CA2228757A1 - Separate gain control and threshold templating - 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 according to a variable gain. 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. Slow gain control circuitry adjusts the variable gain in discrete steps based on stored peak history information representing peak values of the amplified electrical activity. Template generation circuitry responds quickly to set the variable sensing threshold to a level proportional to a peak valve 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.

Description

S~PAR~TF GA~l~ CONTROL
Al~l~ TF~Rli'~HOI,l) T~ IPI,~TING
.
Field of th~ Inventi~ n The present invention relates generally to implantable medical devices, and more particularly, to systems such as automatic gain control systems for ~uLu...~ c~lly adjusting the sensing threshold in cardiac rhythm 10 management devices, such as p~çem~k~rs, cardioverter/defibrillators, and cardioverter/defibrillators with pacing capability.
R~rl~round of the Invention Cardiac rhy~m management devices such as pacem~kers, cardioverter/defibrill~tors, and cardioverter/defibrillators with pacing capability 15 typically include a system for dçtçcting dangerous cardiac ~lhyLlllnia conditions in the heart, such as bradycardia, tachy~;a,dia, and fibrillation by measuring the tinne interval between consecutive 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 20 when an amplitude of the electrical activity eXcee~l~ a pre~1et~rmin~d amplitude level or "sensing threshold." The sensing threshold may be fixed, or may vary over time.
A fixed sensing threshold is not a~pl~liate for detecting certain arrhythmias, such as polymorphic tacLycal-lia and fibrillation, wherein extreme 25 variations occur in the ~nlpl~ de of the electrical activity during the ~lLylhlllia.
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 compared to nolmal cardiac depolarizations.
One approach to compensate for problems associated with a fixed sensing threshold is to program the sensing threshold at a value dt;lt;llllhled by the ~t~etl~ling physician after careful study of the variety of amplitudes in cardiac CA 022287~7 1998-02-0~

signal activity exp~,ic.lced by a patient. In other words, a sensing threshold is programmed into the cardiac rhythm m~n~gçml?nt device, and any cardiac signal amplitude larger than the programme-l sensing threshold is considered a cardiac depol~ri7~tion. If, however, the pro~r~mmrd sensing threshold is set too high S and the cardiac signal amplitude decleases significantly, as is often the case in fibrill~ti~ rl, the cardiac rhythm management device may not sense the arrhythmia. If the pro~ -ed sensing threshold is set too low, the device may over-sense. For example, a system ~ign~d to detect ventricular depolarizations (R-waves) may clluileously detect atrial depolarizations (P-waves) or ventricular 10 recovery (T-waves). R~nflp~cs filtrrin~ can be used to partially elimin~te 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~c filtering is too narrow, certain fibr~ tion signals may not be detecte-l Another approach to colll~e~ le for the above problems is to set 15 the sensing threshold proportional to the arnplitude of the sensed cardiac signal 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 amplitude decreases ~ignifir~ntly, the cardiac rhythm management device is still able to detect the 20 lower level arnplitude of the cardiac signal. Adjusting the sensing threshold to an a~l.ro~l;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 25 invoked response due to a first pacing pulse can cause the sensing threshold to be set so high that a second spontaneous R-wave is not sensed. Because the system does not sense the second spontaneous R-wave, a second pacing pulse is delivered to the patient ina~p~opliately.
One solution to the above problem is found in the Kelly et al.
30 U.S. Patent No. 5,269,300 ~ necl to Cardiac Pacemakers, Inc., the assignee of the present application. The Kelly et al. patent discloses an implantable CA 022287~7 1998-02-0~
WO 97/068~;0 PCT/US96/13095 cardioverter/defihrill~tor with pacing capability wLclcin the sensing threshold is m~ti~lly adjusted to a value proportional to the amplitude of the sensed cardiac signal. The sensing threshold continuously decreases b~ sensed cardiac depol~ri7~tic n~ to ensure that a lower level cardiac signal will be 5 dP,tecte~l However, after a pacing pulse is delivered by the Kelly et al. device, the sPn~ing threshold is set to a fixed value, and held at the fixed value for apre~ f.(l period of ti ne, so that the sensing threshold is not affected by the cardiac r~ollse invoked by the pacing pulse. After a precleterrnined period of time, the sP~ing threshold is decreased, just as after a spontaneous cardiac 1 0 depolarization.
In the Keimel et al. U.S. Patent No. 5,117,824, an R-wave ~lP~tectc r ~utom~tically adjusts the ~letecting threshold in response to the R-wave s~mrlitucle The adjustment of the threshold is disabled for a predetermined period following the delivery of each pacing pulse. Thelearlcl, the sensing 15 threshold is returned to a lower threshold level to allow detection of lower level R waves indicative of tachyrhythmia 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 p~c~orn~k~r, cardioversion and/or cardiac defibrillation device. Initially, a 20 sensing threshold is set to a low value. When the cardiac signal is detected, the amplitude of the R-wave is measured and the sensing threshold is computed as a fimction of the arnplitude of the R-wave. After a refractory period, the sensingthreshold is preferably set to 75% of the :3mplitllcle of the R-wave. The sensing thlreshold is then decreased in uniform steps. The uniform steps may be fixed 25 de~ lc.l~ or percentage reductions.
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 detection of sinus rhythm and ventricular tachycardia. A second, higher sensitivity is designed for dirrc.~ ting ventricular fibrillation from 30 asystole. A third, lower sensitivity is used to differentiate between R-waves and high amplitude current of injury T-waves which occur after shocking. One of CA 022287~7 1998-02-0~

these three sen~ilivilies is selected as a fuIlction of the status of the device, such as during a period of su~pecte~l tachyc~.lia or a post shock period, and the selected sen~ilivily must be ~ at least until the next cycle.
The Dissing et al. U.S. Patent No. 5,370,124 discloses a cardiac S rhythm m~n~gem~nt device having cil~;uill~ for autom~tic~lly adapting the detection se.lsllivily to the cardiac signal. The detection sensitivity is adjusted by either amplifying the electrical signal supplied to the threshold detector with a variable gain given a pc~ -.tly prescribed threshold or by varying the threshold itself. In either case, the effective threshold is based on an average10 value formed over a time interval corresponding to the duration of a few breaths.
A :iwik;hillg hysteresis 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 ~wit~ g hysteresis are varied with the variation of the threshold, but the 15 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 s~ cescive input electrical signals are used for for~ning an average value. The sensing threshold is raised by a preAetermin~ amount if the variance exceeds the pre~llotermined variance 20 value.
The Carroll et al. U.S. Patent No. 4,972,835 discloses an implantable cardiac defibrillator which includes switched capacitor .;ircuill y for amplifying the cardiac electrical signal with non-binary gain ch~n~in~ steps.
Three stages of gain are used to increase the gain approximately 1.5 each 25 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 impl~nt~kle cardiac stim~ tc r. The rate of gain adjustment is dependent on the present sensed conditions and on the prior state of the heart. Different rates of 30 adjl-ctm~nt are selected under varying conditions so that the gain of the sense ~mplifier is adjusted without significant overshoot. Multiple effective time CA 022287~7 1998-02-0~

c~ x are used for di~l~n~ conditions by basing the rate of adj-l~tment of the sense amplifier gain on the path traversed in the state m~l~hinP
Therefore, considerable effort has been ~pPn~lP~l in providing for snltc)mzltic~lly adjustable sensing thresholds through a-lju~ g the threshold level S itself or with ~tom~tiC gain ~;h-;uiLIy in implantable cardiac rhythm management devices for the purpose of Pnh~n-~ing the capability of the device to sense .l.yl~"nia conditions for which therapy is to be applied.
~mmary of the Invention The present invention provides a method and system for 10 automatically 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 amplified according to a variable gain. Cardiac events lep.~senli,lg depol~ri7~tinns in the electrical activity which exceed a variable sensing threshold are detPcte~1 Amplitudes of 15 the amplified electrical activity are tracked, and the variable sensing threshold is adjusted to a level p~ ,o- lional to an amplitude of the amplified electrical activity of a current ~letecte~l 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 variable gain is adjusted based on amplitudes of 20 the amplified electrical activity of at least three ~letecte~l cardiac events.
The variable gain is preferably adjusted with a gain control circuit which includes a storage register or other storage device for storing peak history h~o....~lion ,ep,ese"lalive of peak values of the amplified electrical activity of a first selected number (N) of cardiac events. The gain control circuit includes 25 adjusting circuits for adjusting the variable gain based on the stored peak history information. The adjusting circuitry increases the variable gain if a second selected number (M) of peak values of the N cardiac events are below a selected low threshold and decreases the variable gain if M peak values of the N cardiac events are above a selected high threshold. The value of M is preferably at least 30 3 and the value of N is preferably at least 4. The peak history information from the previous event is updated in the storage register preferably at the beginning CA 022287~7 1998-02-0~

