Factors Determining Vulnerability to Ventricular Fibrillation Induced by 60-CPS Alternating Current

Very weak, 60-cps alternating current applied directly to the heart can cause ventricular fibrillation; greater current strength is required to induce ventricular fibrillation with a single shock delivered during the vulnerable period of the cardiac cycle. The present experiments were designed to examine the basis of this difference. The studies were performed in anesthetized open-chest dogs. The current strength required to induce ventricular fibrillation was 0.52± 0.16 ma with 60 cps for 5 sec, and 15.0±4.3 ma with a single square-wave pulse of 10 msec. However, when the duration of 60 cps was reduced to produced only 4, 3, 2, or1 ventricular response, the threshold for ventricular fibrillation after the last response was 0.4, 2, 18, or 24 ma respectively. After any given number of successive premature ventricular responses, the fibrillation threshold was comparable with either 60 cps or a single pulse. We concluded that, if an accelerating ventricular tachycardia that is produced by 60-cps stimulation is of sufficient duration (5 or 6 beats), the VF threshold is reduced progressively after each premature ventricular response, thus making it possible to include VF with very weak current.