Wenckebach phenomenon within the atria.

rates ranged between I58 and I94 a minute. This phnomenon (also observed by Lewis, Feil, and Stroud, I918-I920) was manifested by a gradual prolongation of the stimulus-to-atrial electrogram intervals until one stimulus failed to reach the recording electrodes. The localization of the site ofdelay and block could be made exclusivelyfrom the intracardiac recordings, since the surface leads showed what appeared to have been only an AV nodal Wenckebach. In Case 2 the intraand interatrial as well as the atrioventricular conduction patterns were studied during intermittent paired high right atrial pacing. Though this patient had complete right bundle-branch block and left anterior hemiblock, the major area of delay was in the atria. The full recovery period of the latter occurred at a longer StI-St2 interval than that of the AV node or His Purkinje system.