Electrophysiologic studies in Mobitz type II second degree heart block

The electrophysiologic basis for Mobitz type II second degree heart block was studied in a patient with documented episodes of complete heart block and Stokes-Adams seizures. Recording of His bundle electrograms demonstrated that all atrial impulses were conducted through the atrioventricular (A-V) node to the bundle of His. Nonconducted P waves were blocked distal to the bundle of His. The frequency of blocked beats increased when the rate was accelerated by atrial pacing. Administration of atropine caused more frequent blocked beats, reaffirming the concept that the block occurred beyond the node. It is concluded that Mobitz type II second degree heart block is a manifestation of bilateral bundle branch block. Testing with atropine in this case was useful in distinguishing block in the bundle branches from that in the A-V node.