2:1 Atrioventricular block:: Order from chaos

2:1 AV block can occur in either the AV node or the His-Purkinje system and cannot be classified into type I or type II second-degree AV block because there is only one PR interval to examine before the blacked P wave. It is inappropriate to use terms such as 2:1 or 3:1 type 1 or type II AV block because this characterization violates the accepted traditional definitions of type I and type II block based on electrocardiographic patterns and not on the anatomical site of block, Type I and type II second-degree AV block can progress to 2:1 AV block, and 2:1 AV block can regress to type I or type II block, Consequently, the site of the lesion in 2:1 block can often be determined by seeking the company 2:1 AV block keeps, An association with type I block and a narrow QRS complex almost always reflects AV nodal block but type I block with a wide QRS complex occurs more commonly in the His-Purkinje system than the AV node, Type II block, if correctly defined, is always infranodal. Outside of acute myocardial infarction, sustained 2:1 and 3:1 AV block with a wide QRS complex occurs in the His-Purkinje system in 80% of cases and 20% in the AV node, Administration of atropine in patients with His-Purkinje disease may increase the degree of AV block, (Am J Emerg Med 2001;19: 214-217, Copyright (C) 2001 by W.B. Saunders Company).