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).
Historical Perspective
First degree AV block was first described by Dr. Engelmann in 1984. Dr. Ashmar further studied the blocked impulses and its impact on the conduction in the myocardium.
Classification
There is no established system for the classification of First degree AV block.
Pathophysiology
The atrioventricular node is a normal electrical pathway between the atria and ventricles and it is located in the right atrium. First degree AV block pathogenesis can be attributed to an electrical conduction delay in the AV node or His-Purkinje system. First degree AV block can be associated with normal QRS complex or wide QRS complex on the ECG.
Causes
An atrioventricular block (or AV block) is a type of heart block involving an impairment of the conduction between the atria and the ventricles of the heart. It usually involves the atrioventricular node, but it can involve other structures too. AV block is categorized according to the degree and the site of conduction block. In first-degree AV block, all atrial impulses are conducted to the ventricles; however, there is a delay in conduction within the AV node resulting in a prolonged PR interval on ECG (>200 msec or >5 small blocks). In other words, first degree block is a slowed conduction without loss of atrioventricular synchrony.
Differentiating Xyz from Other Diseases
Epidemiology and Demographics
Risk Factors
Screening
Natural History, Complications, and Prognosis
Diagnosis
Diagnostic Study of Choice
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.
Laboratory 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.