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Heart | Fetal Arrhythmia

The normal fetal heart has a regular rhythm ranging from 120 -160 beats per minute (bpm). If a fetus’ heart beats at an irregular or abnormally fast or slow rate, serious complications can result.

Essential Information


Fetal tachyarrhythmias such as fetal supraventricular tachycardia (SVT) or fetal atrial flutter (AF) are heart rhythms where the baby’s heart beats too fast. Without proper treatment, these can result in heart failure and/or fetal hydrops (the accumulation of fluid in multiple cavities within the fetus).

In fetal bradycardias or bradyarrhythmias, the heart beats too slowly. An example of this is “congenital heart block”, which may occur when mothers have certain antibodies in their blood that cross the placenta and interfere with the normal functioning of the electrical conduction system in the heart.

If an abnormal heart rate or rhythm is suspected, the diagnosis should be confirmed with a targeted heart ultrasound – a fetal echocardiogram. Left untreated, these arrhythmias could result in a fetus passing away inside the womb. Ongoing close surveillance in a specialized center is be warranted to ensure that, even if the heart rate returns to normal, the fetus continues to thrive.

Fetal Therapy

If there is a tachyarrhythmia (fast heart rate), mothers can be treated with medications that can cross the placenta to reach the fetal heart. These medications are usually effective at converting the heart beat back to a normal rate and rhythm, and can dramatically improve the outcome both before and after birth. Mothers who are being treated with these medications must be followed closely for side effects, and their fetuses should be monitored closely to ensure that they remain healthy.

If there is a bradyarrhythmia (slow heart rate), sometimes mothers may be treated with medication (steroids or Intravenous immunoglobulin (IVIG)) to decrease damage to the developing fetal heart.

Mothers with either tachy- or bradyarrhythmias may require special management at delivery. Their pregnancies are followed jointly though the fetal medicine unit at Mount Sinai and the fetal cardiac clinic at SickKids and are delivered at Mount Sinai hospital.

Referral Information

Please refer your patient as soon as the diagnosis of a fetal arrhythmia is suspected. Mothers may be admitted to the antenatal ward at Mount Sinai for monitoring at the start of treatment (typically for less than one week). Regular fetal echocardiographic follow-up will be organized. Local patients will be delivered at Mount Sinai to facilitate postnatal assessment at SickKids hospital.

Click here for referral to the fetal cardiac program at SickKids.

To date, none of the medications used to treat supraventricular tachycardia (SVT), including atrial flutter (AF) in-utero have been evaluated in a randomized controlled clinical trial (RCT). In the absence of such evidence, there is no consensus as to the optimal treatment. The Fetal Atrial Flutter and Supraventricular Tachycardia (FAST) Therapy Trial is a prospective international multi-center trial designed to address this knowledge gap and to guide future fetal AF & SVT therapy. It is funded by the Canadian Institutes of Health Research (CIHR) and the principal investigator is Dr. Edgar Jaeggi at SickKids hospital, a key member of the OFC team.

Referral criteria

  • Fetal tachyarrhythmia >170 100% of time or >180 for 10% of an observation time of 30 minutes or longer.

Exclusion criteria

  • maternal contra-indication to anti-arrhythmic treatment
  • high maternal creatinine
  • low maternal serum potassium
  • low calcium levels

Current Research Study

Patients will be offered randomization in the FAST-trials. These trials randomize between different types of anti-arrhythmic treatment to determine the most efficient therapeutic regimen (www.clinicaltrials.gov NCT02624765):

  • AF without hydrops will be randomized between digoxin and sotalo.
  • SVT without hydrops will be randomized between digoxin and flecainide.
  • SVT with hydrops will be randomized between digoxin plus sotalol or digoxin plus flecainide dual-therapy.