Interrelationships Between Automaticity and Conduction in Purkinje Fibers

Abstract

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 pacemakers, 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. ADDITIONAL KEY WORDSphase-4 depolarization transmembrane potentials arrhythmia latent pacemakers Purkinje fibers diastolic membrane potential pacemaker threshold potential conduction disturbances cardiac electrophysiology canine heart • Conduction in excitable tissues is determined by many variables. Among these, the amplitude of the action potential and the maximum rate of change of membrane potential during its upstroke (dV/dt, phase 0) are of singular importance (1). In Purkinje fibers, these two variables bear a predictable relationship to the level of membrane poten-

The effect of chemical ablation of the endocardium on ventricular fibrillation threshold.

“…As with any experimental observation, the applica- 30 Although these mechanisms may not have played a major role in the initiation or maintenance of ventricular fibrillation in the animals with nonischemic myocardium in our study, their potential contribution cannot be excluded.…”

Effects of Acetylcholine on Automaticity and Conduction in the Proximal Portion of the His-Purkinje Specialized Conduction System of the Dog

“…Singer et al (4) demonstrated that phase 4 depolarization in Purkinje fibers results in varying degrees of conduction depression. Thus, as impulses arrive at a cell progressively later during spontaneous phase 4 depolarization, take-off potential will be decreased, and hence total action potential amplitude, maximal rising velocity of phase 0, and conduction velocity will fall.…”

Effect of lidocaine on the electrophysiological properties of ventricular muscle and Purkinje fibers

“…of action -potentials evoked in Purkinje fibers are, for practical -purposes, entirely dependent on membrane voltage at the time of activation (24), no matter whether a given membrane voltage has been reached: (a) during repolarization of a normal action potential, (b) by generalized depolarization of the membrane, or (c) by phase 4 depolarization. Augmented automaticity localized -to one branch of a Purkinje fiber could cause delayed Londuction and block of impulses propagating into this branch; this conduction delay can be unidirectional and -cause reentry (20). Application of lidocaine could diminish automaticity and thereby speed conduction and abolish reentry.…”

“…As pointed out previously (18)(19)(20), ventricular arrhythmias are best understood in terms of alterations in automaticity, conductivity, and the associated changes in the electrophysiological properties of the Purkinje fiber. Automatic ventricular rhythms almost certainly originate in the Purkinje fibers (PF) and the impulse in reentrant ventricular arrhythmias frequently must utilize these fibers as a portion of the reentrant circuit.…”