First-degree heart block, also called first-degree atrioventricular (AV) block, is a cardiac conduction abnormality in which electrical impulses from the atria reach the ventricles more slowly than normal. It occurs due to delayed conduction through the AV node or, less commonly, through the His-Purkinje system. Although it is usually a mild form of AV block and may be asymptomatic, careful assessment is important in critically ill patients because it may be associated with underlying cardiac disease, medications, electrolyte abnormalities, or progression to a higher degree of heart block. The main electrocardiographic feature is a prolonged PR interval of more than 0.20 seconds (200 milliseconds), with every atrial impulse still conducted to the ventricles. The PR interval remains relatively constant from beat to beat. Unlike second-degree heart block, there are no dropped QRS complexes. The QRS complex may be normal or widened depending on the location of the conduction delay. The heart rate is generally normal, although bradycardia may occur because of an associated condition. Common causes include increased vagal tone, ischemic heart disease, myocardial infarction, myocarditis, cardiomyopathy, and