CA2121795A1 - Method and apparatus for termination of ventricular arrhythmias - Google Patents

Abstract

An implantable defibrillator provided with a plurality of defibrillation electrodes (500, 502, 504, 506, 508, 510), which may be reconfigured by a switch matrix (512) to define a plurality of defibrillation pathways. The device is capable of measuring the impedance along a selected defibrillation pathway, during delivery of an impedance pulse, and monitoring the success or failure of the pulse to accomplish defibrillation or cardioversion. In response to a detected failure to accomplish cardioversion in conjunction with a measured change of impedance of greater than a predetermined amount, a new defibrillation pathway is selected, which may employ some or all of the electrodes employed to define the original impedance pathway. The device also includes apparatus (524, 554, 550) for varying the relative amplitude of defibrillation pulses applied to individual electrodes used in sequential or simultaneous, multiple electrode pulse regimens, in order to equalize current distribution, in response to measured pathway impedances.

Description

W093-/0~ 2 1 2 1 7 ~ ~ PCr/US92/09~8 ... 1 .
METHOD AND APPARATUS FOR TERMINAT10N OF ~/ENTRICULAR ARRHYI HMIAS

Background of the Invention This invention relates to implantable stimulators generally and more particularly to implantable cardioverters and defibrillators.
Over the past 20 years, there has been substantial work j toward developing a practical, implantable defibrillator.
¦ However, several significant problems still remain. Early conceptions of implantable defibrillators, such as disclosed in U.S. Patent No. RE 27,652 by Mirowski et al., envision a system employing a ventricular endocardial electrode and a plate electrode, mounted directly to the heart, subcutaneously, or applied to the ~kin. However, it was , ~5 recognized early on that a totally transvenous system would be desirable in order to simplify the use of implantable ¢ defibrillators. One such system is suggested in U.S. Patent No. 3,942,536 by Mirowski et al., which discloses a transvenous lead ha~ing electrodes intended for location in the right ventricular apex and superior vena cava. Such ~yste~s were eventually tested in human beings with some success. However, currently available commercial versions of i~plantable defibrillators generally employ epicardial patch I eleetrodes alone or in conjunction with transvenous ~25 electrodes.
- ~ While syste~s eJploying epicardial patch electrodes are workable, a thoracoto~y i8 required in order to apply the epicardial electrode or electrodes. It is generally believed that it would be highly desirable to produce an implantable defibrillator which would entirely avoid the necessity of a thoracoto~y, and there has been substantial work directed ~-- toward such systems, a8 disclosed in U.S. Patent No. 4,727,877 issued to Kallok- and U.S. Patent No. 4,708,145 issued to Tacker et al. Both Tacker et al. and the Kallok patents , ~` W093/0~4 PcT/uss2/o9 ~

di~close the use of a transvenous, two-electrode leaa in combination with a subcutaneous patch electrode.
U.S. Patent No. 4,392,407 i~ued to Williams et al. and co-pending, commonly as~igned applications 284,957 by Mehra S and 284,955 by Bardy, both filed Decembeir 15, 1988 disclose multiple electrode sy~tems employing subcutaneious patch electrodes, coronary sinus/great vein electrodes, and ventricular endocardial electrodes. These electrode systems and other multiple electrode systQms employing endocardial electrodies alone or in conjunction with subcutaneous electrodes appear to hold significant promise.
Where there are electrical conductors there is the possibility of electrical malfunction. In the context of pacing leads, the~e ~alfunctions have often taken the form of open circuits or ~hort circuits, and monitoring systems have been developeid to detect and remedy these problems. U.S.
Patent No. 4,140,131 i~ued to Dutcher, incorporated herein by reference ini it~ entirety, di~cloces a pacemaker which a~certaini the presence of ~hort circuits or open circuits by ~ea~uring the i~pedance between the pacing electrodes and determining whether the aeasured iapedance fa}ls outside a predeterained range. If the aeasured iapedance fall~ out~ide this range, a warning ~ignal i6 coamunicated to the patient in ~, who~ the pacer i~ i~planted by aeans of electrical ~timulation of the tissue ad~acent the pacer. A ~ore recent exa~ple of a pac~ ~ker which ~easure~ i~pedance is disclosed in U.S. Patent No. 4,899,750 issued to Ekwall, and-incorporated herein by reference in its entirety. In this pacer, measure~ents of i-p dance are stored in a log for later review by the physician to allow diagnos~is of lead related problems. In ~o~e pace~aker~, lead configuration i8 programmable between unipolar and bipolar configurations. This feature rai~es the pos~ibility that the pacer may be programmed to a configuration inco~patible with the leads-actually implanted.
The pacer disclosed in published EP0 Patent Application No.

W093/0~4 2 1 2 1 7 9 ~ PCT/US92/09~

338,363, also incorporated herein by reference in its entirety, addresses the problem of inappropriate lead configuration programming by mea~uring impedance between pacing electrodes whenever reprogramming has taken place and reconfigures the programming of the lead configuration if the measured impedance indicates that the expected lead system is not pre~ent. The pacer also mea~ure~ iJpedance in respon~e to a failure to capture or other circumstances indicative of lead Jalfunction and reprograms the lead configuration in response to a Jea~ured iJpedance indicative of an electrical fault.
Lead configurations are programmed and tested until a configuration exhibiting appropriate values of measured iJpedanCe i8 selected.

Summary of the Invention The pre~ent invention provide~ a mechanism for optimizing electrode configuration in the context of an implantable cardioverter/defibrillator provided with a plurality o-defibrillation electrodes and the ability to deliver pulses between differing combinations of individual ones of the ele~trodes, or all of the electrodes together. For example, the invention may u~efully be practiced in the context of an i~plantable cardioverter/defibrillator provided with right -~ ventricular, coronary sinus and ~ubcutaneous electrodes.
Electrode sy~tems consisting entirely of epicardial electrodes 2S and electrode sy~tems employing other transvenously inserted electrodQ~ ~uch as superior vena cava electrodes may also be u~ed beneficially in conjunction with the pre~ent invention.
The cardioverter/defibrillator is provided with an internal therapy menu, ~ listing particular electrode configurations in a predetermined order. Each therapy regimen listed will ~pecify the pathways for pulse delivery employed during that particular regimen. For example, in a system ~ploying right ventricular, coronary sinus and subcutaneous electrode~, a sequential pulse regimen may be selected in W093/O~W 2 1 2 1 7 9 ~ PCT/US92/0936X

