Fetal Arrhythmia

Fetal arrhythmia is any abnormal fetal heart rhythm, or a sustained rate outside 110 to 160 beats per minute, and it includes ectopic beats, tachyarrhythmias and bradyarrhythmias. Most are isolated premature atrial beats that settle without treatment. Sustained tachycardia or complete heart block can cause heart failure, hydrops and fetal death. The rhythm type decides whether to observe, give the mother drugs or plan delivery.

• Types: premature atrial or ventricular beats, supraventricular tachycardia, atrial flutter, ventricular tachycardia, complete heart block, long QT syndrome.
• Risk factors: maternal anti-Ro or anti-La antibodies, maternal thyroid disease, caffeine, beta agonist drugs, structural heart disease, infection.
• Complete heart block with a structurally normal heart is usually caused by maternal antibodies.

Women are usually unaware, and an irregular or abnormal rate is found on auscultation or Doppler at an antenatal visit. Listen to the fetal heart for a full minute and record rate and regularity.

• Irregular rhythm with pauses suggests ectopic beats.
• Persistent rate above 180 beats per minute or below 100 beats per minute is abnormal.
• Differential diagnosis: normal variation, maternal pulse misread as fetal, ectopic beats, supraventricular tachycardia, atrial flutter, complete heart block, long QT syndrome, fetal hypoxia, fetal anaemia, maternal fever.
• Complications: heart failure, hydrops fetalis, preterm birth, stillbirth, neonatal death, cardiomyopathy.

The approach is to define the rhythm on ultrasound and to look for a cause and for heart failure.

• M mode or pulsed Doppler to record atrial and ventricular rates and the timing between them; rate above 180 beats per minute with 1 to 1 conduction suggests supraventricular tachycardia; atrial rate above 300 beats per minute with block suggests flutter; rate below 100 beats per minute with independent atria and ventricles is complete heart block.
• Fetal echocardiography for structure, function, effusions and the PR interval; check for hydrops (ascites, pleural effusion, skin oedema).
• Maternal tests: anti-Ro and anti-La antibodies, TSH and free T4, random glucose, ECG (long QT), electrolytes, full blood count, parvovirus, cytomegalovirus and syphilis tests.
• Where echocardiography is unavailable, auscultate weekly and refer any sustained abnormal rate.

The principle is to observe benign ectopy, treat sustained tachycardia with maternal drugs, and plan delivery in a unit with neonatal cardiology.

• Supportive: stop caffeine, alcohol, smoking, decongestants and beta agonist drugs; weekly fetal rhythm check; daily fetal movement counts; delivery in a facility with neonatal intensive care.
• Pharmacological: ectopic beats need no drug; sustained tachycardia without hydrops: digoxin 0.5 mg oral 8-hourly for 3 doses then 0.25 mg oral 12-hourly, aiming for a maternal serum level of 1.0 to 2.0 nanograms/mL; with hydrops or failure of digoxin: flecainide 100 mg oral 8-hourly (maximum 300 mg daily, stopped if the maternal QRS widens by 25 percent) or sotalol 80 mg oral 12-hourly increased to 160 mg 12-hourly; incomplete or complete heart block with myocarditis or hydrops: dexamethasone 4 mg oral daily until delivery; betamethasone 12 mg IM twice, 24 hours apart, between 24 and 34 weeks and 6 days if preterm delivery is likely.
• Procedures: maternal ECG and electrolyte checks on antiarrhythmic drugs; fetal medicine therapy by direct fetal injection for refractory cases; delivery by induction or caesarean section at term, earlier for hydrops that fails treatment; neonatal pacing for complete heart block.
• Special groups: women with anti-Ro or anti-La antibodies (lupus, Sjogren syndrome) have fetal heart rate and PR interval measured weekly from 16 to 26 weeks and take hydroxychloroquine 200 mg oral 12-hourly throughout pregnancy after an affected baby.
• Red flags: rate above 200 or below 55 beats per minute, hydrops, ascites, reduced fetal movements, maternal palpitations on drugs, QRS widening.
• Follow-up: rhythm check weekly until normal, then monthly; echocardiography every 1 to 2 weeks while on treatment; newborn ECG at birth and at 1 week.