Ventricular Fusion Beats

Relatively precise criteria are proposed for the recognition of ventricular fusion beats. Apart from the obvious and widely accepted criteria involving contour, duration, and time of appearance, other less obvious characteristics have been adduced: (1) the P-S time of the fusion beat must always be considerably longer than the P-R interval of the component sinoatrial beat; (2) the P-R interval of the fusion beat is generally not more than 0.06 second shorter than the sinoatrial P-R; (3) the terminal vector of the fusion beat is always different from that of the sinoatrial beat, while the initial vector may or may not differ. Exceptions to these criteria are seen in the presence of bundle-branch block, during periods of inconstant atrioveutricular conduction times, and presumably in the presence of diffuse intraventricular conduction delays. The shortcomings of conventional diagrams for depicting the mechanisms of arrhythmias are noted, and a plea is made for the use of more accurate symbols in their construction. Fusion beats are not an indispensable criterion for the diagnosis of ventricular parasystole.