The Use and Interaction of PermanentPacemakers and the Automatic ImplantahleCardioverter Defibrillator

ANDREW I. COHEN, MARC H. WISH, ROSS D. FLETCHER,FREDERICK C. MILLER, DANIEL MCCORMICK, JOHN SHUCK,NADIV SHAPIRA, ALBERT A. DELNECRO

The cardiology and surgical services. Veteran's Administration Medical Center Medical Center,Walter Reed Army Medical Center and Georgetown University Hospital, Washington, DC

COHEN, A., ET AL.: The use and interaction of permanent pacemakers and the automatic implantablecardioverter defibrillator. The adverse interactions of permanent pacemakers and automatic implantablecardioverter defihrillators (AICD) were studied in nine patients in whom both devices were implanted. Bothunipolar and bipolar pacemakers were evaluated. The permanent pacemakers were also used to dononinvasive elecfrophysioiogical studies and to induce venfricuiar/ibriJiation. L/ndersensing o/ventricu-lar jibriJiation by the permanent pacemakers caused inappropriate pacemaker stimuli, which causedundersensing of ventricular fibrillation by the AICD in three of four patients with unipolar pacemakers.After an AICD discharge, pacemaker noncapture was seen in eight of 22 episodes for an average 4.9seconds and inability to sense was seen in 11 of 20 episodes for an average 9.0 seconds. Counting ofpacemaker stimuli and QRS by the AICD caused inappropriate discharges. Noninvasive e/ectrophysiologi-cai testing by the pacemakers correlated with invasive testing. Furthermore, induction of ventricuJar/ibriliation was successful in four of five patients attempted, though requiring long bursts at high outputs atthe shortest cycle lengths obtainable by these pacemakers. Operation of the AICD and permanent pace-makers must be clearly understood to avoid adverse interactions of these devices. (PACE, Vol. 11,June 1988]

automatic implantable cardioverter dejibrillator, pacemakers

IntroductionThe automatic implantable defibrillator

(AICD) (CPI, St, Paul Minnesota) has had increas-ingly wide application since its introduction intoclinical use in 1980 by Mirowski.' Several studieshave shown the device to improve survival in pa-tients who have had aborted sudden death epi-sodes or recurrent ventricular tachycardia.^^ Al-though the concurrent need for pacemakers hasbeen rare,^ case reports have noted adverse inter-actions between the implantable defibrillator andimplanted pacemaker, causing both inappropriatedischarges of the AICD as well as inhibition of a

Address for reprints: Andrew Cohen MD, Cardiology (151D),50 Irving St. NW. Washington DC 20422

Received September 10. 1987; accepted September 11. 1987

discharge during ventricular tachycardia or fibril-lation.̂ "^^ No previous study has prospectivelyevaluated the frequency or extent of pacemaker-AICD interactions. We prospectively studied theinteraction of permanent pacemakers with theAutomatic Implantable Cardioverter Defibrillatorin nine patients with a total of 12 pacemakers, andtbe use of permanent pacemakers in these pa-tients to perform noninvasive electrophysiologi-cal studies.

MethodsStudy Patients

The nine patients in this study were from thegroup of all patients who underwent implantationof Automatic Implantable Cardioverter defibrilla-tor studied in three institutions. The patients

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AICD AND PERMANENT PACEMAKERS

were studied under projects approved by the In-stitutional Review Boards of the hospitals (ap-proved June 1984) and informed consent was ob-tained.

The patients were all males. Seven had coro-nary artery disease, one had a dilated cardiomy-opathy and one had no structural heart disease.The mean ejection fraction was 36% with a rangeof 20-70%. Four patients were resuscitated fromsudden cardiac death (mean 1.7 episodes) withventricular fibrillation being the initial rhythmrecorded. One patient was resuscitated from sud-den death with ventricular tachycardia being theinitial rhythm recorded. The other four patientshad recurrent sustained ventricular tachycardiarefractory to multiple medial trials. Seven of thepatients had failed clinical trials of amiodarone.

Eight of the patients had pacemakers im-planted prior to implantation of the AICD device.Three pacemakers were implanted to terminaterecurrent sustained ventricular tachycardia (twowith patient-initiated programming devices, onewith an automatic antitachycardia pacemaker).Two patients had pacemakers implanted for Hisbundle to ventricular activation times (H-V) ofmore than 100 milliseconds [one of these patientshad syncope prior to any documented arrhyth-mias). Two patients had tachycardia/bradycardiasyndrome with prolonged pauses, two patientshad symptomatic bradycardia, one of whom hadprolonged pauses secondary to amiodarone re-quiring permanent pacing.

