Case of the week #8

Fetal SVT in healthy mother with baseline low blood pressure

This is a case of fetal supraventricular tachycardia (SVT) in a healthy mother. Fetal SVT accounts for up to 90% of fetal tachycardias.​ Like any prolonged untreated tachycardia, they can lead to cardiac enlargement, systolic heart failure and in a fetus can ultimately cause hydrops fetalis and death.

The goal in management is to slow the heart rate down to improve the baby's cardiac output and hopefully convert the rhythm back to normal sinus. However since the baby cannot directly ingest medications or undergo direct cardioversion, they must be admistered through the mother, which complicates things.

In this particular patient, her baseline very low blood pressures make anti-arrhythmic administration more challenging.

As with adult SVT, fetal SVT results in reduced cardiac output as well as longterm complications of reduced systolic heart function and ultimately systolic heart failure.

Treatment options depend both on gestational age as well as how long the baby is actually in SVT.

  • > 37 weeks old, delivery may be the best management option followed by evaluation directly by pediatric cardiologist/electrophysiologist
     

  • < 36 weeks with fetal HR between 200-220 bpm are less likely to develop hydrops fetalis and consideration can be made for close monitoring with cardiologist and frequent HR checks.
     

  • < 36 weeks with persistent HR over 220, the mother will likely need to be admitted to the hospital for anti-arhythmic initiation and monitoring, especially if the fetal HR is in SVT >220 bpm more than 50% of the time.

Drug therapy​

 

Transplacental therapy (via the mom) is the standard management. All considerations for treatment need to account for both the fetus and the mother's needs/contraindications and should be monitored in a hospital setting.

Historically digoxin has been the first line choice due to its oral option and generally well tolerated (with treatment goal of 1-2ng/mL maternal level).

2011 multi-center, non-randomized trial looked at fetal SVT and atrial fibrilation suppression with digoxin (24 patients), sotalol (n=52), and flecainide (n=35). They found that flecainide and digoxin were superior to sotalol in converting SVT to a normal rhythm and in slowing both AF and SVT to better-tolerated ventricular rates and therefore might be considered first to treat significant fetal tachyarrhythmia."

In a more recent 2017 meta-analysis of fetal tachycardias, 21 studies were included, and flecainide and sotalol were superior to digoxin for converting to sinus rhythm. The authors of this study recommend against digoxin for first-line therapy.

In this particular patient, she was started on digoxin and titrated to maternal therapeutic level of 1-2ng/mL but the fetus maintained >50% in SVT. It also caused recurrent sinus bradycardia in the mother, though was largely asymptomatic.

After 3 days of digoxin, she was upgraded to the cardiac telemetry unit and started on sotalol, titrated up to 120mg BID. However the mother's blood pressure dropped to as low as 70s/40-50, and the fetal HR remained predominantly in SVT. Mother was then switched to amiodarone 400mg bid and the fetus successfully converted to sinus rhythm.

Have you seen any fetal tachycardias in your practice?

POCUS Med Ed

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