[Severe atrioventricular block in 2 cases with pre-excitation syndrome].

In two patients with WPW syndrome Type A suffering from syncopes and dizziness intermittent high degree A-V block was observed. The analysis of the surface Ecg revealed in the first case a complete A-V block within the normal conduction system at the level of the A-V node. In the second case there was a constant left bundle branch block with intermittent block in the right fascicle (intermittent trifascicular block). In both cases the preexcitation syndromes could be best explained by accessory tracts bypassing the normal nodal system left side. One-to-one conduction through the bypass occurred only at a distinct range of cycle lengths, at lower frequencies the accessory tracts were refractory and a IInd or IIIrd degree A-V block occurred. However, outside this frequency zone some P waves were conducted through the accessory tracts without changes in cycle lengths. The findings support the thesis of at least two functionally different atrioventricular pathways in patients with preexcitation syndrome.