of a new refractory period caused by a cardiac event. The storage register preferably includes a first group of storage locations which store peak i. .fo. . .~tion indicating if the peak values are below the selected low threshold and a second group of storage locations which store peak hlro,lll~Lion indicating S if the peak values are above the selected high threshold.
In a l,ler~ d embodiment of the present invention, the adjusting circ~iLl y increases the variable gain if a stored peak value of a 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 and decreases the variable gain if the 10 stored peak value of the last cardiac event and the M - I peak values of the last N
- 1 cardiac events previous to the last cardiac event are above the selected high threshold.
The variable sensing threshold is preferably adjusted with a tetnrl~te gt;neldlion circuit. The template gen~,.dlion circuit ~-cr~ldbly acquires 15 the depolarization peak value, or after a pace or shock pulse, the variable sensing threshold is set to a selected relatively high threshold value. In one plerel.~dembo-lim~nt the selected relatively high threshold value is one binary number below the m~Lxi--lulll value of the variable threshold. Following a delay, the template gen.,.dlion circuit adjusts the variable sensing threshold to a level which 20 is a ~ ge of a peak value of the amplitude of the amplified electrical activity ofthe current detectecl cardiac event. The ~e..;e--l~ge is typically approximately 75%. The template generation circuit preferably adjusts the variable sensing threshold to the level prior to the end of a new sensing refractory period caused by a detectecl cardiac event. When the 25 cardioverter/defibrillator is embodied in a cardioverter/defibrillator havingpacing capability, the template generation circuit preferably adjusts the variable sensing threshold to the level at the end of a paced/shock refractory period resnltin~ from a pace or shock pulse. The template generation circuit preferablycalculates an amount of drop for a discrete step using integer math to achieve a30 piecewise linear a~,oxil-lation of a geometric progression. The geometric progression preferably represents an exponential decay curve. The template -CA 022287~7 1998-02-0~

generation circuit according to the present invention preferably de-,reases the variable sçn~in~ threshold from a level in discrete steps which are grouped intostep groups. Each step group preferably decreases the variable sensing thresholdby a ~efinPci ~ ge. The defined p~l.;e.ll~ge is typically approximately 5 50%. In one embodiment of the present invention, each step group includes at le~t four discrete steps.
The system of the present invention preferably compri~çc a decay rate controller for varying a time width of each discrete step based on o~t;l~lillg conditions of the cardioverter/defibrillator to control the decay of rate of the10 variable sen~in~ threshold. When the cardioverter/defibrillator is embodied in a cardioverter/defibrillator having pacing capability, the o~la~ g conditions include bradycardia pacing, ta-;l.y-l-yLhl..ia sen~ing, and normal sinus sen~ingWhen the cardioverter/defibrillator with pacing capability embodiment of the present invention is opcldlillg under the bradycardia pacing conditions, the decay 15 of rate controller varies the time width as a function of a bradycardia pacing rate.
In a preferred embodiment of the present invention, the gain control circuit includes a storage device capable of storing peak history information ~~ ,s~ ve of peak values of amplified electrical activity of a third selected number of cardiac events. In this embodiment, far field sense 20 circuitry responds to the stored peak history information to indicate a decrease in the variable gain if the peak values of the amplified electrical activity of the third selected number of cardiac events ~ltt?rn~t~ between clipped peak values and non-clipped peak values. The peak value is determin~d to be clipped if the peak value is at a maximum peak value. In the plc;fell~d embodiment of the present 25 invention, the gain control circuit is implemented in digital ci.~ y, which permits a simple comp~ri~on to a maximum digital value to determine whether or not a peak value has been clipped. The third selected number can be equal to the first selected number. ln the embodiment of the present invention where M
is equal to 3 and N is equal to 4, the third selected number is preferably set to a 30 value of 6 which requires two extra storage locations in the storage device.

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In a ~r~ r~ d embodiment of the present invention, the gain control circuit includes cil~iuilly responsive to the detect signal to set the gain to a selected relatively high s~.lsilivily if a cardiac event is not ~let~cted for a selectçcl time period. The gain control circuit preferably adjusts the variable gain S in discrete gain steps and the selected relatively high sensitivity is preferably at least one discrete gain step from a ~ X;~ se~ ivily. The gain control circuit also preferably incllldes ~ih~;uilly to set the gain to the select~l relatively high sensitivity when the cardioverter/defibrillator delivers a shock pulse to the heart.
In the cardioverter/defibrillator with pacing capability embodiment of the present 10 invention, the gain control circuit also preferably inc.llldes cil~;uilly to set the gain to the selected relatively high sensitivity when the cardioverter/defibrillator delivers a pacing pulse to the heart.
The gain control circuit preferably incl~ s cir~;uill y to decrement the gain from the selected relatively high sensitivity by a selected number of 15 discrete gain steps if the setting of the gain to the selected relatively high se.~ilivily creates a clipped peak value of the amplified electrical activity on the following ~letected cardiac event. The cil~;uilly preferably further decrements the gain by at least one discrete gain step if the peak value of the amplified electrical activity is still clipped on the second ~letected cardiac event following the setting 20 of the gain to the selected relatively high sensitivity. In addition, the ch~;uiLI~y preferably decrements the gain from the selected relatively high sensitivity by a selected number of discrete gain steps if the setting of the gain to the selected relatively high sensitivity does not create a clipped peak value of the amplified electrical activity on the following detected cardiac event and does create a 25 clipped peak value of the arnplified electrical activity on the second detected cardiac event following the setting of the gain to the selected relatively high sensitivity.
P~rief D~ Jlion of the D~ a~
Figure 1 is a block diagram of a dual chamber 30 cardioverter/defibrillator according to the present invention.

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Figure 2 is a logical block diagram of an AGC filter and rli~iti7ing circuit according to the present invention.
Figure 3 is a timing ~ gr~m illu~ lg the sensed refractory used in the cardioverter/defibrillator of Figure 1.
Figure 4 is a timing diagram illu~ hlg the paced/shock refractory used in the cardioverter/defibrill~tor of Figure 1.
Figure 5 is a logical block diagram of a gain control circuit according to the present invention.
Figure 6 is a timing ~ gr~m illustrating a piecewise linear approximation of an exponential decay of the variable sensing threshold according to the present invention.
Figure 7 is a template generation circuit according to the present invention, which achieves the piecewise linear approximation of an exponential decay illustrated in Figure 6.
Figure 8 is a timing diagram illustrating the operation of the slow gain control circuit of Figure 5, in combination with the fast templ~ting generation circuit of Figure 7 in adjusting the gain and the sensing threshold of the cardioverter/defibrillator according to the present invention.
n~ n of the Preferred F'mbo-liments In the following detailed description of the preferred embo-lim~-nt~, lGrGlellce is made to the acc~lllpdnyillg drawings which form a part hereof, and in which is shown by way of illustration specific embo.l;
in which the invention may be practiced. It is to be understood that other embo-li..~k..l~ may be utilized and structural or logical changes may be made 25 without departing from the scope of the present invention. The following detailed description, therefore, is not to be taken in a limiting sense, and thescope of the present invention is defined by the appended claims.
nual Ch~mber Cardioverter/Defibrillator with Pac;n~ Capability A dual charnber cardioverter/defibrillator 20 with pacing 30 capability is illustrated in block ~ gr~m form in Figure 1.
Cardioverter/defibrillator 20 operates as a pulse generator device portion of a CA 022287~7 1998-02-0~

