The definition of first-degree atrioventricular (AV) block is a PR interval of greater than 0.20 seconds on electrocardiography (ECG) without disruption of atrial to ventricular conduction (figure). The normal measurement of the PR interval is 0.12 seconds to 0.20 seconds. When the PR interval prolongs more than 0.30 seconds, the first-degree atrioventricular block is called "marked." In certain situations, the P waves can be within the preceding T waves. It is generally asymptomatic and without significant complications. For the vast majority of patients, no treatment is necessary beyond routine observation for worsening conduction delay. Regular evaluation is essential, as affected patients have demonstrated an increased risk of developing atrial fibrillation or higher degree AV block.