The Ventricular Arrhythmias

Abstract

Ventricular rhythms originate from sites distal to the penetrating bundle of His. In the majority of cases, the resulting QRS complexes are abnormally wide, exhibit axis deviation, and have a range of bizarre morphologies. In some cases, however, ventricular ectopic impulses arise in the distal conduction structures themselves and therefore exhibit narrow QRS complexes. Three basic mechanisms are thought to account for the majority of ventricular rhythms: enhanced automaticity, reentry, and early and late afterdepolarizations. In the case of enhanced automaticity, single or multiple excitable foci spontaneously discharge impulses. In the case of reentry, the sinus impulse traverses a circuit within the distal conduction system or ventricular myocardium, forming a single ectopic ventricular beat if conduction through the circuit is an isolated event, or producing ventricular tachycardia if conduction through the circuit is repetitive. A reentry circuit may consist of anatomically separate pathways with differing conduction properties, or disparate conduction properties (anisotropy) in contiguous myocardium may provide a functional basis for reentry. If the circuit is very small, the mechanism is called micro-reentry. If the circuit consists of larger structures, such as a bundle branch and the contiguous myocardium, the resulting mechanism is called macro-reentry. (The reentry mechanism is described in detail in Chapters 11 and 13.) Early after-depolarization, an early depolarization that occurs before repolarization is complete, is now widely believed to be the physiologic substrate of polymorphic ventricular tachycardia.