cardiac rhythm management system which also includes leads or electrodes (not shown) disposed in the ventricular chamber of the heart to sense electric~l activity r~fes~ re of a R-wave portion of the PQRST complex of a surface EGM indicating depolari7~tione in the ventricle. Cardioverter/defibrillator 20 includes input/output t~rmin~lc 22 which are connectable to the ventricular leads to receive the ventricular electrical activity of the heart sensed by the ventricular leads. A pace pulse circuit 24 provides pacing pulses such as bradycardia and ~ntit~-~l.y~;~dia pacing pulses to input/output termin~lc 22 to be provided to the ventricular chamber of the heart via the ventricular leads to stim~ te excitablemyocardial tissue to treat ~lLylhlllia conditions such as bradycardia and some tachyc~dia. A shock pulse circuit 26 provides shock pulses to input/output termin~lc 22 to be provided to the ventricular chamber of the heart via the ventricular leads to shock excitable myocardial tissue to treat tachyrhythmia conditions. The tachyrhythmia conditions may include either ventricle fibrillation or ventricle tacl,~caldia.
A filter and after potential removal circuit 28 filters the ventricular electrical activity received by inputloutput 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 plcf~.led after potential removal circuit is described in detail in the co-pending and commonly ~cci~necl U.S. patent application Serial No. 08/492,199 entitled "AFTER POTENTIAL REMOVAL IN CARDIAC RHYTHM
MANAGEMENT DEVICE" filed on June 19, 1995.
An automatic gain control (AGC)/filter and ~ligiti7ing circuit 30 according to the present invention a~nplifies the filtered ventricular electrical activity provided from the filter and after potential removal circuit 28.
AGC/filter and digitizing circuit 30 includes circuitry for ~ligiti7in~ 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 ~iigiti7in~ circuit 30 to detect depolarizations in the CA 022287~7 lsss-02-o~
wo 97/068s0 PCT/USg6/13095 amplified ventricular electrical activity ~ selll~ te of R-wave depolarizations when the ~mplifi~Pd ventricular electrical activity exceeds a sPIecte~l amplified level known as the t'sell~iliviLy threshold" or the "sensing threshold" and r~fractory is inactive. A template ~ .dlion circuit 36 ~lltom~ti~ ly selects and5 adjusts the sensing threshold. R-wave detection circuit 34 provides a R-wave depol~ri7~ti~ n signal, indicative of the R-wave depolarizations, to a 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 10 electri~l activity r~le3ell~ e of a P-wave portion of the PQRST complex of a surface EGM indicating depolarizations in the atrium. Cardioverter/defibrillator20 correspondingly also inrhl(lP~Ic input/output termin~l~ 42 which are co~ P.;I~ble 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 15 bradycardia pacing pulses to input/output tPrmin~l~ 42 to be provided to the atrial chamber of the heart via the atrial leads to stim~ te excitable myocardial tissue to treat ~lhyllllllia conditions such as bradycardia or atrial tachycardia. A
filter and after potential removal circuit 48 O~c.aLt;s similar to filter and after potential removal circuit 28 to filter the atrial electrical activity received by 20 input/output tçrmin~lc 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 rli~iti7ing circuit 50 accorlillg to the present invention amplifies the filtered atrial electrical activity 25 provided from the filter and after potential removal circuit 48. AGC/filter and ~iigiti7:in~ circuit 50 includes circuitry for ~Ijgiti7inp the filtered atrial electrical activity. A gain control circuit 52 automatically adjusts the gain of AGC/filterand ~ligiti7ing circuit 50. An P-wave detection circuit 54 is coupled to AGC/filter and digitizing circuit 50 to detect depolarizations in the amplified 30 atrial electrical activity representative of P-wave depolarizations when the amplified atrial electrical activity exceeds a selected amplified level known as -CA 022287~7 1998-02-0~

the "sensitivity threshold" or the "sensing threshold" and the refractory is inactive. A template generation circuit 56 dulo~ ic~lly selects and adjusts the sensing threshold. P-wave detection circuit 54 provides a P-wave depolarization signal, indic~tive of the P-wave depol~ri7~tions~ to microprocessor and memory 5 38.
Microprocessor and memory 38 analyzes the detected P-waves in~lir~ted in the P-wave depolarization signal from P-wave detection circuit 54 along with the R-wave depolarization signal provided from R-wave detection circuit 34 for the detectiQn of ~lhylh--lia conditions based on known algolilllllls.
10 For example, microprocessor and memory 38 can be used to analyze the rate, regularity, and onset of variations in the rate of the reoccurrence of the d~otecte-l P-wave and/or R-wave, the morphology of the cletected P-wave and/or R-wave, or the direction of propagation of the depolarization ,c;~,~st;,,led by the ~letected P-wave and/or R-wave in the heart. In addition, microprocessor and memory 38 l S stores depolarization data and uses known techniques for analysis of the detected R-waves to control pace pulse circuit 24 and shock pulse circuit 26 for deliveryof pace pulses and shock pulses to the ventricle and for analysis of detecte(l Pwaves 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 20 which places various circuits of cardioverter/defibrillator 20 in desired logical states based on various con~litiQns such as when a pace pulse or shock pulse occurs or on ~pc.dlillg conditions of the cardioverter/defibrillator such as bradycardia pacing, tachyrhythmia st?n~ing, and normal sinus sen~ing The dual chamber cardioverter/defibrillator 20 with pacing 25 capability illustrated in Figure 1 includes pacing and shocking capabilities for the ventricle and pacing capability for the atrium. Nevertheless, the present 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 30 pulses to the ventricle only.

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WO 97/0~8SO PCT/US96/13095 In some embo~ of cardioverter/defibrillator 20, inputloutput t~min~le 22 and 42 are each implem~ntPcl to be connt?ct~ble to a co~ ollding single set of electrodes (not shown) used for pacing, shock delivery, and seneing In other embo~1imPnte of cardioverter/defibrillator 20, the S input/output termin~le are implem~nt~l to be co~ hle to separate sets of electrodes for pulse delivery and senein~ In some embc~lim~nte, the input~output terminals are implemented to be corm~ct~hle to sep~le electrodes for pacing and shock delivery. In all of these embo-lim~nte, the electrodes of acardiac rhythm management system are typically implemPntç(l as unipolar or 10 bipolar electrodes.
A unipolar electrode configuration has one pole or electrode (i.e., negative pole or cathode electrode) located on or within the heart, and the other pole or electrode (i.e., positive pole or anode electrode) remotely located fromthe heart. With endocardial leads, for example, the c~thode is located at the 15 distal end of a lead and typically in direct contact with the endocardial tissue to be stim~ t~l thus forming a "tip" electrode. Conversely, the anode is remotely located from the heart, such as comprising a portion of the mçt~llic enclosure which surrounds the implanted device, thus forming a "can" electrode and is often referred to as the "indifferent" 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 lead, referred to as the "tip" electrode. In the bipolar configuration, the anode is usually located approximate to the "tip" electrode spaced apart by 0.5 to 2.5 cm., 25 and typically forming a ring-like structure, referred to as the "ring" electrode.
With respect to sensing, it is well known that bipolar and unipolar electrode configurations do not yield equivalent cardiac EGMs. Each configuration has advantages and disadvantages, for example, with a unipolar-sensing configuration, only the electrical events adjacent to the "tip" electrode 30 control the unipolar EGM, while the remote "indifferent" electrode contributes n.?glip~ihle voltage due to its location being extracardiac.

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With a bipolar-sensing configuration, the m~gnitl~ of the cardiac signal is similar for both the "ring" and the "tip" electrodes, but the resnlting EGM is highly dependent upon the orientation of the electrodes within the heart. Optimal sensing will occur, for example, when the sensing vector 5 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 coullL~.~Ls, the depol~n7~tion signal will be shorter in durationthan that produced from a unipolar configuration. Due to a more restrictive leadfield or ~nt~nn~, bipolar sensing offers improved rejection of electrom~n~tic 10 and skeletal muscle artifacts, and thus provides a better signal-to-noise ratio than unipolar sen~in~
AGC/F;lter ~n~l nU~iti7ir~ Circuit A logical block diagrarn l~ ci,ellL~ e of AGC/filter and digitizing circuit 30 or 50 is illustrated in Figure 2. A programmable gain filter 15 60 filters the electrical activity provided from the filter and after potential removal circuit 28 or 48 of Figure 1. When cardioverter/defibrillator 20 of Figure 1 is irnp!em~nted to be connectable to bipolar electrodes, programmable gain filter 60 comprises an analog differential sense arnplifier to sense and amplify the di~rellce between first and second bipolar electrodes.
20 Programrnable 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 amplified electrical activity from progr~mm~ble gain filter 60 and converts the analog electrir~l activity to ~igiti7f'(l cardiac data, which is stored in a successive 25 al,~roxilllation register (SAR) 64. A/D converter 62 opc;ldles by CO...p~ a sample of "unknown" analog electrical activity from programrnable gain filter 60against a group of weighted values provided from SAR 64 on lines 66. A/D
converter 62 COlll~CS the weighted values on lines 66 in desc~n-ling order, starting with the largest weighted value. A weighted value is not added to the 30 snmm~d digital data stored in SAR 64 if the weighted value, when added to theprevious sun~ned weighted values, produces a sum larger than the sampled _ WO 97/C~68SO PCT/US96/13095 "unknown" analog electrical activity. The sl~mm~d digital data is l~pd~ted in SAR 64 and a new weighted value is col,,p~ucd on each active edge of a SAR
clock on a line 68.
At the end of the ~.,cce~ e a~plo~imalion when balance is achieved, the sum of the weighted values stored as the s~mm~ocl digital data in SAR 64 r~.escn~ the h~lu~ rd value of the sampled "unknown"analog electrical activity. SAR 64 provides the stored digital cardiac data to an absolute value circuit 70. Absolute value circuit 70 provides the absolute value of the arnplitude of the digital cardiac data on a line 72 to be provided to gain control circuit 32/52 and template generation circuit 36/56. Successive approximation A/D conversion as ~clro~ ed by A/D converter 62 and SAR 64 is very fast to permit adequate tracking of the incoming analog cardiac signal. The gain of prograrnrnable gain filter 60 is raised or lowered in discrete gain steps based on outputs from gain control circuit 32/52.
Se~rate G~in Control ~nd Threshold Templ~tin~
Gain control circuit 32/52 and template gc.lc,dlion circuit 36/56 operate with the AGCMlter and digitizing circuit 30/50 to implement two indlependent AGC digital loops according to the present invention. Gain control circuit 32/52 provides slow gain control to AGC/filter and f~igiti7in~ circuit 30/50 to keep sensed depol~ri7~ti~ n~ le~le3clll~ e of cardiac events in a~pluxilllately the upper one third of the dynamic range of A/D converter 62.
Template generation circuit 36/56 provides a fast responding variable sensing threshold to the detection circuit 34/54 for actual sensing of R-wave or P-wave depolari_ations represPnt~tive of cardiac events.
Gain control circuit 32/52, as described in more detail below with rcr~,lcr~ce to Figure 5, stores peak history inforrnation representative of peakvalues of the amplified electrical activity of a selected nurnber (N) of cardiacevents. Gain control circuit 32/52 adjusts the variable gain of AGC/filter and - ~ligiti7ing circuit 30/50 in discrete steps based on the stored peak history information. The stored peak history information is compared against pred~ofin~d levels and a~)pro~l;ate gain changes are initi~te~l based on a second CA 022287~7 1998-02-0~

