Abstract and Introduction
Abstract
Vagally mediated atrioventricular (AV) block is defined as a paroxysmal AV block, localised within the AV node, associated with slowing of the sinus rate. All types of second-degree AV block, including pseudo-Mobitz II block, and complete AV block, may be present. Most of the patients have normal AV conduction. Differential diagnosis with intrinsic AV block is based on the behaviour of the sinus rate. Vagally mediated AV block is benign; it can be recorded as an asymptomatic or symptomatic event (syncope/presyncope). Syncope due to this form of AV block should be diagnosed and managed as neurally mediated syncope. When this block is fortuitously recorded in asymptomatic patients, pacemaker implantation is not indicated.
Introduction
Vagally mediated atrioventricular (AV) block is defined as a paroxysmal first-, second- or third-degree AV block associated with slowing of the sinus rate. This review deals with second- and third-degree AV block. Episodes of vagally mediated AV block occur more often during the night than during the day. It must be underlined that most of the nocturnal AV blocks are secondary to sleep disorders, such as obstructive or central sleep apnoea. Bradyarrhythmias secondary to nocturnal apnoeas involve different mechanisms from the pathophysiological and therapeutic points of view. Therefore, AV blocks due to these disorders should be discussed separately; consequently, they are not dealt with in this review.
A vagal input depresses sinus node function and AV nodal conduction, but does not influence the velocity of conduction in the His-Purkinje system. It is therefore extremely likely that the site of vagally mediated AV block is within the AV node; the results of some electrophysiological studies during which this form of AV block has been reproduced confirm this site. There is little published information on vagally mediated AV block, and in clinical practice it often goes unrecognised. In this review, we discuss the diagnosis, pathophysiology and clinical significance of vagally mediated AV block.