3. Incidence of adverse outcomes in infants treated with propranolol or digoxin. BACKGROUND SUPRAVENTRICULAR TACHYCARDIA In infants and children, SVT results primarily from an accessory connection between the atrium and ventricle [5, 8-10, 13]. This provides the substrate for a reentrant circuit utilizing the normal conduction system and the accessory pathway, resulting in an atrioventricular reciprocating tachycardia (AVRT). A similar reentrant circuit can exist within the atrioventricular node, resulting in an atrioventricular nodal reentrant tachycardia (AVNRT); however, this is much less common in infants [5, 13, 14]. Both AVRT and AVNRT result in a very rapid heart rate with an attendant decrease in diastolic filling time and cardiac output. The differentiation of these two mechanisms is not always easily made on the surface electrocardiogram (ECG) [8, 15], and currently the same therapies are applied for both conditions [16].