Interrelationships Between Automaticity and Conduction in Purkinje Fibers

Microelectrode studies of transmembrane potentials of canine Purkinje fibers show that phase-4 depolarization causes voltage-dependent changes in conduction and responsiveness similar to those occurring during repolarization at comparable levels of potential. Abnormalities ranged from simple slowing of conduction to decrement, unidirectional and bidirectional block, and unexcitability. Reentrant excitation also developed. Significant conduction disturbances usually appeared at −75 to −70 mv; decrement and advanced block at −65 to −60 mv, or below. Because the threshold potential of normal Purkinje cells is approximately −70 mv, depolarization to lower levels implies shifts in this variable toward 0. Determinations of threshold potential confirm such shifts. It may be further inferred that significant abnormalities would most likely occur in fibers in which threshold potential is shifted toward 0 or membrane responsiveness impaired. Alterations in conduction due to phase-4 depolarization provide a reasonable explanation for various peculiarities of cardiac rhythm, including occurrence of conduction disturbances and reentrant rhythms at low heart rates, exit and entry block about parasystolic foci, instability of peripheral Purkinje pacemaakers, and supernormal conduction. Circumstances that enhance phase-4 depolarization are common in diseased hearts, indicating that this mechanism may be a significant factor in human arrhythmias.