selected number (M) of peak values of the N cardiac events being outside of a selected range.
Tem~ te generation circuit 36/56, as described in more detail below with l~,l'elence to Figure 7, provides a time varying sensing threshold toS detect circuit 34/54 for co,l.~;son to the ~1igiti7.Pf1 cardiac data provided on line 72 from AGC/filter and (1igiti7in~ circuit 30/50. Detection circuit 34/54 provides a detection signal indicating R-wave or P-wave depolari_ations ~ pl~,Sc~ e of cardiac events when the value of the in~,oming digital cardiac data is greater than the sensing threshold level provided that the refractory 10 windows are inactive. Template generation circuit 36/56 includes cil.;uiLly for selecting and adjusting the variable sensing threshold to a level ~r~olLional tothe amplitude of the digital cardiac data on line 72. Typically, template gPnt?rzltiorl 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 15 threshold is held at the peak value for a selected period of time after which the variable sensing threshold drops to a pelcc;~ ge of the peak value. The variablesensing threshold is then allowed to slowly decay from this p~,..;~llL~ge of peak value in discrete steps until the variable sensing threshold is at a low threshold value. Template generation circuit 36/56 preferably employs integer math to 20 achieve a piecewise linear approximation of a geometric progression such as an expon~nti~l decay curve with minim:~l error between piecewise steps.
Refractory Periods Cardioverter/defibrillator 20 utilizes ventricular and atrial l~;rla~;luly periods to determine which sensed events are R-waves or P-waves 25 lespe-;Li~ely. The active sensed refractory periods are illustrated in timingdiagram form on line 73 at 74 in Figure 3. Any sensed event that occurs when 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 measurement. Typical sensed 30 events occurring on the lead are represented on line 75 at 76. As illustrated, the start of the active ventricular or atrial refractory period is syllcl-loni~d with the CA 022287~7 1998-02-0~

start of the cardiac cycle. An absolute refractory interval is indicated on line 77 at 78. The absolute le~d~ilol,~ interval starts at the beginning of the cardiac cycle cimlllt~n~ous with the start of the active sensed refractory period. The absolute refractory interval disables all sensing. The operation of template g~l.c.dlion 5 circuit 36/56 based on the absolute refractory interval is f,urther described below under the T,hreshold Templating for a Fast Digital AGC Circuit h~ inp 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 ~ltili7.od instead of the sensed refractory period. Similar to the sensed refractor,v 10 period, any sensed event that occurs when the paced/shock refractory period is inactive is con~idçred to be a R-wave or P-wave. Typical pace pulses on the leadare ~ ;s~.t~d for illu~l,dli~e purposes on line 83 at 85. A typical 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 15 pacing or shocking conditions. The time duration of the paced refractor.,v period is preferably prog~ able, while the time duration of the shock refractory period is typically not programmable. The paced refractory period can be selected by the physician and programmed into cardioverter/defibrillator 20 when the cardioverter/defibrillator is op~l~Lillg in a pacing mode. The operation 20 of template generation circuit 36/56 based on the paced/shock refractory period is further described below under the Threshold Templating for a Fast Digital AGC Circuit h~ 1in~
Slow G~in Control Circuit Gain control circuit 32, or alternately gain control circuit 52, is 25 l~lese~ rely illustrated in Figure S in logical block diagram form. A
connp~r~tor 80 receives the digital cardiac data on line 72 and co",pa,c:s the peak value of the digital data reprçs~nting the current cardiac event to a selected low threshold and a selected high threshold. For example, in one prer~"~d embodiment of the present invention where the maximum value of the peak 30 value of the digital cardiac data is 7F hex, the selected low threshold is 52 hex and the st?lectecl high threshold is 7E hex. A first storage register 82 includes a CA 022287~7 1998-02-0~

first group of storage locations which store peak history information provided by co. . .~ or 80 on a line 84 indicative of whether the peak values are below the selected low threshold (52 hex in the example embodiment). A second storage register 86 includes a second group of storage locations which store peak history info,lndlion provided by CO~ alOl 80 on a line 88 indicative of whether the peak values are above the selected high threshold (7E hex in the example embodiment).
An M/N circuit 90 receives peak history information from storage register 82 and ~ . . . i n~s if M peak values of N cardiac events are below the10 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 increment signal on line 92 when M out of N peak values are below the selected low threshold (52 hex) to in~1ir,~te that the gain of AGC/filter and ~ligiti~ingcircuit 30/50 is to be incl~llle~lled by at least one discrete gain step. In one15 embodiment of the present invention, the discrete gain step is approxim~t~?lyequal to 1.25. An M/N circuit 96 receives peak history information from storage register 86 and clet~ s if M peak values of N cardiac events are above the selected high threshold (7E hex). M/N circuit 96 provides a dec~ ellt signal on a line 98 to gain control clock circuit 94. M/N circuit 96 activates the decrement 20 signal on line 98 when M out of N peak values are above the selected high threshold (7E hex) to inflir,~te that the gain of AGC/filter and ~ligiti7in~ circuit 30/50 is to be decrem~ntec~ by at least one discrete gain step. The decrementin~discrete gain step is preferably equal to the incremPnting discrete gain step and is &~llo~illlately 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 t1ipiti7in~ circuit 30/50 by c~lleing the gain to be incremented or decremented in discrete gain steps based on the increment signal on line 92 and the decrement signal on line 98. The gainof AGC/filter and ~igiti7ing circuit 30/50 can be increased or decreased by a 30 fixed number of steps or amount, or the level of the discrete gain step is optionally made prograrnmable via microprocessor and memory 38. In addition, -CA 022287~7 1998-02-0~
WO 97/~6850 PCT/US96/13095 gain control and clock circuit 94 optionally causes increments or dec~ elll~ of gain in mllltiple discrete gain steps. Since the increment signal on line 92 andthe decrement signal on line 98 are never activated at the sarne tirne due to the dual low threshold (52 hex) and high threshold (7E hex), no ~bil.d~ion cil~iuil S is .~cec~s.. y to ~billdl~ b~ n the h~clelnelll or decrement signals to indicate which direction to proceed. Gain control circuit 32/52 preferably keeps the peakvalues of atrial or ventricle sensed cardiac events in approximately the upper one third of the dynarnic range of A/D converter 62. As a result, the lower approxim~tely two thirds of the dynamic range of A/D converter 62 is available 10 for s~n~in~ low amplitude signals such as occurring during fibrillation.
The above referenced number of M peak values is preferably odd to prevent lock-up of the AGC loop. For eY~mrle, in a p.~fc.lc:d embodiment of gain control circuit 32/52, M is equal to three and N is equal to four. In this embodiment, storage register 82 stores peak history information for four cardiac15 events in four col.~s~ollding storage locations each ,c~ se~ e of whether thecollcs~onding one of the last four values for peak values was below the selectedlow threshold (52 hex). In this embo~iiment, storage register 86 stores peak history inforrnation for four cardiac events in four coll~ onding storage locations each representative of whether the coll~onding one of the last four 20 valiues for peak values was above the selected high threshold (7E hex).
The peak values in storage register 82 and storage register 86 are ~-~rl;ldbly updated at the beginning of a new refractory period for a previous sensed event. As new peak value information is ac.lLIh~d from con~ald~or 80, the old peak history inform~tion is shifted one value to the right. If storage 25 registers 82 and 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 ~ and lost.
In a pl~ r~lled embodiment of the present invention, M/N circuit 90 activates the increment signal on line 92 only if the stored peak value of the 30 last cardiac event and M - I peak values of the last N - 1 cardiac events previous to the last cardiac event are below the selected low threshold (52 hex). In this CA 022287~7 1998-02-0~

