Arrival of excitation at the left ventricular apical endocardium in Wolff-Parkinson-White syndrome type B.

Electrograms were recorded from the His bundle area, right ventricular apex, right ventricular inflow tract, and left ventricular apical endocardium in four patients (aged, 1, 1, 1.5, and 16 years) with Wolff-Parkinson-White syndrome type B. In beats without preexcitation: (a) delayed activation of the right ventricular inflow tract reflected the occurrence of a conduction disturbance through the "distal" or "peripheral" ramifications of the right bundle branch; and (b) the slightly earlier activation of the left ventricular apical endocardium (in reference to the right ventricular apex) may have been due to an earlier emergence from the divisions of the left bundle branch, presumably due to the greater length of the right bundle branch. In beats with preexcitation: (a) the "incomplete" right bundle branch block pattern was concealed because the right ventricular inflow tract was activated before the right ventricular apex; (b) the delta-right ventricular apical intervals were shorter than those of adults with Wolff-Parkinson-White type B; and (c) arrival of excitation at the left ventricular endocardium was a function, either of the impulse emerging from the left bundle branch, or of that propagating from the preexcited site. Therefore, the delta-left ventricular apical endocardial intervals were considered to have represented conduction time from preexcited region to endocardium of left ventricle only when it could be proven that the conduction time (from atrial site of origin to left ventricular apical endocardium) was shorter through the right sided accessory pathway than through the normal pathway. This study suggests that some beats, which may be interpreted as representing "pure" Wolff-Parkinson-White type B complexes from epicardial maps, may in reality be "fusion" complexes.