which pulses are delivered sequentially along a first pathway between the right ventricular electrode and the coronary sinus electrode and along a ~econd pathway between the right ventricular electrode and the subcutaneous electrode. During delivery of the pul~es, impedance for each pathway is measured, and is compared to the previously recorded measured impedance for that pathway. Following delivery of the pulses, the underlying cardiac rhythm is assessed to determine whether the pulses were successful in terminating the cardiac arrhythmia that led to the delivery of the cardioversion or defibrillation pulies. In the event that the impedance along ~ at least one of the pathways involved in the pulse regimen $ delivered differs more than a predetermined amount from the previously measured impedance along the same pathway or from s 15 a predefined impedance baseline, and the pulse regimen was f unsuccessful in terminating the arrhythmia, the pulse pathway is marked as "bad" in an internal impedance history log within ~ the cardioverter/defibrillator.
i~ Following the marking of a delivery pathway as "bad", thedevice scans the therapy ~enu to find the next available s therapy, checking to determine whether it employs pathways ~arked as "badn. When it locates a therapy regimen which has no pathways marked as "bad~, it schedules this pulse regimen or therapy for delivery following the next detection of an arrhythmia, or following a redetection of arrhythmia following the delivery of the preceding cardioversion or defibrillation pul~es. In more advanced e~bodi~ents, it is anticipated that the device ~ay automatically inventory the available - electrode~ and generate its own alternative therapy regimens if the phy~ician's ~pecified therapy menu is exhausted.
In practical implementations of the invention, it is anticipated that the physician will prefer that the pulse a~plitude associated with the next available therapy will be determined using the same criteria that would apple to control delivery of successive attempts using the original electrode , ,, ~ WOg3/~n~4 2 1 2 1 7 ~ ~ PCT/US92/09~8 configuration and pulse regimen. Generally, therefore, the pulse amplitude will increase with each successive attempt, . ~
even when the electrode configuration has been altered.
However, in some cases, physicians ~ay wish to begin using a new electrode configuration and/or pulse regime at the same pulse amplitude as used with the previous unsuccessful attempt using the original electrode configuration or t the pulse amplitude specified for the initial attempt to cardiovert or defibrillate. Therefore, it is anticipated that this aspect of the device's functioning will be made subject to external programmer control.
The present invention, unlike systems directed toward detection of shorts and open circuits in pacing leads does not require that the next therapy selected necessarily cease to employ any of the defibrillation electrodes associated with ~; the pathway marked "bad". For example, let it be assu~ed thatthe initially ~elected therapy comprises a simultaneous pulse, ~ multiple electrode regimen in which the coronary sinus and -~ ~ubcutaneous plate electrodes are tied together and a pulse is - 20 delivered between these two electrodes and the right ventricular electrode. Upon detection that this pathway (CS+SQ - RV) is bad, the device may then move on to try the next scheduled therapy, for example a ~imultaneous pulse multiple electrode regimen in which the right ventricular and coron~ry sinus leads are tied together, and a pulse is ~a delivered between the~e two electrodes and the subcutaneous plate electrode ~RV+CS - SQ). ~-Alternatively, let it be assu~ed that the initial therapy ~elected i~ a sequential pulse regimen in which pulses are deli~ered fir~t between the coronary sinus electrode and the right ventricular electrode and subsequentIy between the right - ventricular electrode and the subcutaneous electrode, and that the pathway between the coronary sinus electrode and right ventricular electrode (RV - CS) is marked as bad. The next WOg3/0~4 PCT/US92/09~*
212179~ 6 subsequent therapy may be a multiple pulse regimen in which pulses are delivered sequentially between the coronary sinus 1 and subcutaneous electrodes (CS - SQ) and between the right E~ ventricular and subcutaneous electrodes (RV - SQ), and not employing the CS - RV pathway.
Unlike the reconfiguration of pacing systems as described in the above-cited references, the present invention is also capable of responding to change6 in the pulse delivery pathways other than short circuits and open circuits within individual leads. For example, in either of the two examples ~et forth above, the change in impedance might be due to migration or poor initial location of either the right ventricular electrode or the coronary sinus electrode such that the electrodes are in excessively close proximity to one another at some point. This problem, while it may preclude .
the use of the RV - CS pathway, does not necessarily preclude u6age of the electrodes in other pulse delivery regimens which do not use this pathway. Similarly, even if two or more electrodes are located on the same defibrillation lead, a ~hort circuit or a failure in the insulation separating the conductors coupled to the two electrodes need not entirely preclude 'their use in delive~y of subsequent therapies, 80 long as the therapies delivered do not employ the pathway between the two electrodes.-Similar problems associated with epicardial lead ~y~tem~ may al~o be addressed~.
~he present invention is particularly optimized for u~e in conjunction with an iDplantable cardioverter/
defibrillator. It is ~ub~tantially more important in cardioverters and defibrillators than in pacemakers that each individual defibrillation pu}~e or pulse regimen delivered be 'i' effective. Seguential un~uccessful defibrillation attempts are painful, and in the wor~t case may result in failure to terminate fibrillation, leading to serious injury or death.
For this reason, even in the ca~e where a significant change in impedance is noted which would trigger a change in ,...

W093/0~4 2 1 2 1 7 9 '~ PCT/US92/09368 electrode configuration if - the delivered therapy is ineffective, the electrode configuration will remain unaltered if the therapy proves to be effective. In this fa~hion, a known electrode configur~tion which ha~ proven to be effective i S is not prematurely discarded. Further, if preimplant testing ~ of the patient indicates, for example, that the patient is E generally more ea6ily cardioverted or defibrillated using ; multiple electrode configurations, the present invention makes ¦ it possible to check for other available multiple electrode configurations prior to abandoning one of the electrodes and reverting to a single pair of electrodes, which may require higher amplitude pul~es in order to succe~sfully terminate detected arrhythmias.
The invention al~o a6sist~ in accomplishing cardioversion or defibrillation with the least possible energy expenditure.
By reducing the number of shocks given, less energy ic used per cardioversion or defibrillation attempt. By selecting shock pathway~ which are determined to be usable, the unneces~ary repetition of unsucce~sful pulse regimens is avoided. Reducing the nu~ber of unsuccessful defibrillation or ~diover~ion pulses should also result in a shortening of the average duration of cardioversion and defibrillation atte~pts. This should benefit the patient by reducing the time during which the heart is ischemic and should thus reduce the potential da~age to heart tis6ue due to lack of blood 6upply.
The present invention ~ay also be employed in conjunction with an irpedance sensing system specifically directed to detection of open circuits or dead short~, as in the prior art. In thi~ ca~e, open circuit or short detection should require a change in ~ea~ured impedance substantially greater than the increase in i~pedance neces~ary to trigger a change in the ~elected pul~e regi~en. For example, a ~oderate but ~ignificant change in impedance, e.g., S0~, in conjunction with failure to defibrillate may trigger a change in electrode configuration, while a ~ubstantially greater change in , W093/ON~4 2 1 2 1 7 9 ~ PCT/US92/09~

impedance may be used to detect an actual open circuit or fractured lead conductor. Alternatively, measured impedances ~i outside of a predeter ined range may be used as indicative of a short or open circuit. Detection of a short or open circuit, may result in the pathway 80 measured being ~ abandoned, regardless of the efficacy of the delivered therapy.
The present invention measures the impedance during ;' delivery of high voltage cardioversion or defibrillation pulses to detect overall changes in the performance of the c defibrillation pathway between the electrodes, rather than simply detecting a mechanical or electrical failure o~ the electrodes and associated leads. This a~pect of the invention is directed toward optimization of the electrode configuration and pulse regimen then mere operability and should be kept in ~,.J mind when reading the more detailed disclosure of the ~7' invention, below.
The impedance measureaent of the p~esent invention may also be eaployed to adjust the relative amplitude of the defibrillation pulses delivered along the individual shock pathways. For exaaple, in the context of a therapy regimen _ploying multiple pathways, it is dete D ined that one pathway has an acceptable, but significantly higher impedance than the other pathway, a higher voltage pulse may be delivered across ! the high impedance pathway. This should result in a reduced pul~e width for a given pulse energy, and a relatively iincre~sed current density during the pulse. Thi~ aspect of the~invention is best practiced in~a device which employs ~ultiple, independently chargeable capacitor banks 80 that the capacitor banks coupled to individual shock pathways may be charged to different amplitude~ in order to accomplish a more unifo.~ current density throughout the heart during delivery of the defibrillation pulse. Thi8 approach is believed to provide substantial advantages due to the ability to reduce the o~erall energy expenditure required to achieve a current ,.~

W093/0~4 2 1 2 1 7 9 ~ PCT/US92/09368 ~ density across the heart that is sufficient to cause depolarization of a sufficient percentage of heart tissue to terminate the tachycardia of fibrillation episode in progress.

