[Wenckebach periods of alternate beats].

Wenckebach periods of alternate beats (AW) can be described as a 2:1 atrioventricular (A-V) block in which the conducted P waves show progressive prolongation of the P-R interval of the Wenckebach type. However, while classical Wenckebach periods terminate with a single blocked P wave, AW necessarily ends with (or begins from) two consecutive blocked P waves. Five clinical cases and several experimental examples of AW are reported. Recovery curves of A-V conduction were constructed, and it was demonstrated that AW is related to a marked prolongation of both the absolute and relative refractory periods. All the cases were associated with intraventricular block. In addition, recording of His bundle potentials in one case, histological study of the conduction system in another, and the experimental observations, support the view that AW tends to occur below the A-V node, in one of the main ventricular conducting fascicles. Four of the five patients developed complete heart block and Adams-Stokes seizures.