves) or ventricularleco~,c.y (T-waves). R~n-lr~s filtering can be used to partially elimin~te 10 erroneous detection of the P-waves and T-waves in a R-wave detection system.
If, however, the band of frequencies passed by the b~n-lp~cc filtering is too narrow, certain fibrillation signals may not be ~letecte-l Another approach to col.,~ells~Le for the above problems is to set the sensing threshold ~l~U~JOl ~ional to the amplitude of the sensed cardiac signal 15 each time a cardiac depolarization is sensed. The sensing threshold is then allowed to decrease over time between consecutively sensed cardiac depolarizations so that if the sensed cardiac signal ~mplih~le decreases significantly, the cardiac rhythm management device is still able to detect the lower level amplitude of the cardiac signal. Adjusting the sensing threshold to 20 an a~plo~,;ate level with this approach becomes difficult if the patient requires pacing due to a bradycardia condition. For example, in a system that senses R-waves according to this approach, the sensing threshold may be adjusted to one-half of the R-wave amplitude when an R-wave is sensed. However, the invoked response due to a first pacing pulse can cause the sensing threshold to be 25 set so high that a second ~o"l~leous R-wave is not sensed. Because the systemdoes not sense the second spont~neous R-wave, a second pacing pulse is delivered to the patient in~l.pl~,iately.
One solution to the above problem is found in the Kelly et al.
U.S. Patent No. 5,269,300 ~ n~d to Cardiac Pacemakers, Inc., the assignee of t 30 the present application. The Kelly et al. patent discloses an implantable cardioverter/defihrill:3tor with pacing capability wherein the sensing threshold is ~llt~ m~tically adjusted to a value ~.opollional to the arnplitude of the sensedcardiac signal. The sensing threshold continuously decreases between sensed cardiac depol~ri7~tion~ to ensure that a lower level cardiac signal will be ~ietecteA However, after a pacing pulse is delivered by the Kelly et al. device,~ S the sen~in~ threshold is set to a fixed value, and held at the fixed value for a pred~ d period of time, so that the sensing threshold is not affected by the cardiac response invoked by the pacing pulse. After a predet~ormin~d period of time, the sensing threshold is de~ ased, just as after a ~o~ eous cardiac depol~ri7~ti-~n In the Keimel et al. U.S. Patent No. 5,117,824, an R-wave ~lc~e~or ~ulo~ ;G~lly adjusts the detecting threshold in response to the R-wave ~mplitu~llo. The adj . .~ . .l of the threshold is disabled for a predetçrmined period following the delivery of each pacing pulse. ThelearL~l, the sensing threshold is returned to a lower threshold level to allow detection of lower level R-waves indicative oftachy-llyll-ll~ia conditions.
In the Henry et al. U.S. Patent No. 5,339,820, a sensitivity control is used for controlling a sensing threshold in a cardiac control device such as a pacemaker, cardioversion and/or cardiac defibrillation device. Initially, a sensing threshold is set to a low value. When the cardiac signal is d~tectç-l the ~mplihlde of the R-wave is measured and the sensing threshold is computed as a function of the ~mpli~ e of the R-wave. After a refractory period, the sensing threshold is preferably set to 75% of the amplitude of the R-wave. The sensing threshold is then decreased in ul~ir~llll steps. The uniform steps may be fixed dew~ or ~ ct;lllage reducti~ nc The Gravis et al. U.S. Patent No. 4,940,054 discloses a cardioversion device having three sensitivities. A first, medium sensitivity is used for the detecti-)n of sinus rhythm and ventricular tachycaldia. A second, higher se~ ivily is designed for dirr~ tin~ ventricular fibrillation from asystole. A third, lower sensitivity is used to dir~le.lli~le between R-waves and high amplitude current of injury T-waves which occur after shocking. One of these three sensilivilies is selected as a function of the status of the device, such CA 02228~78 1998-02-04 W O 97/06851 PCTrUS96/13181 as during a period of ~.l~c~,L~d taclly.;a dia or a post shock period, and the selected s~;l~ilivily must be ~ od at least until the next cycle.
The Dissing et al. U.S. Patent No. 5,370,124 discloses a cardiac rhythm management device having ch.;uik.~ for ~lltom~tir~lly adapting the detection sensitivity to the cardiac signal. The detection sensilivily is adjusted by either amplifying the electrical signal supplied to the threshold detector with a variable gain given a p~rrn~n~ntly prescribed threshold or by varying the threshold itself. In either case, the effective threshold is based on an averagevalue formed over a time interval CG~ onding to the duration of a few breaths.
A switching hy~l~-e~is is generated having a lower limit value and an upper limit value, where the threshold is reset only when the average value falls below the lower limit value or exceeds the upper limit value. The limit values of the switching hysteresis are varied with the variation of the threshold, but the relationship of the limit values to the threshold remain unvaried. In one embodiment of the Dissing device, when the threshold is set below a minimum value, a beat-to-beat variance of signal heights of succe~sive input electrical signals are used for forming an average value. The sensing threshold is raised by a pre~iet~rminPc~ amount if the variance excee~ the prec~etermin~ variance value.
The Carroll et al. U.S. Patent No. 4,972,835 discloses an implantable cardiac defibrillator which includes switched capacitor cil-;uilly for arnplifying the cardiac electrical signal with non-binary gain ch~nging steps.
Three stages of gain are used to increase the gain approximately 1.5 each increment.
The Baker et al. U.S. Patent No. 5,103,819 discloses a state m~chine for automatically controlling gain of the sensing function in an implantable cardiac stim~ tor. The rate of gain adjustment is dependent on the present sensed conditions and on the prior state of the heart. Different rates of adj~ are selected under varying conditions so that the gain of the sense amplifier is adjusted without significant overshoot. Multiple effective time CA 02228~78 1998-02-04 CO1AAjL~AA1k; are used for different conditions by basing the rate of adjlletmçnt of the sense amplifier gain on the path traversed in the state m~ in~
Therefore, considerable effort has been expended in providing for ~ukJlrl~Lically adjustable sensing thresholds through adjusting the threshold level ~ 5 itself or with ~ulolllaLic gain cil-;uilly in impl~nt~hle cardiac rhythm management devices for the purpose of enhancing the capability of the device to sense LyLlAAllia conditions for which therapy is to be applied.
~ ry of the Invention The present invention provides a method and system for ~AulolllaLically adjusting a sensing threshold in a cardioverter/defibrillator which receives electrical activity of the heart and provides shock pulses in response to the received electrical activity. The electrical activity is arnplified. Cardiacevents l~le,,~ ;n~ depolarizations in the electrical activity which exceed a variable sensing threshold are detectç~l ~mplih~-les of the amplified electricall S activity are tracked. The variable sensing threshold is adjusted to a level proportional to the amplihude of the ~mplifie~l electrical activity of a current~1etecterl cardiac event. The variable sensing threshold is decreased from the level in discrete steps until the variable sensing threshold is at a low threshold value. The discrete steps are grouped into step groups. Each step group decleases the variable sensing threshold by a defined percentage.
The variable sensing threshold is preferably adjusted with a template ge~ ion circuit. The template gçn~r~tion circuit preferably acquires the depol~ri~~ti~-n peak value, or after a pace or shock pulse, the variable sensing threshold is set to a selected relatively high threshold value. In one preferredembodiment, the selected relatively high threshold value is one binary number below the m~xi.,.~l... value of the variable threshold. Following a dlelay, the template generation circuit adjusts the variable sensing threshold to a level which is a ~cl~ lL~Age of a peak value of the amplitude of the amplified electrical activity of the current ~let~cte~l cardiac event. The percentage is typically 30 a~ xi.~ t~ly 75%. The template generation circuit preferably adjusts the variable sensing threshold to the level prior to the end of a new sensing CA 02228~78 1998-02-04 -~~Ld;lul,~ period caused by a det~cted cardiac event. When the cardioverter/~efihrill~tor is embodied in a cardioverter/defibrillator having pacing capability, the t~mpl~te generation circuit preferably adjusts the variable sensing threshold to the level at the end of a paced/shock refractory period 5 reSlllting from a pace or shock pulse. The template generation circuit preferably calculates an amount of drop for a discrete step using integer math to achieve apiecewise linear a~)pr~"~il,lation of a geometric progression. The geometric progression preferably lc;~le3t;l.ls an exponential decay curve. The defined p~ lldge drop for each step group is typically a~rux~ dl~ly 50%. In one 10 embodiment of the present invention, each step group includes at least four discrete steps.
The system of the present invention preferably compri~es a decay rate controller for varying a time width of each discrete step based on op~ldlillg c~ n~liti- ns of the cardioverter/defibrillator to control the decay of rate of the 15 variable sensing threshold. When the cardioverter/defibrillator is embodied in a cardioverter/defibrillator having pacing capability, the opeldlillg conditions include bradycardia pacing, tachyllly~ ia SÇn.cing, and normal sinus SÇncin~
The decay rate controller preferably varies the time width to produce a relatively fast decay rate when the cardioverter/defibrillator is op~,ldlillg under 20 tachy~llylh,llia sensing conditions. When the cardioverter/defibrillator withpacing capability embodiment of the present invention is o~ lh~g under the bradyc~dia pacing conditions, the decay of rate controller varies the time widthas a function of a bradycardia pacing rate. A pl~rell~,d embodiment of the decayrate controller varies the time width under bradycardia pacing conditions based 25 on the time between each pacing pulse, a programmed paced refractory period, and the number of discrete steps to go from said level to the low threshold value.
P~rief Description of the Dl ~tw;.. ~ -Figure 1 is a block diagram of a dual chamber cardioverter/defibrillator according to the present invention.
Figure 2 is a logical block diagram of an AGC filter and ~1igiti7ing circuit according to the present invention.

