Relationship between shock energy and postdefibrillation ventricular arrhythmias in patients with implantable defibrillators

Postdefibrillation Ventricular Arrhythmias. Background: The relationship between postdefibriliation ventricular arrhythmias and shock strength is poorly understood in patients with implantable defibrillators. The purpose of this study was to characterize the relationship between postdefibrillation ventricular arrhythmias and shock strength. Methods and Results: Forty-three patients with an implanted defibrillator underwent six separate inductions of ventricular fibrillation (VF) after a step-down defibrillation energy requirement (7.3 ± 4.6 J) was determined. For each of the first three inductions of VF, the first two shocks were low energy and equal to approximately 75% of the defibrillation energy requirement (5.4 ± 3.3 J), or to the defibrillation energy requirement plus 10 J (17.5 ± 4.3 J). After the first two shocks, subsequent shocks were programmed to the maximum available energy (29.0 ± 2.5 J). The alternate technique was used for the subsequent three inductions of VF. Postdefibrillation ventricular arrhythmias were noted. Postdefibrillation ventricular arrhythmias with a cycle length ≤ 300 msec were more frequent after a low-energy shock (19%), than after a high- energy shock (1.5%; P = 0.005). Postdefibrillation ventricular arrhythmias with a cycle length > 300 msec were more frequent after a high-energy shock (32%), than after a low-energy shock (7.1%; P = 0.002). A relationship between the cycle length of the postdefibrillation ventricular arrhythmias and the absolute defibrillation energy was observed (P < 0.001; r = 0.6), and ventricular arrhythmias with a cycle length > 300 msec were uncommon after shocks ≤ 10 J (P = 0.001). The characteristics of ventricular arrhythmias after maximum-energy shocks were similar to those that occurred after high- energy shocks. Conclusions: Postdefibrillation ventricular arrhythmias with a cycle length ≤ 300 msec are more common after shocks of strength associated with a low probability of successful defibrillation. Postdefibrillation ventricular arrhythmias with a cycle length of > 300 msec are more common after high- and maximum-energy shocks, and are directly related to the absolute defibrillation energy.