The Mechanism of the Wenckebach Type of a-v Block

The Wenckebach type of auriculo-ventricular block is characterized by a progressive lengthening of the A-V conduction time, ending in a completely blocked auricular complex- the Wenckebach period.These periodically recurring dropped beats were first recognized clinically by Wenckebach in 1899 from a careful analysis of radial arteriograms; Engelmann (1896) had noted the same phenomenon in the frog's heart a few years earlier.Changing conduction times, due to varying degrees of recovery of the A-V junction, were postulated by them to account for the dropped beats.A typical feature of the progressive prolongation of the P-R interval is that the most marked increase in the conduction time is seen in the second complex of the group.Later ones show only slightly greater prolongation.The shortest A-V conduction time is seen in the first complex after the dropped beat; i.e. following the longest pause or the longest R-P interval.Clinically, the Wenckebach type of A-V block is seen especially in digitalis poisoning, in infective myocarditis, due particularly to rheumatic fever and diphtheria, in myocardial fibrosis or infarction due to coronary artery disease, and occasionally in other lesions of the conduction system.It occurs at low auricular rates, most often under the influence of digitalis, or at the high auricular rates of paroxysmal tachycardia (Wenckebach and Winter- berg, 1927;Decherd, et al., 1943).A-V block, often of the Wenckebach type, has been produced experimentally by increasing the auricular rate (von Kries, 1902;