EFFECTS OF PREMATURE RESPONSES ON VULNERABILITY TO FIBRILLATION IN A COMPUTER-MODEL

The purpose of this study was to determine the effects of premature re sponses on vulnerability to fibrillation using a computer model based on the wavelet hypothesis. The model simulated propagation, nonuniform recovery of excitability, and slow propagation during incomplete reco very. Vulnerability was assessed as the fibrillation threshold, which was defined as the duration of train stimulation required to initiate self-sustained reentrant excitation with multiple excitation fronts. T he fibrillation threshold was determined at various premature cycle le ngths in the presence of refractory periods of varied range and durati on, at various rates, and after compensatory pauses and varied pattern s of consecutive premature responses. The fibrillation threshold was f ound to be reduced by premature responses, and with increasing prematu re cycle length, there was an initial decrease followed by an increase of fibrillation threshold. The fibrillation threshold was directly re lated to the duration and indirectly related to the range of the refra ctory period. The time phase of curves relating premature cycle length and fibrillation threshold was such that premature responses at some cycle lengths were associated with a lower fibrillation threshold in t he presence of longer refractory periods, with slower rates, and with an immediately preceding compensatory pause. The mechanism may be impo rtant in the proarrhythmia effects of drugs that prolong repolarizatio n and in the bradycardia-tachycardia syndrome. Consecutive premature r esponses at a constant rate increased the fibrillation threshold in co mparison with the initial response, while consecutive responses at an accelerating race decreased the fibrillation threshold.