Heterogeneous electrical memory in restitution of action potential duration in pig ventricles

Abstract

Electrical stability in the heart is affected by restitution of action potential duration (APD), i.e. dependence of APD on preceding diastolic interval (DI) and memory, i.e. dependence of an APD on previous activations. Heterogeneity in APD restitution has been recently demonstrated by a number of studies. In the present study we explored whether memory in restitution is also expressed heterogeneously. Transmembrane potentials were recorded from endocardial and epicardial sides of both ventricles using 15 farm pigs. Oscillatory changes in DIs with a period of 100 beats revealed hysteresis in restitution from which quantitative measures of memory were computed. Results showed that hysteresis loop thickness, overall tilt and areas were larger in the endocardium than the epicaridum, 24.5 msec vs 14.2msec, 0.36 vs 0.23 and 6866 msec2 vs 4122 msec2 (p < 0.05). Except for overall tilt, memory was not significantly different between the two ventricles. Memory has been hypothesized to increase stability, thus, our results suggest that in the endocardium a decrease in stability predicted by increased slope may be offset by an increase in memory. Memory may provide an explanation for why experimental observations of alternans have not been consistent with the stability predicted by heterogeneous restitution.