The 12 pacemakers implanted were: Med-tronic Spectrax (8420 one unit, 5984 two units),Medtronic Symbios [7005 three units, 7006 threeunits). Pacesetter AFP (283 one unit, 262 oneunit), and Intermedics Intertach [one unit). Fourpacemakers were unipolar, eight bipolar fivewere single chamber, and seven were dualchamber pacemakers. Three patients with unipo-lar pacemakers had pacemakers subsequentlychanged to bipolar units.

Preoperative Electrophysioloqical Evaluation

All patients underwent invasive electro-physiological testing with programmed extrastim-uli prior to implantation of the AICD. Ventricularpacing was done at drive cycles of 600 msec. 500msec and 400 ms with three premature extra-

stimuli at each drive cycle. If pacing did not in-duce sustained ventricular tachycardia (30 sec-onds or requiring intervention because of hemo-dynamic instability) or ventricular fibrillation,then the protocol was repeated at the right ven-tricular outflow tract. Two of the three patientswhose arrhythmia was not induced from thesetwo sites were also stimulated from the left ven-tricle and during isoproterenol infusion.

Intraoperative Evaluation

Implantation of the AICD was done via one ofthree approaches: an epigastric transdiaphrag-matic thoracotomy [1), a median sternotomy (5) orleft lateral thoracotomy (3). One patient also un-derwent multiple vessel coronary artery bypassgrafting as part of the procedure. All patients hadepicardial placement of two patch electrodes (fordelivery of the discharge and morphology sens-ing) and two epicardial electrodes for rate sensingby the AICD unit. The epicardial electrodes wereplaced 1-2 cm apart, at sites that minimized pace-maker potential recordings [usually on the leftventricular apex). In one patient the epicardialelectrodes were placed on the basal anterior leftventricle in an attempt to minimize recordings ofthe ventricular stimulus from a dual chamberunipolar pacemaker. The two epicardial patcheswere placed to include as much of the heart aspossible in the defibrillation current pathway.They were always placed on nonscarred myocar-dium as judged by prior contrast left ventriculog-raphy and visually at the time of surgery.

Simultaneous recordings were made of atleast three surface electrocardiographic leads, thebipolar electrogram of the rate sensing epicardialelectrodes and the bipolar electrogram of the epi-cardial patches. Recordings were made on a VR16multichannel recorder [Electronics for Medicine,Pleasantville, New York) at paper speeds of50-100 mm/s. Filters were set for frequencies be-tween .04 and 2500 Hz on the multichannel re-corder with preset filters of 0.8-750 Hz on theexternal cardioverter-defibrillator (CPI, St. Paul,Minnesota) through which the epicardial leadand patch recordings were made.

The QRS and pacemaker spike amplitudeswere measured from the epicardial electrodes

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COHEN, ET AL.

and patches. The time from pacemaker spike tuQRS on the epicardial leads was recorded on allpatients who had pacemakers implanted prior toAICD insertion.

All patients were induced into ventricular fi-brillation a minimum of two times for a determi-nation of defibrillation threshold during AICDimplantation. Induction of ventricular fibrillationwas accomplished by pacing with a train of 80beats at 16 ms cycle length and 8 ms pulse width.Pacemaker sensing function during ventricular fi-brillation was analyzed as well as pacing andsensing function after the discharge of both theexternal cardioverter defibrillator and of theAICD. Pacemaker stimuli were set at approxi-mately four times diastolic threshold (with pulsewidths set between 0.5-1.0 ms).

Postoperative Electrophysiological Studies

Seven of the patients had electrophysiologi-cal testing in an eiectrophysiology laboratory afterAICD implantation. At this time, a phonogram ofthe AICD audible signal, synchronous withsensed events, was recorded while the unit was inan "EP" mode with the magnet in place over theAICD generator. Invasive induction of ventricularfibrillation was accomplished using the sameprotocol as in the operating room.

In five patients, the implanted pacemakerswere used for noninvasive induction of ventricu-lar fibrillation using customized programmers aspreviously described.^'^ Standard programmedextrastimulus studies were used.^ If ventricularfibrillation was not induced, long trains of extra-stimuli at maximum pacemaker output were de-livered if the standard noninvasive protocol wasunsucccessful in inducing ventricular fibrillation.To do these noninvasive studies the AICD wasplaced in an "EP" standby mode because thesepacing stimuli and resulting ventricular depolar-izations could satisfy tachycardia detection crite-ria and cause an AICD discharge if left in an activemode.

