Effect of Extrasystoles on Idioventricular Rhythm

Abstract

Ventricular extrasystoles exert a variable effect on idioventricular rhythm. The postextrasystolic, or the "returning," cycle may be unchanged, or it may be shorter or longer than the dominant cycle. The factors responsible for these different responses have never been investigated. Therefore, the effect of electrically induced extrasystoles was studied by microelectrode techniques in an in vitro model of idioventricular rhythm consisting of spontaneously firing Purkinje fiber bundles with their attached ventricular segments. Late extrasystoles lengthened the Purkinje returning cycle, primarily by transiently increasing the duration of spontaneous phase-4 depolarization. In contrast, early extrasystoles produced shortening of the returning cycle, primarily because of the marked abbreviation of the duration of the extrasystolic action potential. Other factors affecting the ultimate duration of the Purkinje returning cycle were delineated. The effect of extrasystoles on the electrical activity of the ventricular segments was studied by surface electrograms. This effect was complex and was modified by the direction of propagation of the extrasystole with respect to the pacemaker site and by the degree of conduction delay. Ventricular electrical activity usually reflected the electrical events occurring in the pacemaker cells, but frequent exceptions occurred: the ventricular returning cycle could be prolonged while the corresponding Purkinje returning cycle was shortened and vice versa. "Interpolation" of an extrasystole did not necessarily indicate failure of the extrasystole to discharge the pacemaker site. The effects of extrasystoles on idioventricular rhythm which have been described in the earlier literature are easily understood from the findings of this study. KEY WORDS electrocardiogram premature ventricular contraction cardiac electrogramHis-Purkinje system complete heart block transmembrane potentials spontaneous phase-4 depolarization postextrasystolic cycle pacemaker cells returning cycle• Relatively little information exists in the literature concerning the effect of ventricular extrasystoles on idioventricular rhythm in

Phase resetting in a model of cardiac Purkinje fiber

“…Addition of a net outward current of <1 ,gA/cm2 would convert the system, via a saddle-node bifurcation, into one containing three equilibrium points. In that circumstance, very long prolongations of cycle length showing oscillatory activity in the pacemaker range of potentials similar to those experimentally seen in Purkinje fiber (Klein et al, 1972;Antzelevitch and Moe, 1983) and in heart cell aggregates (Guevara et al, 1986) would then result if the new equilibrium point in the pacemaker range of potentials were to be unstable and have at least one pair of complex conjugate eigenvalues (see also Clay et al, 1984;Reiner and Antzelevitch, 1985).…”

“…At low pulse amplitudes, there are two different well-separated ranges of tc, over which prolongation can be seen, with the first (Fig. 3, middle; arrow labeled a) being due to prolongation of action potential duration (Klein et al, 1972;Jalife and Moe, 1976) and the second (Fig. 3, middle; arrow labeled b) being due to prolongation of the duration of diastole (Weidmann, 1951;Jalife and Moe, 1976).…”

“…This second range occurs just before shortening of the cycle length takes place. At a sufficiently high pulse amplitude, this second range of prolongation is not seen, since the stimulus is then suprathreshold throughout diastole (Weidmann, 1951;Kao and Hoffman, 1958;Klein et al, 1972). Phase-dependent prolongation and shortening of cycle length can also be seen in clinical cases of modulated junctional (Moe et al, 1977) and ventricular (Castellanos et al, 1984) parasys-…”

Efficacy of Precordial Percussion Pacing Assessed in a Cardiac Standstill <i>Microminipig</i> Model

“…Efficacy of Precordial Percussion Pacing idioventricular rhythm, of which automaticity usually originated from the pacemaker cells in the His-Purkinje system. 22 In addition, the width and amplitude of the QRS complex during PPP, as well as ventricular electrical pacing, were greater than those during the sinus rhythm, as shown in Figure 4. Thus, PPP may have induced the electrical activity in the ventricular contractile myocardium rather than the cardiac conduction system.…”

The effects of single premature stimuli on automatic and triggered rhythms in isolated canine Purkinje fibers.

“…Klein et al 3 reported on the effects of "extrasystoles" on the return cycle of automaticity in isolated canine Purkinje fibers. They found that premature action potentials elicited during the late stages of repolarization usually produced a shortened "return cycle" (the interval from the upstroke of the premature impulse to the upstroke of the subsequent, or returning, automatic beat) in fibers with normal maximum diatolic potentials "because of the marked abbreviation of the duration of the [premature] action potential."…”