Mechanisms of Intermittent Ventricular Bigeminy

Abstract

An analysis is presented of 5 selected cases exemplifying a group characterized by a regular dominant rhythm and variable coupling of the ectopic beats. Under such circumstances, ventricular bigeminy may be due either to re-entry or parasystole. Criteria for the differentiation between the 2 mechanisms are based upon (a) the presence or absence of fixed coupling and (b) the numerical relationship of the interectopic intervals which may point to the presence of a simple parasystole or one complicated by a conduction disturbance of the Wenckebach type. When spacing of the ectopic beats characteristic of a parasystole was found only in the individual runs of bigeminy, but not throughout the record, an intermittent form of parasystole was diagnosed. Under such circumstances the coupling of the first premature beats after the intermittences of bigeminy was fixed. The latter phenomenon supported the diagnosis of intermittent parasystole. Intermittent parasystole appears to represent a link between the two fundamental mechanisms of re-entry and parasystole. Intermittent bigeminy in parasystole occurs when the rate of the parasystolic pacemaker appraoches half the rate of the dominant pacemaker. If it is slightly faster the coupling of successive ectopic beats exhibits progressive shortening; if it is slightly slower, the coupling shows progressive lengthening. Intermittence of bigeminy occurs as a result of (a) the normal ventricular refractory period, (b) a conduction disturbance of the parasystolic impulses, (c) intermittence of ectopic impulse formation, and (d) a combination of these mechanisms. Differentiation between a re-entry mechanism and intermittent parasystole is not always possible. This is true for both short runs of bigeminy with fixed coupling and certain cases with progressive lengthening of the coupling. The literature is reviewed for cases with intermittent ventricular bigeminy, and some unusual instances are pointed out, revealing mechanisms similar to those presented above. A classification is proposed of intermittent bigeminy due to ventricular premature systoles based on genetic principles.