Repolarization Inhomogeneities in Ventricular Myocardium Change Dynamically With Abrupt Cycle Length Shortening

Background. In single heart cells, abrupt changes in stimulation rate elicit complex alterations in repolarization. The effects of rate change on dispersion of repolarization, however, have not been well characterized. Methods and Results. To determine the effects of abrupt cycle length (CL) shortening on spatial inhomogeneity of repolarization in a syncytium ofventricular cells, 124 action potentials were simultaneously recorded from Langendorff-perfused guinea pig hearts using high-resolution optical mapping with voltage-sensitive dye. The distribution of ventricular action potential durations (APDs) mapped during each cardiac cycle was used to calculate mean APD and repolarization dispersion index (DI), defined as the variance of the distribution. After abruptly shortening CL from 500 to 300 msec, mean APD declined exponentially in normoxic controls (by 23±3 msec, p<O.OOOl). This response was characterized by beat-to-beat oscilla-tions of APD that were synchronized at all ventricular sites. After 30 minutes of hypoxia, mean APD decreased from 175.0+13.3 to 76+25.7 msec. However, during hypoxia, abrupt CL shortening lowered mean APD by only an additional 6±6 msec, and APD oscillations were no longer synchronized throughout the ventricle. In controls, beat-to-beat DI decreased signifi