p.~fe.lGd embodiment of the present invention, M/N circuit 96 a ;liv~Lt:s the decrement signal on line 98 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 above the selected high threshold (7E hex).
S Gain control circuit 32/52 o~ LGs ~ described above to the possibility of i~ Jr~GI sensing by not allowing AGC/filter and digiti_ing circuit 30/50 to go to low sensitivity if large R-waves or P-waves are present or to go to full sen~ilivily in the presence of slow R-waves or P-waves.ropc;l sensing can cause therapy to be delivered to a patient at hla~ pliate 10 times as a result of false indications of &l,LyLhll.ia conditions. Oversensing is reduced because the full sensitivity of AGC/filter and digiti7ing circuit 30/50 is not reached bcl~ slow beats as a result of gain control circuit 32/52 keeping the amplified depol~ri7~tion electrical activity in the upper approximately one third of the dynamic range of A/D converter 62. The reduced o~ ;..g 15 greatly inclc;ases the comfort level of a patient having the cardioverter/defibrillator according to the present invention implanted in his or her body. Undel~ellsillg is reduced because minimllm sensitivity will not occur due to a single large R-wave or P-wave.
In addition, as in-~iç~ltPd above, gain control circuit 32/52 20 ~ les the need for a high precision A/D CO11VG1L~I implemçnt~tion of A/D
converter 62, bec~use the entire dynamic range of the inconlillg cardiac signal does not need to be sp~nn~l Thus, in the l,rG~ ,d embodiment of the present invention, A/D converter 62 is implemented in 8 bits or less. The dynamic range of the incoming cardiac signal from the atrial and/or ventricular channels of the 25 heart ranges from 0.1 mV to 25 mV 1G~1G~ )g a 250 to 1 dynamic range.
Lower precision A/D converters consume less power, convert the incoming analog signal to representative digital data more quickly, and allow more cost effective silicon processes to be utili7~d Moreover, m~nllf~ctllrability of the cardioverter/defibrillator is improved since no external parts are required to 30 control the gain of the AGC/filter and digitizing circuit 30/50. Testing and r.h~r~c~ ;on ofthe cardioverter/defibrillator is also improved since the digitallogic of the gain control circuit 32/52 iS easily f~ult graded.
AGC Turn Down ~echanism for Far Field Sencin~
The ~r~rc.~d embodiment of gain control circuit 32/52 illu~trslt~cl ~ S in Figure S 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 sensing. Forexarnple, when sensing events in the ventricle c-h~nnel of the heart, P-waves, l~les~ g far field events, can be sensed during nor nal sinus rhythms at maximum sensitivitv. Under this example condition, instead of AGCing on the R-wave peaks, which are clipped, the P-wave peak level ~ltern~titlg with the clipped R-wave peak le~el combine to inhibit gain changes. The clipped R-wave peaks indicate that the R-wave peaks are above the m~ximllm digital value for a peak signal. In this case, the M of N algorithm 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 norrnal modes of operation to prevent this lock-up condition from occ~lrring.
In the embodiment illustrated in Figure 5, two additional history storage locations are provided in storage register 86 to extend the peak historyinformation to N + 2 storage locations. Far field sense circuit 102 responds to the last N + 2 sensed events stored in storage register 86 to deterrnine if the storage inform~tion ~Iternz~s bclw~en clipped peaks and non-clipped peaks for the last N + 2 sensed events. Far field sense circuit 102 de~ermin~s that a peak is clipped when the peak is at the m~ximl-m value (7F hex) which co.l~ onds to peak values greater than the lligh tllreshold value used by co,.-p~dlor 80 (7E
hex). Far field sense circuit 10 ~ pro~ides a decrement signal on a line 104 to 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 ~ altern~tes ~clwc;ell clipped peaks and non-clipped peaks for the last N + 2 sensed 30 cvcnts. In one embodiment, the decrement signal on line 104 indicates that gain of AGC/filter and digitizing circuit 30/50 is to be decremented by one discrete CA 022287~7 1998-02-0~

gain step, but can alternatively indicate any number of discrete gain step changes.
Far field sense circuit 102 operates in the cases where the actual depolarization of the incoming cardiac signal is clipped to prevent the digital S AGC loop from locking up under the condition of far field sensed events. If far field events are detected in the above manner, the gain of AGC/filter and ~iigiti7:ing 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 10 according to the present invention greatly improves sensing disclil--ina~ion by minimi7inL~ or subst~nti~lly elimin~ting oversensing in the presence of far field 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.
15 Sl~w G~in Jnn~ E~ack for AGC
Previous gain cil~;uill y reaches maximum sensitivity in a single cardiac cycle. Unlike previous gain circuitry, the slow gain cil~;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 20 of the AGC/filter and ~ligiti7ing circuit 30/50 is not reached between cardiac depolarizations, which can cause un~C. ,~ g of cardiac events. As illustrated in Figure 5, additional ci~cuill.y is preferably added to gain control circuit 32/52 to prevent undersensing of cardiac events.
Exception ci.~;uiL-y 106 detects any one ofthree conditions which 25 inr1ic~te that the gain of the AGC/filter and digitizing circuit 30/50 is to be set to a selected relatively high sensitivitv. Exception circuitry 106 provides a set gain signal on a line 108 to cause the gain of AGC/filter and digitizing circuit 30/S0 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 30 for a selecte(i time period (i.e., a R-wave or P-wave depolarization is not sensed for the selected time period). Typically, the selected time period is equal to -a~pro~ ely 1.5 secon-l~, col-~,*)o--ding to a heart rate of less than 40 beats per mimlte The second condition occurs after the cardioverter/defibrillator deliversa shock pulse. The third condition occurs after the cardioverter/defibrill~tor delivers a pacing pulse.
~ S In any of the three conditions, it is desirable to prevent und~ e;..... ~ by setting the gain of the AGC/filter and ~ligiti7ing circuit 30/50 to the selçcte~l relatively high sensitivit,v to quickly increase the sensitivity of the cardioverter/defibrillator. A/D converter 62 (shown in Figure 2) typically O~,.d~s in bands of an a~,ro~ ately 10:1 dynamic range. The combined 10:1 10 dynamic range bands create a total 250:1 dynamic range of A/D converter 62.
The three exception conditions are conditions where A/D converter 62 needs to operate near m~ximllm sensitivity, or in other words, near the upper portion of the highest 10:1 dynamic range band to adequately prevent undersensing.
In the ~er~cd embodiment of the present invention, the selected 15 relatively high sc;,,~ilivily is two gain steps from a m~x;...~l-.. sensitivity to prevent mi.ct~kin~ P-wave depolarizations and T-wave repolarizations for R-wave depolarizations. If the selected relati~ely high sensitivity creates a clipped signal on the following depolarization, having its peak at the m~ u~l~ value (7F hex), as indicated from col"~aldLor 80 on line 88, a jump back compare 20 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 gate1 16 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 indicate that the gain of AGC/filter and ~1igiti7ing circuit 30/50 is to 25 be decrementç~l by at least one discrete gain step during the current refractory period. The offset value stored in register 112 is preferably programmable and is provided to gain control clock circuit 94. In one embodiment, the offset value is prograrnmed to equal three discrete gain steps.
If the peak value of the digital cardiac data on line 72 is still 30 clipped on the next depolarization after the gain has been decreased by the offset value stored in offset register 112, co"lp~dlor 80 indicates on line 88 that the CA 022287~7 1998-02-0~

peak of the cardiac signal is still clipped. Jump back co~ pal~, circuit 11 0 then in(1iC~tes that the gain is to be decrementçd by at least one discrete gain step by activating a line 118 to the other input of OR gate 116, which co--e~polldingly activates enable line 120 to gain control clock circuit 94. If the peak value is still 5 clipped, normal AGC action as described above res-lmes. This two staged back off n~erl~ iem after a jurnp out or escape to the selected relatively high sensitivity due to lack of sensing reduces oversensing reslllting from the clipped peak of the cardiac signal.
If the peak value of the digital cardiac data on line 72 is not 10 clipped on the first depolarization after the gain is set to the relatively high sensitivity, but the peak value is clipped on the second depolarization after the gain is set to the relatively high sensitivity by having its peak at the ~ X i ~value (7F hex), as indicated from co...~.A~ r 80 on line 88, jump back compare circuit 110 activates line 1 14 to two input OR gate 116 to indicate that the gain is 15 to be reduced by the offset value stored in offset register 1 12. OR gate 1 16 provides the decrement signal on enable line 120 to gain control clock circuit 94 which is activated when either of the two inputs to the OR gate are activated toindicate that the gain of AGCMlter and digitizing circuit 30/50 is to be decr~nt~d by at least one discrete gain step during the current refractory 20 period. If the peak value is still clipped, normal AGC action as described above resl-mt?s. This situation, where the peak value of the first detected depolarization is not clipped and the peak value of the second detected depolarization is clipped after the gain is set to the relatively high sensitivity, results when the firstdepolarization ~ ,lesents a far field sensed event such as described above. For 25 example, when sensing events in the ventricle channel of the heart, P-waves, lc~ s~ g far field events, can be sensed during normal sinus rhythms at maximulll sensitivity.
Thrl?~hold Tem~l~tir~ for a Fast D;~ital AGC Circuit Figure 6 illustrates, in timing diagram form, the variable sensing 30 threshold generated by template generation circuit 36/56 and provided to detection circuit 34/54. The variable sensing threshold is indicated by line 130.