Brief Description of the Drawin~s S The above and still further objects, features and advantages of the present invention will become apparent from the following detailed description of a presently preferred embodiment, taken in conjunction with the accompanying drawings, and, in which:
- 10 Figure 1 is an illustration of an implantable pacemaker/cardioverter/defibrillator of the type in which the pre~ent invention may be embodied, employing a transvenou~/subcutaneous electrode system.
Figure 2 illu~trates a myocardial/epicardial electrode -~ 15 8 y 8 t e m a p p r o p r i a t e f o r u s e w i t h a pacemaker/cardioverter/defibrillator embodying the present invention.
Figures 3a and 3b are schematic block diagrams illustrating the structure of two embodiment~ of an implantable pace~aker/cardioverter/defibrillator in which the present invention may be practiced.
Figures 4a, 4b and 4c are functional flow charts illustrating the method of operation of the present invention, as e~bodied in microproces~or based devices as illustrated in Figure~ 3a and 3b.
Figures Sa and 5b are examples of therapy menus illu~tr~tive of the operation of the present invention.
Figures 6a and 6b are examples of impedance history records illustrative of the operation of the present invention.
. ~ ' De~ailed Description of the Preferred Embodiment Figure 1 illustrate~ an implantable pacemaker/
cardioverter/defibrillator 100 and its associated lead system, .~,.................... .

~,~

~ W093/0~4 2 1 2 1 7 9 5 lo PCT/US92/09~8 as implanted in and adjacent to the heart. AS illustrated, the lead system comprises a coronary sinus lead 110, a right ventricular lead 120, and a subcutaneous lead 130. The coronary sinus lead is provided with an elongated electrode - 5 located in the coronary sinus and great vein region 112, extending around the heart until approximately t~e point at which the great vein turns downward, toward the apex of the heart. The right ventricular lead 120, correspond~ to the lead illustrated in Figure 1, and includes an elongated defibrillation electrode 122, a ring electrode 124, and helical electrode 126, which is screwed into the tissue of the right ventricle at the right ventricular apex. Leads 1~0 and 120 may correspond to the leads disclosed in allowed U.S.
Patent Serial No. 07/284,955 by Bardy for an "Endocardial Defibrillation Electrode Systemn, filed December 15, 1988 and incorporated herein by reference in its entirety. A
subcutaneous lead 130 is also illustrated, generally implanted ~ubcutaneously in the left chest. Lead 130 includes a large surface electrode pad 132, carrying elongated electrode coils 136, 138 and 140. Electrode 132 ~ay correspond to the electrode illustrated in allowed U.S. Patent Application Serial No. 07/376,730, by Lindemans et al. for a Medical Electrical Lead, filed July 7, 1989 and incorporated herein by reference in its entirety.
Figure 2 illustrates an epicardial and myocardial electrode ~ystem for use in conjunction with an implantable paceraker/cardioverter/defibrillator. In this case, two unipolar ~yocardial electrodes 200 and 202 are located on the left ventricle of t~e heart. The~e electrodes may correspond to those illustrated in U.~S. Patent No. 3,737,579, issued to 8O1duc, on June 5, 1973, and incorporated herein by reference in its entirety. Al~o illustrated are three large surface electrodes 204, 206 and 208, spaced around the ventricles of the heart. These electrodes may correspond to the electrodes di~closed in U.S. Patent No. 4,817,634, issued to Holleman et ~ ` .
, ~.
J
'',1 , ~J¢
:",~,, W093/0~4 2 1 2 1 7 9 ~ PCT/US92/~ ~8 ;

al. on April 4, 1989, also incorporated herein by reference in its entirety.
Figure 3d is a functional schematic diagram of an implantable pacemaker/cardioverter/defibrillator in which the present invention may u~efully be practiced. This diagram ~hould be taken a~ exemplary of the type of device in which the invention may be embodied, and not as limiting, a8 it is believed that the invention may usefully be practiced in a wide variety of device implementations, including devices having functional organization similar to any of the implantable pacemaker/defibrillator/cardioverters presently being implanted for clinical evaluation in the United Stites.
The invention is also believed practicable in conjunction with implantable pacemaker/cardioverters/ defibrillators as disclosed in prior U.S. Patent No. 4,548,209, issued to Wielders et al. on October 22, 1985, U.S. Patent No.
4,693,253, issued to Adams et al. on September 15, 1987, U.S.
Patent No. 4,830,006, i~sued to Haluska et al. on May 6, 1989, and U.S. Patent No. 4,949,719, issued to Pless on August 21, 1990, all of which are incorporated herein by reference in their entireties.
The device is illustrated as being provided with cix electrodes, 500, 502, 504, 506, 508 and 510. Electrodes 500 and 502 may be, for example, a pair of electrodes located in the ventricle, for exa~ple, corresponding to electrodes 124 and 126 in Figure 1. Electrode 504 may correspond to a re~ote, indifferent electrode located on the housing of the i~plantable pace~aker/cardioverter/defibrillator.
Electrode~ 506, 508 and 510 may correspond to the large surface area electrodes located on the ventricular, coronary sinu~ and subcutaneous leads illustrated in Figure 1 or to the epi~a~dial electrodes 204,206 and 208 of Figure 2.
Electrodes 500 and 502 are coupled to the R-wave detector circuit, co~prising bandpass filter circuit 514, an automatic 3S gain control circuit 516 for providing an adjustable sensing ~, , ; W093/og ~ PCT/USg2/09368 threshold as a function of the measured R-wave amplitude and a comparator 518. A signal is generated on R-out line 564 whenever the signal sense between electrodes 500 and 502 exceeds the present sensing threshold defined by the automatic ~ 5 threshold adjustment circuit 516. As illustrated, the gain on 'J, the band pa~s amplifier 514 is al~o adjustable by means of a signal from the pacer timing and control circuitry 520 on GAIN
ADJ line 566.
; The operation of this R-wave detection circuitry may correspond to that disclosed in commonly a~signed, copending U.S. Patent Application Serial no. 07/612,760 by Keimel, et al., filed November 15, 1990, for an "Apparatus for Electrical Physiologic Signals, incorporated herein by reference in its entirety. However, alternative R-wave detection circuitry such as that illustrated in U.S. Patent No. 4,819,643, issued to Menken et al. on April 11, 1989 or U.S. Patent No.
4,800,004, i~sued to Baker on November 14, 1989, all incorporated herein by reference in their entireties, may also u~efully be e~ployed to practice the present invention.
For purposes of the present application, it should be under~tood that the threshold adju~t~ent circuit 516 sets a thre~hold corre~ponding to a predeterminea percentage of the a~plitude of a ~en~ed R-wave, which threshola decays to a ~ini~um tbreshold level over a period of less than three ~econd~ thereafter, si~ilar to the automatic sensing threshold circuitry illustrated in the article "Reliable R-wave DQtection from A~bulatory Subjects~, by Thakor et al, publi~hed in 8io~edical Science Instrumentation, Vol. 6, pp 67-72, 1978, incorporated herein by reference in its entirety.
However, in the context ~of the present invention, it is preferable that the threshold level not be adjusted in re~pon~e to paced R-waves, but instead should continue to - approach the minimum threshold level following paced R-waves ~:; to enhance ~en~ing of low level ~pontaneous R-waves associated with tachyarrhythmias. The invention ~ay also be practiced in .

