Atrioventricular (AV) block is defined as a delay or interruption in the transmission of an impulse, either transient or permanent, from the atria to the ventricles due to an anatomic or functional impairment in the conduction system. The conduction can be delayed, intermittent, or absent. The commonly used terminology includes first degree AV block (slowed conduction without missed beats), second degree AV block (missed beats, often in a regular pattern, eg, 2:1, 3:2, or higher degrees of block), and third degree or complete AV block.
In second degree AV block, some atrial impulses fail to reach the ventricles. Wenckebach described progressive delay between atrial and ventricular contraction and the eventual failure of an atrial beat to reach the ventricle . Mobitz subsequently divided second degree AV block into two subtypes, as determined by the findings on the electrocardiogram (ECG) :
- Mobitz type I AV block, in which progressive PR interval prolongation precedes a nonconducted P wave.
- Mobitz type II AV block, in which the PR interval remains unchanged prior to a P wave that suddenly fails to conduct to the ventricles.
- Advanced second degree AV block, in which two or more consecutive P waves are nonconducted. In contrast to third degree or complete heart block, however, some P waves continue to be conducted to the ventricle.
Mobitz type II AV block will be reviewed here. Mobitz type I (Wenckebach) AV block and the etiology of AV block are discussed separately. (See "Second degree atrioventricular block: Mobitz type I (Wenckebach block)" and "Etiology of atrioventricular block".)
SECOND DEGREE AV BLOCK: TYPE I VERSUS TYPE II
For patients with second degree AV block with a ratio of atrial to ventricular conduction that is not 2:1, the PR interval on the standard ECG can be used to distinguish Mobitz type I AV block and Mobitz type II AV block: