Relation of vulnerability to ECG and action potential characteristics of premature beats

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Abstract
Premature stimuli applied to the ventricles of isolated perfused rabbit hearts induced vulnerability in 39% of 33 hearts tested with 1 premature stimulus and in 86% of 28 hearts tested with 2 premature stimuli. The stimuli that induced the multiple and the earliest single responses had similar strength. However, the stimuli that induce multiple responses could be applied earlier. We compared the electrocardiograms and the ventricular transmembrane action potentials, recorded distally to the stimulating electrodes, in the earliest single responses and in the responses preceding multiple spontaneous beats (MSB) or ventricular fibrillation. The QRS interval was significantly longer in the responses preceding MSB. The temporal relation between the ORS duration and the effective refractory period of the single fiber suggested that re-entry could occur during the propagation of an early premature impulse. During inscription of R and S in the early premature beats, depolarization of ventricular fibers began before they had been completely repolarized. This could induce vulnerability by slowing the conduction and shortening the refractory period.