The effective refractory period (ERP) of His Purkinje tissue is known to oscillate following sudden changes in stimulus frequency. It has been proposed that the oscillation of ERP is caused by oscillation of the diastolic interval. This hypothesis was tested in vitro in canine cardiac Purkinje fibers superfused with normal and hyperkalemic (10 mM KCl) Tyrode's solution. The ERP of responses to premature stimuli, responses to postextrastimuli, and responses to postmature stimuli were measured. ERP of premature responses and postextrasystolic responses oscillated in normal Tyrode's solution. Oscillations of ERP of premature responses and postextrasystolic responses were damped in hyperkalemic Tyrode's solution. Determinations of the restitution of ERP as a function of diastolic interval revealed that oscillation of ERP of premature and postextrasystolic responses was for the most part predictable from the diastolic interval during superfusion with normal Tyrode's solution. In contrast, both hyperkalemic Tyrode's solution and postmature stimuli shifted the restitution curves, resulting in ERPs that were markedly longer than those predicted from the diastolic interval alone. Shifts in the restitution curves completely accounted for damping of the oscillation of ERP in hyperkalemic Tyrode's solution and with postmature stimuli. These data may relate to alterations of refractoriness during sudden changes in ventricular rate, such as occur during atrial fibrillation or ventricular tachycardia.
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