Pacemaker waveforms were recorded on anoscilloscope (Tektronics, Beaverton, Oregon) fromskin electrodes prior to and immediately (within 3minutes) after an AICD discharge. Waveformswere also recorded 5 and 15 minutes after dis-charge. Pacemaker capture thresholds were mea-

sured prior to induction of ventricular fibrillationand immediately (within 3 minutes) after defi-brillation with the implanted AICD.

Results

Of the nine patients, eight had pacemakersimplanted prior to insertion of either their initialAICD generator or a second unit which was im-planted for end of life parameters. Two patientshad one pacemaker in place during the initialAICD implantation and a different, second pace-maker in place at the time of AICD replacement.Three of the four patients with nnipolar pace-makers (Pacesetter 283, or Medtronic 7005) hadgenerator charges to bipolar because of pace-maker-AICD interactions. The fourth patient diedin the immediate postoperative period due tocomplications of surgery.

Electrophysiological Measurements

The time from pacemaker ventricular stimu-lus to onset of rapid defiection of the QRS was 50± 29 ms on the patch leads and 90 ± 35 ms on thebipolar epicardial leads in eight of the patientsrecorded. In one patient the time from stimulus toonset of rapid deflection of the QRS was 150 ms.

The amplitude of the QRS complexes andpacemaker artifacts were recorded as noted onTable I. The amplitude of the atrial stimulus wasgenerally small on the epicardial rate sensingleads. The atrial stimulus on the patches tended

Table I.

Amplitude of Pacemaker Stimuli and QRS Potentials

Rate MorphologySensing SensingLeads Patches

QRS AMPLITUDE (MV)

ATRIAL STIMULUS (MV)unipolarbipolar

VENT. STIMULUS (MV)unipolarbipolar

11.0 ±6.99 10.5+ 4.32

1.9± 1.151.5 ±0.70

13.9 ± 6.573.9 ± 2.00

17.8 ± 15.704.7 ± 4.60

13.0 ± 2.603.4 ± 2.20

MV = millivolts; Vent. = ventricular values are mean ± standarddeviation.

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AICD AND PERMANENT PACEMAKERS

to be larger with patches that were placed on theright atrium (two patients), or on the right ventri-cle (six patients). The ventricular stimulus wassignificantly larger in patients with unipolarpacemakers than bipolar pacemakers on both therate sensing leads (13.9 mV versus 3.9 mV. p< .004) and on the morphology sensing patches(13.0 mV, vs 3.4 p < .0003).

Pacemaker Inhibition of the AICD

Thirty episodes of ventricular fibrillationwere analyzed. Lack of appropriate sensing by thepacemaker caused pacemaker stimuli during thefibrillation, during 24 episodes in seven patients.In three patients (all with unipolar pacemakers),the inappropriate pacing caused inhibition of theAICD discharge. In two of these three patients,pacemakers were implanted prior to the AICD al-lowing recordings of the pacemaker stimuli am-plitudes and QRS. In these two patients, the pace-maker stimulus to QRS amplitude ratios were 3.5and 2.1 on the bipolar epicardial leads and 1.2 and1.1 on the patch leads respectively. The mean forall other patients was 0.2 on the bipolar leads and0.3 on tbe patches.

Five patients had ventricular tachycardia in-duced: all episodes were appropriately sensed bythe pacemakers causing inhibition of output (ex-cept in the patient with an Intertach which re-verted to its antitachycardia mode).

Pacemaker Function After AICD Discharge

Pacemaker function after AICD dischargesfor ventricular tachycardia or ventricular fibrilla-tion was analyzed after 37 episodes. For 31 ana-lyzable tracings, the immediate rhythm afterAICD discharge was paced in ten episodes, ven-tricular in 18 episodes and supraventricular intwo episodes. Pacemaker nonsensing of ventricu-lar complexes post defihrillation as evidenced byan inappropriate pacemaker stimulus was presentin 11 of 20 analyzable episodes (Fig. 1). The meanduration of lack of sensing was 9.1 ± 11.6 secondsin those units which failed to sense appropriately.

Lack of ventricular capture was observedeight times from 22 analyzable tracings (Fig. 1).The mean duration of lack of capture was 4.9± 5.07 seconds, one patient had no capture for upto 16 seconds after discharge.

