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

Postdefibrillation Ventricular Arrhythmias. Background: The relationship be tween postdefibrillation ventricular arrhythmias and shock strength is poor ly understood in patients with implantable defibrillators. The purpose of t his study was to characterize the relationship between postdefibrillation v entricular arrhythmias and shock strength. Methods and Results: Forty-three patients with an implanted defibrillator u nderwent six separate inductions of ventricular fibrillation (VF) after a s tep-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 require ment (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 progr ammed to the maximum available energy (29.0 +/- 2.5 J). The alternate techn ique was used for the subsequent three inductions of VF. Postdefibrillation ventricular arrhythmias were noted. Postdefibrillation ventricular arrhyth mias,vith a cycle length less than or equal to 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 defibrillat ion energy was observed (P < 0.001; r = 0.6), and ventricular arrhythmias w ith a cycle length > 300 msec were uncommon after shocks less than or equal to 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 less than or equal to 300 msec are more common after shocks of strength as sociated with a low probability of successful defibrillation. Postdefibrill ation ventricular arrhythmias with a cycle length of > 300 msec are more co mmon after high- and maximum-energy shocks, and are directly related to the absolute defibrillation energy.