WHAT IS IT?

A fetal arrhythmia is an abnormality in fetal heart rate and/or cardiac rhythm. Most often, the unusual rhythm is detected during an otherwise normal prenatal visit.

Arrhythmias presenting with an irregular rhythm include:

• Premature Atrial Contractions (PACs)
  • PACs are common in fetuses and may be seen in up to 51% of healthy newborns
    • A premature atrial contraction is usually followed by a ventricular contraction that will also be premature, but will occur at the appropriate time in the cycle
    • There is a low risk of fetal tachycardia
    • Considered normal at birth unless other problems are found in the nursery
  • PACs will usually spontaneously resolve
• Premature Ventricular Contractions (PVCs)
  • Fetal PVCs are considerably less common than PACs
  • The PVC is not preceded by a normal atrial beat
  • In isolation, PVCs should be monitored regularly, but intervention is usually not required

Arrhythmias presenting with a slow heart rate include:

• Sinus Bradycardia – 80-110 bpm (beats per minute)
  • The rhythm appears normal but the rate is slow with respect to gestational age
  • May be well tolerated and require monitoring but no intervention
  • Underlying causes can include non-reassuring fetal status or cardiac structural defects, or maternal antibodies such as anti-SSA (Ro), which may require anti-inflammatory therapy
• Complete Heart Block (CHB) – 40-80 bpm
  • This is a complete failure of normal atrial impulses to the ventricles such that the atria and ventricles are no longer coordinated
  • Underlying causes can include cardiac structural defects (50%)
  • Maternal anti-SSA (Ro) anti-SSB (La) autoantibodies are  strongly associated with CHB and there is an associated risk of fetal loss, especially with fetal heart rate below 55 bpm
    • Mothers are often asymptomatic with no evidence of autoimmune disease such as lupus
      • antibodies first detected upon evaluation for fetal arrhythmia
    • The risk for acquired heart block in the fetus in the setting of maternal anti-SSA and anti-SSB antibodies is approximately 2–3%, with a recurrence risk of 14–17%
    • In newborn with otherwise normal cardiac exam but CHB, likely cause is autoantibodies
    • Cutaneous findings consistent with neonatal lupus erythematosus are more common (16%), than CHB with anti-SSA/SSB
  • Medical therapy may be used prenatally in the case of antibody related CHB with pacing after delivery

Arrhythmias presenting with an increased heart rate include: 

• Fetal heart rate above 180 bp should be considered critical
  • Supraventricular Tachycardia (SVT) – 190-300 bpm
    • May be monitored if infrequent but otherwise maternal anti-arrhythmic therapy (e.g. digoxin) may be required
  • Atrial Flutter (FT) – ventricular rate – 150-250 bpm
    • Almost exclusively seen in the 3rd trimester or at birth
    • Usually well tolerated and can be treated with maternal anti-arrhythmic agents (e.g. digoxin)
  • Sinus Tachycardia – 200+ bpm
    • Look for and manage underlying cause such as infection or non-reassuring fetal status

KEY POINTS:

• Identifying an unusual fetal cardiac rhythm or rate during a normal antepartum visit may be a key moment in detecting a significant cardiac arrhythmia
• Even though fetuses may easily tolerate irregular rhythms such as PACs, any change from a normal heart rate or regular rhythm pattern should trigger a referral to a high risk specialist who can make the diagnosis and with a multidisciplinary team, can determine whether to monitor, treat or deliver
• Fetal echocardiography remains the mainstay of diagnosis, as well as a detailed fetal anatomy scan to determine if any other structural abnormalities exist
  • Refer to high risk center for diagnosis and management plan
• The above list constitutes the more common types of fetal arrhythmias, but more complex and rare forms may exist

Learn More – Primary Sources:

Fetal arrhythmias: diagnosis and treatment

Fetal and Neonatal Arrhythmias

Diagnosis and treatment of fetal arrhythmia

Anti-SSA/Ro antibodies and the heart: more than complete congenital heart block? A review of electrocardiographic and myocardial abnormalities and of treatment options

Incidence and spectrum of neonatal lupus erythematosus: a prospective study of infants born to mothers with anti-Ro autoantibodies

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