Diagnostics
2:1 Atrioventricular block: Order from chaos*,**,*,★★

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Abstract

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 blocked P wave. It is inappropriate to use terms such as 2:1 or 3:1 type I 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 © 2001 by W.B. Saunders Company)

Section snippets

Incorrect use of type I and type II designations in 2:1 AV block

It is inappropriate to describe 2:1 or n:1 nodal AV block (n > 2) as type I block and 2:1 or n:1 infranodal block (n > 2) as type II block. This practice violates the well-accepted traditional definitions of type I and type II blocks based on electrocardiographic patterns rather than the anatomical site of block.9 Perhaps those who cling to the above designations still believe that all type I blocks are confined to the AV node (incorrect)10, 11, 12, 13 and all type II blocks are infranodal

Site of block

The significance of 2:1 AV block can be evaluated during changes in conduction to 3:2, 4:3, and so on. AV block that permit the emergence of the electrocardiographic patterns of type I or type II block. Type I block can occur either in the AV node or the His-Purkinje system. Type I block with a wide QRS complex (≥ 0.12 sec) is localized in the His-Purkinje system in 60% to 70% of cases outside of acute myocardial infarction.14, 16 Type I block with a narrow QRS complex occurs almost always in

Source of confusion

Mobitz30 in his original description of type II block included 2:1, 3:1, and so on AV block as a form of type II block but the renowned Chicago School of Electrocardiography in the mid 1950s seems responsible for the continuing misconceptions about 2:1 AV block.31, 32 These highly respected experts advocated calling 2:1 AV block type I block if the ECG showed evidence of decremental conduction with changing circumstances (when the AV conduction ratio does not change to at least 3:2 AV block so

Illogical arguments

Marriott, a renowned electrocardiographer,32, 33, 34, 35, 36 has repeatedly stated that (1) 2:1 AV block may be either type I (AV nodal with a narrow QRS) or type II (infranodal with a wide QRS). (2) Type I 2:1 AV block is more common than type II 2:1 AV block. (3) Type I and type II 2:1 AV block can often be differentiated by evaluating the “presence or absence of bundle branch block, the associated patterns of conduction and the clinical context.” Marriott's contentions really apply to the

Implications for emergency care

The reflex use of atropine in 2:1 AV block is strongly discouraged. Atropine can be administered to hemodynamically unstable patients with narrow QRS 2:1 AV block because the block is often AV nodal. Most of these patients respond to atropine by increasing the sinus rate and improving AV nodal conduction with a resultant increase in the ventricular rate. A paradoxical reaction to atropine is unusual in the setting of a narrow QRS complex. In His-Purkinje disease atropine increases the sinus

Bedside diagnostic approach

All the arguments about 2:1 AV block are also applicable to n:1 AV block where n > 2.

When faced with 2:1 AV block, bear in mind the following relatively simple points. (1) Avoid the “reflex” labels of type I or type II block. (2) Remember that the anatomical site of block should not be characterized in terms of either type I or type II because these designations refer only to electrocardiographic patterns. (3) The company 2:1 AV block keeps can provide information about the site of block. Wait

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    Returned July 27, 2000.

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    Address reprint requests to S. Serge Barold, MD, 6237 NW 21st Court, Boca Raton, FL 33496. E-mail: ssbarold\@aol.com

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    Am J Emerg Med 2001;19:214-217.

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