Atrioventricular (AV) Block

Abstract

There are three categories of atrioventricular (AV) block: first-, second-, and third-degree. First-degree AV block is manifest only by a prolongation of the PR interval beyond 0.2 s. The essential element of both second- and third-degree AV block is a “dropped beat,” in this case meaning an atrial depolarization (P wave) not followed by a ventricular depolarization (QRS complex). Second-degree AV block is subdivided in Mobitz I (or Wenckebach) block or Mobitz II. Mobitz I has the characteristic of increasing prolongation of sequential PR intervals until there is a “dropped beat,” a P wave not followed by a QRS. Mobitz II is a higher degree of block where only a fraction (for example, 1 of 3) of the P waves is followed by a QRS, and the PR interval for that conducted beat may be normal or prolonged but is generally fairly constant. Third-degree AV block (also known as complete heart block) is manifest by the absence of conduction from atria to ventricles and the presence of atrioventricular dissociation, where the atrial depolarization (P waves) occurs at one rate and the ventricular depolarization (QRS complexes) appear at a totally unrelated, slower rate.