First degree AV block overview

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Cafer Zorkun, M.D., Ph.D. [2]

Overview

First degree AV block is a disease of the electrical conduction system of the heart in which the PR interval is lengthened or prolonged. In first degree heart block, the disease is almost always at the level of the atrioventricular node (AV node). It is defined as PR prolongation of more than 200 milli seconds (normal PR interval is between 120 and 200 msec).

Diagnosis

History and Symptoms

First degree AV block patients are usually asymptomatic at rest. In the setting of left ventricular dysfunction markedly prolonged PR interval can causeexercise intolerance and syncope.

Physical Examination

First degree AV block is an incidental finding on an EKG and is not associated with specific physical examination findings.

Electrocardiogram

In normal individuals, the AV node slows the conduction of electrical impulse through the heart. This is manifest on a surface EKG as the PR interval. The normal PR interval is from 120 milliseconds (ms) to 200 milliseconds (ms) in duration. This is measured from the initial deflection of the P wave to the beginning of the QRS complex.

In first degree heart block, the diseased AV node conducts the electrical activity slower. This is seen as a PR interval greater than 200 milliseconds (ms) in length on the surface EKG. It is usually an incidental finding on a routine EKG.

First degree heart block does not require any particular evaluation except for electrolyte and drug screens especially if an overdose is suspected.

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