Figure 3 is a timing rliQ~rQm ilh~ g the sensed ler,dc~o.
used in the cardioverter/defibrillator of Figure 1.
Figure 4 is a timing diagram ill~ g the paced/shock refractory used in the cardioverter/defibrillator of Figure 1.
Figure 5 is a logical block diagr_m of a gain control circuit according to the present invention.
Figure 6 is a timing ~liQgr~m illustrating a piecewise linear rox;---Qt~ of an GA~one~lial decay ofthe variable sensing threshold according to the present invention.
Figure 7 is a ttonnpl~te gellG~dlion circuit according to the present invention, which achieves the piecewise linear ~ Ahl,ation of an expon~nti~l decay ill--etrAt~d in Figure 6.
Figure 8 is a timing diagram illu~Lldlillg the operation of the slow gain control circuit of Figure 5, in combination with the fast templating gel,GldLion circuit of Figure 7 in adjusting the gain and the sensing threshold of the cardioverter/defibrillQtnr according to the present invention.
ior~ of the r~f~ Fmbodimen~
In the following detailed description of the pler~ d embo.l;...~ , reference is made to the accc,l,l~ ying drawings which form a 20 part hereof, and in which is shown by way of illustration specific embo-iiment~
in which the invention may be practiced. It is to be understood that other embo~1im~nt~ rnay be utilized and structural or logical çhQng~s may be made without departing from the scope of the present invention. The following detQiled description, therefore, is not to be taken in a limiting sense, and the25 scope of the present invention is defined by the appended claims.
1 )ual ChQmber Cardioverter/DefibrillQtor with Pacin~p Capability A dual chamber cardioverter/defibrillator 20 with pacing capability is illustrated in block diagram form in Figure 1.
Cardioverter/defibrillator 20 O~clales as a pulse generator device portion of a 30 cardiac rhythm management system which also includes leads or electrodes (notshown) disposed in the ventricular chamber of the heart to sense electrical CA 02228~78 1998-02-04 activity .~,~se.~ e of a R-wave portion of the PQRST complex of a surface EGM in-lir~tin~ depol~ri7~tion~ in the ventricle. Cardioverter/defibrillator 20 includes input/output t~rmin~l~ 22 which are c~ le to the ventricular leads to receive the ventricular electrical activity of the heart sensed by the ventricular 5 leads. A pace pulse circuit 24 provides pacing pulses such as bradycardia and chycarlia pacing pulses to input/output t~rmin~l~ 22 to be provided to the ventricular chamber of the heart via the ventricular leads to sfim~ te excitablemyocardial tissue to treat ~hylh~llia conditions such as bradycardia and some tachy-;ar~lia. A shock pulse circuit 26 provides shock pulses to input/output 10 termin~l~ 22 to be provided to the ventricular chamber of the heart via the ventricular leads to shock excitable myocardial tissue to treat tachyrhythmia con-1iti~ n~. The tachylhyll~ ia conditions may include either ventricle fibrillation or ventricle tachycardia.
A filter and after potential removal circuit 28 filters the 15 ventric~ r electrical activity received by input/output terminals 22 and the pacing pulses provided from pacing pulse circuit 24. In addition, filter and after potential removal circuit 28 removes after potential created by a pacing pulse from pacing pulse circuit 24 or a shock pulse delivered by shock pulse circuit 26.
A ~.~f~ ..Gd after potential removal circuit is described in detail in the co-pending 20 and commonly ~si~ned U.S. patent application Serial No. 08/492,199 entitled "AFTER POTENTIAL REMOVAL IN CARDIAC RHYTHM
MANAGEMENT DEVICE" filed on June 19, 1995.
An ~utom~tic gain control (AGC)/filter and ~i~iti7ing circuit 30 according to the present invention amplifies the filtered ventricular electrical25 activity provided from the filter and after potential removal circuit 28.
AGC/filter and digitizing circuit 30 includes circuitry for digitizing the filtered ventricular electrical activity. A gain control circuit 32 automatically adjusts the gain of AGC/filter and digitizing circuit 30. An R-wave detection circuit 34 is coupled to AGC/filter and ~ligiti7ing circuit 30 to detect depolarizations in the 30 amplified ventricular electrical activity representative of R-wave depolarizations when the amplified ventricular electrical activity exceeds a selected amplified CA 02228~78 1998-02-04 W O 97106851 PCTrUS96/13181 level known as the lls~ iLivily threshold" or the "sensing threshold" and refractory is ina~;live. A temrl~te generation circuit 36 ~tom~tic~lly selects and adjusts the sensing threshold. R-wave detection circuit 34 provides a R-wave depol~ri7~tion signal, indicative of the R-wave depolari_ations, to a - 5 microprocessor and memory 38.
The cardiac rhythm management system also includes leads or electrodes (not shown) disposed in the atrial chamber of the heart to sense electrical activity ,~p ~st;lll~live of a P-wave portion of the PQRST complex of a surface EGM in-lic~tin~ depolari_ations in the atrium. Cardioverter/defibrillator 20 colle.,l,olldingly also includes input/output termin~l~ 42 which are conn~ct~hle to the atrial leads to receive the atrial electrical activity of the heart sensed by the atrial leads. A pace pulse circuit 44 provides pacing pulses such as bradycardia p~cing pulses to input~output te~min~l~ 42 to be provided to the atrial chamber of the heart via the atrial leads to stimul~tP excitable myocardial tissue to treat arrhythmia conditions such as bradycardia or atrial tachycardia. A
filter and after potential removal circuit 48 operates similar to filter and after potential removal circuit 28 to filter the atrial electrical activity received by input/output te~min~l~ 42 and the pacing pulses provided from pacing pulse circuit 44. In addition, filter and after potential removal circuit 48 removes after potential created by a pacing pulse from pacing pulse circuit 44.
An automatic gain control (AGC)/filter and ~ligiti7ing circuit 50 accol.ling to the present invention amplifies the filtered atrial electrical activity provided from the filter and after potential removal circuit 48. AGC/filter and fli~iti7in~ circuit 50 includes cil~ y for /ligiti7ing the filtered atrial electrical activity. A gain control circuit 52 automatically adjusts the gain of AGC/filterand ~iigiti7ing circuit 50. An P-wave detection circuit 54 is coupled to - AGC/filter and ~iigiti7ing circuit 50 to detect depolarizations in the arnplified atrial electriç~l activity le~.ese~ live of P-wave depolari_ations when the amplified atrial electrical activity exceeds a selected amplified level known as30 the "s~,nsilivily threshold" or the "sensing threshold" and the refractory isinactive. A template generation circuit 56 autom~tic~lly selects and adjusts the CA 02228~78 1998-02-04 W O 97/06851 PCT~US96/13181 sçn~in~ threshold. P-wave detection circuit 54 provides a P-wave depolarization signal, indicative of the P-wave depolarizations, to microprocessor and memory 38.
Microprocessor and memory 38 analyzes the ~letecte~l P-waves 5 indicated in the P-wave depolari_ation signal from P-wave detection circuit 54along with the R-wave depolarization signal provided from R-wave detection circuit 34 for the ~letectiQn of a-,l.ylh"lia conditions based on known algorithms.
For example, mi.lu~"oce3~0l and memory 38 can be used to analyze the rate, regularity, and onset of v~ri~ti~n~ in the rate of the reoccurrence of the rletecte~l 10 P-wave and/or R-wave, the morphology of the detected P-wave and/or R-wave, or the direction of propagation of the depolari_ation represented by the detected P-wave and/or R-wave in the heart. In addition, microprocessor and memory 38 stores depol~ri7~tic~n data and uses known techniques for analysis of the ~letecte-l R-waves to control pace pulse circuit 24 and shock pulse circuit 26 for delivery15 of pace pulses and shock pulses to the ventricle and for analysis of rletected P-waves to control pace pulse circuit 44 for proper delivery of pace pulses to theatrium. In addition, microprocessor and memory 38 controls a state machine 39 which places various circuits of cardioverter/defibrillator 20 in desired logical states based on various cc-n~liti--ns such as when a pace pulse or shock pulse 20 occurs or on opc.~ling conditions of the cardioverter/defibrillator such as bradycardia pacing, l~cl,y,l,yll~,lia sen~ing, and normal sinus sensing.
The dual chamber cardioverter/defibrillator 20 with pacing capability illustrated in Figure 1 includes pacing and shocking capabilities forthe ventricle and pacing capability for the atrium. Nevertheless, the present 25 invention can be embodied in a single chamber cardiac rhythm management device having a single one of these capabilities. For example, the present invention can be embodied in a ventricle defibrillator device for providing shock pulses to the ventricle only.
In some embolliment~ of cardioverter/defibrillator 20, 30 input/output termin~l~ 22 and 42 are each implem~ntçc~ to be connectable to aco,,ci~onding single set of electrodes (not shown) used for pacing, shock delivery, and sen~in~ In other embo-limPnt~ of cardioverter/defibrillator 20, the input/output t.. ;.. ~1~ are impl~mented to be co~ c~lhle to s~dle sets of electrodes for pulse delivery and sçn~ing In some embodim~nt~, the input/output 1~ -...;..Al~ are implem~nt.od to be conn~ct~ble to s~dlt; electrodes 5 for pacing and shock delivery. In all of these embo-liment~, the electrodes of a cardiac rhythm ..~ ge...~ -.1 system are typically implem~?nt~d as unipolar or bipolar electrodes.
A unipolar electrode configuration has one pole or electrode (i.e., negative pole or ciqtho~e electrode) located on or within the heart, and the other 10 pole or electrode (i.e., positive pole or anode electrode) remotely located from the heart. With endocardial leads, for example, the cathode is located at the distal end of a lead and typically in direct contact with the enl1Oc~rdial tissue to be stimulated, thus forming a "tip" electrode. Conversely, the anode is remotelylocated from the heart, such as comprising a portion of the met~ c enclosure 15 which surrounds the impl~ntto~ device, thus forming a "can" electrode and is often referred to as the "indi~ " electrode.
A bipolar electrode configuration has both poles or electrodes typically located within the atrial or ventricular chamber of the heart. With endocardial leads, for example, the cathode is located at the distal end of the 20 lead, referred to as the "tip" electrode. In the bipolar configuration, the anode is usually located ~ hllate to the "tip" electrode spaced apart by 0.5 to 2.5 cm., and typically f~lrminE a ring-like structure, referred to as the "ring" electrode.
With respect to sen~ing, it is well known that bipolar and unipolar electrode configurations do not yield equivalent cardiac EGMs. Each 25 configuration has advantages and disadvantages, for example, with a unipolar-sensing configuration, only the electrical events adjacent to the "tip" electrode control the unipolar EGM, while the remote "indifferent" electrode contributes n~ ihle voltage due to its location being extracardiac.
With a bipolar-sensing configuration, the magnitude of the 30 cardiac signal is similar for both the "ring" and the "tip" electrodes, but the reslllting EGM is highly dependent upon the orientation of the electrodes within W O 97/06851 PCT~US96/13181 the heart. Optimal sensing will occur, for t ~mrlP~7 when the sensing vector defined by the sensing electrodes is parallel with the dipole defined by the depolarization signal. Since bipolar electrodes are more closely spaced than their unipolar cuw~t~ , the depol~ri7~tinn signal will be shorter in duration S than that produced from a unipolar configuration. Due to a more restrictive lead field or ~ bipolar sensing offers improved rejection of electrom~gnPtic and skeletal muscle artifacts, and thus provides a better signal-to-noise ratio than unipolar sen~in~.
AGC/Filter ~n-l D~iti7in~ Circuit A logical block diagram .ep,. ;,e.~ ive of AGC/filter and iti7ing circuit 30 or 50 is illustrated in Figure 2. A programmable gain filter 60 filters the electrical activity provided from the filter and after potential removal circuit 28 or 48 of Figure l . When cardioverter/defibrillator 20 of Figure 1 is implPmPntP~l to be conn~ct~ble to bipolar electrodes, prog~ able 15 gain filter 60 compri~es an analog di~c~ ial sense amplifier to sense and amplify the dirrcle.lce bclwccn first and second bipolar electrodes.
Pro~ l,able gain filter 60 has a programmable gain to initially amplify the incoming electrical activity.
An analog to digital (A/D) converter 62 receives the filtered and 20 amplified electrical activity from programmable gain filter 60 and converts the analog electrical activit.v to .ligiti7P-1 cardiac data, which is stored in a successive ~p,o,~i",ation register (SAR) 64. A/D converter 62 ol,~.ales by col~ g a sample of "unknown" analog electrical activity from programmable gain filter 60 against a group of weighted values provided from SAR 64 on lines 66. A/D
25 cullvclh- 62 COIllpal'cS the weighted values on lines 66 in ~lesc~ncling order, starting with the largest weighted value. A weighted value is not added to the summed digital data stored in SAR 64 if the weighted value, when added to the previous summed weighted values, produces a sum larger than the sampled "unknown" analog electrical activity. The sllmmecl digital data is updated in 30 SAR 64 and a new weighted value is compared on each active edge of a SAR
clock on a line 68.

