Mechanism of the Vulnerable Period in a Model of Cardiac Fibrillation

Vulnerable Period . The mechanism of the vulnerable period for induction of fibrillation was investigated with a computer model of propagated excitation. The model included nonuniform cycle length dependent refractoriness and slow propagation during incomplete recovery. Recovery properties that permitted induction of fibrillation by a single stimulus were identified, and fibrillation usually occurred after stimuli later than the earliest stimulus resulting in propagation. The responsible mechanism was slow propagation near the stimulus site during excitation from the earliest effective stimulus. The time required for That propagation permitted sufficient recovery in surrounding regions that excitation propagated without reentry. Later stimulation was associated with more rapid propagation near The stimulus site, so surrounding regions were invaded while recovery was markedly nonuniform. This resulted in nonuniform propagation and self sustained reentrant excitation simulating fibrillation. Nonuniformity of premature propagation was quantitated and increased with increasing stimulus cycle length before progressively decreasing. The time of maximal increase of nonuniformity became later with increasing mean value of disparate recovery, but was not affected by The range of disparity. Findings provide a possible explanation for failure of The earliest propagated premature response to initiate fibrillation and suggest That prolonged recovery is associated with vulnerability to late premature responses. ( J Cardiovasc Electrophysiol, Vol. l, pp. 303–308, June 1990 )