Prospective Randomized Clinical Trial of Fetal Atrial Flutter & Supraventricular Tachycardia Therapy (FAST RCT)

Inclusion Criteria:

1. Mother has provided written informed consent to participate

2. Either fetal AF without hydrops, SVT without hydrops or SVT with hydrops

3. Tachyarrhythmia that is significant enough to justify immediate transplacentalpharmacological treatment:

• Tachycardia ≥ 180 bpm during at least 10% of observation time of 30 minutes orlonger

• Tachycardia ≥ 170 bpm during +100% of time (≤ 30 0/7 weeks of gestation)

• Tachycardia ≥ 280 bpm (irrespective of SVA duration)

• SVT with fetal hydrops (irrespective of duration)

4. Gestational age > 12 0/7 weeks and <36 0/7 weeks at time of enrollment

5. Untreated tachycardia at time of enrollment

6. Singleton Pregnancy

7. Healthy mother with ± normal pre-treatment cardiovascular findings:

• ECG without significant abnormalities (sinus rhythm; QTc ≤ 0.47; PR ≤ 0.2 sec;QRS: ≤ 0.12 sec; isolated PACs or PVCs or isolated complete right bundle branchblock allowed)

• Resting heart rate ≥ 50 bpm

• Systolic BP ≥ 85 bpm

Exclusion Criteria:

1. AF with hydrops (eligible for FAST Registry only)

2. Any maternal-fetal conditions associated with high odds of premature delivery ordeath other than tachycardia (e.g. severe IUGR; premature rupture of membrane;life-threatening maternal disease (incl. pre-eclampsia; HELLP syndrome); severecongenital fetal abnormalities (T 13 or 18; surgery or death expected < 1 month)

3. History of significant maternal heart condition (open heart surgery; sick sinussyndrome; channelopathy (long QT, Brugada syndrome); ventricular tachycardia; WPWsyndrome; high-degree heart block; cardiomyopathy)

4. Relevant preexisting maternal obstructive airway disease including asthma

5. Current therapy with the following medications:

• Antiarrhythmic drugs

• Pentamidine

6. Maternal serum potassium level <3.3 mmol/L / <3.3 mEq/L (at start of treatment)

7. Maternal ionized serum calcium level of <1 mmol/L / <4 mg/dL) or total serum calciumlevel <2 mmol/L / <8mg/dL (at start of treatment)

8. Maternal serum creatinine level > 97.2 µmol/L (>1.1 mg/dl)