First-degree atrioventricular (AV) block refers to delayed electrical conduction between the atrium and the ventricle. It is identified by a long PR interval (> 200 msec). In adults, the conduction delay most commonly occurs within the AV node.

Potential underlying causes for first-degree AV block include myocardial infarction, coronary artery disease, neuromuscular disorders, and infiltrative disorders such as sarcoidosis, systemic lupus erythematosus, bacterial endocarditis, and Lyme carditis. Medications that block the AV node such as beta blockers, non-dihydropyridine calcium channel blockers, and digoxin may also cause first-degree AV block.

First-degree AV block is associated with increased vagal tone in younger patients (which reduces the rate of impulse conduction) and age-related conduction abnormalities in the elderly. Most patients are asymptomatic, but rarely patients may present with dyspnea, lightheadedness, presyncope, syncope, fatigue, or exercise intolerance, particularly if the patient has intermittently occurring second-degree or third-degree AV block.