Computer analysis of prolongation of cardiac action potential duration

Using Beeler and Reuter's mathematical model of action potential in myocardial fibers, a similar extent of prolongation of action potential duration at -60 mV (APD -60) was produced at the infinite interval of stimulation by altering parameter values for slow inward current (is) time-independent (ik1) and time-dependent (ix1) outward currents. Among five types of prolongation of APD -60 thus produced, the plateau height is high in two types at this interval, but in three at a shorter interval. The degree of prolongation varies between these five types at this shorter interval. The stimulus interval-action potential duration (interval-duration) relation curves reflect changes in time-dependent parameters of these currents. Thus various types of APD prolongation may be distinguished from each other by the different behavior of action potentials at low and high frequencies of stimulation. ix1 increases by 40-fold at the moment of the maximum rate of depolarization at an interval of 330 msec, but is still negligibly small as compared with sodium current.