2:1 AV block

Definition

A second-degree AV block in which every other P wave conducts to the ventricles, so conducted and blocked P waves alternate and there are two P waves for each QRS complex. Because no two consecutive conducted beats are available to compare PR intervals, 2:1 block cannot be classified as Mobitz type I or type II; the block can lie in the AV node or in the His-Purkinje system. A narrow QRS suggests block in the AV node, whereas wide QRS complexes suggest significant His-Purkinje disease.

Clinical note2:1 conduction is neither Mobitz I nor Mobitz II by definition, although it is often mislabelled Mobitz II. Exercise helps locate the block: it improves conduction through the AV node, whereas block below the AV node (infranodal) does not resolve and is likely to worsen as the sinus rate rises.