Supernormal excitability as a mechanism of chaotic dynamics of activation in cardiac Purkinje fibers.

Supernormality, which can be defined as greater than normal excitability during or immediately after action potential repolarization, has been observed in a variety of cardiac preparations. However, as yet, no description of the dynamics of tissue response to repetitive stimulation in the presence of supernormal or relatively supernormal excitability has appeared. Isolated sheep cardiac Purkinje fibers (2-5 mm in length) were superfused with Tyrode's solution and stimulated with depolarizing current pulses through a suction pipette. Recovery of excitability, restitution of the action potential duration, and response patterns were measured in each fiber for a wide range of current amplitudes and stimulation frequencies. When the potassium chloride concentration of the Tyrode's solution was decreased from 7 to 4 mM, the excitability recovery function consistently changed from monophasic (normal) to triphasic (supernormal'). During repetitive stimulation at increasing rates, normal preparations responded ...