CA 022287~7 1998-02-0~

As illustrated, the variable sensing threshold 130 follows a piecewise linear appro~im~tion of an ~ onel~lial decay curve with minim~l error bcLv~ steps.
The template gelle.~lion circuit 36/56 forces the variable sensing threshold 130to rapidly follow the l~h~ --~- peak level of the ~1igiti7Pd cardiac data. When ~ S the incoming digitized cardiac data is greater than the current sensing threshold, template g. I~ Lion circuit 36/56 raises the variable sensing threshold 130 to apeak threshold value apl)loxi..-ately equal to the peak value of the in- oming iti7Pd cardiac data as indicated at time T(0) After a shock or pace pulse is delivered by the cardioverter/defibrillator, template generation circuit 36/56 sets 10 the variable sensing threshold 130 to a selected relatively high threshold value The selected relatively high threshold value is preferably 7E hex in the exampleembo-lim~nt or one binary number below the m;- xi- ~ ~- ~- ~ ~ value of the variable threshold.
The variable sensing threshold 130 remains at the peak threshold 15 value through a refractory or a portion of a refractory period in~ t~l at 132.
When the cardioverter/defibrillator is op~,ldlillg in pacing mode, the period indicated at 132 is a prograrnmed paced refractory period that is selected by the physician and pro~-dnl,--ed into the cardioverter/defibrillator, such as the paced/shock refractory period indicated at 81 is Figure 4. ~hrhen the 20 cardioverter/defibrillator is ope.d~ g in shocking mode, the period in~lic~te~l at 132 is a shock refractory period, such as the paced refractory period indicated at 81 is Figure 4 When the cardioverter/defibrillator is operating in sensing mode,the period indicated at 132 is a portion of a sensed refractory period, such as the absolute refractory period indicated at 78 is Figure 4. In addition to the 25 refractory or the portion of a refractory period indicated at 132, the variable sPn~ing threshold does not begin to decay from the peak threshold value attainedat time T(0) for an additional drop time indicated at 134 The drop time is a normal template hold time for the peak converter ~;h.;uiLIy of template generation circuit 36/56, and is empirically ~letçrmine~l A suitable value for the drop time 30 in one embodiment of the present invention is approximately 13 7 msec.

CA 022287~7 1998-02-0~

After the lcfia-;lol.~ period and the drop time have elapsed, at time T(l,0) the variable sensing threshold 130 drops by an initial drop percentage, in-lic~tecl by arrows 136. The initial drop ~crcelllage is preferably approximately 25% of the peak threshold value so that the level of the variable sensing 5 threshold obtained at time T(l ,0) is ~ xilndlely 75% of the initial peak threshold value. As indicated at time T(l,l), the variable sensing threshold starts to decay in discrete steps such as indicated at 138. The step time size is c~.r~c..~ ely indicated by arrows 140 between time T(l,l) and T(1,2). The level of the variable sensing threshold 130 decays from a percentage of the peak10 threshold value to step over wide depolarizations or T-waves in the incoming electrical activity.
In the preferred embodiment of the present invention, template generation circuit 36/56 drops the variable sensing threshold 130 in step groupscomprising multiple discrete steps. In the embodiment illustrated in Figure 6, 15 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( l ,0) and T(2,0), arrows 144 for a four step group betweenT(2,0) and T(3,0), and arrows 146 for a four step group between T(3,0) and T(4,0). The defined ~e.cclll~ge for each step group is preferably approximately 20 50%. For example, in the ple~llcd embodiment of the present invention, the value of the variable sensing threshold at time T(2,0) is al,~n~ ately 50% of the value of the variable sensing threshold at time T(l ,0), and the value of the variable sensing threshold at time T(3,0) is approximately 50% of the value of the variable sensing threshold at time T(2,0) or 25% of the value of the variable 25 sensing threshold at time T(l,0).
Wllen the variable sensing threshold 130 decays to a programmable final value, as indicated at 148, template generation circuit 36/56holds the variable sensing threshold at the programmable final value until a newsensed event occurs. The prograrnmable final value is programmable to 30 compensate for noise which is inherent in the sense amplifiers and other AGC
system circuits of the AGC loop.

The initial drop p~lcel,lage to achieve a~ i,l,alc;ly 75% of the peak threshold value, and the four discrete steps in each step group to drop thevaliable sensing threshold to approxim~tely 50% of the level of the start of thefour-step group realizes a piecewise geometric progression linear al)plo,~i.nation - S l~,~leSr~ g an ~olle~lial decay curve with minim~l error between piecewise steps. Since the sensing threshold drops in discrete steps as indicated at 138, integer math can be utilized in template generation circuit 36/56. For example in the embodiment of template generation circuit 36/56 illustrated in Figure 6, floating point numbers are not required because the m~imnm di~ ee/error between any two discrete steps in a four step group is one bit. The present invention can be e~ctçn~ed to use any size integer value or number of steps or step groups to achieve the linear approximation of the exponential decay curve.
In fact, floating point numbers are optionally used, but are not desirable because of the increased silicon area needed to implement floating point logic circuits. In addition, by implçmenting the template generation circuit with integer values, the resulting temrl~te generation circuit consumes a relatively small amount of power.
A pl~erel..d algol;lllll, for calculating the drop in amplitude for each of the discrete steps is shown in TABLE I below.

TART.li', I

I~T~T~VAT STFP CAT CUT A,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
ELSE
IF (FINAL THRESHOLD > TEMPl ) THEN
FINAL ELSE TEMPl 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 (FlNAL 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 1,2,3,4 - Decay Period Referring to TABLE I above, in the interval T(X,0), TEMP is calculated to 75% of the initial peak threshold value, and TEMP 1 is calculated to 50% of a previous step group value. lf the step group is tlle first drop from the peak threshold value, then T(l ,0j is equal to TEMP or 75% of the peak value. InS succes~ive drops, T(X,0) is equ,al to TEMP1 or a j0% drop from tlle level at the be3~innin~ of the previous step grou~.
In all ofthe T(X,l), T(X,2), and T(X,3) intervals, the variable sensing threshold obtains the TEMP value u~less the TEMP value is less than FINAL THRESHOLD which is the ~Inal programmable value indicated at 148 in 10 Figure 6. ~or example~ in the T(X,l) interval, T(X,1) is set to TEMP which iscalculated to 87.~% of the T~X,0) ~alue. In thc T(X,2) interval, T(X,2) is set to TEMP which is calculated to 75% orthe T(X,0) value. ln the T(X,3) interval, T(X,3) is set to TEMP which is calculated to 62.5% of the T(X,0) value.
A logical block diagram of a ~ le~ed embodiment of template 15 generation circuit 36/56, which uses integer values for calcul~tin~ the variable sensing threshold, is illustrated in Figur~ 7. A peak detection circuit 160 detects the peak value of the digitized cardiac data provided on line 72 from AGC/filterand cli~iti7in~ circuit 30,'~0. Peak detection circuit 160 provides a peak threshold value which is equal to the peak value of the digiti~d data to a threshold register 20 162 if the ~lig~iti7.od peak is greater than the current threshol~i value. Threshold register 162 stores and provides the culTent variable sensin~ threshold on line 164 to the detection circuit 34/54. Peak de~ection circuit 160 also provides thepeak threshold value to l (X,0) register 166.
If the step group is ns~t ~he ~irst drop from the peak thresholcl 25 value the TEMPl calculation must be im.pleDlented for the T(X,0) interval of the discrete step calculation algorithm in r.~L E I abovc. Tc implement the TE~IPl calculation, the T(X-1,0! va]ue stored in the T(X,Q~ register 166 from the previous step group is dividcd by :2 t3~-ougll a hard S31i~t of one lo the right as ~ intli~ted by line 1~8 to place the shifted data in both the thresho!d l~ister 162 30 and the T(X.0) register 166.

