Second-degree atrioventricular block.

Abstract

by analyzing the A, C and V waves in the jugular venous pulse, described two types of second-degree atrioventricular (AV) block. After the introduction of the electrocardiograph, Mobitz4 classified the two types of block as type I and type II. Electrocardiographically, typical type I second-degree AV block is characterized by progressive PR prolongation culminating in a nonconducted P wave, while in type II second-degree AV block, the PR interval remains constant before the blocked P wave. In both instances, the AV block is intermittent, generally repetitive and may involve several P waves in a row. Usually, the eponym Mobitz type I and Mobitz type II AV block is applied to the two types of block, while the term "Wenckebach block" refers to type I AV block only.The purpose of this paper is to provide both a brief review of clinically relevant electrophysiologic and electrocardiographic concepts of AV conduction and block, and a critical evaluation of the present classification of second-degree AV block.Electrophysiological Characteristics of the AV Node and His-Purkinje System Since the difference between type I and type II AV block, in most instances, probably relates to the site at which the block occurs, it is important to review the electrophysiological properties of the tissues involved.Transmembrane action potentials recorded from cells within the AV node differ markedly from those recorded in the His-Purkinje system. AV nodal cells display reduced action potential amplitude, duration, resting potential, conduction velocity, maximum rate

Atrioventricular nodal accommodation in isolated guinea pig hearts: physiological significance and role of adenosine.

“…In fact, administration of atropine to patients with type I AV block (e.g., in the case of inferior myocardial infarction) can increase the degree of AV conduction delay and block as a result of the faster atrial rate, 37 which is consistent with what has been observed in isolated perfused guinea pig hearts during hypoxia. 38 Based on the results of the present study, restoration of 1:1 AV conduction may not always be beneficial because the AV block can be a "protective" mechanism for the ventricles from excessive demand when O 2 supply is limited.…”

ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities

“…However, it is not always possible to determine the site of AV block without electrophysiological evaluation, because type I second-degree AV block can be infranodal even when the QRS is narrow. 80 If type I second-degree AV block with a narrow or wide QRS is found to be intra-or infra-Hisian at electrophysiological study, pacing should be considered.…”

ACC/AHA/HRS 2008 Guidelines for Device-Based Therapy of Cardiac Rhythm Abnormalities

“…Thus, type II second-degree AV block with a wide QRS typically indicates diffuse conduction system disease and constitutes an indication for pacing even in the absence of symptoms. However, it is not always possible to determine the site of AV block without electrophysiological evaluation, because type I second-degree AV block can be infranodal even when the QRS is narrow (80). If type I second-degree AV block with a narrow or wide QRS is found to be intra-or infra-Hisian at electrophysiological study, pacing should be considered.…”