Alterations induced by a single defibrillating shock applied through a chronically implanted catheter electrode.

In ten beagles ranging in weight from 7.4 to 13.0 kg, a defibrillating shock of 10 A (three dogs), 20 A (four dogs), or 30 A (three dogs) intensity was applied through a chronically implanted right ventricular catheter electrode. Ten-lead ECG, right ventricular electrogram, and right ventricular impedance were recorded prior to, immediately following, and 48 hours post-shock. A single shock of 10 A, 20 A, and 30 A intensity succeeded in defibrillating nine of ten dogs. One dog required two 20 A shocks to defibrillate. No shock was fatal. Post-shock arrhythmias increased in duration and severity as the shock strength increased. ECG vector analysis suggested damage to the right ventricle in eight of ten dogs. The impedance signal amplitude increased directly after the shock, but dropped below control level by 200 seconds post-shock and remained below control by 48 hours post-shock. Pale areas of shock-induced myocardial necrosis were concentrated in the right ventricular walls adjacent to the distal electrode. The mean weight of necrotic myocardium was 0.043 + 0.006 grams at 10 A, 1.203 + 0.268 grams at 20 A, and 1.397 + 1.218 grams at 30 A (mean + sd). Defibrillation was effective after long-term implantation. The alterations sustained from defibrillation were minimized by using a low intensity shock.