Showing papers by "Charles Antzelevitch published in 1983"

Parasystole, reentry, and tachycardia: a canine preparation of cardiac arrhythmias occurring across inexcitable segments of tissue.

Abstract: A protected ectopic focus created in tissue excised from one heart was allowed to interact with the activity of the intact heart of another animal The protected focus consisted of a Purkinje fiber in which a narrow central zone was rendered inexcitable The model permitted us to study parasystole, modulated parasystole, reentry, and tachycardia in the same preparation At moderate levels of electrotonic influence across the region of block, frequency scans revealed wide zones of pacemaker entrainment The incidence and pattern of premature ventricular contractions generated were always a sensitive function of heart rate Parasystolic patterns could be converted to apparent reentrant patterns by simple alteration of the atrial driving rate or the level of block Suppression of pacemaker automaticity converted a modulated parasystole model to one of pure reentry Reciprocation of the impulse across the inexcitable tissue segment generated a ventricular tachycardia that could be initiated and terminated by a single properly timed event Our observations suggest that ectopic activity that behaves like parasystole and activity characteristic of what is commonly diagnosed as reentry, including tachycardia and idioventricular rhythms, may be a manifestation of a common mechanism whose arrhythmic expression differs as a continuous function of heart rate, level of block, or level of automaticity

Electrotonic inhibition and summation of impulse conduction in mammalian Purkinje fibers

Abstract: Models of electrotonically mediated transmission were created by superfusion of the central fiber segment of a three-compartment Purkinje fiber preparation with either an "ion-free" or "ischemic" solution or by localized application of pressure. The frequency-dependent impairment of impulse conduction across such inexcitable gaps was found to be the result of influences exerted not only by impulses transmitted across the area of block but also by impulses blocked at the proximal border of the inexcitable zone. The electrotonic image of a nonconducted response was observed to exert an important inhibitory effect on the electronically mediated transmission of a subsequent impulse, thus causing block or delay. This phenomenon, which we have termed electrotonic inhibition, shows both time and voltage dependence. The related phenomenon of electrotonic summation describes the facilitation of conduction that occurs when two subthreshold potentials occur close enough in time to fuse. These data provide a demonstration of Wedensky inhibition in heart tissues and point to electrotonic inhibition as a mechanism of concealed conduction. Electrotonically mediated phenomena may explain both temporal and spatial inhibition and summation previously described in nodal or depressed tissues and ascribed to partial active invasion of intermediary tissue.

Rate-dependent changes in excitability of depressed cardiac Purkinje fibers as a mechanism of intermittent bundle branch block.

Abstract: When the heart rate is accelerated, rate-dependent intraventricular block may occur. This block has been attributed to abnormal action potential prolongation in a diseased conducting pathway. Less often, intraventricular block develops during slowing of the heart rate and has been explained in terms of phase 4 depolarization in potentially automatic cells within the diseased fascicle. We tested these hypotheses in isolated bundles of Purkinje fibers placed in a three-chambered tissue bath. In one group of experiments, conditions of localized injury and depressed excitability were mimicked by superfusing the central segment with sucrose solution. Action potentials were initiated in the proximal segment while the slope of phase 4 of cells in the distal end was controlled by intracellular ramps of current of either polarity. In these preparations, phase 4 depolarization facilitated rather than retarded propagation across the depressed segment, even at takeoff potentials as low as -45 mV. In a second group, depressed excitability was induced by exposing the three fiber segments to Tyrode's solution that contained high concentrations of KCl and CaCl2 or isoproterenol (0.1 microgram/ml). Under these conditions, Purkinje fibers did not undergo phase 4 depolarization and did not generate abnormally prolonged action potentials. These preparations showed a biphasic time dependence of conduction during premature stimulation or in response to changes in the basic cycle length. Conduction impairment and block were manifest at either side of an optimal interval or cycle length. Our results suggest that phase 4 depolarization and abnormally prolonged action potentials are not necessary conditions for intermittent block. Both tachycardia and bradycardia-dependent intraventricular conduction abnormalities may be associated with time-dependent variations in the excitability of depolarized conducting fibers as well as in the amplitude of the slow responses generated by these fibers. These alterations can be explained in terms of regulation of slow inward current by the intracellular calcium concentration.

Frequency-Dependent Alterations of Conduction in Purkinje Fibers

Abstract: Rate-dependent intraventricular conduction disturbances are of two basic types: 1) those which become manifest when the heart rate is accelerated, and 2) those appearing as a consequence of deceleration. To explain these phenomena, Rosenbaum and coworkers introduced the concept of phase-3 and phase-4 block [1]. Conduction failure of closely coupled beats was termed phase-3 block and was attributed to normal refractoriness or its extension in disease states. Conduction failure of an impulse arriving late in diastole (phase-4 block) was attributed to slow diastolic depolarization (SDD) at the depressed site. In both cases, conduction impairment was attributed to a more positive take-off potential giving rise to responses of lesser amplitude and (dV/dt)max and therefore a diminished conductivity [2–6].

Models of Parasystole and Reflection

Abstract: It is our major hypothesis that parasystolic activity (traditionally rare and “benign”) and reentrant activity (including fixed and closely coupled “dan-gerous” activity) are points on a continuous spectrum that includes almost all idioventricular activity. The sucrose gap preparation can serve as a model of parasystole when a shunt resistance bridging the gap permits one-way “conduction” (entrance block), or as a model of reflection when the shunt resistance is reduced to a level at which impulse transmission is possible in both directions, but only with major delay (1st degree block) or with delay and intermittent failure (2nd degree block). We shall review some of the major aspects of the studies that have lead to this contention.