All forms of cardiac arrhythmias can occur in the fetus or newborn. Those most commonly encountered include sinus tachycardia and bradycardia, premature atrial depolarizations, supraventricular tachycardia; and less commonly, atrial flutter, ventricular arrhythmias, and complete heart block. Many arrhythmias are benign, occurring in otherwise normal hearts, and are of no hemodynamic consequence. Others may result in significant acute cardiovascular compromise, particularly if they are very rapid or there is coexistent structural or functional heart disease. Sustained tacharrhythmia can lead to reversible dilated cardiomyopathy (95). Thus, in evaluating patients with arrhythmias it is important to assess the hemodynamic status and cardiac structure and function. Rarely, arrhythmias are the presenting sign of underlying cardiac abnormality such as cardiomyopathy, Ebstein anomaly, or l-transposition of the great arteries. Arrhythmias may also result from noncardiac disease; in neonates, ventricular tachycardia, ventricular fibrillation, sinus arrest, and extreme bradycardia usually occur in association with preceding severe hypoxemia, hypotension, acidosis, electrolyte disturbance, or drug toxicity (e.g., digitalis).