Electrophysiologic Studies on Mobitz Type II Second-Degree Heart Block

Experimental studies were carried out in animals with intracellular and extracellular electrodes used to record simultaneously from the atrium, atrioventricular (A-V) node, bundle of His, bundle branches, Purkinje fibers, and ventricles to determine the site of conduction delay and block in Mobitz type II second-degree heart block. Type II block with block occurring within the His-Purkinje system was observed only rarely in normal animals; however, Mobitz type II block could be induced in normal hearts by slight but undetectable variations in the P-P interval. Sudden conduction failure was usually due to block within the His-Purkinje system. Type II block also resulted from failure of conduction within the A-V node during depressed A-V conduction due to rapid atrial pacing, vagal stimulation, or ouabain infusion. The classification of second-degree heart block according to Mobitz type I (increasing P-R interval) and Mobitz type II (constant P-R interval) block should take into account that the site of block in both cases can occur either in or below the A-V node. Admittedly, in man Mobitz type I block is most often intranodal while type II block is usually infranodal following myocardial infarction. His bundle electrocardiography may therefore be indicated to determine the site of block in patients with Mobitz type II block, particularly in those with prolonged P-R intervals.