R. F. Gilmour, Jr., N. F. Otani, and M. Watanabe, Memory and Complex Dynamics in Cardiac Purkinje Fibers, American Journal of Physiology 272 (Heart and Circulatory Physiology 41), H1826-H1832 (1997).
The contribution of cumulative changes in action potential duration (memory) to complex cellular electrophysiological behavior was investigated in canine cardiac Purkinje fibers. Complex behavior induced during constant pacing was caused by reciprocal interactions between the the time to full repolarization (TFR), where TFR = response duration + latency, and the diastolic interval (DI). The relationship between TFR and the preceding DI during complex behavior differed from that obtained using a standard restitution protocol. In particular, higher-order periodicities and chaos were produced in fibers in which the restitution curve lacked the prerequisites for such behavior. To investigate whether shifts in the restitution curve might be expected during rapid pacing, the relationship betweeen TFR of a test response (TFRn+1) and the immediately preceding response (TFRn) was determined. For any fixed DIn, reduction of TFRn from 240 to 130 ms was accompanied by a corresponding reducion of TFRn+1, whereas as TFRn was reduced further to 120 msec, TFRn+1 increased. Because of the dependence of TFRn+1 on TFRn (memory) and on the preceding DIn (restitution), the slope of the low-dimensional relationship between TFRn+1 and DIn at a constant pacing cycle length depended on the slopes of the restitution and memory functions. These results suggest that rapid accumulation and dissipation of memory may contribute importantly to complex electrical behavior in cardiac tissue.