Frequency-Dependent Alterations of Conduction in Purkinje Fibers. In: Rosenbaum MB, Elizari MV, editors. Frontiers of Cardiac Electrophysiology. Dordrecht: Springer Netherlands; 1983. pp. 397-415. [DOI: 10.1007/978-94-009-6781-6_20]

Functional dissociation of cellular activation as a mechanism of Mobitz type II atrioventricular block

BACKGROUND

Several mechanisms have been advanced to explain Mobitz type II atrioventricular block in the ischemically damaged His-Purkinje system. Only recently, however, has an animal model been developed to study this form of conduction defect in vivo and in vitro.

METHODS AND RESULTS

Conduction defects were induced in anesthetized dogs by ischemic damage to the proximal His-Purkinje system after anterior septal artery ligation. Stable 2:1 atrioventricular block, localized within the His bundle or in the proximal bundle branches, was obtained in each dog by atrial pacing at an average rate of 239 +/- 20 beats per minute (n = 12). In vitro studies were then performed from the same hearts. Action potentials and electrograms were simultaneously recorded from the His bundle and the proximal right bundle branch at the site of damage. At slow rates of pacing (40-60 beats per minute), the action potential amplitude was 85 +/- 4 mV, and some cells (10 +/- 3%) showed dissociation from the electrical activity in the bundle. At fast rates (149 +/- 11 beats per minute), during 1:1 conduction, the frequency of cellular dissociation increased to 57 +/- 6% (p < 0.001), and the action potential amplitude decreased (-31 +/- 4%, p < 0.001). The frequency of dissociation closely correlated with the reduction in action potential amplitude (r = 0.87, p < 0.001). These changes were markedly attenuated once 2:1 block developed. The site of block was not constant but rather showed a dynamic behavior with spatial shifting in response to changes in pacing rate or the introduction of extrastimuli.

CONCLUSIONS

These results indicate that in the ischemically damaged proximal His-Purkinje system, an increase in rate leads to reduced and asynchronous cellular activation before 2:1 block. The latter provides a more stable activation pattern, because the frequency of dissociation is markedly reduced.

Mechanisms underlying the antiarrhythmic and arrhythmogenic actions of quinidine in a Purkinje fiber-ischemic gap preparation of reflected reentry

The effects of therapeutic levels of quinidine were studied in an ischemic gap preparation of reflected reentry. The preparation consisted of a Purkinje fiber mounted in a three-compartment chamber. A narrow central compartment was perfused with a solution prepared to mimic the extracellular milieu at a site of ischemia. Quinidine in concentrations that exert little effect on normal Purkinje tissue, 1 to 2 micrograms/ml, greatly impaired conduction and markedly prolonged refractoriness across the ischemic gap. The drug effected these changes by (1) extending the inexcitable zone within the depressed region, (2) decreasing the amplitude of the input signal entering this zone, and (3) decreasing the excitability of the tissue beyond the depressed zone (evaluated by current clamp techniques). These actions of the drug produced both antiarrhythmic and proarrhythmic effects. When the initial level of conduction impairment was high, quinidine totally suppressed reflected reentry at all frequencies by precipitating complete anterograde conduction block. At intermediate levels of block, the drug generally caused a prominent shift of the frequency dependence of reentrant activity to lower stimulation rates. Finally, when conduction was relatively less impaired, quinidine created the conditions for reflected reentry to occur. Our results suggest that the heart rate dependence of reentrant arrhythmias might be of prognostic value in the administration of antiarrhythmic drugs.

Rate-related suppression and facilitation of conduction in isolated canine cardiac Purkinje fibers

Previous studies have shown that antegrade conduction through damaged His Purkinje tissue may be suppressed following rapid ventricular pacing (overdrive suppression of conduction). We studied this phenomenon using isolated Purkinje fibers placed in a three-chamber bath. Superfusates for the left, middle, and right segments of the fiber were altered to produce action potentials that resembled those of normal bundle branch, damaged His bundle, and normal His bundle, respectively. To produce anisotropic conduction, the left segment of the fiber was adjusted to be three to four times longer than the right segment. Pacing the right segment at intermediate rates produced maximal action potential amplitude in the middle segment and 1:1 right-to-left conduction, whereas pacing at faster or slower rates reduced action potential amplitude and produced block. Pacing the left segment at fast or slow rates also reduced action potential amplitude in the middle segment, but conduction was maintained (anisotropy). After rapid or slow left segment pacing, action potential amplitude in the middle segment remained low during subsequent right segment pacing at intermediate rates, and transient block occurred (overdrive or underdrive suppression of conduction). With time, action potential amplitude normalized and conduction resumed. In other more severely depressed preparations, conduction block occurred even at intermediate right segment pacing rates prior to left segment pacing. Under these conditions, pacing the left segment at intermediate rates increased action potential amplitude in the middle segment and temporarily permitted 1:1 conduction at intermediate right segment pacing rates (overdrive facilitation of conduction).(ABSTRACT TRUNCATED AT 250 WORDS)

Determinants of postrepolarization refractoriness in depressed mammalian ventricular muscle

Functional determinants of postrepolarization refractoriness were studied with microelectrodes in isolated cat and dog ventricular muscle preparations mounted in a three-chambered bath. Frequency-dependent conduction delay and block were readily manifested when the central segment (1 mm) was superfused with high potassium (20-30 mM) Tyrode's solution. Conduction disorders were attributed to postrepolarization refractoriness involving slow recovery in the amplitude of elicited subthreshold depolarizations in depressed fibers distal to the central blocked zone. Investigations of subthreshold phenomena in homogeneously depressed tissues indicated that a relatively large local response participated in the voltage displacement induced by subthreshold depolarizing currents. The local response was blocked by tetrodotoxin (10 micrograms/ml) or verapamil (2 micrograms/ml) when resting membrane potential was near -70 or -50 mV, respectively. At either level of reduced membrane potential, gradual recovery in diastolic excitability correlated closely with time-dependent recovery of the local response, the rate of which was also proportional to the current intensity applied. Thus, postrepolarization refractoriness in depressed ventricular muscle fibers is a function of the time for recovery of active subthreshold properties (the local response), as well as intensity of excitatory current input. These factors may play a role in the development of delayed conduction and reentry that occur at faster heart rates under ischemic conditions.

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

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.