Apparent bradycardia-dependent advanced second-degree atrioventricular block

A 65-year-old man with repeated chest discomfort and dizzy spells was transferred by an emergency car. On the way to hospital, his pulse was palpable as regular 4 to 5 beats followed by an unpalpable period of about 4 s. His electrocardiographic monitor showed that 4 to 5 sinus QRS complexes were followed by consecutive 3 to 4 blocked sinus P waves, which occurred repeatedly. When PP intervals gradually shortened during inspiration, sinus impulses were conducted to the ventricles, whereas when PP intervals lengthened during expiration, 3 to 4 sinus impulses were blocked in succession. An attempt was made to explain the mechanism for such apparent bradycardia-dependent atrioventricular block by using the concepts of periodic increases in vagal tone due to respiration and concealed electrotonic conduction of blocked impulses. Such a peculiar form of advanced second-degree atrioventricular block has never been reported before.