First degree A-V block is characterized by an abnormally prolonged P-R interval for age and heart rate. Most commonly, this is as a result of enhanced vagal tone and seen during sleep. Rarely this can be secondary to antiarrhythmic medications (e.g., digoxin), hypothermia, increased parasympathetic tone, hypothyroidism, or electrolyte disorders (hypo- or hyperkalemia, hypo- or hypercalcemia, hypoglycemia, and hypomagnesemia). First-degree AV block is generally well tolerated and requires no specific therapy. However, an infant with a muscular dystrophy, Kearns-Sayre syndrome, neonatal lupus, a family history of complete heart block or maternal connective tissue disease should be followed for possible progression of the conduction abnormality.