Pacemaker waveform analysis revealed nodifferences in the pre- to postdischarge trailing toleading edge ratios or voltage amplitudes. Simi-larly, no differences in ventricular pacing thresh-olds were noted after discharge when measuredwithin 3 minutes.

Inappropriate AICD Discharges

AICD inappropriate discharges occurred in avariety of circumstances because of sensing of oneor more pacemaker stimuli and the ventriculardepolarization. If the sensed events exceeded therate cutoff of the AICD, it would discharge. In onepatient with a dual chamber pacemaker, the timedelay from ventricular stimulus to ventricular de-polarization exceeded the refractory period of theAICD (150 ms) and double counting of each pacedbeat (sensing of both the pacemaker spike andresultant QRS) would occur in the VVI mode. Tri-ple counting of atrial stimulus, ventricular stimu-lus and ventricular depolarization would occur inthe DVI mode if the atrial to ventricular delayexceeded 150 ms in this patient (Fig. 2). Thisproblem persisted after the pacemaker systemwas converted from unipolar to bipolar. All pa-tients with unipolar dual chamber pacemakersand one with a bipolar dual chamber pacemakerhad double counting with AV delays that ex-ceeded 150 ms. The patient with the dualchamber pacemaker whose atrial spike wassensed by the AICD had an atrial spike to QRSratio of 1.3 on the rate sensing leads. The atrialspike to QRS ratio was .12 in patients whose atrialspike was not sensed by the AICD. InappropriateAICD discharges also occurred during capturethreshold testing of the pacemaker when lack ofcapture occurred. This was because of AICDcounting of pacemaker stimuli and underlyingrhythm. Also, application of a magnet to the pace-maker which caused fixed rate pacing caused aninappropriate discharge in one patient becauseasynchronous pacing allowed the resulting rate ofpacing artifacts and native depolarizations to ex-ceed the rate limit of the AICD.

Noninvasive Induction of VentricularFibrillation

Noninvasive induction of ventricular fibrilla-tion was attained in four of the five patients it wasattempted. This required decremental ramp pac-

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COHEN, ET AL.

PACEMAKER MALFUNCTION AFTER AICD DISCHARGE

" I I fl I I I I I I I I I I I I I Mil

RATE LEAD

PATCHES

J\J\J\J\J

PACEMAKER

LACK OF SENSING

LACK OF CAPTURE

Figure 1. Pacemaker malfunction after AICD discharge. Illustrated are leads I, II, II VI, V6,epicardial bipolar leads used for AICD rate sensing and two epicardial patches used for AICDdischarge delivery. After delivery of an AICD discharge for induced ventricular tachycardia, twoventricular pacemaker spikes are seen farrows), Both spikes occur too soon after previous QRS andare consistent with pacemaker lack of sensing. The ^rst spike also occurs after the expectedventricular refractory period and is consistent with lack of capture.

TRIPLE SENSING

w

6

MARKERPHONOGRAM

Figure 2. Triple sensing of the atrial paced and ventricular paced rhythm by the AICD. TheMARKER CHANNEL™ is telemetered by the permanent pacemaker illustrating atrial and ven-tricular pacing. The phonogram records the audible signal emitted from the AIGD when a sensedevent occurs. The atrial spike, ventricular spike and resultant QRS are all being sensed by theAICD.

708 June 1988 FACE. Vol. 11

AICD AND PERMANENT PACEMAKERS

NONINVASIVE INDUCTION OF VF AND DELAYED DISCHARGE

cont.'^\,HSj\/V^^^V)/S/-f\/\f->^-/^^

cont.

cont.^WV\^^A/WVwvv.,,,A/^/^AVV^^

cont.

AID DISCHARGE

EP MODE VF DUR. 31.4 S

Figure 3. Noninvasive induclion of VF and delayed discharge. Lead V6 is illustrated, withconlinuous tracings. A decrementoi ramp of eight beats at 351 ms. jive beats at 225 ms, five beatsat 177 ms and 14 beats at 162 ms are iJJustrafed with resultant ventricular /ibriliation. AICDdischarge was delayed because of use of the "EP" mode and the discharge was delivered at 31.4seconds after onset of ventricular jibriliation.

ing (Fig. 3) at cycle lengths as short as 127 ms. Thetime to discharge the AICD in the active modeduring invasive EPS was almost half that of theAICD after the EP mode (15.8 ± 3.8 seconds activemode, 31.5 ± 1.6 seconds after " EP" standbymode).

