Atrioventricular block overview

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]

Overview

An atrioventricular block (or AV block) is a type of heart block involving an impairment of the conduction between the atria and ventricles of the heart.[1] It usually involves the atrioventricular node, but it can involve other structures too.

Classification

AV block is categorized by degree and site of conduction block. In first-degree AV block, all atrial impulses are conducted to the ventricle. However, there is a delay within the AV node, resulting in a prolonged PR interval on ECG (>200 msec or >5 small blocks). Second-degree AV block can be of one of the two types: Mobitz type I and Mobitz type II. Mobitz I, or Wenckebach block , consists of progressive prolongation of PR interval, until loss of conduction to the ventricle occurs (missed beat). Mobitz I block is rarely symptomatic and does not require treatment. On the other hand, Mobitz II AV block is characterized by a constant PR interval with intermittent missed beats. The missed beats can occur with varying frequency such as occasional to 3:1 or 2:1. Complete heart block (third-degree heart block) is characterized by a complete lack of conduction from the atria to the ventricles. The ventricular rates in complete heart blocks are slower than the atrial rate. A junctional escape rate is generally between 40 and 60 beats/min and shows narrow QRS complex on EKG, whereas a ventricular escape rate is slower with a wide QRS complex.

Pathophysiology

Atrioventricular (AV) block is caused by one of the following mechanisms i.e. fibrosis or degeneration of the conduction system, ischemic heart disease, or medications.

Diagnosis

Electrocardiography

The main diagnostic modality used in determining whether a person has heart block, is the electrocardiogram. First degree heart block consists of a prolonged PR interval of more than >200msec. Second degree heart block consists of Mobitz type I and Mobitz type II heart block. Mobitz I or Wenckebach block will show a progressive prolongation of the PR interval, until a ventricular beat is missed. Mobitz II AV block consists of a constant PR interval with intermittent missed beats. Complete heart block or third degree heart block will be depicted by a complete disassociation of atrial and ventricular beats.

Treatment

Medical Therapy

Treatment goal is to remove the extrinsic causes and treat reversible intrinsic causes. Drugs used in the treatment of AV nodal blocks include atropine, isoproterenol, theophylline, antibiotics treatment for lyme disease and treatment of ischemia. A permanent pacemaker is indicated for symptomatic bradycardia due to advanced second- or third-degree heart blocks.

References

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