CA 022287~7 1998-02-0~

T(X,0) register 166 provides its cull~;l,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 calculation by shifting the current T(X,0) value by one bit or two bits to the right, les~ecliv~ly. Subtractor 170 subtracts the value stored in the T(X,0) register 166 from a shifted output provided from shifter 172. The shifted 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, le~e~ilively. A difference output of subtractor 170 is provided to an adder 176. A shifted output of shifter 174 is provided to the other input of adder 10 176. Adder 176 adds the difference 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), 15 and T(X,3) intervals of the discrete step calculation algorithm 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 properly combined according to the algorithm in TABLE I with subtractor 170 and adder 176.
A final threshold register 178 stores the pro~,ldmlllable final value, in~ ted at 148 in Figure 6, of the variable sensing threshold. The pro~,ld,l.~llable final value is provided to a threshold collll)al~or 180. Threshold colllp~dtor 180 compares the prograrnmable final value stored in final thresholdregister 178 with the current variable sensing threshold value on line 164.
25 Threshold colllpa-d~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 progl~ nable 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 30 incoming digiti7~d cardiac data exceeds the final value indicating a new sensed event. In fact, a new sensed event occurs any time the incoming ~ligiti7Pcl CA 022287~7 1998-02-0~
WO 97/(76850 PCT/US96/13095 cardiac data peak value çYcee~1c 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 depo1~ri7~tion in the ~ iti7~d cardiac data.
The above described threshold templating algo,ilh.. l for a fast digital AGC system is completely cont~in~d in digital logic as irnpl~n~nte~l in the ~rert;ll~d embo-1im~nt The digital logic implement~tion is easily ch~ i~d, tested, and achieves repeatable results. In addition, external parts are el;...in;.l~d from the silicon chip implementation ofthe AGC circuitry to reduce cost and increase the manufacturability of the AGC silicon chip. Testing and characterization of the cardioverter/defibrillator devices is uniform from one device to another. In this way, it is easier for the physician to ~ietennine how to implement the cardioverter/defibrillator device in a patient, because the devicereacts c~ nei~tlontly from one unit to another.
~ilorable AGC Decay ~t~
No single decay rate (attack rate) is optimal for all operating conditions of a cardioverter/defibrillator with pacing capability for the above described fast response AGC circuit. The typical operating conditions encountered include bradycardia pacing, tachyrhythmia sensing, and normal sinus s~n~in~ Therefore, according to the present invention, the step time size int~ tto(l by arrows 140 in Figure 6 is prograrnmable 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 Opt;ldlillg conditions, the dec~y rate is customized to optimally meet the selected operating condition.
For normal sinus sensing, a single attack rate is utilized that covers most of the incoming cardiac signals. In one embodiment of the present invcntion, 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 response rate is desirable in order to IJlUp~ lly track the higher tachyrhythmiarates, such as during fibrillation or tachycardia. This is especially true in the CA 022287~7 1998-02-0~

atrium of the heart, where t~chyll,ylll,llia rates run in excess of 300 beats per minute. In one embodiment of the present invention, step size 140 is set to approximately 17.5 mSec/step for atrial tacl,yll-ylh.nia conditions, and is set to approxim~tely 23.5 mSec/step for ventricle tachyrhythmia conditions. By 5 switching to this faster decay rate for tachyrhythmia conditions, cases of undc;l~e~ g a l~cLy.llyllll~lia condition which needs to be treated is reduced.
Bradycardia pacing is a special opel~tillg condition wherein the decay rate of the sensing threshold is tied to the bradycardia pacing rate to help minimi7~ ove.~ensillg and unde, i~nshlg 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/defibrillator with pacing capability l~tili7ing AGC according to the present invention does not have time to decay to m~ki"lulll sensitivity. If the decay rate is not sufficiently sped up along with the high pacing rates 15 undc.~ g occurs and the cardioverter/defibrillator continues pacing in the presence 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 pr~)pelly respond to the fibrillation condition. In 20 addition, when pacing rates are low, a longer decay rate is desirable to minimi the possibility of O~ g The forrnula for calculating the post pace template step time size 140 for bradycardia pacing conditions is as follows:

CA 022287~7 1998-02-0~
WO 97/0~850 PCT~US96/13095 STEP TIME SIZE = ~ ~
(CYCLE LENGTH - REFRACTORY - DROP TIME - MINIMUM TIME) /X
where:
CYCLE LENGTH = pacing cycle length ~ 5 REFRACTORY = programmed paced refractory DROP TIME = normal template hold time for peak collvelL~l (appr xim~tely 13.7 mSec in a preferred embodiment) MINIMUM TIME = minimum time allowed for template at final value (approximately 100 mSec in a preferred embodiment) X = number of steps to go from seed value to final value (equal to 12 steps in the embodiment illustrated in Figure 6) Referring to Figure 6, the cycle length is equal to the pacing cycle length or from time T(0) of one pacing pulse to time T(0) of the next 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 20 programmable final value before the next pacing pulse is indicated by arrows 150. Since multiple pacing rates are :lc~ignPcl the sarne step size, the time in~ te~l at 150 varies from a~>plvxhllately 100 mSec to 200 mSec in the embodiment illustrated. The ~ llll time is the lll;ll;~ llll time allowed for the time inr~ t~P~l by arrows 150, or approximately 100 mSec. X represents the 25 12 steps (i.e., the 3 X four step groups) to go from the peak sensing value at time T(0) to the programmed final value of the variable sensing threshold achieved atT(4,0).
A look-up table stored in microprocessor and memory 38 is forrned by dividing the cycle length by 64, which results in a shift of six bits to 30 the right. In one impl~nnrnt~tion of the present invention, the cycle length is equal to 12 bits, which results in six bits being shifted off in the divide by 64 CA 022287~7 1998-02-0~

formation of the look-up table in microprocessor and memory 38, resultin~ in 64 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,.cs~ollding to the above step time size formula.
The digital embodiment of the AGC loop as described above allows the above described rn...w~e implemt?nted in the look-up table in the microprocessor and memory 38 to dynamically adjust the sensing ch~r~c t~ristics of the cardioverter/defibrillator according to the present invention. By sensinghigh rates di~rer~,-Lly than low rates, the tailorable AGC decay rate according to 10 the present invention can be utilized to orthogonally optimize sensing characteristics of bradycardia and tach~,h)/lll.llia signals, which have mutually exclusive sensing re4~ l..ents. In this way, the physician controls a better-behaved cardioverter/defibrillator. In addition, patient comfort is increased, due to reducing ove.~ g and undersensing of treatable arrhythmia conditions in 15 the patient.
Tnteractioll of T~i~ital AGC Usin~ Se~rate Gain Control and Threshold Tenu~l~ti~
Figure 8 illustrates in timing diagram form depolarization cycles in the electrical activity of the heart. The incoming electrical activity at 20 input/output terminals 22 or 42 is inrlir~tecl by waveform 200. The filtered and gain controlled digitized cardiac signal is indicated by waveform 202. The variable sensing threshold is indicated by waveform 204. The absolute value of the ~iigiti7PCl and gain controlled cardiac signal is in~lic~te-l by waveform 206 underneath the variable sensing threshold waveform 204. The refractory period 25 is indicated by waveform 208. The discrete stepped slow gain is indicated by waveform 2 10.
As indicated by waveform 204, the variable sensing threshold waveform responds to the absolute value of the digiti7ed cardiac signal to assume the peak value of the digitized cardiac signal. The variable sensing 30 threshold then decays according to a piecewise linear approximation of an exponential decay curve to step over wide depolari7~ations or T-waves.

_ CA 022287~7 I sss - 02 - o~
wo 97/06850 PCT/US96/13095 The influence of the slow gain control on the fast templating circuit is illustrated at time 212. As is indicated, the gain is decreased at time 212, which correspondingly results in a reduced filtered and gain controlled iti7f-CI cardiac signal in~ ted at 202, which col-c~Jondingly reduces the 5 variable sensing threshold indicated at 204 as the variable sensing threshold follows the peak value of the absolute value of the .ligiti7Pd and gain controlled cardiac signal inl1ir~t~-1 at 206.
Conclll~ion By utili7ing this present invention, which incorporates two 10 independent loops in a cardioverter defibrillator with pacing capability which are both implem~nting digital logic circuits, the AGC~ response is effectively movedfrom analog circuits into the digital logic circuits, where it is easier to test and çh~r~cteri7e Design of the sense amplifier is simplified, due to the digital control of the sense amplifier. It is easier to test and characterize the analog15 sense amplifier, since the AGC Cil~;ui~ / iS no longer in the analog domain. The cardioverter/defibrillator device is more uniform from device to device, which greatly inclcases the physician's ease of predicting device behavior. In addition, the patient comfort is increased due to reduced oversensing and undc.~ensillg.
Althoùgh specific embodillle~ have been illustrated and 20 described herein for purposes of description of the preferred embodiment, it will be appreciated by those of ordinary skill in the art that a wide variety of sllt~rn~te andl/or equivalent implem~ont~tion~ calculated to achieve the same purposes may be :~ub~ ulcd for the specific embo-lim~nt~ shown and described without departing from the scope of the present invention. Those with skill in the 25 mechanical, electro-mechanical, electrical, and computer arts will readily appreciate that the present invention may be implcmented in a very wide variety - of embo-lim~ntc This application is int~n-l.?d to cover any adaptations or variations of the preferred embo~imente discussed herein. Therefore, it is manifestly intended that this invention be limited only by the claims and the equivalents thereof.

