Defibrillation Thresholds in ICDs

Abstract & Commentary

By John P. DiMarco, MD, PhD, Professor of Medicine, Division of Cardiology, University of Virginia, Charlottesville. Dr. DiMarco is a consultant for Novartis, and does research for Medtronic and Guidant.

Synopsis: DFT reassessment after the institution of antiarrhythmic drug therapy with amiodarone or sotalol is not routinely required.

Source: Hohnloser SH, et al. Effect of Amiodarone and Sotalol on Ventricular Defibrillation Threshold: The Optimal Pharmacological Therapy in Cardioverter Defibrillator Patients (OPTIC) Trial. Circulation. 2006;114:104-109.

The Optimal Pharmacological Therapy in Cardioverter Defibrillator Patients (OPTIC) trial was a study which compared the effects of beta blockers, beta blockers plus amiodarone, and sotalol on arrhythmia frequency in implantable cardioverter defibrillator (ICD) recipients. OPTIC evaluated the effects of the 3 treatment arms on defibrillation energy requirements. At 4 centers, all OPTIC patients were approached to participate in the defibrillation threshold (DFT) substudy. Patients were excluded from participation in the substudy if they had signs of class IV congestive heart failure, if their left ventricular ejection fraction was less than 0.20, or if they had received amiodarone or sotalol before participation in the trial. DFT testing was performed at the time of device implantation and approximately 2 months after initiation of drug therapy. A stepdown protocol was used with 50V decrements, and the voltage threshold converted to joules for comparison with other published studies. All shocks were biphasic, and the shock duration was automatically adjusted to deliver 65% tilt shocks for each phase.

The majority of patients in the DFT substudy were male, with a mean age of 65 years. Most had a history of myocardial infarction, with only 7 of 94 patients having nonischemic cardiomyopathy. Baseline DFT values were low in all 3 groups: 8.77 ± 5.1 in the beta blocker, 8.53 ± 4.3 in the amiodarone plus beta blocker group, and 8.09 ± 4.8 in the sotalol group. At follow-up DFT testing, there were slight decreases in both the beta blocker and sotalol groups and a slight increase (1.29 ± 4.4 joules) in the amiodarone group. Of all patients who had a baseline DFT greater than or equal to 10 joules, none had an increase of more than 10 joules on repeat testing. One patient who had a baseline DFT of 2.5 joules had an increase of greater than 10 joules, with a follow-up DFT of 19.5 joules. This patient had been assigned to receive amiodarone. All patients had an adequate safety margin of at least 10 joules maintained during drug therapy. Eight clinical variables (age, left ventricular function, sex, type of heart disease, spontaneous vs induced ventricular arrhythmia, unmonitored syncope, ventricular effective refractory period, and intracardiac QRS duration) were tested to see if they could serve as predictors of baseline DFT. None of the variables were independent predictors.

Hohnloser and colleagues conclude that their results do not support the practice of DFT reassessment after the institution of antiarrhythmic drug therapy with amiodarone or sotalol.


When ICDs were first introduced, it was noted that many antiarrhythmic drugs, particularly those with sodium channel blocking activity, raised defibrillation thresholds. This led to a recommendation that defibrillation threshold be repeated if an antiarrhythmic drug was introduced after initial testing. Since that time, however, there have been numerous improvements in ICD technology. Virtually all systems now use active biphasic defibrillation waveforms with transvenous lead systems. Charge times up to maximum energy have also been greatly improved, and now there is only a trivial delay even if the maximum shock energy is programmed. The data presented here further suggest that routine testing, if an antiarrhythmic drug is introduced after implant, is not necessary.

One limitation to this study is the relatively small number of patients with nonischemic cardiomyopathy or those with very advanced heart failure. In patients with very large hearts, particularly, those undergoing resynchronization therapy, higher defibrillation thresholds than those seen in this study are not uncommon. Although the data here are reassuring, we still should remain cautious about whether these observations can be applied.