At the end of the su~ces~ive a~pl~xi.nation when balance is achieved, the sum of the weighted values stored as the s~mme~l digital data in SAR 64 l~plc:sellls the al",rok; . . .~te-l value of the sampled "unknown"analogclc~ l,ical activity. SAR 64 provides the stored digital cardiac data to an absolute - 5 value circuit 70. Absolute value circuit 70 provides the absolute value of the amplitude of the digital cardiac data on a line 72 to be provided to gain control circuit 32/52 and t~mrl~te ge~ lion circuit 36/56. Successive a~pruxillla~ion A/D conversion as p - rn....~d by A/D converter 62 and SAR 64 is very fast to perrnit adequate tracking of the h~o~ lg analog cardiac signal. The gain of 10 pro~r~mm~ble gain filter 60 is raised or lowered in discrete gain steps based on outputs from gain control circuit 32/52.
Se~rate G~in Con~ol ~nrl Threshold T~n~ tin~
Gain control circuit 32/52 and template ~llel~lion circuit 36/56 operate with the AGC/filter and ~1igiti7ing circuit 30/50 to implement two 15 in~ ,pPndent AGC digital loops accor lillg to the present invention. Gain control circuit 32/52 provides slow gain control to AGC/filter and ~ligiti7ing circuit 30/50 to keep sensed depolarizations representative of cardiac events in approximately the upper one third of the dynamic range of A/D converter 62.
Template gen~ldlion circuit 36/56 provides a fast responding variable sensing 20 threshold to the d~tection circuit 34/54 for actual sensing of R-wave or P-wave depol~ri7~tions lel)r~ e of cardiac events.
Gain control circuit 32/52, as described in more detail below with reference to Figure 5, stores peak history h~llll~lion ~ est;~ e of peak values of the amplified electrical activity of a selected number (N) of cardiac 25 events. Gain control circuit 32/52 adjusts the variable gain of AGC/filter and tii~iti7in~ circuit 30/50 in discrete steps based on the stored peak history information. The stored peak history information is CO-Il~ d against pre~l~fin~d levels and ~plo~,liate gain changes are initi~tPd based on a second selected number (M) of peak values of the N cardiac events being outside of a 30 selected range.