~, .
,.,,,~ .

W093/0~4 2 1 2 1 7 9 ~ PCT/US92/09~8 . 13 conjunction with more traditional R-wave sensors of the type comprising a band pass amplifier and a comparator circuit to determine when the bandpassed signal exceeds a predetermined, fixed sensing threshold.
Switch matrix 512 is used to select which of the available electrode6 are coupled to amplifier 534. Selection of which two electrodes are e~ployed is controlled by the microproces~or 524 via data/address bus 540. Signals from the selected electrodes are passed through bandpass ~mplifier 534 and into multiplexor 532, where they are converted to multibit 'F~~ digital ~ignals by A/D converter 530, for storage in random access ~emory 526 under control of direct memory access circuit 528. Microproce~sor 524 may analyze the digitized ECG signal stored in random access memory 526 to identify waveform characteristics, if desired. The remainder of the circuitry i8 dedicated to the provision of cardiac pacing, cardioversion and defibrillation therapies. The pacer ti ing/control circuitry 520 includes programmable digital counters or ti~ers which control the b~sic time intervals a~ociated with VVI ~ode cardiac pacing, including the pacing os Q pe intervals, the refractory periods during which sensed . R-wave~ are ineffective to re~tart timing of the escape interval~ and the pulse width of the pacing pulses. The duration~ of these intervals are determined by microprocessor 526, and are co~unicated to the pacing circuitry 520 via address/data bus 540. The counters and timers within pacing control circuitry 520 are also used to control the timing and duration of cardioversion and defibrillation pulses under , s control of ~icroprocessor 524. Pacer timing/control circuitry 520 also determines the ampl~itude of the cardiac pacing pulses and the gain of bandpass amplifier, under control of ~icroprocessor 524.
During W I ~ode pacing, the escape interval counter within pacer timing/control circuitry 520 is reset upon ~ensing of an R-wave as indicated by a signal on line 564, and ~ W093/0~4 PCT/US92/09~
; ` 2121795 14 ~i its timeout triggers generation of a pacing pulse by pacer output circuitry 522, which is coupled to electrodes 500 and 502. The escape interval counter is also reset on generation of a pacing pulse, and thereby controls the basic timing of cardiac pacing functions, including anti-tachy pacing. The duration of the interval defined by the escape interval timer i~ determined by microprocessor 524, via data/address bus 540.
The value of the count pre~ent in the escape interval counter when reset by ~en~ed R-waves may be used to measure the duration of R-R intervals, to detect the presence of tachycardia and to determine whether the minimum rate criteria are met for detection of tachycardia or fibrillation.
Microprocessor 524 operates as an interrupt driven device, and is awakened by interrupts from pacer ti~ing/control circuitry 520 corresponding to the occurrence of sensed R-waves and corresponding to the generation of cardiac pacing pul~es. -Thece interrupts are provided via data/addre~ bus 540. Any nece~ary mathematical calculations to be performed by ~icroprocessor 524 and any updating of the values or intervals controlled by pacer timing/control circuitry 520 take place following such interrupts.
In the event that a tachyarrhythmia i8 detected, and an antitachyarrhyth~ia pacing regimen is desired, appropriate ti~ing intervals for controlling generation of antitachy pacing therapies are loaded from ~icroprocessor 524 into the 7 pacer tioing and control circuitry 520, to control the operation of the e~cape interval counter and to define 1 refractory period~ during which detection of an R-wave by the -~ R-wave detection circuitry is ineffective to restart the escape interval counter. Similarly, in the event that generation of a cardioversion or defibrillation pulses -~ required, ~icroprocessor 524 employs the escape interval counter in pacer timing and control circuitry 520 to control timing of ~uch cardioversion and defibrillation pulses, as , W093/0~4 2 1 2 1 7 9 `~ PCT/USg2/09368 well as assoeiated refractory periods during whidh sensed ~-waves are ineffective to reset the timing circuitry.
In re~ponse to the deteetion of fibrillation or a taehyeardia reguiring delivery of a eardioversion pulse, mieroproeessor 524 aetivates eardioversion/defibrillation eontrol eireuitry 554 whieh initiates eharging of the high voltage eapaeitors 556, 558, 560 and 562 via charging eireuit 550, under eontrol of high voltage eharging line 552. The voltage on the high voltage eapaeitors is monitored via VCAP
0 line 538, whieh is passed through multiplexer 532, and, in respon~e to reaehing a predetermined value set by mieroproeessor 524, results in generation of a logie signai on CAP FULL line 542, terminating eharging. Thereafter, timing of the delivery of the defibrillation or eardioversion pulse .5 is eontrolled by paeer timing/eontrol eireuitry 520 under eontrol of mieroproeessor 524. One embodiment of an appropriate system for delivery and synchronization of eardiover~ion and defibrillation pulses, and eontrolling the timing funetions related to them is diselosed in more detail ~0 in eopending, eoDmonly a~signed U.S. Patent Applieation Serial No. 07/612,761, by Keimel, for an ~Apparatus for Deteeting and Treating a Taehyarrhythmia~, filed Nove~ber 15, 1990, ineorporated herein by referenee in its entirety. However, any known eardioversion or defibrillation pulse generation ~5 eireuitry whieh allows ~eleetion among the available large ~urfaee eardioversion or defibrillation eleetrodes is believed u~b}e in eon~unetion with the pre~ent invention. For exa~ple, eireuitry eontrolling the generation of eardioversion and defibrillation pulses as diselo~-d in U.S. Patent No.
~0 4,384,585, i~sued to Zipes on May 24, 1983, U.S. Patent No.
4,949,719, issued to Pless et al. on August 21, 1990, and U.S.
Pate~nt No. 4,357,817, is~ued to Engle et al. on Mareh 8, 1983, all ineorporated herein by referenee in their entireties may al~o be employed. Similarly, known eireuitry for eontrolling ~5 the generation of antitaehyeardia paeing pulses as deseribed , W093/~4 PCT/USs2/09~8 ~ 212179`5 16 in U.S. Patent No. 4,577,633, is~ued to Berkovit~ on March 25, , 1986, U.S. Patent No. 4,880,005, i~sued to Pless et al. on November 14, 1989, U.S. Patent No. 7,726,380, issued to Vollmann et al. on February 23, 1988 and U.S. Patent No.
4,587,970, issued to Holley et al. on May 13, 1986, all of which are incorporated herein by reference in their entireties may also be u~ed.
In the present invention, sequential or ~imultaneous discharging of the first and second capacitor banks (capacitors 556, 558, 560, 562) through one or more pathways defined by electrode~ 506, 608, 510 is accomplished by output circuit 548, under control of cardiovercion/ defibrilla~ion control circuitry 524 via control bus 546. output circuit 548 determines which of the high voltage electrodes 506, 508 and 510 will be employed in delivering the defibrillation or cardioversion pulse regimen, and may also be used to specify a multielect~ode, simultanQous pulse regimen, a multielectrode ~equential pul~e regimen or a pul~e regimen employing only a ~ingle pair of electrode. One example of circuitry which may be u~ed to perfor~ this function i8 set forth in commonly a~signed copending Patent Application Serial No. 07/612,758, for an ~Apparatu~ for Delivering Single and Multiple Cardiover~ion and Defibrillation Pul~e~n, filed by Keimel on November 15, 1990, incorporated herein by reference in its entirety. However, alternative output control circuitry as di~clo~ed in U.S. Patent No. 4,953,551, i~ued to ~ehra et al.
on September 4, 1990 or U.S. Patent No. 4,800,883 issued to Win~tro~ et al. on January 31, 1989, both incorporated herein by reference in their entiretie~, may al~o be u~ed in the context of the pre~ent invention.
Mea~urement of the impedance of an electrode pathway may be perfor~ed u~ing any of a nu~ber of impedance measurement technique~ known to the art. For example, in the case of an implantable cardioverter/defibrillator which regulates the en rgy deliv r d oy controlling the voltage to vhich the W093/09844 2 1 2 1 7 9 ~ PCr/USg2/09368 , . .

