The inclusion of low energy cardioversion capability into modern implantable antiarrhythmic devices, although an appealing idea, is nevertheless unproven with regard to its potential benefits. Moreover, since occasional reports have surfaced suggesting that ineffective application of low energy shocks may prejudice subsequent arrhythmia reversion, we examined the effectiveness and risks of this feature in a large series of patients performed as part of a US Food and Drug Administration clinical trial performed under an investigational device exemption. A total of 813 induced monomorphic ventricular tachycardias were studied in 244 patients. We found that many of the arrhythmias could be reverted to sinus rhythm with small amounts of energy. Cardioversion energy was less-than-or-equal-to 6 joules (J) for 84 (53.2%) and less-than-or-equal-to 14 joules in 105 (66.4%) of the 158 patients tested at implant and subsequently remained unchanged through > 4 months follow up. The incidence of noncardioversion, acceleration or both occurred in 12.7%, 5.7%, and 13.1%, respectively on a per patient basis. On a per episode basis, nonconversion occurred in 51 (6.3%) and acceleration in 61 (7.5%) of the 813 inductions. There was no correlation between the occurrence of nonconversions and accelerations. The devices were allowed to recycle in the event the arrhythmia was not reverted. The subsequent shock was almost always effective, and in any event, no patient failed to be reverted by the second 30-J rescue shock. Over the entire follow-up period as long as 17 months, there were 17 deaths. Neither the incidence nor the mode of death was correlated with nonconversion or acceleration. We conclude that low energy cardioversion is effective in treating monomorphic ventricular tachycardia and if unsuccessful, does not prejudice the success of subsequent higher energy therapy.