CA 02228~78 1998-02-04 Template gelh~.dlion circuit 36/56, as described in more detadil below with reference to Figure 7, provides a time varying sensing threshold to detect circuit 34/54 for c~ -p~- ;eon to the ~ iti7~?~1 cardiac data provided on line 72 from AGC/filter and ~ligiti7ing circuit 30/50. Detection circuit 34/54 5 provides a detectiQn signal indicating R-wave or P-wave depolari7ations cples~ ;v-e of cardiac events when the value of the incoming digital cardiac data is greater than the sensing threshold level provided that the refractory windows are inactive. Template gcllc~alion circuit 36/56 includes Chcuill,y for selecting and adjusting the variable sensing threshold to a level ~lopollional to 10 the amplitude of the digital cardiac data on line 72. Typically, temrl~te gt?tler~til n circuit 36/56 responds very quickly to change the sensing threshold to the peak value of the digital cardiac data on line 72. The variable sensing threshold is held at the peak value for a selected period of time after which the variable sensing threshold drops to a p.,.cellldge of the peak value. The variable 15 sensing threshold is then allowed to slowly decay from this percentage of peak value in discrete steps until the variable sensing threshold is at a low threshold value. Template ~scllclalion circuit 36/56 preferably employs integer math to achieve a piecewise linear approximation of a geometric progression such as an c~oncll~ial decay curve with minim~l error between piecewise steps.
20 Refractory Periods Cardioverter/defibrillator 20 utilizes ventricular and atrial refractory periods to d~ which sensed events are R-waves or P-waves ,e~ livcly. The active sensed refractory periods are illustrated in timing rli~m forrn on line 73 at 74 in Figure 3. Any sensed event that occurs when 25 the sensed refractory period is inactive is considered to be a R-wave or P-wave.
Any events sensed during the active sensed refractory period are ignored and do not affect the ventricular or atrial cycle length mea~ulclllcnt. Typical sensed events occ~ ng on the lead are represented on line 75 at 76. As illustrated, thestart of the active ventricular or atrial refractory period is synchronized with the 30 start of the cardiac cycle. An absolute refractory interval is indicated on line 77 at 78. The absolute refractory interval starts at the beginning of the cardiac cycle W O 97/06851 PCTrUS96/13181 ~imlllt~nPous with the start of the active sensed refractory period. The absolute l~ Ld ;lol~ interval disables all sPncing The operation of template ~r~ l ion 0 circuit 36/56 b~ed on the absolute refractory interval is further described below under the Threshold Tetnr!~tin~ for a Fast Digital AGC Circuit hP~-1ing ~ 5 During pacing or shock delivery from cardioverter/defibrillator 20 a paced/shock refractory period, as indicated on line 79 at 81 in Figure 4, is ll-tili7P(l instead of the sensed refractory period. Similar to the sensed lc;rldclul y period, any sensed event that occurs when the paced/shock refractory period is inactive is considered to be a R-wave or P-wave. Typical pace pulses on the leadare l~res~l.ted for illustrative purposes on line 83 at 85. A t.,vpical shock pulse is not shown. The paced/shock refractory period is started with the delivery of the pace or shock pulse. Absolute refractory intervals are not utilized during pacing or shocking con-lition~ The time duration of the paced refractory period is preferably prc!~r~nnm~ble, while the time duration of the shock refractory period is typically not prog.d lll-lable. The paced refractory period can be selected by the physician and prograrnmed into cardioverter/defibrillator 20 when the cardioverter/defibrill~t~r is op~ g in a pacing mode. The operation of template g~ wdLion circuit 36/56 based on the paced/shock refractory period is further described below under the Threshold Templating for a Fast Digital 2û AGC Circuit hP~ing.
Slow Gain Control Circuit Gain control circuit 32, or ~ltPrn~t~ly gain control circuit 52, is repres~nt~tively illustrated in Figure 5 in logical block diagram form. A
co l~alor 80 receives the digital cardiac data on line 72 and co~pal~es the peakvalue of the digital data lel)les~ g the current cardiac event to a selected lowthreshold and a selected high threshold. For example, in one ~,er~ d embodiment of the present invention where the maxi.nu-ll value of the peak value of the digital cardiac data is 7F hex, the selected low threshold is 52 hex and the selected high threshold is 7E hex. A first storage register 82 includes a first group of storage locations which store peak history information provided by co~ udlor 80 on a line 84 indicative of whether the peak values are below the CA 02228~78 1998-02-04 W O 97/06851 PCTrUS96/13181 selected low threshold (52 hex in the example embodiment). A second storage register 86 in~ es a second group of storage locations which store peak history information provided by co~ ~dlor 80 on a line 88 indicative of whether the peak values are above the selected high threshold (7E hex in the ex~mrle S embo-1im~nt).
An M/N circuit 90 receives peak history information from storage register 82 and det~rrnin~os if M peak values of N cardiac events are below the selected low threshold (52 hex). M~N circuit 90 provides an increment signal on a line 92 to a gain control clock circuit 94. M/N circuit 90 activates the 10 increment signal on line 92 when M out of N peak values are below the selected low threshold (52 hex) to indicate that the gain of AGC/filter and digitizing circuit 30/50 is to be incrempntpd by at least one discrete gain step. In one embodiment of the present invention, the discrete gain step is approximately equal to 1.25. An M/N circuit 96 receives peak history il~fo~ ation from storage15 register 86 and determinçs if M peak values of N cardiac events are above theselected high threshold (7E hex). M/N circuit 96 provides a decrement signal on a line 98 to gain control clock circuit 94. M/N circuit 96 activates the decrement signal on line 98 when M out of N peak values are above the selected high threshold (7E hex) to indicate that the gain of AGC/filter and digitizing circuit 20 30/50 is to be decr~Tnlo-nted by at least one discrete gain step. The decrem~nting discrete gain step is preferably equal to the incr~menting discrete gain step and is a~rox; . . .~tely equal to 1.25 in one embodiment of the present invention.
Gain control clock circuit 94 provides a gain control signal on a line 100 which controls the gain of AGC/filter and ~ iti7in~ circuit 30/50 by 25 causing the gain to be increment~d or decrçmente~1 in discrete gain steps based on the increlllelll signal on line 92 and the decrement signal on line 98. The gain of AGC/filter and ~ligiti7in~ circuit 30/50 can be increased or decreased by a fixed number of steps or amount, or the level of the discrete gain step is optionally made programmable via microprocessor and memory 3 8 . In addition, 30 gain control and clock circuit 94 optionally causes increments or decrements of gain in multiple discrete gain steps. Since the increment signal on line 92 and W o 97/06851 PCTAUS96/13181 the de.ilcl~ll signal on line 98 are never activated at the same time due to thedual low threshold (52 hex) and high threshold (7E hex), no ~I,iL~ ion Cil~;Uilly iSlRCe~S;~,y to a~ dLe b~,lw~ell the increment or decrement signals to indicate which direction to proceed. Gain control circuit 32/52 preferably keeps the peakS values of atrial or ventricle sensed cardiac events in a~loxil,lately the upper one third of the dynamic range of A/D converter 62. As a result, the lower oxilllately two thirds of the dynamic range of A/D converter 62 is available for sçn~ing low ~mrlit~l-le signals such as occllrring during fibrillation.
The above referenced number of M peak values is preferably odd 10 to prevent lock-up of the AGC loop. For exarnple, in a plerc.led embodiment of gain control circuit 32/52, M is equal to three and N is equal to four. In this embo~lim~nt storage register 82 stores peak history inforrnation for four cardiac events in four col,~,~ponding storage locations each ~ selll~live of whether thecoll~ ,ollding one of the last four values for peak values was below the selected 15 low threshold (52 hex). In this embodiment, storage register 86 stores peak history inform~tion for four cardiac events in four COll~ spollding storage locations each ~ e.ll~ e of whether the colle~onding one of the last four values for peak values was above the selected high threshold (7E hex).
The peak values in storage register 82 and storage register 86 are 20 preferably ~-p~te~ at the beginning of a new lc;rld.;lol y period for a previous sensed event. As new peak value information is acquired from conll)aldlor 80, the old peak history il~i ".~tion is shifted one value to the right. If storage regi~ters 82 andl 86 only contain four storage locations, the peak history values older than the last four cardiac events are shifted out of the registers to the right 25 and lost.
In a pl~ r~.l, d embodiment of the present invention, M/N circuit 90 activates the increment signal on line 92 only if the stored peak value of the last cardiac event and M - 1 peak values of the last N - 1 cardiac events previous to the last cardiac event are below the selected low threshold (52 hex). In this30 preferred embodiment of the present invention, M/N circuit 96 activates the declGlllwll signal on line 98 only if the stored peak value of the last cardiac event W O 97/06851 PCTrUS96/13181 and M - 1 peak values of the last N - 1 cardiac events previous to the last cardiac event are above the selected high threshold (7E hex).
Gain control circuit 32/52 operates as described above to ~.,;..;...;,~ the pos~ibility of ill~)l'Upel sensing by not allowing AGC/filter and 5 ~1igiti7in~ circuit 30/50 to go to low sensitivity if large R-waves or P-waves are present or to go to full st;nsilivily in the presence of slow R-waves or P-waves.
r~. sensing can cause therapy to be delivered to a patient at ina~p.oplia times as a result of false indications of a"l,ylil."ia conditions. Oversensing is reduced bec~use the full sensitivity of AGC/filter and digitizing circuit 30/50 is 10 not reached bc;l~ ell slow beats as a result of gain control circuit 32/52 keeping the amplified depolarization electrical activity in the upper approximately one third of the dynamic range of A/D converter 62. The reduced oversensing greatly increases the comfort level of a patient having the cardioverter/defibrillator acco,-lil,g to the present invention implanted in his or 15 her body. Un-l~rs~ncing is reduced because minimllm sensitivity will not occur due to a single large R-wave or P-wave.
In addition as indicated above gain control circuit 32/52 elimin~tçs the need for a high precision A/D converter implementation of A/D
converter 62, bec~llse the entire dynamic range of the incoming cardiac signal 20 does not need to be sr~nne~l Thus, in the ~,~;fe~ embodiment of the present invention, A/D converter 62 is implemented in 8 bits or less. The dynamic range of the incollling cardiac signal from the atrial and/or ventricular channels of the heart ranges from 0.1 mV to 25 mV ~ es~ ;,.g a 250 to 1 dynamic range.
Lower precision A/D converters consume less power, convert the incoming 25 analog signal to l~lese.,l~ Je digital data more quickly, and allow more costeffective silicon processes to be lltili7~ (1 Moreover m~nllf~çturability of thecardioverter/defibrillator is improved since no external parts are required to control the gain of the AGC/filter and ~1igiti7ing circuit 30/50. Testing and çh:-r~ lion of the cardioverter/defibrillator is also improved since the digital 30 logic of the gain control circuit 32/52 is easily fault graded.