~ output capacitors are charged and by regulating the width of the pulse, impedance can be mea~ured by mea~uring the voltage differential between the leading and trailing edges of the pul~e, as set forth in U.S. Patent No. 4,776,338, issued to Lekholm et al. on October 11, 1988 and in U.S. Patent No.
4,140,131, issued to Dutcher on February 20, 1979; both of which are incorporated herein by reference in their entireties. A signal reflecting the voltage on the output capacitors after delivery of the defibrillation pulse i8 readily available on VCAP line 538, accessible to the microprocessor 524 via the A/D converter 530 and data/address bus 540. Following delivery of the defibrillation pulse, the ~icroprocessor may co~pare the amplitude to which the output capacitors were initially charged, typically controlled by the progra~ming of the device, to the voltage remaining after ter~ination of delivery of the pulse, and calculate the iapedance of the pathway over which the pulse was delivered.
Alternatively, the invention may be practiced in cardioverters and defibrillators which regulate the energy delivered by the defibrillation pulse by means of a pulse tilt control, which ter~inates delivery of the pulse when the voltage on the output capacitor either reaches a predetermined threshold or reaches a predeter~ined percentage of the initial - charging voltage. Such systems are di~closed in U.S. Patent No. 4,850,357, issued to Bach on July 25, 1989 and in the above-citea U.8. Patent No. 4,800,883, issued to Winstrom, both of which are incorporated herein by reference in their ntiretie~. In such a~ system, the ~icroprocessor 524 may - either _ploy the counter within the pacer timing/control circuitry e~ployed to regul~ate pacing pulse width and check the count on defibrillation pulse termination or may note the actual ti~es of occurrence of pul~e initiation and pulse ter~ination, and ~ay use the ~easured pulse width in con~unction with the known capacitance of the output capacitors and the known initial charging voltage to calculate WOg3/ON~4 PCT/USg2/09~
` 212179S 18 the impedance of the pathway over which ~the pulse was delivered.
As noted above, pacer timing and control circuitry 520 includes a plurality of counters which time out intervals a~sociated with the bradycardia pacing. These intervals ~ include a bradycardia pacing escape interval, representing the s interval between succes~ive cardiac pacing pulses and between sensed R-waves and the next subsequent cardiac pacing pulses.
At the expiration of the brady pacing escape interval, a ventricular pacing pulse i8 delivered between electrodes 500 and 502. In response to sensing of an R-wave, timing of the q e~cape interval i~ re-initiated. Pacer circuitry 520 also q defines a blanking period, during which R-waves are not sensed by the R-wave amplifier 514 and a refractory period, during ~jlS which R-waves are sensed, but are ineffective to re-initiate timing of the brady pacing escape interval. Signals indicative of the occurrence of sensed R-waves and cardiac pa~ing pulses are passed to microprocessor 524 as interrupts, ; awakening the microprocessor and allowing it to perform any ~20 necessary calculations. Microprocessor 524 specifies the values ti~ed by the timers in pacer circuitry 520 by ~eans of control/data bu~ 540.
R-waves sen~ed by amplifier 514 are employed by ~icroprocessor 524 in performing tachycardia and fibrillation ~25 detection. Tachycardia and fibrillation detection algorithms believed appropriate for u~e in conjunction with the present invention are disclosed in the article nOnset and Stability for Ventricular Tachyarrhythmia Detection in an Implantable Pacer-Cardioverter-Defibrillatorn, by Olson et al., published in Co~puters in Cardiology, October, 7-10, 1986, Pages 167-172, IEEE Computer Society Press and incorporated herein by reference in its entirety. However, the pre~ent invention is al~o believed workable in conjunction with any of the numerous alternative fibrillation and tachycardia detection algorithms known to the art, including those disclosed in the above-cited W093/0~4 1~ 1 2 1 7 9 ~ PCT/USg2/09368 .
U.S. Patent number 4,726,380 issued to Vollmann, U.S. Patent number 4,880,005 is~ued to Pless et al., U.S. Patent number 4,830,006 issued to Haluska et al., and U.S. Patent number 4,523,595 issued to Zipes. Moreover, it is within the scope of the invention to use physiologic sensors to accomplish detection and characterization of tachyarrhythmias to trigger delivery of cardiover~ion or defibrillation pul~es.
~icroproce~sor 524 al~o re~pond~ to interrupts indicating the occurrence of ~en~ed R-waves to determine whether ~L0 previously ~ensed fibrillation or tachycardias which led to the delivery of cardioversion or defibrillation pulses have terminated. In the context of the present invention, ter~ination of tachycardia can be verified by the sensing of a sequence R-R intervals (intervals ~eparating R-waves), each LS of which exceeds a predetermined duration indicative of sinus rhythm. Detection of fibrillation termination may be ~i~ilarly accomplished. Alternatively, any other method of detection of termination of the detected tachyarrh~thmia may be e~ployed, including the use of physiologic ~ensors to ~0 detect a return to normal hemodyna~ic functioning.
Figure 3b illustrate~ the high voltage cardiover~ion/defibrillation pul~e generation pulse generation circuitry and a~sociated control circuitry of an alternative ~bodiment of the device illustrated in Figure 3a. The alternative e~bodiment illustrated is functionally similar to that illu~trated in Figure 3a, with one major difference. In the device as illustratea in Figure 3b, the two capacitor b~nks are provided with independently controllable charging circuits, allowing for them to be charged to different ~0 voltages. As di~cussed above, it is believed desirable to be able to regulate the voltage of defibrillation pulses applied acros~ a defibrillation pathway as a function of the measured impedance of the pathway. In order to accomplish this, it is de~irable to be able to specify independently controllable ~5 charging a~pIitudes for the capacitor banks couples to the ~' .

