Digoxin and propranolol have a wide range of reported efficacy for preventing recurrence of SVT [5, 17] and neither digoxin nor propranolol is fully effective; treatment failures can reach 26% [17] and additional antiarrhythmics are often required [4, 8, 16, 26, 30-36]. Digoxin and propranolol are often referred to as "first-line agents", i.e. the medications that will be started initially to prevent recurrences of SVT. Several studies have reported on the use of medications in combination without specifying details (see Table 1). There are limited data on side effect profiles of digoxin and propranolol [16, 37, 38]. Despite this lack of data, digoxin and propranolol are used over other agents whose side effect profiles have been the subject of greater scrutiny, perhaps due to their relative "newness" [30-33, 35, 36, 39]. Both are frequently used for chronic therapy of infant SVT, but they have never been compared in a controlled trial.