Epicardial monophasic action potentials exhibited a rate-dependent spike-and-dome morphology, indicating that the transient outward current contributes to ventricular repolarization in vivo.
The transient outward current (Ito) contributes to ventricular repolarization in vivo, predominantly in the epicardium, resulting in unequal action potential duration shortening across the ventricular wall at short diastolic intervals.
A transient outward current (Ito), long considered to be a unique feature of Purkinje fiber tissue, has recently been demonstrated in dog ventricular tissue in vitro and most prominently in the epicardium. To investigate its possible contribution to ventricular repolarization in vivo, we recorded right ventricular endocardial and epicardial monophasic action potentials (MAP) simultaneously in pentobarbital-anesthetized open-chest dogs. Epicardial MAP had lower phase 1 than phase 2 amplitude at both spontaneous heart rate and paced cycle length of 300 and 400 ms. This "spike-and-dome" morphology of the epicardial MAP, possibly attributable to Ito, progressively disappeared at shorter extrastimulus intervals. In endocardium the phase 1 amplitude was always higher or equal to phase 2 amplitude and was not affected by shorter extrastimulus intervals. The action potential duration (APD) was shorter in epicardium than in endocardium. Both endocardial and epicardial APD shortened as the premature intervals were reduced, but the shortening was not parallel. The restitution curves converged so that, at the shortest intervals (160 ms), there were no longer any significant differences in APD between endocardium and epicardium. This study indicates that Ito contributes to ventricular repolarization in vivo, and most prominently in the epicardium. Unequal shortening of APD between endocardium and epicardium after progressively shorter diastolic intervals may thus partly result from uneven distribution of Ito across the ventricular wall.
Tande et al. (Fri,) conducted a other in Ventricular repolarization physiology. Epicardial vs endocardial monophasic action potential recording vs. Endocardial recording was evaluated on Monophasic action potential (MAP) configuration and action potential duration (APD). Epicardial monophasic action potentials exhibited a rate-dependent spike-and-dome morphology, indicating that the transient outward current contributes to ventricular repolarization in vivo.