Durations of Transmembrane Action Potentials and Functional Refractory Periods of Canine False Tendon and Ventricular Myocardium: Comparisons in Single Fibers.

• The relationship between the functional refractory periods of the specialized cardiac conduction system and of the ventricular muscle is of major importance in the genesis of ventricular fibrillation by a single premature beat arising in the specialized conducting system. Wiggers and Wegria demonstrated that premature ventricular excitation must occur in late systole during the inscription of the T wave in order for ventricular fibrillation to be initiated by a single premature stimulus, i.e., excitation must occur in the functional refractory period of ventricular fibers. If the functional refractory period of Purkinje tissue exceeds that of the ventricular muscle, an early premature discharge in the Purkinje system or an impulse transmitted from the A-V node could not excite the ventricles sufficiently early to cause ventricular fibrillation; excitation of ventricular muscle via the Purkinje system would by necessity occur after expiration of the ventricular vulnerable period. However, if the Purkinje tissue has a functional refractory period equal to or less than that of ventricular muscle, a premature systole conducted over