W O 97/06851 PCTrUS96/13181 AGC Tllrr~ nown Mecll ...;~... for F~r Field S~n~i~
The ~ f~,l,ed embodiment of gain control circuit 32/52 illustrated in Figure 5 includes a far field sense circuit 102. Far field sense circuit 102 provides a solution to a possible AGC loop lock-up due to far field sPn~ing For 5 example, when sensing events in the ventricle channel of the heart, P-waves, cpl~,s~ g far field events, can be sensed during normal sinus rhythms at X;~ II sensitivity. Under this P~rnrle condition, instead of AC;Cing on the R-wave peaks, which are clipped, the P-wave peak level alt~-rn~ting with the clipped R-wave peak level combine to inhibit gain changes. The clipped R-wave 10 peaks indicate that the R-wave peaks are above the m~xim~lm digital value for a peak signal. In this case, the M of N algo~ .l is never met in M/N circuit 96, which causes a lock-up condition in the AGC loop. Far field sense circuit 102 provides an additional gain decrease option to gain control circuit 32/52 in addition to the normal modes of operation to prevent this lock-up condition from15 occ lrring In the embodiment illustrated in Figure 5, two additional history storage locations are provided in storage register 86 to extend the peak historyinf rm~tion to N + 2 storage locations. Far field sense circuit 102 responds to the last N + 2 sensed events stored in storage register 86 to determine if the 20 storage information ~ltern~t~s between clipped peaks and non-clipped peaks for the last N ~ 2 sensed events. Far field sense circuit 102 ~l~terrnines that a peak is clipped when the peak is at the ..~xi.~n..., value (7F hex) which co.le;,~onds to peak values greater than the high threshold value used by Cc:~lllpald~ - 80 (7E
hex). Far field sense circuit 102 provides a de~ nelll signal on a line 104 to 25 gain control clock circuit 94. Far field sense circuit 102 activates the decrement signal on line 104 when the peak history information in storage register 86 - ~It~rn~tes between clipped peaks and non-clipped peaks for the last N + 2 sensed events. In one embodiment, the decrement signal on line 104 indicates that gain of AGC/filter and ~i~iti7inp circuit 30/50 is to be decremented by one discrete 30 gain step, but can altt-rnSltively intlir~te any number of discrete gain step chdllges.

CA 02228~78 1998-02-04 W O 97/06851 PCT~US96/13181 Far field sense circuit 102 operates in the cases where the actual depolarization of the incoming cardiac signal is clipped to prevent the digital AGC loop from locking up under the condition of far field sensed events. If far field events are rl~ect~ in the above manner, the gain of AGC/filter and 5 rli~iti7inp~ circuit 30/50 is decreased to the point that far field events no longer are sensed. Previous cardioverter/defibrillator devices all oversense (double count) under this far field sensing condition. The far field sense circuit 102 according to the present invention greatly improves sensing ~ crimin~tion by minimi7in~ or sllbst~nti~lly eli...i~ g OVtils~.~7illg in the presence of far field 10 events. Accordingly, the cardioverter/defibrillator according to the present invention provides a patient and his or her physician a cardioverter/defibrillator which senses the R-wave depolarizations more reliably.
Slow G~in Jnn~p Rack for AGC
Previous gain ~ cuil"r reaches .,.i~Xi..,.",. sensitivity in a single 15 cardiac cycle. Unlike previous gain chcuilly, the slow gain ch~;uill y according to the present invention described above makes discrete step gain changes of oneor more discrete gain step per cardiac depolarization cycle, so that full sensitivity of the AGC/filter and ~ iti7in~ circuit 30/50 is not reached between cardiac depolarizations, which can cause undersensing of cardiac events. As illustrated 20 in Figure 5, additional ~ih~iuill y is preferably added to gain control circuit 32/52 to prevent undc,~ g of cardiac events.
Exception cil~;uiLIy 106 detects any one of three conditions which in~ te that the gain of the AGC/filter and digitizing circuit 30/50 is to be set to a selected relatively high sensitivity. Exception circuitry 106 provides a set gain 25 signal on a line 108 to cause the gain of AGC/filter and digitizing circuit 30/50 to be set to the selected relatively high sensitivity when any of the three conditions occur. The first condition occurs when a cardiac event is not detected for a selected time period (i.e., a R-wave or P-wave depolarization is not sensed for the selected tiIne period). Typically, the selected time period is equal to 30 approximately 1.5 seconds, corresponding to a heart rate of less than 40 beats per mimlte The second condition occurs after the cardioverter/defibrill~tQr delivers CA 02228~78 1998-02-04 a shock pulse. The third condition occurs after the cardioverter/defibrillator delivers a pacing pulse.
In any of the three conditions, it is desirable to prevent lmf~ X;I-g by setting the gain of the AGC/filter and ~igiti7ing circuit 30/50 to- S the selecte~l relatively high sensitivity to quickly hlc-~;ase the sensitivity of the cardioverter/~efibrill~tor. ~n~con~.L~162 (shown in Figure 2) typically op~.dtes in bands of an approxim~t~ly 10:1 dynamic range. The combined 10:1 dynamic range bands create a total 250:1 dynarnic range of A/D converter 62.
The three exception conditions are conditions where A/D converter 62 needs to 10 operate near mz.x;.nl.ln sensitivity, or in other words, near the upper portion of the highest 10:1 dynamic range band to adequately prevent undersl~n~ing In the pler~ d embodiment of the present invention, the selected relatively high sensitivity is two gain steps from a llldXill~ sensitivity to prevent mi~t~k;ng P-wave depolari_ations and T-wave repol~r-7~tion~ for R-15 wave depol~ri7~ti- ne. If the selected relatively high sensitivity creates a clipped signal on the following depolarization, having its peak at the maxhnul,l value (7F hex), as indicated from c-,ln~udlor 80 on line 88, a jump back compare circuit 1 10 activates a line 1 14 to a two input OR gate 1 16 to indicate that the gain is to be reduced by an offset value stored in offset register 1 12. OR gate20 116 provides a decrement signal on an enable line 120 to gain control clock circuit 94 which is activated when either of the two inputs to the OR gate are activated to in-lic~te that the gain of AGC/filter and rligiti7.ing circuit 30/50 is to be decrem~nt~ by at least one discrete gain step during the current refractory period. The offset value stored in register 1 12 is preferably programmable and is 25 provided to gaim control clock circuit 94. In one embodiment, the offset value is prograrnrned to equal three discrete gain steps.
If the peak value of the digital cardiac data on line 72 is still clipped on the next depolari_ation after the gain has been decreased by the offset value stored in offset register 112, co~ )aldlor 80 indicates on line 88 that the 30 peak ofthe car~liac signal is still clipped. Jump back compare circuit l l0 then in~ tes that the gain is to be decremented by at least one discrete gain step by CA 02228~78 1998-02-04 W O 97/OC851 PCTAUS96tl3181 activating a line 118 to the other input of OR gate 1 16, which colle~ondingly a~iliv~Lles enable line 120 to gain control clock circuit 94. If the peak value is still clipped, normal AGC action as described above le;~ lcs. This two staged back offmech~ni~m after a jump out or escape to the selected relatively high 5 sensitivity due to lack of sensing reduces ov~ i..g resllltin~ from the clipped peak of the cardiac signal.
If the peak value of the digital cardiac data on line 72is not clipped on the first depol~ri7~tion after the gain is set to the relatively highsensitivity, but the peak value is clipped on the second depolarization after the 10 gain is set to the relatively high sensitivity by having its peak at the m~ximum value (7F hex), as indicated from colnpd.dlor 80 on line 88, jump back compare circuit 1 10 activates line 1 14 to two input OR gate 1 16 to indicate that the gain is to be r~duced by the offset value stored in offset register 112. OR gate 116 provides the decrement signal on enable line 120 to gain control clock circuit 94 15 which is activated when either of the two inputs to the OR gate are activated to indicate that the gain of AGC/filter and tli~iti7ing circuit 30/50 is to be de~;re~ by at least one discrete gain step during the current refractory period. If the peak value is still clipped, normal AGC action as described aboveres~lm~s This situation, where the peak value of the first ~letecte~l depolarization 20 is not clipped and the peak value of the second ~etecte(l depolarization is clipped after the gain is set to the relatively high sensitivity, results when the firstdepolarization ,~l~,se~ a far field sensed event such as described above. For example, when sensing events in the ventricle channel of the heart, P-waves, l~,lP~S~ ;..g far field events, can be sensed dluring normal sinus rhythms at ... ~,~;.. sensitivity.
~rhr~hold Tem~ qtin~ for a Fast Di~ital AGC Circuit Figure 6 illustrates, in timing diagram form, the variable sensing threshold generated by template generation circuit 36/56 and provided to ~letection circuit 34/54. The variable sensing threshold is indicated by line 130.
30 As illu~te-l, the variable sensing threshold 130 follows a piecewise linear ap~lv~nalion of an expon~nti~l decay curve with minim~l error between steps.

