Interference Dissociation

Abstract

Contradirectional interference results when 2 stimuli arising in different foci in any part of the heart spread in opposite directions toward each other. Interference dissociation is defined as that type of dissociation which is due to repetitive contradirectional interference. The electrocardiograms of 12 cases have been selected for analysis to illustrate many of the important features of interference dissociation. The various centers between which dissociation may theoretically occur are listed for reference. This series includes examples of dissociation due to interference between the following pairs of rhythms: Normal sinus rhythm and A-V nodal rhythm, normal sinus rhythm and multifocal idioventricular rhythms, sinus tachycardia and paroxysmal ventricular tachycardia, paroxysmal atrial tachycardia with block and idioventricular rhythm, simultaneous dissociated paroxysmal A-V nodal and ventricular tachycardias, and normal sinus rhythm and A-V nodal rhythm in the presence of the Wolff-Parkinson-White syndrome. Interference may occur at any level between the points of origin of the 2 rhythms, and the most common sites of interference are illustrated diagrammatically. Interference at the upper end of the A-V junction is of particular importance, and a clear understanding of isolated interference in this region is of help in resolving some of the more difficult problems of dissociation. For this reason the concept of the zone of potential interference is introduced. This is the time interval in the cardiac cycle during which interference between an A-V nodal, or ventricular, and oncoming S-A beat may be anticipated. Certain important deductions are derived from theoretical considerations of the zone of potential interference. The various mechanisms leading to the onset and termination of interference dissociation are described, and the rates of the dissociated rhythms are compared. Aberrancy of the ventricular response to A-V nodal impulses is discussed. This phenomenon is ascribed to uneven spread of the excitation wave from an eccentrically located focus, due to the fact that in the A-V node longitudinal is faster than horizontal transmission. Ventricular and atrial captures are considered in detail. Of particular interest is the occasional occurrence of both types of capture in the same case. This paradox is explained on the basis of either intermittent retrograde block or simultaneous bidirectional conduction through the A-V node in the presence of functional longitudinal dissociation. The relationship of other forms of heart block to interference dissociation is pointed out. Heart block predisposes to, and frequently complicates, interference dissociation. A differential diagnosis between interference dissociation and other disturbances of rhythm is presented. Interference dissociation is one of the most complex arrhythmias and familiarity with all its possible variations is essential to the correct interpretation of any given tracing.