Diastolic intervals accompany increased cycle length following two minutes fibrillation

Following short fibrillation durations, fibrillation action potentials occur immediately upon repolarization from the previous action potential. However, Fourier analysis shows that frequency of fibrillation decreases with fibrillation duration and that the decrease is correlated with the difficulty of defibrillation. We tested the hypothesis that the decrease in fibrillation frequency is accompanied by the appearance of diastolic intervals between fibrillation action potentials. Fibrillation was induced in isolated rabbit hearts (n=5) and perfusate immediately clamped to make the hearts ischemic. ECGs and monophasic action potentials (MAPs) were recorded. Cycle length increased from 81.8±6.5 following 5 seconds fibrillation to 111.6±5.9 msec following 1.5 minutes fibrillation (p<0.004). Action potential duration (APD90) was 73.5±8.3 msec at two minutes. The period of diastolic resting potential was 42.8±6.7 msec. Our results suggest that the mechanism underlying fibrillation changes as fibrillation duration increases due to the developing ischemia