The template ~ ~n~lalion circuit 36/56 forces the variable sensing threshold 130to rapidly follow the ln~?u~ulll peak level of the r1igiti7~cl cardiac data. When the incoming di~iti7~d cardiac data is greater than the current sensing threshold, temrl~te pf --f~ ;on circuit 36/56 raises the variable sensing threshold 130 to a 5 peak threshold value a~l )~ tely equal to the peak value of the incoming .1ipiti7~d cardiac data as in~ te~i at time T(0). After a shock or pace pulse isdelivered by the cardioverter/defibrill~tor, template p~ -.f ~ ;on circuit 36/56 sets the variable sensing threshold 130 to a selected relatively high threshold value.
The selected relatively high threshold value is preferably 7E hex in the example10 embo~liment or one binary number below the m~ximllm value of the variable threshold.
The variable sensing threshold 130 remains at the peak threshold value ~hr~ gll a feLa;~ol ~ or a portion of a l~,LacLOl y period indicated at 132.
When the cardioverter/defibrillator is Oyc~alillg in pacing mode, the period 15 inAic ~ted at 132 is a pro~,,alnl"cd paced refractory period that is selected by the physician and programmed into the cardioverter/defibrillator, such as the paced/shock ~ a~lol~ period indicated at 81 is Figure 4. When the cardioverter/defibrillator is Op~àling in shocking mode, the period indicated at132 is a shock refractory period, such as the paced refractory period indicated at 20 81 is Figure 4. When the cardioverter/defibrillator is ~lJ~ldLillg in sensing mode, the period in-lic~ted at 132 is a portion of a sensed refractory period, such as the absolute refractory period in~lic~ted at 78 is Figure 4. In addition to the refractory or the portion of a refractory period indicated at 132, the variable s~n~in~ threshold does not begin to decay from the peak threshold value ~tt~inecl 25 at time T(0) for an additional drop time indicated at 134. The drop time is anormal template hold time for the peak coll~,el l. . Cil.;uiLIy of template generation ~ circuit 36/56, and is empirically deterrnine-l A suitable value for the drop time in one embodiment ofthe present invention is ~ro~illlately 13.7 msec.
After the refractory period and the drop time have elapsed, at time 30 T(1,0) the variable sensing threshold 130 drops by an initial drop perc~ ge, inf1ir~tecl by arrows 136. The initial drop pc;-.;c;ll~ge is preferably approximately CA 02228~78 1998-02-04 25% of the peak threshold value so that the level of the variable sensing threshold obtained at time T(l ,0) is a~ oxilllal~ly 75% of the initial peak threshold value. As indicated at time T(1,1), the variable sensing threshold starts to decay in discrete steps such as in-lir~tecl at 138. The step tirne size is 5 l~.cs~-.tS~I;v~ly indicated by arrows 140 between time T(l,l) and T(1,2). The level of the variable sensing threshold 130 decays from a p~ lce~ ge of the peakthreshold value to step over wide depol~ri7~tions or T-waves in the inco.llillg electrical activity.
In the pler~.led embodiment of the present invention, template 10 genel~lion circuit 36/56 drops the variable sensing threshold 130 in step groups coll~lisillg multiple discrete steps. In the embodiment illustrated in Figure 6,the step group size is four. Each step group decreases the variable sensing threshold by a defined percentage, as indicated by arrows 142 for a four step group between time T(1,0) and T(2,0), arrows 144 for a four step group between 15 T(2,0) and T(3,0), and arrows 146 for a four step group between T(3,0) and T(4,0). The defined pe~ age for each step group is preferably approximately 50%. For example, in the pler...led embodiment of the present invention, the value of the variable sensing threshold at time T(2,0) is approximately 50% of the value of the variable sensing threshold at time T(1,0), and the value of the20 variable sensing threshold at time T(3,0) is approximately 50% of the value o~
the variable sensing threshold at time T(2,0) or 25% of the value of the variable sensing threshold at time T(l,0).
When the variable sensing threshold 130 decays to a progr~mm~ble final value, as indicated at 148, template generation circuit 36/5625 holds the variable sensing threshold at the programmable final value until a new sensed event occurs. The progld,lllllable final value is programmable to colllpellsate for noise which is inherent in the sense amplifiers and other AGC
system circuits of the AGC loop.
The initial drop pt;l~;e~ ge to achieve approximately 75% of the 30 peak threshold value, and the four discrete steps in each step group to drop the variable sensing threshold to approximately 50% of the level of the start of the -four-step group realizes a ~iec~ ~. ise geometric progression linear a~p~ ti~ n ~JlGS~ an exponential decay curve with minim~l error bGIWGe1I piecewise steps. Since the sensing threshold drops in discrete steps as indicated at 138, integer math can be utilized in template gGn~.dlion circuit 36/56. For example in ~ 5 the embodiment of template gGl~alion circuit 36/56 illustrated in Figure 6, floating point r~lmhçr.~ are not required bec~use the ma~h.lulll dirr~.~nce/error bGIwGGll any two discrete steps in a four step group is one bit. The present invention can be ~xt~n-le-l to use any size integer value or number of steps or step groups to achieve the linear a~pl~,xi.llation of the exponential decay curve.
In fact, floating point numbers are optionally used, but are not desirable because of the increasedl silicon drea needed to implement flo~ting point logic circuits. In addition, by implementing the template gen~ .dlion circuit with integer values, the resulting template g~ -lGldlion circuit c~-n~--mes a relatively small amount of power.
A preferred algolil~LIll for calculating the drop in amplitude for each of the discrete steps is shown in TABLE I below.

W O 97/06851 PCT~US96/13181 TARTJh', I

T~TF.RVAT STF.P CAT CUT ~TION

S TEMP = T(0) - T(0) /2 + T(0) /4 TEMPl = T(X-1,0) /2 T(X,0) = IF (X= 1) THEN
IF (FINAL THRESHOLD > TEMP) THEN
FINAL ELSE TEMP

IF (FINAL THRESHOLD > TEMPl) THEN

TEMP = T(X,0) - T(X,0) /4 + T(X,0) /8 T(X,l) = IF (FINAL THRESHOLD > TEMP) THEN FINAL
ELSE TEMP

TEMP = T(X,0) - T(X,0) /2 + T(X,0) /4 T(X,2) = IF (FINAL THRESHOLD > TEMP) THEN FINAL
ELSE TEMP

TEMP = T(X,0) - T(X,0) /2 + T(X,0) /8 T(X,3) = IF (FINAL THRESHOLD > TEMP) THEN FINAL
ELSE TEMP

Where:
T0 = PEAK THRESHOLD VALUE
T(X, 13)...... = One of Four Steps X = 1,2,3,4 - Decay Period pceferring to TABLE I above, in the interval T(X,0), TEMP is calculated to 7~% of the initial peak threshold value, and TEMPl is calculated to 50% of a previous step group value. If the step group is the first drop from thepeak ~reshold value, then T(1,0) is equal to TEMP or 75% of the peak value. In - 5 ~1cce~ive drops, T(X,0) is equal to TEMPl or a 50% drop from the level at the bee;nl~;..g ofthe previous step group.
In all ofthe T(X,l), T(X,2), and T(X,3) intervals, the variable sen~ing threshold obtains the TEMP value unless the TEMP value is less than FINAL THRESHOLD which is the final programtnable value indicated at 148 in 10 Figure 6. For example, in the T(X,l) interval, T(X,1) is set to TEMP which isc~lr~ ted to 87.5% of the T(X,0) value. In the T(X,2) interval, T(X,2) is set toTEMP which is calculated to 75% of the T(X,0) value. In the T(X,3) interval, T(X,3) is set to TEMP which is ç~lc~ ted to 62.5% of the T(X,0) value.
A logical block diagram of a plGr~ d embodiment of template 15 L~ on circuit 36/56, which uses integer values for calc~ ting the variable sensing threshold, is illustrated in Figure 7. A peak detection circuit 160 detects the peak value of the ~igiti7~1 cardiac data provided on line 72 from AGC/filterand ~ iti7ing circuit 30/50. Peak detection circuit 160 provides a peak threshold value which is equal to the peak value of the ~ligiti7~d data to a threshold register 20 162 if the tli~iti7e-1 peak is greater than the current threshold value. Threshold register 162 stores and provides the current variable sensing threshold on line 164 to the ~letectif)n circuit 34/54. Peak detection circuit 160 also provides the peak threshold value to T(X,0) register 166.
If the step group is not the first drop from the peak threshold 25 value the TEMPl calculation must be impl~?m~nte~l for the T(X,0) interval ofthe discrete step calculation algorithm in TABLE I above. To implement the ~ TEMPl calculation, the T(X-1,0) value stored in the T(X,0) register 166 from the previous step group is divided by 2 through a hard shift of one to the right as inrlic?lted by line 168 to place the shifted data in both the threshold register 162 30 and the T(X,0) register 166.

