Inclusion Criteria:

1. Presentation with SVT due to AVRT or AVNRT.
2. Age 4 months or less at presentation.
3. No major structural heart disease (patent foramen ovale and patent ductus arteriosus are allowable.
4. No other significant co-morbid condition likely to result in non-compliance or death in next 6 months.

The SVT mechanism will be presumed to be AVRT or AVNRT if the ECG shows:

• Normal complex tachycardia with abrupt onset and offset;
• The RR interval remains relatively constant during tachycardia with heart rates of 220-310 bpm;
• VA (ventriculo-atrial) association [i.e., there is a 1:1 AV relationship (except for cases of proven AV nodal reentry with a 2:1 relationship between atrium and ventricle)]; and
• Termination of tachycardia with vagal maneuvres or adenosine with AV block or VA block.

Additional supportive information:

• The presence of a P wave in either the ST segment or T wave, or the presence of a P wave altering the terminal portion of the QRS complex;
• Spontaneous termination of the tachycardia with a P wave;
• Onset with prolongation of the PR interval;
• Altered rate with resolution of temporary bundle branch block;
• Esophageal or electrophysiology study confirming tachycardia mechanism.

Exclusion Criteria:

1. Failure to obtain consent;
2. Known hypersensitivity to either study medication or suspension;
3. Structural heart disease other than a patent foramen ovale or patent ductus arteriosus;
4. Persistent abnormal cardiac function documented by echocardiogram (shortening fraction <28%) in sinus rhythm;
5. Pre-excitation (Wolff Parkinson White syndrome);
6. Permanent junctional reciprocating tachycardia;
7. Ectopic atrial tachycardia;
8. Atrial flutter;
9. Sick sinus syndrome or significant bradycardia;
10. Long QT syndrome;
11. Digoxin > 40 micrograms/kg total received within past 7 days
12. Amiodarone >50 milligrams/kg total received within past month
13. Asthma or obstructive airway disease;
14. Renal failure.