Excitation of a cardiac muscle fiber by extracellularly applied sinusoidalcurrent

Sinusoidal Excitation of Cardiac Muscle Fiber. Introduction: The goal of th is study was to examine the effect of AC currents on a cardiac fiber. The s tudy is the second in a series of two articles devoted to the subject. The initial study demonstrated that low-strength sinusoidal currents can cause hemodynamic collapse without inducing ventricular fibrillation. The present modeling study examines possible electrophysiologic mechanisms leading to such hemodynamic collapse. Methods and Results: A strand of cardiac myocytes was subjected to an extra cellular sinusoidal current stimulus. The stimulus was located 100 mum over one end. Membrane dynamics were described by the Luo-Rudy dynamic model. E xamination of the interspike intervals (ISI) revealed that they were depend ent on the phase of the stimulus and, as a result, tended to take on discre te values. The frequency dependency of the current threshold to induce an a ction potential in the cable had a minimum, as has been found experimentall y. When a sinus beat was added to the cable, the sinus beat dominated at lo w-stimulus currents, whereas at high currents the time between action poten tials corresponded to the rate observed in a cable without the sinus beat. In between there was a transition region with a wide dispersion of ISIs. Conclusion: The following phenomena observed in the initial study were repr oduced and explained by the present simulation study: insignificant effect of temporal summation of subthreshold stimuli, frequency dependency of the extrasystole threshold, discrete nature of the ISI, and increase in regular ity of the ISI with increasing stimulus strength.