CA 02228~78 1998-02-04 W O 97tO6851 PCT~US96/13181 T(X,0) register 166 provides its ~ lly stored value to a subtraction circuit 170 and a shifter 172. Shifter 172 provides either a divide by 2 or a divide by 4 c~lc~ tion by shifting the current T(X,0) value by one bit ortwo bits to the right, re~e~;lively. Subtractor 170 subtracts the value stored in S the T(X,0) register 166 from a shifted output provided from shifter 172. Theshifted output of shifter 162 is also provided to a shifter 174. Shifter 174 provides an additional divide by 2 or divide by 4 through shifts of 1 bit or 2 bits to the right"~e~ ely. A di~t;l~.lcc output of subtractor 170 is provided to an adder 176. A shifted output of shifter 174 is provided to the other input of adder 176. Adder 176 adds the di~tlc.lce output of subtractor 170 and the shifted output of shifter 174 and provides the added value to threshold register 162.
The shifters 172 and 174 can~ in combination, achieve shifts of 1, 2, 3, or 4 bits to produce divide by 2, divide by 4, divide by 8, or divide by 16 calculations. The TEMP calculations required for the T(X,0), T(X,l), T(X,2), and T(X,3) intervals of the discrete step calculation algo~ n in TABLE I above are all achieved through shifters 172 and 174 in combination with subtractor 170and adder 176. Shifters 172 and 174 calculate the desired divide by values which are then plo~clly combined according to the algorithm in TABLE I with subtractor 170 and adder 176.
A final threshold register 178 stores the progld~ able final value, indicated at 148 in Figure 6, of the variable sensing threshold. The pro"~ ble final value is provided to a threshold colll~ lo~ 180. Threshold colll~ Lor 180 culllpales the progr~mm~ble final value stored in final thresholdregister 178 with the current variable sensing threshold value on line 164.
Threshold conl~ua~or 180 indicates to threshold register 162, on a line 182, whether the current variable sensing threshold value is greater than the programmable final value. If the programmable final value is greater than the calculated sensing threshold value, then the final value is stored in threshold register 162. The sensing threshold value stays at the final value until the illcolllillg ~ iti7~d cardiac data exceeds the final value indicating a new sensed event. In fact, a new sensed event occurs any time the incoming ~ligiti7~d W O 97106851 PCT~US96/13181 cardiac data peak value excee-l~ the current variable sensing threshold value online 164. With the new sensed event, the variable sensing threshold obtains a new T(0) peak threshold value equal to the peak value of the sensed depol~ri7~tio~ in the ~1igiti7ed cardiac data.
~ 5 The above described threshold t~mpl~ting algc.~ . for a fast digital AGC system is completely contained in digital logic as impl~mented in the ~ ,r~ll.,d embo~lim~nt The digital logic implem~nt~tion is easily characterized, tested, and achieves repeatable results. In addition, e~ct~rn~l parts are eli...il-~lç(l from the silicon chip impl~om~?nt~tion of the AGC Cil~;UiLly to 10 reduce cost and increase the m~nllf~ctllrability of the AGC silicon chip. Testing andchar~ r.;~i.l;onofthecardioverter/defibrillatordevicesis .. irc.. fromone device to another. In this way, it is easier for the physician to cletçrrnin~ how to implement the cardioverter/defibrillator device in a patient, because the devicereacts cor~ t~ntly from one unit to another.
1~ T~ilorable AGC necay ~te No single decay rate (attack rate) is optimal for all O~ld~ g conditions of a cardioverter/defibrillator with pacing capability for the above described fast response AGC circuit. The typical operating conditions encountered include bradycardia pacing, tachylllylllmia s~n~ing, and normal sinus sçn~in~. Therefore, according to the present invention, the step time sizeindicated by anows 140 in Figure 6 is programmable to achieve a tailorable AGC decay rate for the variable sensing threshold 130. In this way, by varying the step time size 140 for each of the defibrillator's Op~ldlillg conditions, the decay rate is customized to optimally meet the selected op~ .alillg condition.
For normal sinus sen~ing, a single attack rate is utilized that covers most of the incoming cardiac signals. In one embodiment of the present invention, the step time size 140 is set to 29.3 mSec/step to achieve the normalsinus sensing decay rate.
~ Tachyrhythmia sensing is a special condition under which a fast .c~ollse rate is desirable in order to plvp~lly track the higher tachyrhythmia rates, such as during fibrillation or tachycardia. This is especially true in the CA 02228~78 1998-02-04 atriurn of the heart, where tacl-yll,yLlL.,lia rates run in excess of 300 beats per min~te In one embodiment ofthe present invention, step size 140 is set to a~ x;~ ly 17.5 mSec/step for atrial tachylllyLl~llia conditions, and is set to a~plo~i.n~tl?ly 23.5 mSec/step for ventricle Ldchylllyll~lllia conditions. By S :iwiLcl~illg to this faster decay rate for tachylllyLl,lllia conditions, cases of un~ a ~chylllylhlllia condition which needs to be treated is reduced.
Bradycardia pacing is a special op~-dling condition wherein the decay rate of the sensing threshold is tied to the bradycardia pacing rate to help minimi7~ ovel~.lsing and un~iersen~ing conditions. In prior 10 cardioverter/defibrillator devices with pacing capability, the sensing template attack rate is fixed. Under situations of high pacing rates, the cardioverter/defibrill~tor with pacing capability lltili7ing AGC according to the present invention does not have time to decay to mz-x i- .~n ~ ~- sensitivity. If the decay rate is not sufficiently sped up along with the high pacing rates 15 undel~e~ g occurs and the cardioverter/def1brillator continues pacing in the sencc of fibrillation. With the decay rate varied as a function of the bradycardia pacing rate under bradycardia pacing conditions, the decay rate is sufficiently sped up to enable the cardioverter/defibrillator according to the present invention to sense and IJlu~clly- respond to the fibrillation condition. In 20 addition, when pacing rates are low, a longer decay rate is desirable to minimi the possibility of overs~n~ing The formlll~ for calculating the post pace template step time size 140 for bradycardia pacing conditions is as follows:

W O 97106851 PCTrUS96/13181 STEP TIME SIZE =
(CYCLE LENGTH - REFRACTORY - DROP TIME - MINIMUM TIME) /X
where:
CYCLE LENGTH = pacing cycle length - 5 REFRACTORY = programmed paced l~id~;Lol,~
DROP TIME = normal t~mpl~te hold time for peak CO~ .t~ plox;~ y 13.7 mSec in a yl~re.lcd embodiment) MINIMUM TIME = n.;.. il~.. time allowed for template at final value (approximately 100 mSec in a ~ler~lled embodiment) X = number of steps to go from seed value to final value (equal to 12 steps in the embodiment ilhl~t~t~d in Figure 6) Referring to Figure 6, the cycle length is equal to the pacing cycle length or from time T(0) to T(0) b.,lwe~ll each pacing pulse. The paced refractory period is indicated by arrows 132. The drop time is indicated by arrows 134. The time the variable sensing threshold is at the pro~lcul~llable final 20 value before the next pacing pulse is intlie~ted by arrows 150. Since multiple pacing rates are ~.ci~nçd the same step size, the time indicated at 150 varies from ~plu~iln~tely 100 mSec to 200 mSec in the embodiment illustrated. The ;lI;llllli~ time is the ~--i-~;-------- time allowed for the time indicated by arrows 150, or a~pr~x;.~tely 100 mSec. X le~l~se.l~ the 12 steps (i.e., the 3 X four 25 step groups) to go from the peak sensing value at time T(0) to the programmedfinal value of the variable sensing threshold achieved at T(4,0).
A look-up table stored in microprocessor and memory 38 is formed by dividing the cycle length by 64, which results in a shift of six bits to the right. In one implçment~tion of the present invention, the cycle length is 30 equal to 12 bits, which results in six bits being shifted off in the divide by 64 formation of the look-up table in microprocessor and memory 38, resulting in 64 CA 02228~78 1998-02-04 W O 97/06851 PCT~US96/13181 entries in the look-up table. Thus, the current cycle length is divided by 64 toindex the look-up table to access the values stored in the look-up table co..e~onding to the above step time size formula.
The digital embodiment of the AGC loop as described above S allows the above described r,-ll,w~e implernt?nt~d in the look-up table in thernicroprocessor and memory 38 to dynamically adjust the sensing characteristics of the cardioverter/defihrill~h~r according to the present invention. By sensinghigh rates di~ lLly than low rates, the tailorable AGC decay rate according to the present invention can be utilized to orthogonally optimize sensing 10 ch~.~ ;etice of bradycardia and tachyrhythmia signals, which have m~ y exclusive sensing requirements. In this way, the physician controls a better-behaved cardioverter/defibrillator. In addition, patient comfort is increased, due to redllcing o~ ing and nn~ e~ g of treatable arrhythmia conditions in the patient.
15 Int~ tion of ni~ital AGC U.cir~ Se~rate G~in Control ~ntl Threshold T~ tin~
Figure 8 illustrates in timing diagram form depolarization cycles in the electrical activity of the heart. The incoming electrical activity at input/output t~rrnin~le 22 or 42 is in~licated by waveform 200. The filtered and20 gain controlled ~ligiti7~cl cardiac signal is indicated by waveform 202. The variable sensing threshold is indicated by waveform 204. The absolute value of the .ligiti7~d and gain controlled cardiac signal is indicated by waveform 206 m-1ernt?~th the variable sensing threshold waveform 204. The refractory period is indicated by waveform 208. The discrete stepped slow gain is indicated by 25 waveform 210.
As indicated by waveform 204, the variable sensing threshold waveform responds to the absolute value of the digitized cardiac signal to assume the peak value of the digitized cardiac signal. The variable sensing threshold then decays according to a piecewise linear approximation of an 30 exponential decay curve to step over wide depolarizations or T-waves.