W093/o9 ~ PCT/US92/Os ~
21217~ 20 individual pathways. In the context of the embodiment of Figure 3b, it is to be understood that the microprocessory 524 (Fig. 3a) ~pecifies voltages for each charging circuit independently, and a~ a function of the mea~ured impedance of the defibrillation pulse pathways. In the device as J~ illustrated, the cardioversion/defibrillation control circuitry 551 corresponds to the cardioversion/ defibrillation control circuit 554 (Fig. 3a), with the exception that it is provided with two outputs corresponding to HVCH line 552 in ~,10 Figure 3a. These are designated as HVCH 1 line 557 and HVCH
'~ 2 line 559. Signals on these lines activate charging circuits 549 and 547, respectively, each of which corresponds to . ., charging circuit 550 as illustrated in Figure 3a. The voltage on the first capacitor bank (556 and 558) is provided on VCAP
1 line 539. The voltage from the second capacitor bank (560 and 562) is provided on VCAP 2 line 537. Like VCAP line 538 Figure 3a, these lines provide inputs to the multiplexor 532 (Fig. 3a) whereby they may be provide inputs to the multiplexor 532 (Fig. 3a) whereby they may be provided to the microprocessor 534 via AlD converter 530 (Fig. 3a).
Cardioversion/defibrillation control circuitry 551 also have two inputs corresponding to CAPFULL line 542 in Figure ~ 3a. These are designated CAPlFULL line 543 and CAP2FULL line '~'J 541. These lines contain signals corresponding to that on CAPFULL line 542, and are provided by A/D converter 530, as di~cu~ed above in con~unction with Figure 3a. These signals indicate that the first capacitor bank (556, 558) and the ~econd capacitor bank (560, 562), respectively have reached the voltage specified by the microprocessor, and function to turn off the charging signal~ on HVCH 1 line 557 and NVCH 2 line 559, respectiVely. Also provided as an input to cardiover~ion/defibrillation control circuitry 551 is the dataladdre~ bus 540 from the microprocessor 524 (Fig. 3a).
By means of signals applied on this bus, the microprocessor control~ the pulse regimen (e.g., sequential, simultaneous, I` wo93/On~4 2 1 2 1 7 g ~ PCT/US92/09368 single) to be provided by the output circuit. This ~- ~ information i8 passed through the eontrol circuit 551 to the output eireuit 553 via eontrol bus 535, which eorresponds to . eontrol bus 546 in Figure 3a. As in the ease of the output eireuit 548 illustrated in Figure 3a, output cireuit 553 may eouple one of the eapaeitive banks aeross output lines HVA and HVC, and the other of the eap~eitor banks aeross output lines HVB and HVC. However, beeau~e the eharging eireuits 547 and 549 are independent from one another, the voltage is applied aeross lines HVA-HVC and HVB-HVC may differ from one another, as a funetion of the impedanee of the defibrillation pathway defined by the eleetrodes to whieh output lines HVA, HVB and I HVC are eoupled.
fj Also illustrated is an optional switeh matrix 555, eontrolled by mieroproeessor 524, (Fig. 3a) via data/address bus 540. Switeh matrix 555 is an optional feature which allows seleetion of whieh of the eleetrodes 506, 508 or 510 are e~upled ~o output lines HVA, HVB and HVC. In an embodi~ent as illustrated in Figure 3b, it is expeeted that the 6witeh ~atrix 555 may be employed to reeonfigure the eleetrode delivery sy6tem, and that the 6tored information as to the eleetrode pathways to be used will be defined in terms of the eleetrodes to be employed, with the output lines HVA, HVB and HVC from output eireuit 553 eoupled aeeordingly.
The embodi~ent illustrated in Figure 3b is believed to be workable in eonjunetion with either a sequential or a si~ult~neou6 pulse regimen, and, as di6eussed below in eon~unetion with the de~eription of Figure 4b, should provide for an inerea~e in the uniformity of eurrent density, a8 well a8 an inerease in the overall flexibility of the system. It is also believed desirable to regulate the voltage applied as a funetion of ~ea~ured impedanee in the eontext of a therapy regi~en whieh employs only a single eleetrode pair. The e~kodi~ent of Figure 3b is of eourse eapable of providing this fe~ture as well.

5.;1 ,~; .
,, ~ wo93/os&w PCT/US92/09~
~ 1217~.) 22 Basic operation of the invention can be understood by reference to the flow chart illustrated in Figures 4a, 4b and 4c. These flow charts are intended to reflect the overall function of the device, rather than any particular software or firmware which must be employed in the device. Because the invention is not dependent upon any particular ~aftw~re or hardware configuration in order to be usefully practiced, the flow charts focus on the important functional aspects of the invention and it6 interrelation to an implantable ; 10 pacemaker/cardioverter/defibrillator which includes fibrillation and tachycardia detection functions and hardware for initiation of pacing, cardiovercion and defibrillai~ion ~ pul~es typical of tho~e in products currently in clinical ;~ investigation in the United States.
~ 15 The flow chart of Figure 4a i~ entered in response to an i interrupt to the microproce~or 524 indicative of a sensed R-wave or the delivery of a pacing-pulse which awakens the ~t ~icroproce~or from it~ ~leep ~tate at 600. One of the functions performed in re~ponse to ~uch an interrupt is the determination at 602 of whether a tachyarrhythmia is pre~ent in the form of either fibrillation or a tachycardia requiri~g delivery of a cardioversion pulse. In the absence of ~uch detection, the ~icroproces~or goes on to update control function~ and time interval6 a~sociated with bradycardia or anti-tachycardia pacing at 604, as may be appropriate. In the pre~ence of a tachyarrhythmia requiring delivery of cardioversion or defibrillation pul~e, the random acce~
~e~ory ~526 is checked at 606 to determine the currently ~chedulQd electrode configuration and defibrillation pul~e regi~en. On initial implant or following reprogramming of the device, the scheduled therapy will be the fir~t therapy on the therapy menu. For example, as illu~trated in Figure 5a, the q~ device could be progra _ ed to initially deliver a simultaneous pul~e defibrillation regimen, with a ~econd simultaneous pulse 3S defibrillation regimen and ~ingle pulse regimens as fallback W093/0~4 ~ 1 2 1 7 g i PCT/US92/09 therapies. Alternatively, the device may be initially programmed to provide a sequential pulse therapy as illustrated in Figure 5b, with a second sequential pul8e regimen and single pulse regimens as backups.
S The microproce~sor 524 determines the pulse pathways associated with the scbeduled therapies at 608 and scans the appropriate i~pedance histories stored in random acce~s memory i 526, as indicated at 610. If the currently scheduled therapy includes a pathway marked "bad", as indicated at 612, the currently scheduled therapy is canceled, and the therapy i8 either deleted from the therapy menu, or otherwise designated as unavailable at 614. In the therapy menus illustrated in Figures Sa and Sb, both of the first listed therapies are designated as unavailable.
lS The microprocessor 524 next checks to see whether any available therapies remain on the therapy menu at 616. If not, the ~icroproce~sor returns to the portions of its ~oftware dedicated to control of bradycardia and tachycardia pacing functions at 532. If an available therapy is found, it is retsieved at 618 and it too is checked to deter~ine whether the pathways as~ociated with the therapy have been ~arked ~bad~ at 608. A~su~ing that no pathways employed in the new therapy have been arked as ~bad~, the therapy is designated a~ the currently ~cheduled therapy regimen at 620 and is delivered at 622, Nea~urement of the impedance along the pathways e~ployed in delivering the therapy is taken at 624.
Thi~ rea~ured i~pedance i~ ~tored in an i~pedance hi~tory log of the type illustrated in figures 6a and 6b, along with the ti-e of therapy delivery as indicated in figures 6a and 6b.
At this point, the ~icroproces~or awaits ~ubsequent ventricular sensing interrupte and ventricular pacing interruptC in order to allow it to deter~ine whether the delivered therapy was successful in terminating the tachyarrhyth-ia. As discussed above, a typical mechanism for detection of ter~ination is the presence of a predetermined W093/09~ PCT/US92~09368 21217~ 24 number of sequential measured R-R intervals in excess of either the detection criteria indicative of the occurrence of the tachyarrhythmia, or a ~eries of R-R intervals otherwi~e indicative of a return to normal sinus rhythm.
Alternatively, termination may be detected using a hemodynamic ~ensor, such as a pressure sensor, which may be used to identify a return to a normal cardiac output. If the measured impedances did not deviate more than the desired predetermined percentage at 626 from the previously measured impedances, and d~ 10 the therapy was ineffective to terminate the tachyarrhythmia at 628, the therapy will typically be reapplied with the energy level incremented until the maximum available energy level has been reached, as indicated at 642.
; In the event that the measured impedance change did exceed the predetermined percentage at 626, and the tachyarrhythmia was redetected at 630, the microprocessor ~ marks the pathway displaying the excessive impedance change as r~. ~bad" at 636. Optionally, the pul~e amplitude for the next ~., therapy is incremented at 637. The previously delivered therapy is then marked unavailable at 614.
In the event that tachyarrhythmia is not redetected following delivery of the therapy, regardless of whether the detected change in impedance exceeded the predetermined percentage, the therapy delivered may remain scheduled as the current therapy and rerains available on the therapy menu.
The ~icroprocessor, in this case, may return to that portion ¦ Of it~ prograrming devoted to tachycardia and bradycardia pacing. However the measured impedances may optionally also b2 co~puted to predetermined impedances "A~ and "B", as illu~trated at 638 and 640. These impedances are either fixed iopedance~ which are felt to conclu~ively indicate a short circuit or an open circuit or impedances reflecting a percentage change substantially in excess of the impedance change threshold at 626. In response to such a detected extre~e impedance, the microprocessor may optionally label the W093/o~W 2 1 2 1 7 ~ ~ PCT/US92/09~8 pathway involved as bad at 636 and îndicate the therapy involved to be unavailable at 614 regardless of the euccess of the therapy in terminating the arrhythmia.
In Figure 4a, at 626, 638 and 640, the measured impedances are compared to previously measured impedances in order to determine whether a substantial change ha~ occurred.
These previously measured impedances may be impedances as initially measured in the fir~t time the pathway i8 used, for example impedance measurement~ taken during initial testing .0 associated with the implant of the device. Alternatively the prior impedance measurements may be made after implant and may represent the most recent measurement or the average of the mo~t recent set of measurements. Yet another alternative would be to use programmed reference values set by the .S physician in place of actual measurements, and compare the current mea~ured impedances to these reference values.
Figures 5a and 6a, together, provide an illustrative example of the operation of the present invention. As indicted in 5a, the phy~ician has programmed the therapy menu ~0 by specifying two ~i~ultaneous pulse regimens and two single pulse regiaens. The impedance hi~tory in Figure 6a illustrates the re~ults of applying the therapies on the therapy menu. The first two ti es that therapy number one is applied, it is succesQful, and the measured variation in ~5 iapedance is le~s than the predetermined percentage of change ~pecified at 626 (Figure 4). The third time the simultaneous pul~e regimen i~ delivered, the impedance shows a ~ignificant change, being reduced from 60 to lS ohms for the combined i~pedance acro~ the electrode ~ystem and the delivered pul~e~
~0 are un~uccessful in terminati~ng the detected tachyarrhythmia.
Rather than retry the therapy at a higher amplitude, the device instead changes its electrode configuration and pulse regi~en to correspond with theràpy number two, marking therapy number one as unavailable in Figure Sa and marking the current ~5 pathway as~ociated with the therapy as bad in Figure 6a.

