Heart | Fetal Arrhythmia

Within the heart lies a highly complex conduction system that allows the heart to beat at a normal rate. In unborn babies, a normal heart rate has a regular rhythm and ranges from 120 -160 beats per minute. If an unborn baby’s heart beats at an irregular or abnormally fast or slow rate inside the womb, serious complications can result.

Essential Information

  • Fetal tachyarrhythmias, such as fetal supraventricular tachycardia (SVT) or fetal atrial flutter (AF) refer to irregular heart rhythms where the fetus’ heart beats too fast.
  • Fetal bradycardias, such as congenital heart block, refer to irregular heart rhythms where the fetus’ heart beats too slowly.
  • These rhythms may be identified at a regular prenatal appointment, when listening to the unborn baby’s heartbeat. The abnormalities, and their potential consequences, may also be discovered on ultrasound. These should be confirmed and investigated in specialized centres like the Ontario Fetal Centre.
  • Medications provided to mom can cross the placenta and reach baby’s circulation. These are highly effective treatments to return the fetus’ heartbeat to a normal rhythm.
  • If left untreated, these abnormal heart rhythms can result in heart failure or may lead to baby passing away.

Overview

Within the heart lies a highly complex conduction system that allows the heart to beat at a normal rate. In unborn babies, a normal heart rate has a regular rhythm and ranges from 120 -160 beats per minute. If an unborn baby’s heart beats at an irregular or abnormally fast or slow rate inside the womb, serious complications can result.

Fetal tachyarrhythmias, such as fetal supraventricular tachycardia (SVT) or fetal atrial flutter (AF) refer to irregular heart rhythms where the fetus’ heart beats too fast (sometimes twice as fast as it should). Without proper treatment, fetal tachyarrhythmias can result in heart failure and/or fetal hydrops (a term that refers to the accumulation of fluid in multiple cavities within the fetus, such as the heart, the lungs or the abdomen). If left untreated, these arrhythmias might result in the fetus passing away inside the womb.

Fetal bradycardias refer to irregular heart rhythms where the unborn baby’s heart beats too slowly. An example of this is congenital heart block, which may result, in some cases, if mothers carry certain antibodies in their blood that can interfere with the normal functioning of the fetus’ conduction system.

If an abnormal heart rate or rhythm is suspected, the diagnosis should be confirmed with a targeted ultrasound of fetus’ heart, called an echocardiogram. Ongoing close surveillance in a specialized centre will be warranted to ensure that, even if the fetus’ heart rate returns to normal, the unborn baby continues to thrive inside the womb.

 

Treatment

Mothers will be treated with medications that travel through the placenta to reach the unborn baby directly. These medications are highly effective at converting the fetus’ heartbeat back to a normal rate and rhythm, and can dramatically improve the outcomes for babies after they are born. Mothers who are being treated with these medications will be monitored for side effects, and the babies should be watched closely to ensure they continue to appear healthy on ultrasound. The mother may be admitted to our high-risk antental unit for monitoring for approx. one week and may be required to plan to birth at Mount Sinai Hospital. The multidisciplinary team will determine the care plan.

If the unborn baby’s heart rate remains high post-medication, surgical intervention after birth may be required.

Referral Information (for physicians)

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

Inclusion criteria

Fetal tachyarrhythmia >170 bpm 100% of time or >180 bpm for 10% of 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 (for physicians)

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