[SHORT REPORTS]Effects of Low-Intensity Field Stimulation on Action Potential Repolarization and Refractory Period in Guinea Pig Ventricular Muscle: The Influence of Electrical Axis with Respect of Fiber Orientation

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For clarifying the mechanism which underlies re-entrant arrthymias induced by DC-defibrillation, we investigated the effects of low-intensity field stimulation on action potentials repolarization in guinea pig ventricular muscle, and their modulation by electrical axis of stimulation. A 10 msec monophasic shock (S2) was applied during the repolarization phase of action potentials induced by basic stimulus at 10. Hz (S1). S2 of field intensity at 1-6 V/cm caused a repolarization delay of S1 action potential and a prolongation of effective refractory period (ERP). These changes were greater at higher S2 intensities and at the later phase of S2 application. S2 applied in a longitudinal (L) direction to the fiber orientation caused larger repolarization delays and greater ERP prolongation than that of transverse (T) direction. This anisotropic characteristic of the low-intensity shock may set the stage for re-entry of excitation through an enhancement of spatial inhomogeneity of refractoriness.
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