Claims (47)

WHAT WE CLAIM IS:

1. A method of automatically adjusting a sensing threshold in a cardioverter/defibrillator, the method comprising the steps of:
amplifying the electrical activity according to a variable gain;
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 an acquired amplitude of the amplified electrical activity of a current detected cardiac event;
decreasing the variable sensing threshold from the level in discrete steps until the variable sensing threshold is at a low threshold value, and adjusting the variable gain based on amplitudes of the amplified electrical activity of at least three detected cardiac events.

2. The method of claim 1 wherein the step of adjusting the variable gain comprises the steps of:
increasing the variable gain if a first selected number (M) of peak values of the amplified electrical activity of a second selected number (N) of cardiac events are below a selected low threshold; and decreasing the variable gain if M peak values of the amplified electrical activity of the N cardiac events are above a selected high threshold.

3. The method of claim 2 wherein the variable gain is increased in the increasing step if a 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 and the variable gain is decreased in the decreasing step 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 above the selected high threshold.

4. The method of claim 1 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.

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 step of decreasing is performed by decreasing the variable sensing threshold in step groups, wherein each step group comprises multiple discrete steps, and wherein each step group decreases the variable sensing threshold by a defined percentage.

8. The method of claim 1 wherein the step of decreasing includes the step of calculating an amount of drop for a discrete step using integer math.

9. The method of claim 1 wherein the variable sensing threshold is decreased in the decreasing step according to a decay rate and wherein the method further comprises the step of 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.

10. The method of claim 9 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, tachyrhythmia sensing, and normal sinus sensing.

11. The method of claim 10 wherein the time width is varied as a function of a bradycardia pacing rate when the cardioverter/defibrillator is operating underthe bradycardia pacing conditions.

12. 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.

13. The method of claim 2 further comprising the step of decreasing the variable gain if the peak values of the amplified electrical activity of a thirdselected number of cardiac events alternate between clipped peak values and non-clipped peak values.

14. The method of claim 1 wherein the step of adjusting the variable gain includes the step of setting the variable gain to a selected relatively high sensitivity based on a certain condition occurring.

15. The method of claim 14 wherein the certain condition occurs when a cardiac event is not detected within a selected time period.

16. The method of claim 14 wherein the certain condition occurs after a shock pulse is delivered by the cardioverter/defibrillator.

17. The method of claim 14 wherein the method automatically adjusts the sensing threshold in a cardioverter/defibrillator having pacing capability, and wherein the certain condition occurs after a pacing pulse is delivered by the cardioverter/defibrillator.

18. The method of claim 14 wherein the step of adjusting the variable gain is performed in discrete steps and the selected relatively high sensitivity is at least one discrete step from a maximum sensitivity.

19. The method of claim 18 further comprising the step of decrementing the variable gain from the selected relatively high sensitivity by a selected number of discrete gain steps if the setting of the variable gain to the selected relatively high sensitivity creates a clipped peak value of the amplified electrical activity on the following detected cardiac event.

20. The method of claim 19 further comprising the step of further decrementing the variable gain by at least one discrete gain step if the peak value of the amplified electrical activity is still clipped on the second detected cardiac event following the setting of the variable gain to the selected relatively highsensitivity.

21. The method of claim 18 further comprising the step of decrementing the variable gain from the selected relatively high sensitivity by a selected number of discrete gain steps if the setting of the variable gain to the selected relatively high sensitivity does not create a clipped peak value of the amplified electrical activity on the following detected cardiac event and does create a clipped peak value of the amplified electrical activity on the second detected cardiac event following the setting of the variable gain to the selected relatively high sensitivity.

22. A system for automatically adjusting a sensing threshold in a cardioverter/defibrillator having a pulse circuit for generating shock pulses based on a detect signal representing cardiac events indicated in electrical activity of a heart, the system comprising:
an amplifier (30/50) for amplifying the electrical activity of the heart according to a variable gain;
a cardiac depolarization (34/54) detector for detecting depolarizations in the amplified electrical activity of the heart and providing the detect signal representing a cardiac event indicative of a depolarization when the amplified electrical activity exceeds a variable sensing threshold;
gain controller (32/52) for adjusting the variable gain of the amplifier based on amplitudes of the amplified electrical activity of at least three detected cardiac events; and threshold controller (36/56) for acquiring amplitudes of the amplified electrical activity and for adjusting the variable sensing threshold to a level proportional to the acquired amplitude of the amplified electrical activity of acurrent detected cardiac event and for decreasing the variable sensing thresholdfrom said level in discrete steps until the variable sensing threshold is at a low threshold value.

23. The system of claim 22 wherein the gain controller comprises:
storage means for storing peak history information representative of peak values of the amplified electrical activity of a first selected number (N) of cardiac events; and adjusting means for adjusting the variable gain based on the stored peak history information.

24. The system of claim 23 wherein the adjusting means adjusts the variable gain only if a second selected number (M) of peak values of the N cardiac eventsare outside of a selected range.

25. The system of claim 23 wherein the adjusting means increases the variable gain if a second selected number (M) of peak values of the N cardiac events are below a selected low threshold and decreases the variable gain if M
peak values of the N cardiac events are above a selected high threshold.

26. The system of claim 25 wherein the adjusting means increases the variable gain if a 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 and decreases the variable gain 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 above the selected high threshold.

27. The system of claim 25 wherein the storage means comprises a first group of storage locations which store peak history information indicating if the peak values are below the selected low threshold and a second group of storage locations which store peak history information indicating if the peak values areabove the selected high threshold.

28. The system of claim 23 wherein the peak history information from a previous cardiac event is updated at the beginning of a new sensed refractory period caused by a cardiac event.

29. The system of claim 22 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.

30. The system of claim 22 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.

31. The system of claim 22 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.

32. The system of claim 22 wherein said discrete steps are grouped into step groups, wherein each step group decreases the variable sensing threshold by a defined percentage.

33. The system of claim 22 wherein the threshold controller calculates an amount of drop for a discrete step using integer math to achieve a piecewise linear approximation of a geometric progression.

34. The system of claim 33 wherein the geometric progression is an exponential decay curve.

35. The system of claim 22 wherein the threshold controller decreases the variable sensing threshold according to a decay rate and wherein the system further comprises decay rate control means for 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.

36. The system of claim 35 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, tachyrhythmia sensing, and normal sinus sensing.

37. The system of claim 36 wherein the decay rate control means varies the time width as a function of a bradycardia pacing rate when the cardioverter/defibrillator is operating under the bradycardia pacing conditions.

38. The system of claim 22 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.

39. The system of claim 23 wherein the storage means is capable of storing peak history information representative of peak values of the amplified electrical activity of a second selected number of cardiac events, and the system further comprises:
gain turndown means responsive to the stored peak history information to decrease the variable gain if the peak values of the amplified electrical activity of the second selected number of cardiac events alternate between clipped peak values and non-clipped peak values.

40. The system of claim 22 wherein the gain controller includes means responsive to the detect signal to set the variable gain to a selected relatively high sensitivity based on a certain condition occurring.

41. The system of claim 40 wherein the certain condition occurs when a cardiac event is not detected within a selected time period.

42. The system of claim 40 wherein the certain condition occurs after a shock pulse is delivered by the cardioverter/defibrillator.

43. The system of claim 40 wherein the system automatically adjusts the sensing threshold in a cardioverter/defibrillator having pacing capability, and wherein the certain condition occurs after a pacing pulse is delivered by the cardioverter/defibrillator.

44. The system of claim 40 wherein the gain controller adjusts the variable gain in discrete gain steps and the selected relatively high sensitivity is at least one discrete step from a maximum sensitivity.

45. The system of claim 44 further comprising means for decrementing the variable gain from the selected relatively high sensitivity by a selected number of discrete gain steps if the setting of the variable gain to the selected relatively high sensitivity creates a clipped peak value of the amplified electrical activity on the following detected cardiac event.

46. The system of claim 45 further comprising means for decrementing the variable gain by at least one discrete gain step if the peak value of the amplified electrical activity is still clipped on the second detected cardiac event following the setting of the variable gain to the selected relatively high sensitivity.

47. The system of claim 44 further comprising means for decrementing the variable gain from the selected relatively high sensitivity by a selected number of discrete gain steps if the setting of the variable gain to the selected relatively high sensitivity does not create a clipped peak value of the amplified electrical activity on the following detected cardiac event and does create a clipped peak value of the amplified electrical activity on the second detected cardiac event following the setting of the variable gain to the selected relatively high sensitivity.