Discussion

AICD pacemaker interactions were initiallynoted during the clinical investigation phase ofthe AICD prompting the manufacturer to recom-mend against using unipolar pacemakers in com-bination with the AICD. Several of our patientshad pacemakers implanted prior to implantationof the AICD allowing evaluation of these interac-tions. Additionally, we found the use of bipolar

pacemakers did not uniformly eliminate theproblems found with concurrent use of both de-vices, AICD pacemaker interactions fell into threecategories: AICD oversensing of normal pacedrhythms, pacemaker undersensing of tachyar-rythmias. and pacemaker malfunction after AICDdischarge.

AICD Oversensing of Paced Rhythms

Oversensing of pacemaker stimuli occurredmainly, although not exclusively, in patients withunipolar pacemakers. Ideally, a ratio of the pace-maker stimulus amplitude to ventricular depolar-ization could be recommended which wouldavoid these problems. However, the automaticgain control on the AICD changes the filter fre-

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COHEN, ET AL,

quency as well as the gain of the electrogramsfrom both the epicardial rate sensing leads andepicardial patches. The ratio of the QRS and pace-maker stimulus amplitude may not be constant asseen by the AICD. Therefore, recordings madethrough the leads and external cardioverter defi-hrillator only approximate the values seen by theAICD.

Undersensing of Ventricular Tachyrhythmias

In the three patients whose pacemaker stim-uli during ventricular fibrillation inhibited theAICD discharge, the ratio of pacemaker artifact toQRS as recorded on the bipolar epicardial leadswas two to three times that of the remainder ofthe patients. The automatic gain control, adjustedfor the high amplitude pacemaker stimuli,blinded the AICD to the lower amplitude fibrilla-tory waves. The amount of time delay of the AICDdischarge would depend on the frequency ofpacemaker stimuli as well as the relative ampli-tudes of stimuli and fibrillatory waves. No pa-tients with bipolar pacemakers had any apparentdelay to discharge of the AICD hecause of the rel-atively small stimulus artifact.

All cases of induced ventricular tachycardiawere adequately sensed by the permanent pace-maker in the present series. However, we havepreviously presented data on certain pacemakermodels which may fail to adequately sense ven-tricular tachycardia and thus could potentiallydelay or inhibit AICD discharge.

Pacing and Sensing Problems After AICODischarge

Pacemaker function after AICD dischargewith lack of adequate sensing and capture wasfrequent in both unipolar and bipolar pace-makers. Waveform analysis of the pacemakerstimuli failed to show any changes after the AICDdischarge, implying that the problem was not inthe pacemaker or lead. However, most waveformswere obtained approximately between 1-3 min-utes after AICD discharge and malfunction wasgenerally seen for less than 10 seconds. Transientabnormalities of the waveform may have beenmissed. Another possibility is that the high cur-rent of the AICD is partially shunted through thepacemaker electrodes because of the zener diodes

present in permanent pacemakers.The zenerdiode prevents current from flowing hack to thepacemaker when critical voltages are exceeded(after the time constant of the zener diode isreached).Although this may prevent damage tothe pacemaker from defibriUation energy, it mayalso allow high currents to flow through the elec-trodes. Studies by Yee et al,^°''' have shown thatpacing capture thresholds may double and Rwave amplitudes may half when electrodes areused for both defibrillation and pacing. In theirstudy, abnormalities persisted for up to 10 min-utes when 5-50 joules were delivered. Presum-ably the current shunted through pacemakerelectrodes would be lower and the abnormalitiesless pronounced in the patients in this study, Yeepostulated that polarization of the electrodes, gasbubble formation or electrical injury to the myo-cardium may be the mechanism of amplitude andthreshold changes. Polarization of titanium- orplatinum-tipped electrodes is likely to be lessthan .5 volts {because of the material) and there-fore unlikely to cause the abnormahties noted. Itis unlikely that enough energy is shunted to causegas bubble formation, and the bubbles them-selves are too transient to account for the changesseen. We therefore postulate that changes occurat the tissue level after defibrillation which causethe pacing and sensing changes seen,

Noninvasive Induction of VentricularFibrillation

The induction of ventricular fibrillationusing permanent pacemakers was difficult in thispopulation of patients. Long trains of rapid burstsat the minumal cycle lengths allowed by theseunits (as short as 127 ms) at the maximum pace-maker output were required. The induction wassuccessful in four of the five patients attemptedafter multiple attempts.

