Dynamics of, and alternating Wenckebach periods during, 4:1 and 6:1 atrioventricular block

In summary, the irregular dynamics of progression of 4:1 AV block in atrial flutter, presumably different from those observed in similar degrees of rate-dependent block, most likely reflected the complex electrophysiologic mechanisms operating during the highest degrees of AV nodal block. Occurrence of previously undescribed arrhythmias, namely Wenckebach periods during 4:1 and 6:1 block, tends to support the multilevel block hypothesis.