1168 July 2001 PACE, Vol. 24

Case ReportA 19-year-old man with sustained ventricular

tachycardia secondary to a benign left ventriculartumor underwent transvenous defibrillator im-plantation using a single coil, dedicated bipolarlead (Medtronic Model 6936, Minneapolis, MN,USA) in the right ventricular apex and a singlecoil lead (Medtronic Model 6937) in the left in-nominate vein. A Medtronic 7219 Jewel generator(a nonactive can device) was implanted in a leftprepectoral pocket. Using a step-down protocol,the defibrillation threshold was 12 J using theright ventricular electrode as the anode (B . AX).The high voltage lead impedance at 12 J was 59ohms. During follow-up the patient had multipleshocks and was started on amiodarone with amaintenance dose of 400 mg/day.

In June 1999 the patient underwent generatorreplacement due to normal battery depletion. AMedtronic 7227Cx generator (an active can device)was connected to the chronic leads. Defibrillationthreshold testing revealed that the defibrillatorthreshold was greater than the maximum output ofthe device (35 J) using either polarity. The highvoltage lead impedance was 40 ohms. Comparisonof the current chest X ray with one done followingthe original implant 4 years ago did not show anyevidence of lead migration. The 7227Cx generatorwas implanted despite the unacceptably high de-fibrillation thresholds. Amiodarone was discon-tinued and the patient was started on sotalol. Re-peat noninvasive defibrillation testing 6 weeks

later demonstrated that the defibrillation thresh-olds were still greater than 35 J.

Revision of the defibrillator system was per-formed the next day. The chronically implantedleads were connected to a Medtronic DefibrillatorImplant Support Device (DISD), which mimicsthe function of the original nonactive can 7219generator. Defibrillation threshold testing via theDISD done using a step-down protocol and theright ventricular electrode as the anode (B . AX)demonstrated successful defibrillation at 24 J and18 J. A Medtronic 7221 (nonactive can) generatorwas then implanted. Defibrillation threshold test-ing via the implanted device was repeated using astep-down protocol. Defibrillation was successfulat 24 J and 18 J but unsuccessful at 12 J. The highvoltage lead impedance with an 18 J shock was 61 ohms.

DiscussionTransvenous lead configurations using an ac-

tive can result in significantly lower defibrillationthresholds compared to lead configurations usingan inactive can.1,2 The mechanism by which theactive can lowers defibrillation thresholds is prob-ably due in large part to the larger surface area ofthe can, which decreases the impedance. In addi-tion, the left-sided position of the can may shiftthe electrical fields to encompass more of the leftventricular mass. The incidence of not being ableto successfully implant an active can system dueto high defibrillation threshold was 0% in oneprospective series1 and , 2% in two other se-ries.2,3 The majority of the patients who had unac-ceptably high defibrillation thresholds with an ac-tive can system were able to have a nonactive cansystem successfully implanted although the finallead configuration (including the number andtype of leads) and the final defibrillation thresh-olds were not described.2,3

Unexpectedly High Defibrillation Thresholds inan Active Can Implantable CardioverterDefibrillator SystemARUN RAO, DWIGHT W. REYNOLDS, and KAREN J. BECKMANFrom the Cardiac Arrhythmia Research Institute and the University of Oklahoma Health SciencesCenter, Oklahoma City, Oklahoma

RAO, A., ET AL.: Unexpectedly High Defibrillation Thresholds in an Active Can Implantable CardioverterDefibrillator System. This case report describes a patient undergoing defibrillator generator replacementin whom the defibrillation thresholds were significantly higher with a can-active system than with a non-active can system. (PACE 2001; 24:1168–1169)

defibrillation thresholds, implantable cardioverter defibrillators

Address for reprints: Karen J. Beckman, M.D., Cardiac Ar-rhythmia Research Institute, University of Oklahoma HealthSciences Center, 1200 Everett Dr. (TUH 6E-103), OklahomaCity, OK 73104. Fax: (405) 271-7455; e-mail: karen-beck-man6ouhsc.edu

Received September 26, 2000; revised December 7, 2000; ac-cepted January 26, 2001.

Reprinted with permission fromJOURNAL OF PACING AND CLINICAL ELECTROPHYSIOLOGY , Volume 24, No. 7, July 2001Copyright © 2000 by Futura Publishing Company, Inc., Armonk, NY 10504-0418.

HIGH DFT IN ACTIVE CAN ICD

PACE, Vol. 24 July 2001 1169

In this patient, the addition of the active canto the already implanted transvenous lead systemraised the defibrillation threshold by almost 20 J,despite the fact that the high voltage leadimpedance had decreased by 19 ohms. The reasonfor this is unclear. The 7219, 7221, and 7227CxMedtronic generators all use a biphasic waveformwith a 65% tilt. The 7219 and the 7221 generatorshave minimally larger capacitors than the 7227Cxgenerator (132 uF vs 130 uF using a DC rating, per-sonal communication from Medtronic TechnicalSupport); however, this is unlikely to account forsuch a large difference in defibrillation efficacy,especially since the delivered energy was actuallyhigher with the active can system. The most likelyexplanation is that the active can shifted the areaof peak current density superior and/or anterior

compared to the original configuration, resultingin ventricular areas with low voltage gradients in-sufficient to defibrillate a critical mass of my-ocardium or residual high voltage gradients al-lowing fibrillation to be reinitiated.4

As the older nonactive can defibrillator gen-erators that have been implanted prepectorallyreach their elective replacement indicators, mostof them will be replaced with active can genera-tors with the goal of further reducing defibrillationthresholds. Virtually all new defibrillator im-plants use the active can system. In patients whereunexpectedly high defibrillation thresholds areencountered using an active can system, it may beworth evaluating a nonactive can configurationbefore adding additional leads or subcutaneouspatches.

References1. Haffajee C, Martin D, Bhandari A, et al. A multi-center randomized

trial comparing an active can implantable defibrillator with a pas-sive can system. PACE 1997; 20: 215–219.

2. Gold MR, Foster AH, Shorofsky SR. Effects of an active pectoral-pulse generator shell on defibrillation efficacy with a transvenouslead system. Am J Cardiol 1996; 78: 540–543.

3. Bardy GH, Yee R, Jung W, et al. Multicenter experience with a pec-toral unipolar implantable cardioverter-defibrillator. J Am Coll Car-diol 1996; 28: 400–410.

4. Hillsley RE, Wharton MH, Cates AW, et al. Why do some patientshave high defibrillation thresholds at defibrillator implantation?Answers from basic research. PACE 1994; 17: 222–239.