COMPARISON OF THE VULNERABLE PERIODS AND FIBRILLATION THRESHOLDS OF NORMAL AND IDIOVENTRICULAR BEATS

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Abstract
The spans of the reactive and vulnerable periods of large and small beats of left ventricular origin were studied. Such left ventricular beats were occasionally or continuously evoked by excitation of the left ventricle. D.C. shocks (.01-.02 sec.) of varying intensity were applied at various moments of such beats. Results indicated that even in fresh hearts the vulnerable period of premature beats extends nearly to the end of the isometric relaxation process; and that the fibrillation threshold is essentially the same as for beats of supraventricular origin. Since such idioventricular beats probably have a greater asynchronicity in termination of fractionate contractions, this suggests that sensitivity to fibrillation is not related to the degree of asynchronicity in the termination of contraction in ultimate cardiac units.