Fetal arrhythmias

Definition

• A fetal arrhythmia is present when the heart rate lies outside the normal range of 110 to 160 beats per minute or the rhythm is irregular.

Classification

• A distinction is made between extrasystoles, tachycardias and bradycardias.
• A pathological tachycardia is a sustained rate above 180 bpm; atrial tachyarrhythmias are much more common than ventricular ones, the most frequent being supraventricular re-entry tachycardias (60–70%) and atrial flutter (25–30%).
• Bradycardias below 110 bpm result from sinus bradycardia, blocked atrial bigeminy or AV block; sustained bradycardia is most commonly due to congenital complete heart block.

Occurrence & epidemiology

Epidemiology

• The commonest rhythm abnormality is premature atrial contractions (about 1 in 400 pregnancies, 2% in the second and third trimesters), which are usually benign; complete heart block is reported in about 1 in 20,000 births.

Aetiopathogenesis

Aetiology

• Complete heart block is associated with complex heart defects in up to 53%, for example congenitally corrected transposition or heterotaxy with left atrial isomerism.
• Without heart defects it is usually caused by maternal anti-Ro/SSA antibodies, which cross the placenta from about 11 weeks and cause inflammation, fibrosis and calcification of the conduction system; onset is typically in the second trimester.
• Sinus bradycardia may indicate fetal distress, sinus node dysfunction (for example with anti-Ro antibodies or left isomerism) or long QT syndrome.

Clinical features

Clinical features and complications

• Sustained arrhythmias can lead to haemodynamic decompensation with hydrops fetalis and fetal death; sustained tachycardia with hydrops has a mortality of up to 30–50%, without hydrops about 4%.
• Anti-Ro-mediated heart block may be accompanied by cardiomyopathy, valvulitis, endocardial fibroelastosis, pericardial effusion and hydrops.

Diagnosis

• The key method is echocardiography: M-mode through the atria and ventricles and pulsed Doppler, for example of left ventricular inflow and outflow or simultaneously of the superior vena cava and aorta, determine atrial and ventricular rates and their relationship.
• Electrical activity is inferred from mechanical events; fetal magnetocardiography and fetal ECG are being developed as complementary methods.

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Cross-references