W093/09&~ PCT/US92/09368 212179~ 26 After redetection of the tachyarrhythmia, therapy number two i~ applied, and it is successful in terminating the tachyarrhythmia, allowing the pathway to remain marked Hgood"
in the impedance history, and allowing therapy number two to remain available on the therapy menu.
For example, the therapies referred to in Figure Sa may correspond to therapies available for delivery using an electrode ~ystem having a coronary sinus electrode (HVA), a subcutaneous plate electrode (HVB) and a right ventricular electrode (HVC). In response to the failure to terminate in conjunction with a measured impedance change exceeding the predetermined percentage specified, the device reconfigures its electrode configuration to deliver pulses using the right ventricular and coronary sinus electrodes tied together, and ~lS a pulse delivered between these two electrodes and the subcutaneous plate electrode (HED), indicated as therapy two.
Because the coronary sinus and right ventricular electrodes are tied together during delivery of thi~ therapy anyway, their clo~e ~pacing or contact is not problematic in the context of this particular pulse regimen. Figures Sband 6b may illu~trate a corre~ponding therapy menu and impedance hi~tory for device progra~med by the physician to initially deliver pul~es in a ~eguential pulse, multi-electrode regimen as ~et forth at 5b. Again, it may be assumed that a coronary ~inus (HVA), a subcutaneous (HVD) and a right ventricular electrode (HVC) are u~ed. Similar to the sequence illustrated in con~unction with Figures 5a and 6a, the first two attempts to deliver therapy number one are successful, and the third a ff empt i~ unsuccessful, coupled with a measured increase in the impeaance along one of the two defibrillation pathways, as illustrated in Figure 6b. In response to detection that the pulse pathway between the coronary sinus and right ventricular electrode has developed a rapid increase in impedance, in conjunction with a failure to terminate the sensed tachyarrhythmia, the device changes to a ~econd sequential ;
~ W093/0~4 2 1 2 1 7 9 ~ PCr/US92/09368 pulse defibrillation therapy number two, in which the HVA-HVC
pathway is not used. As indicated in Figure 6b, the first time this therapy is tried, it is successful, allowing both pathways associated with delivery of the therapy to remain - 5 marked as "goodn.
~;~ It should be noted with regard to Figures 6a and 6b that the impedance histories are illustrated as retaining only the three most recent impedance measurements along the particular pathway involved. However, a more lengthy measurement of the ~ 10 impedance record may also be provided if desired. Further, `~ while the method discussed above envisions comparing the measured impedance with the i D ediately preceded impedance, it ~ay in some cases be desirable to compare the measured impedance with an average of two or more previously measured impedances to determine whether the change in impedance should be considered ~ignificant.
Figures 4b and 4c illustrate optional additional portions of the operative flowchart of Figure 4a. As illustrated, the flowcharts of Figures 4b and 4c would be inserted between blocks 620 and 622 in Figure 4a. The flowcharts of Figures 4b and 4c illustrate the additional processing required in the ca~e of an erbodiment as illustrated in Figure 3b, in which pulse amplitudes are independently selectable for individual defibrillation pathways. For purposes of the discussion of ~25 Figure 4b, it ~hould be assu~ed that in addition to ~ programming a therapy menu indicating a preferred order of ¦ pathways and pulse regimens to be employed, the device also works in the fashion of presently available implantable c~rdioverter~/defibrillators, and provides a specified pulse amplitude for ~ach selected therapy, which pulse amplitude increases in response to the failure of a delivered therapy to accomplished cardioversion or defibrillation. This is reflected at 642 in Figure 4a.
The initial amplitude for each defibrillation therapy 3S type and the suCceeding~ increased amplitudes are typically .. . .

