Ventricular Fusion Beats.

VENTRICULAR FUSION, summation or combination beats occur when two separate pacemakers compete for control of the ventricles. The usual pair of pacemakers are the sinoatrial node and an ectopic ventricular focus; but any pair of pacemakers, whose impulses invade the ventricular myocardium more or less simultaneously but at different points, can produce fusion beats. An ectopic atrial focus or the atrioventricular node, for example, may discharge an impulse that fuses with an ectopic ventricular one. Again, two ectopic ventricular foci can compete for control of the ventricles and produce fusion beats; this is often seen in complete atrioventricular block when more than one idioventricular center is active. Theoretically the same sinus impulse may divide above the ventricles and then reunite within them to produce a fusion beat; indeed, this is precisely the mechanism of the Wolff-ParkinsonWhite syndrome according to the popular bundle-of-Kent theory. Malinow and Langendorfl have compiled an exhaustive list of the possible pacemaker combinations that niay produce fusion beats. We shall confine our attention to the common form of ventricular fusion resulting from the summation of sinoatrial and ectopic ventricular impulses. Criteria for the recognition of such fusion beats, so far as we are aware, have not been precisely formulated. We have attempted to fill this gap and in the process have again been impressed with the shortcomings of the conventional bar-andladder diagram for depicting arrhythmias.