Wenckebach phenomenon within the atria

A patient with a history of fainting and a bifacicular block (CRBBB plus LPH) was analyzed by His Bundle electrograms. During atrial pacing at a rate of 138/min, a Wenckebach type of block occurred within the atrium or at the electode-atrium junction, or both. The sinus intra-atrial conduction time (P-A inverval) was normal. During atrial pacing (pacing rate of 138 to 148/min), St-A interval progressively lengthened and ultimately a second degree Wenckebach type of block was obtained. The surface electrocardiogram (ECG) was normal and only electrical pacing brought on the intra-atrial conduction defect. Simultaneous with this atrial block, a second degree (Mobitz II) block developed.