Transthoracic defibrillation in 100-kg calves with sequentially applied pulses.

The effectiveness of up to five 50-A, 4-ms rectangular pulses (each nominally 50% successful) followed, when required, by up to six 70-A, 6-ms pulses applied at 15-s intervals in reversing ventricular fibrillation in 100-kg calves was studied in 600 episodes in which 50-A, 6-ms; 70-A, 3-ms; 70-A, 6-ms; 70-A, 12-ms; or 100-A, 6-ms prefibrillation shocks had been delivered 5 s before the induction of fibrillation and in another 600 episodes in which the prefibrillation shock was omitted. We found that 1) there was a modest adverse influence of the prefibrillation shock upon the outcome of the 50-A, 4-ms portion of the sequential shock effort; 2) The 50-A, 4-ms shocks remained reasonably successful throughout the five-shock sequence; 3) when the prefibrillation shock was omitted, the time intervals for a return of a ventricular complex and normal sinus rhythm in the postdefibrillation electrocardiogram increased rapidly with the number of shocks required for defibrillation; and 4) if defibrillation was not achieved with the five-shock sequence, a single 70-A, 6-ms shock was about 94% successful and the sequence of up to six shocks was 100% successful.