Paroxysmal vagally mediated AV block with recurrent syncope

Paroxysmal complete atrioventricular (AV) block without associated electrocardiographic (ECG) abnormality is not a well recognized entity. A mother and her daughter had recurrent syncopal episodes, but a normal ECG. The episodes were preceded by nausea and vomiting. ECG during these episodes revealed complete heart block. In the mother, one episode was promptly reversed by atropine. Electrophysiological evaluation of the sinus and AV nodal function and atrial and ventricular effective refractory periods before and after autonomic blockade was normal. Provocative manoeuvres failed to induce AV block. Paroxysmal AV block was vagally mediated in one of the patients, as indicated by prompt response to atropine. In the second case, the vagal dependence could not be proved but appears to be the most likely explanation. It thus appears that paroxysmal, vagally mediated complete AV block should be seriously considered in patients with unexplained syncope.