Atrioventricular (AV) block is defined as a delay or interruption in the transmission of an impulse from the atria to the ventricles due to an anatomical or functional impairment in the conduction system. The conduction disturbance can be transient or permanent. The conduction can be delayed, intermittent, or absent. The commonly used terminology includes first degree (slowed conduction without missed beats), second degree (missed beats, often in a regular pattern, eg, 2:1, 3:2, or higher degrees of block), and third degree or complete AV block. (See "ECG tutorial: Atrioventricular block".)
The clinical manifestations and management of third degree AV block will be discussed here. Congenital complete heart block, the etiology of AV block in general, and the management of other specific types of AV block are discussed separately. (See "Etiology of atrioventricular block" and "Congenital third degree (complete) atrioventricular block" and "First degree atrioventricular block" and "Second degree atrioventricular block: Mobitz type I (Wenckebach block)" and "Second degree atrioventricular block: Mobitz type II".)
THIRD DEGREE AV BLOCK
No atrial impulses reach the ventricle in third degree or complete heart block (waveform 1). The block can exist in the AV node or in the infranodal specialized conduction system [1-3]. A His-bundle electrocardiographic study can determine the site of block quite accurately, but the escape rhythm provides important clues.
Escape rhythms — Escape rhythms occur when a pacemaker other than the sinus node has sufficient time to depolarize, attain threshold, and produce a depolarization. In complete heart block, the escape rhythm that controls the ventricles can occur at any level below that of the conduction block and the morphology of the QRS complex can help to determine the location at which this is occurring.
- If third degree AV block occurs within the AV node, about two-thirds of the escape rhythms have a narrow QRS complex, ie, a junctional or AV nodal rhythm (waveform 2) [4-6].
- Block at the level of the bundle of His is also typically associated with a narrow QRS complex.
- Patients with trifascicular block have a subjunctional escape rhythm with a wide QRS complex (waveform 3).