Excerpt from Heart Block, First Degree

Synonyms, Key Words, and Related Terms: atrioventricular block, first-degree atrioventricular block, AV block, first-degree AV block, first-degree heart block, prolongation of the PR interval, P wave, PR interval

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Background: On an electrocardiogram (ECG), the PR interval, which is defined as the time from the initial deflection of the P wave to the start of the QRS complex, should be between 120 and 200 msec. First-degree heart block, or first-degree atrioventricular (AV) block, is defined as prolongation of the PR interval on the ECG to more than 200 msec.

Pathophysiology: With first-degree AV block, every atrial impulse is transmitted to the ventricles, resulting in a regular ventricular rate. This type of AV block can arise from delays in the conduction system in the AV node itself, the His-Purkinje system, or a combination of both. Overall, dysfunction at the AV node is much more common than dysfunction at the His-Purkinje system. If the QRS complex is of normal width and morphology on the ECG, then the conduction delay is almost always at the level of the AV node. If, however, the QRS demonstrates a bundle-branch morphology, then the level of the conduction delay is often localized to the His-Purkinje system.

Frequency:

In the US: In the United States, the prevalence of first-degree AV block among young adults ranges from 0.65-1.6%. Higher prevalence is reported in studies of trained athletes (8.7%) and medical students (8%). The incidence is 1.13 per 1000 persons. It is more common among African Americans compared with Caucasian populations. The prevalence of first-degree AV block increases with advancing age.

Mortality/Morbidity: In and of itself, first-degree AV block is a benign condition, with no associated increase in morbidity or mortality.