The Spectrum of Acquired Atrioventricular Block in Clinical Practice

Abstract

ABSTRACT: Type I and type II second-degree AV block characterize block of a single sinus P wave:Type I block describes visible, varying and generally decremental AV conduction and type II block describes apparent all-or-none conduction without visible changes in AV conduction time before and after the blocked impulse. Absence of sinus slowing is an important criterion of type II block because a vagal surge (generally benign) can superficially resemble type II block. The diagnosis of type II block cannot be established if the first postblock P wave is followed by a shortened PR interval or is not discernible. All correctly defined type II blocks are infranodal. A pattern resembling narrow QRS Type II block together with an obvious type I structure in the same recording effectively rules out type II block because the co-existence of both types of narrow-QRS block is rare. Narrow QRS type I block is almost always AV nodal whereas type I block with bundle branch block outside acute myocardial infarction is infranodal in 60-70 % of cases. 2:1 AV block cannot be classified in terms of type I or II blocks and can be nodal or infranodal. Pacing is indicated in symptomatic marked first-degree AV block (>0.30 sec.), but patients with systolic heart failure might benefit more with biventricular pacing. Permanent pacing is almost never needed after inferior myocardial infarction and narrow QRS AV block. It should be considered only if second- or third-degree AV block persist for 14-16 days. Patients with bundle branch block and transient secondand third-degree AV block during anterior myocardial infarction have a high risk of sudden death after hospital discharge usually from ventricular tachyarrhythmias rather than AV block. They should receive an implantable cardioverter- defibrillator rather than a stand-alone pacemaker in the setting of severely depressed systolic left ventricular function. There are many causes of atrioventricular (AV) block but progressive idiopathic fibrosis of the conduction system related to an aging process of the cardiac skeleton is the most common cause of chronic acquired AV block. Barring congenital AV block, Lyme disease is the commonest cause of reversible third-degree AV block in young individuals and it is usually AV nodal. Before implantation of a permanent pacemaker, reversible causes of AV block such as Lyme disease, hypervagotonia, athletic heart, sleep apnea, ischemia, and drug, metabolic, or electrolytic imbalance must be excluded. Table 1 outlines the format used in the 2002 American College of Cardiology/American Heart Association/ North American Society of Pacing and Electrophysiology (ACC/AHA/NASPE) guidelines for pacemaker implantation [1]. The indications for permanent pacing in second- or third- degree AV block unlikely to regress are often straightforward in symptomatic patients but they are more difficult in asymptomatic patients. Some of the ACC/AHA/NASPE guidelines appear somewhat dogmatic. The final responsibility rests with the physician in terms of decisions, and the guidelines only represent a basic framework to start from.