Future Devices

Future devices will likely combine pacing ca-pabilities, including antitachycardia modes, withimplantable automatic defibrillators. In those de-vices, the pacemaker spike could be eliminatedfrom the tachycardia detection algorithm. How-ever, further work may be necessary to elucidatethe pacing capture and sensing problems that

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AICD AND PERMANENT PACEMAKERS

occur on pacemaker leads after a high energyshock is delivered.

Recommendations

We recommend that all patients who requireimplantation of permanent pacemakers andAICDs should be carefully evaluated at the timeimplantation. Only bipolar pacemakers should beused, although both single and dual chamberpacemakers may be implanted. Lead potentials ofthe pacemakers and QRS amplitudes sbould berecorded and an attempt should be made to mini-mize the relative amplitude of tbe pacemakerstimulus (or stimuli with dual chamber pace-makers) to the QRS amplitude. In our group ofpatients, a pacemaker artifact of 5 the height ofthe native QRS or less was adequate to avoid lackof AICD recognition of ventricular fibrillation.This may be done by closely spacing the leads ofthe pacemaker (if placed epicardially) and AICD(if placed epicardially) and by attempting to placethe two pairs of leads distant from each other. The

time delay from pacemaker artifact to resultantQRS must be measured and kept below 150 ms toavoid double sensing. Pacemaker function duringventricular fibrillation should be appropriate. Ifpacemaker stimuli (implying lack of sensing) arefrequent during the arrhythmia, the sensitivity ofthe pacemaker may have to be increased. Pace-maker function after AICD discharge should beevaluated and the stimulus output or sensingthreshold sbould be adjusted if problems occurafter discharge of the AICD. Ventricular fibrilla-tion should be induced while tbe pacemaker is inan asynchronous mode to test the ability of theAICD to recognize the arrhythmia during pace-maker firing. The operation of both the AICD andthe pacemakers must be completely understoodand tested to avoid adverse interactions of thesedevices.

Acknowledgmenf: We thank Dr, Stan Bach and Dr. EdPlatia for their thoughtful reviews and comments of thismanuscript.

References

Mirowski M, Reid PR, Mower MM. et al. Termina-tion of malignant ventricular arrhythmias with animplanted automatic defibrillator in humanbeings. N Engl I Med 1980; 303:322-4.Mirowski M, Reid PR, Winkle RA, et al. Mortalityin patients with implanted automatic defibrilla-tors. Ann Intern Med 1983; 98:585-8.Echt DS, Armstrong K, Schmidt P, et al. Clinicalexperience, complications and survival in 70 pa-tients with the automatic implantable cardio-verter/defibrillator. Circulation 1985; 71:289-96.Kun SG, Furman S, Waspe LE, et al. Unipolarpacer artifacts induced failure of an automatic im-plantable cardioverter/defibhllator to detect ven-tricular fibrillation. Am ] Cardiol 1986; 57:880-1.Chapman PD, Troup P, The automatic implant-able cardioverter, defibrillator: evaluating sus-pected inappropriate shocks, JACC 1986; 7:1075-8Fletcher RD. Cohen Al, DelNeggro AA, Noninva-sive electrophysiologic studies using implantedpacemakers. In Barold SS (ed): Mount Kisco, NewYork, Futura Publishing Company, 1985, pp.421-38,Wish MH. Fletcher RD, Cohen AI, et al. A new

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advancement in noninvasive electrophysiology: Astandard laboratory stimulator pulse coupled withan implanted pacemaker. PACE 1986; 9:1089-1100,Cohen AI, Fletcher RD, Wish MH, et al. Noninva-sive electrophysiology using standard permanentpacemakers in patients with spontaneous sus-tained ventricular tachycardia or ventricular fi-brillation. Am I Cardiol 1987; 59:564-67DelNegro AA, Cohen A, Miller F, et al. Automaticextension of the ventricular refractory period: acause of sensing failure during tachycardia in dualchamber pacemakers,(abstract) PACE 1986; 9:304.Yee R, Joses DL, Klein GJ, Pacing thresholdchanges after transvenous catheter countershock.Am J Cardiol 1984; 53;503-7,Yee R, Jones DL, Jarvis E, et al. Changes in pacingthreshold and R wave amplitude after trans-venous countershock, lACC 1984; 4:543-9.Bardy GH, Coltori F, Ivey TD, et al. Some factorsaffecting bubble formation with catheter me-diated defibrillator pulses. Circulation 1986;73:525-38.

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