~ W093/09 ~ PCT/US92/09~

preset by the physician by programming. Alternatively, the device may simply automatically increase the amplitude of predetermined percentage until such time as the maximum available a plitude has been reached. In either ca6e, a defined series of pulse amplitudes is provided, which may be used in conjunction with the circuitry of Figure 3b in two alternative methods to control the voltage of the defibrillation pulses actually delivered across the pathways employed in the selected therapy regimen.
The first alternative approach is illustrated in Figure 4b. In Figure 4b, it is to be assumed that the defined voltage i8 intended to be the maximum voltage available for application. In this case, the software of Figure 4b is entered following block 620 in Figure 4a. The microprocessor checks at 700 to determine whether a multiple path pulse regimen (e.g., simultaneous or sequential) has been selected.
If not, a single pulse pathway regimen has been selected, and the microproces~or returns to the flowchart of Figure 4a at 622, allowing for delivery of the single pathway, single pulse regimen using the predefined voltage. However, if a simultaneou~ or ~equential pulse regimen has been selected, the microproce~sor checks at 702 to determine whether impedance mea~ure ents have been made for both pathways to be e~ployed. If 80, the microprocessor adjusts the pulse amplitude at 704 using the measured impedance values to provide a more uniform current distribution. For example, the prograr~Qd pul~e amplitude may constitute the maximum available pul~e ~plitude, which would be applied across the h~gher i~pedance pathway, with the voltage applied acro~s the lower iqpedance pathway equal to the maximum voltage mNltiplied by the ratio of the lower pathway impedance to the higher pathway i~pedance. Alternatively, the programmed defined voltage may constitute the minimum voltage, to be ~ appliéd acros~ the lower impedance pathway, with the voltage to be applied across t:e higher impedance pathway equal to the , ~, W093/0~4 2 1 2 1 7 ~ ~ PCT/US92/0936$

programmed voltage multiplied by the impedance for the high impedance pathway divided by the impedance for the low impedance pathway. In either case, a more egual current density should be accomplished.
A second approach is illustrated in Figure 4c. The flowchart of Figure 4c presumes that the microprocessor will ~ adjust the voltage of the defibrillation pulse regimen, 2 regardless of whether it is a single or multiple pathway regimen. After selection of a therapy type at 620 (Fig. 4a), ~ 10 the microproces~or may check to ~ee whether the impedance of ¦ the pathway or pathways involved in the defibrillation pulse regimen selected have been previously measured. If so, t~ese measured values are used to adjust the output voltage. In this case, the microprocessor may assume that the programmed ~ 15 or physician specified voltage for the therapy is based upon ¦ an assumption of a reference impedance value, for example 50 or 100 ohms. The actual impedance across the pathway may be co~pared to the mea~ured impedance, and the voltage of the defibrillation pulse to be applied across the pathway recalculated to provide a pulse correeponding to a pulse of the programmed amplitude and pulse duration or tilt, applied across the reference i~pedance value. Thus, if the measured impedance is less than the reference impedance, the microprocessor will specify a lower voltage to be applied across that pathway than the prograr~ed voltage. If the i~pedance of the pathway is higher than the reference impedance, the microprocessor will specify a higher voltage than program~ed. This voltage adjusted system is as applicable to single pulse, single pathway defibrillation pulse therapies as to multi-electrode, multiple path defibrillation pulse therapies.
Turning to the flowchart of Figure 4c, the flowchart is entered following selection of the therapy type to be delivered at 620, and, if impedance amplitude measurements are , found to be present for all pathways at 710, new values for . .

W093/0~4 PCT/USg2/09~
2121795 30 ` '~"
~ the pulse voltages are calculated at 712. If, on the other ~;
hand, there are no pre-existing measurements for the impedance, the programmed pulse amplitudes will be employed and the impedance mea~urement taken in conjunction with S delivery of the therapy at 622 and 624 (Fig. 4a) will be used to allow for adjustment of the defibrillation pulse voltage in subsequent applications of the same therapy or other therapies employing the measured pathways.
The above specification and the embodiments disclosed are intended to allow one of skill in the art to incorporate the present invention into a modern implantable cardioverter/defibrillator. However, it is of course understood that the particular implementation of the invention '~ will vary depending upon the particular underlying circuitry types and software systems employed. As such, the above disclosure should be considered exemplary, rather than limiting with regard to the claims that follow.

~`
~A
~, , ~'' : .
~"~
~,.
,.

", ,.;
:.,. i

Claims (7)

1. An implantable or defibrillator comprising means (514, 520, 524) for detecting the presence of a tachyarrhythmia, a pulse generator (548) for delivering a cardioversion or defibrillation pulse regimen in response to detection of a tachyarrhythmia by said detection means, electrode means (506, 508, 510) for delivering said cardioversion or defibrillation pulse regimen to a heart, said electrode means comprising a plurality of electrodes (506, 508, 510), said electrodes defining a plurality of pulse pathways between ones of said electrodes, and means (554) for selecting which of said defibrillation pathways are to be employed during delivery of said defibrillation or cardioversion pulse regimen, characterized in that:
said cardioverter or defibrillator further comprises means (537, 537, 539, 532, 530) for measuring the impedance of each of said selected pathways during delivery of said cardioversion or defibrillation pulse regimen, means (524, 526) for comparing each said measured impedance associated with delivery of said cardioversion or defibrillation pulse regimen with a reference impedance for each said selected pathway to determine whether said measured impedance differs by more than a predetermined amount from said previously measured impedance, means (524) for determining whether said cardioversion or defibrillation pulse regimen was effective to terminate said detected tachycardia, and means (524, 526, 554) for rendering any selected pathway for which said measured impedance differs by more than said predetermined amount from said reference impedance unavailable for delivery of future cardioversion or defibrillation regimens, provided that said cardioversion or defibrillation pulse regimen is determined to be ineffective to terminate said detected arrhythmia.

2. An implantable cardioverter or defibrillator according to claim 1 wherein said means for selecting further comprising means for selecting a defibrillation or cardioversion pulse regimen for subsequent use which does not employ said unavailable pathway.

3. An implantable cardioverter or defibrillator according to claim 1 or claim 2 further comprising memory means (526) for storing said measured impedances and wherein said reference impedance comprises an impedance measurement stored in said memory means.

4. An implantable cardioverter or defibrillator according to claim 1 or claim 2 further comprising means (554, 530) for incrementing the amplitude of a subsequent cardioversion or defibrillation pulse regimen in response to said determining means determining that a previous cardioversion or defibrillation pulse regimen was ineffective to terminate said detected tachyarrhythmia.

5. An implantable cardioverter or defibrillator according to claim 1 or claim 2 further comprising means (554, 530) for adjusting the amplitude of a subsequent cardioversion or defibrillation pulse regimen in response to the measurement of the impedances of said selected pathways during delivery of said cardioversion or defibrillation pulse regimen.

6. An implantable cardioverter or defibrillator according to claim 1 or claim 2 wherein said selecting means comprises means responsive to said comparing means to define an alternative cardioversion or defibrillation pulse regimen employing all of said plurality of electrodes previously selected to deliver said cardioversion or defibrillation regimen and which does not employ any said selected pathway for which said measured impedance differs by more than said predetermined amount from said reference impedance.

7. An implantable cardioverter or defibrillator comprising means (514, 520, 524) for detecting the presence of a tachyarrhythmia, pulse generator means (548) for delivering a cardioversion or defibrillation pulse in response to detection of tachyarrhythmia by said detection means, and electrode means (506, 508, 510) for delivering said cardioversion or defibrillation pulse to a heart, said electrode means comprising a plurality of electrodes (506, 508, 510), said electrodes defining a plurality of pulse pathways between ones of said electrodes, means (524) for defining a first cardioversion or defibrillation pulse regimen and for selecting (554) a set of said electrodes and a set of said defibrillation pathways to be employed during the delivery of said first cardioversion or defibrillation pulse regimen, characterized in that:
said defibrillator or cardioverter further comprises means (537, 538, 539, 530) for measuring the impedance of each of said selected pathways during delivery of said first cardioversion or defibrillation pulse regimen and means (524, 554) for setting the relative amplitudes of subsequent cardioversion or defibrillation pulses applied along said selected pathways as a function of the relative impedances measured by said measuring means.