The influence of the slow gain control on the fast templating circuit is ill~ lef1l at time 212. As is indicated, the gain is decreased at time 212, which colrc~ondingly results in a reduced filtered and gain controlled f~igiti7~d cardiac signal inl1ic~ted at 202, which co,~ ,pol1dingly reduces the ~ S variable sensing threshold inflir?tefl at 204 as the variable sensing threshold follows the peak value of the absolute value of the fli~iti7f-d and gain controlled cardiac signal ;nflic~tf~d at 206.
Cnnr,lll~ion By lltili7in~ this present invention, which inco,po.~les two independent loops in a cardioverter defibrillator with pacing capability which are both impl~ m~ntin~ digital logic circuits, the AGC response is effectively movedfrom analog circuits into the digital logic circuits, where it is easier to test and çh*~ lr~ ;~e Design of the sense ~mplifif-r is simplified, due to the digital control ofthe sense amplifier. It is easier to test and cll~cLe-;~ the analog sense amplifier, since the AGC ~h~iuilly is no longer in the analog domaim The cardioverter/defibrillator device is more uniform from device to device, which greatly h,-,n,ases the physician's ease of predicting device behavior. In addition, the patient comfort is increased due to reduced oversensing and undersen~in~
Although specific embo~ ; have been illustrated and described herein for purposes of description of the ~l~relled embodiment, it will be appreciated by those of ordin~u y skill in the art that a wide variety of alternate and/or equivalent implf~mf~nt~tions calculated to achieve the same purposes may be :jub~ uled for the specific embo~lim~-nt~ shown and described without departing from the scope of the present invention. Those with skill in the mechanical, electro-mechanical, electrical, and computer arts will readily appreciate that the present invention may be implemented in a very wide variety of embo~liment~ This application is intf-nflf d to cover any adaptations or variations of the ~ d embodiments discussed herein. Therefore, it is .lirP~Iy inten(lefl that this invention be limited only by the claims and the equivalents thereof.

Claims (28)

WHAT IS CLAIMED IS:

1. A method for automatically adjusting a sensing threshold in a cardioverter/defibrillator, which receives electrical activity of the heart and provides shock pulses in response thereto, the method comprising the steps of:
amplifying the electrical activity of the heart;
detecting cardiac events representing depolarizations in the electrical activity which exceed a variable sensing threshold;
acquiring amplitudes of the amplified electrical activity;
adjusting the variable sensing threshold to a level proportional to the amplitude of the amplified electrical activity of a current detected cardiac event, and decreasing the variable sensing threshold from said level in discrete steps until the variable sensing threshold is at a low threshold value, wherein said discrete steps are grouped into step groups, wherein each step group decreases the variable sensing threshold by a defined percentage.

2. The method of claim 1 wherein the adjusting and decreasing steps achieve a piecewise linear approximation of a geometric progression.

3. The method of claim 2 wherein the geometric progression is an exponential decay curve.

4. The method of claim 1 wherein said level is a percentage of a peak value of the amplitude of the acquired amplified electrical activity of the current detected cardiac event.

5. The method of claim 1 wherein the step of adjusting the variable sensing threshold to said level occurs prior to the end of a new sensed refractory period caused by a cardiac event.

6. The method of claim 1 wherein the method automatically adjusts the sensing threshold in a cardioverter/defibrillator having pacing capability, and wherein the step of adjusting the variable sensing threshold to said level occurs at the end of a paced/shock refractory period resulting from a pace or shock pulse.

7. The method of claim 1 wherein the decreasing step uses integer math to calculate an amount of drop for a discrete step.

8. The method of claim 1 further comprising the step of controlling the decay rate of the variable sensing threshold by varying a time width of each discrete step based on operating conditions of the cardioverter/defibrillator tocontrol the decay rate of the variable sensing threshold.

9. The method of claim 8 wherein the method automatically adjusts the sensing threshold in a cardioverter/defibrillator having pacing capability, and wherein the operating conditions of the cardioverter/defibrillator include bradycardia pacing, tachyarrhythmia sensing, and normal sinus sensing.

10. The method of claim 1 wherein the method automatically adjusts the sensing threshold in a cardioverter/defibrillator having pacing capability, and wherein the method further comprises the steps of:
setting the variable sensing threshold after a pace or a shock pulse to a selected relatively high threshold value, and holding the variable sensing threshold at the selected relatively high threshold value through a paced/shock refractory period resulting from the pace or shock pulse.

11. A system for automatically adjusting a sensing threshold in a cardioverter/defibrillator, which receives electrical activity of the heart and provides shock pulses in response thereto, the system comprising:

an amplifier for amplifying the electrical activity of the heart;
a cardiac depolarization detector for detecting depolarizations in the amplified electrical activity of the heart and providing a detect signal representing a cardiac event indicative of a depolarization when the amplified electrical activity exceeds a variable sensing threshold; and threshold controller for acquiring amplitudes of the amplified electrical activity and for adjusting the variable sensing threshold to a level proportional to the amplitude of the amplified electrical activity of a current detected cardiacevent and for decreasing the variable sensing threshold from said level in discrete steps until the variable sensing threshold is at a low threshold value,wherein said discrete steps are grouped into step groups, wherein each step group decreases the variable sensing threshold by a defined percentage.

12. The system of claim 11 wherein the threshold controller adjusts the variable sensing threshold to achieve a piecewise linear approximation of a geometric progression.

13. The system of claim 12 wherein the geometric progression is an exponential decay curve.

14. The system of claim 11 wherein said level is a percentage of a peak value of the acquired amplitude of the amplified electrical activity of the current detected cardiac event.

15. The system of claim 11 wherein the threshold controller adjusts the variable sensing threshold to said level prior to the end of a new sensed refractory period caused by a cardiac event.

16. The system of claim 11 wherein the system automatically adjusts the sensing threshold in a cardioverter/defibrillator having pacing capability, and wherein the threshold controller adjusts the variable sensing threshold to said level at the end of a paced/shock refractory period resulting from a pace or shock pulse.

17. The system of claim 11 wherein the threshold controller calculates an amount of drop for a discrete step using integer math.

18. The system of claim 11 further comprising a decay rate controller for varying a time width of each discrete step based on operating conditions of the cardioverter/defibrillator to control the decay rate of the variable sensing threshold.

19. The system of claim 18 wherein the system automatically adjusts the sensing threshold in a cardioverter/defibrillator having pacing capability, and wherein the operating conditions of the cardioverter/defibrillator include bradycardia pacing, tachyarrhythmia sensing, and normal sinus sensing.

20. The system of claim 11 wherein the system automatically adjusts the sensing threshold in a cardioverter/defibrillator having pacing capability, and wherein the system further comprises:
means for setting the variable sensing threshold after a pace or a shock pulse to a selected relatively high threshold value and for holding the variablesensing threshold at the selected relatively high threshold value through a paced/shock refractory period resulting from the pace or shock pulse.

21. A method for automatically controlling a decay rate of a sensing threshold in a cardioverter/defibrillator with pacing capability, which receiveselectrical activity of the heart and provides shock pulses and pacing pulses in response thereto, the method comprising the steps of:
amplifying the electrical activity of the heart;
detecting cardiac events representing depolarizations in the electrical activity which exceed a variable sensing threshold;

acquiring amplitudes of the amplified electrical activity;
adjusting the variable sensing threshold to a level proportional to the amplitude of the amplified electrical activity of a current detected cardiac event;
decreasing the variable sensing threshold from said level in discrete steps until the variable sensing threshold is at a low threshold value; and controlling the decay rate of the variable sensing threshold by varying a time width of each discrete step based on operating conditions of the cardioverter/defibrillator including bradycardia pacing, tachyarrhythmia sensing, and normal sinus sensing.

22. The method of claim 9 or claim 21 wherein the controlling step varies the time width as a function of a bradycardia pacing rate when the cardioverter/defibrillator is operating under bradycardia pacing conditions.

23. The method of claim 22 wherein the controlling step varies the time width under bradycardia pacing conditions based on the time between each pacing pulse, a programmed paced refractory period, and the number of discrete steps to go from said level to the low threshold level.

24. The method of claim 21 wherein the controlling step varies the time width to produce a relatively fast decay rate when the cardioverter/defibrillator is operating under tachyarrhythmia sensing conditions.

25. A system for automatically controlling a decay rate of a sensing threshold in a cardioverter/defibrillator with pacing capability, which receives electrical activity of the heart and provides shock pulses and pacing pulses in response thereto, the system comprising:
an amplifier for amplifying the electrical activity of the heart;
a cardiac depolarization detector for detecting depolarizations in the amplified electrical activity of the heart and providing a detect signal representing a cardiac event indicative of a depolarization when the amplified electrical activity exceeds a variable sensing threshold;
threshold controller for acquiring amplitudes of the amplified electrical activity and for adjusting the variable sensing threshold to a level proportional to the amplitude of the amplified electrical activity of a current detected cardiacevent and for decreasing the variable sensing threshold from said level in discrete steps until the variable sensing threshold is at a low threshold value; and decay rate controller for controlling the decay rate of the variable sensing threshold by varying a time width of each discrete step based on operating conditions of the cardioverter/defibrillator including bradycardia pacing, tachyarrhythmia sensing, and normal sinus sensing.

26. The system of claim 19 or claim 25 wherein the decay rate controller varies the time width as a function of a bradycardia pacing rate when the cardioverter/defibrillator is operating under bradycardia pacing conditions.

27. The system of claim 26 wherein the decay rate controller varies the time width under bradycardia pacing conditions based on the time between each pacing pulse, a programmed paced refractory period, and the number of discrete steps to go from said level to the low threshold value.

28. The system of claim 25 wherein the decay rate controller varies the time width to produce a relatively fast decay rate when the cardioverter/defibrillator is operating under tachyarrhythmia sensing conditions.