An analysis of Wenkebach periodicity during atrial pacing

Seventy-six atrial pacing induced Wenckebach periods in 20 patients were analyzed. They were divided into 5 types according to the manner of increment change of P'V prolongation. Fifty-four % of the periods were atypical (Types II-V). There was a significant increase in the incidence of atypical periods as the length of the periods increased. The recovery curve of the A-V node was obtained by plotting P'V against preceding VP'. The curve was convex and the slope became greater than 45 degrees near the absolute refractory period in the majority of the cases, although there was 1 case in which the slope was less than 45 degrees throughout the curve. The P'V interval was found to be determined mainly by the preceding VP' interval. The mechanism by which these different types of Wenckebach periods develop can be explained by the relationship between the shape of the recovery curve and the pacing rate. The types II, III, and IV periods which have been dealt with as atypical periods were understood to develop by the same mechanism as type I (typical) periods.