Idiopathic Paroxysmal Atrio-Ventricular Block. What is The Mechanism?

Abstract

Idiopathic paroxysmal atrioventricular (AV) block poses a true diagnostic challenge. What is clear about this entity is the confusion about its definition and consequently about its etiology. According to certain sources, the diagnosis of this block requires the lack of a structural cardiac pathology that justifies the observed manifestations and an absence of electrocardiographic disorders prior to an episode. The clinical presentation of idiopathic paroxysmal AV block does not differ from that of another cardiogenic syncope or of a vasovagal syncope with a significant cardioinhibitory component. With respect to the mechanism that explains this block, it has been postulated that patients with low basal adenosine levels exhibit hyperaffinity of the A2 receptors of the AV node. Variations in plasma adenosine levels may favor episodes of paroxysmal AV block. The diagnosis of this block is complex and can require years to determine. Routine electrophysiological examination of these patients is not cost effective due to the low sensitivity and specificity of this approach. Numerous groups have supported the use of an implantable loop recorder to substantiate AV block paroxysms and assess their clinical correlations. Permanent stimulation devices are utilized to reduce syncopal recurrence.

Keywords: Adenosine Plasma Level; Atrioventricular Block Idiopathic; Syncope.

Figure 1.. Extrinsic vagal AV block registered during implantable loop recorder monitoring in a patient without heart disease, normal ECG and recurrent syncope. A) implantable holter histogram and B)electrocardiogram. Initial sinus rhythm slowing is present, there is minimal PR interval variation in the last beat previous to 2:1 AV block, followed by complete AV block with very long pause. During AV block p-p interval shortens due to sympathetic activation. Restoration of AV conduction is without PR prolongation

Figure 2.. Baseline electrocardiogram, sinus bradycardia at 48 beats per minute, normal axis, PR normal, narrow QRS with no repolarization abnormality. Figure 2B: External monitoring, third-degree atrioventricular block with delay of 4320 ms and QRS escape of similar morphology to the baseline electrocardiogram. As can be seen, the paroxysmal third-degree AV block does not exhibits abrupt onset, prolonging the PR interval. So, this is not an idiopathic paroxysmal AV block. This record shows just how problematic this clinical entity

Published by authority of the editor. Original source: Guerrero Márquez FJ, Gonzales Vargas-Machuca M, Pérez Cano B, Revello Bustos A, Marín Morgado J, Ruiz Borrell M. Aneurysmal membranous septum as an exceptional cause of paroxysmal atrioventricular block. Rev EspCardiol 2016; 69(2): 226-8. Copyright © 2016 Sociedad Española de Cardiología. Edited by Elsevier Spain, S.L. All rights reserved