Fetal Arrhythmia

Definition

Fetal arrhythmias are rhythm disturbances detected in the fetus characterised by a heart rate outside the normal range of 110–160 bpm, or by an irregular rhythm within that range. They are diagnosed in approximately 1% of all fetuses and up to 49% of referrals for fetal echocardiography. The hemodynamic consequences depend on the heart rate, duration, mechanism, and degree of irregularity. Management is primarily transplacental (maternal systemic drug administration), with direct fetal drug injection reserved for refractory cases.

Key Concepts

Diagnosis

• Screening: fetal auscultation, non-stress testing, fetal doppler
• Fetal echocardiography: key tool to characterise rhythm and assess hemodynamic consequences (ventricular function, presence of hydrops) sources/arrhythmia-pregnancy-hrs-2023 (rating: very high)
• Fetal magnetocardiography (fMCG): non-invasive; provides electrogram-equivalent signal; useful for precise arrhythmia classification
• Normal fetal HR: 110–160 bpm; rhythms in normal range but irregular can go undetected until birth
• Cardio-obstetrics team: pediatric cardiologist or EP, MFM subspecialist, neonatologist sources/arrhythmia-pregnancy-hrs-2023

Classification

• Tachyarrhythmias: SVT, atrial flutter (AFL), atrial tachycardia (AT), ventricular tachycardia (VT), junctional tachycardia
• Bradyarrhythmias: congenital heart block (CHB), sinus bradycardia, sinus node dysfunction
• Ectopy: PACs (most common irregular finding; generally benign)
• Intermittent (<50% of the time or <12 hours/day) vs incessant (>50% of the time or >12 hours/day) — critical distinction for management

Fetal Atrial Tachyarrhythmias

Epidemiology and Mechanisms

• SVT: most common sustained fetal tachyarrhythmia; re-entrant (AVRT) most frequent mechanism
• AFL: accounts for ~30% of fetal tachyarrhythmias; often associated with structural CHD sources/arrhythmia-pregnancy-hrs-2023
• Incessant fetal SVT or AFL: associated with fetal hydrops and ventricular dysfunction; mortality 5–30% for incessant AFL with hydrops
• PACs in fetus: common, benign; monitor with serial weekly doppler auscultation for 1–2 weeks after resolution (1–3% progress to SVT) sources/arrhythmia-pregnancy-hrs-2023

Management Principles

• Intermittent SVT or AFL (no hydrops): monitoring without pharmacotherapy; excellent prognosis sources/arrhythmia-pregnancy-hrs-2023
• Incessant SVT or AFL (with or without hydrops): pharmacological therapy indicated sources/arrhythmia-pregnancy-hrs-2023
• Treatment is transplacental (maternal drug administration) — exposes mother to antiarrhythmic side effects
• Close-to-term fetus with incessant flutter/SVT + hydrops: delivery preferred over ongoing maternal pharmacotherapy sources/arrhythmia-pregnancy-hrs-2023

Drug Hierarchy for Fetal SVT

1. Flecainide (first-line): Superior to digoxin and sotalol for fetal SVT; advantage most pronounced when hydrops is present — meta-analysis of 10 studies, 537 patients sources/arrhythmia-pregnancy-hrs-2023
2. Digoxin: Less effective when hydrops present (impaired transplacental transfer); may combine with direct intramuscular injection if transplacental delivery fails sources/arrhythmia-pregnancy-hrs-2023
3. Sotalol: Alternative; trend toward better termination rates vs digoxin in some studies; data conflicting for AFL specifically sources/arrhythmia-pregnancy-hrs-2023
4. Amiodarone: Last resort for drug-refractory SVT; converts 14/15 in one series; transient adverse effects in 5 infants and 9 mothers sources/arrhythmia-pregnancy-hrs-2023
5. Direct fetal administration (intraperitoneal or umbilical): Reserved for refractory SVT with hydrops when transplacental approach fails sources/arrhythmia-pregnancy-hrs-2023

• Verapamil: avoid — reports of unexpected fetal death, fetal bradycardia, hypotension, myocardial depression, asystole; safer alternatives exist sources/arrhythmia-pregnancy-hrs-2023

Drug Hierarchy for Fetal AFL

1. Sotalol or digoxin (first-line for AFL): No strong evidence favouring one over the other; sotalol may have slight advantage for incessant flutter sources/arrhythmia-pregnancy-hrs-2023
2. Flecainide: less evidence for AFL than SVT
3. Multiple drug combinations for refractory cases

• Infants with AFL respond well to cardioversion after delivery; low risk of postnatal recurrence sources/arrhythmia-pregnancy-hrs-2023

Fetal Ventricular Tachycardia

Causes

• Most common: Inherited arrhythmia syndromes (especially LQTS) — see concepts/LQTS-Pregnancy-Management
• Non-IAS causes: AV block, cardiac tumours, myocarditis, ventricular aneurysms, cardiomyopathy sources/arrhythmia-pregnancy-hrs-2023
• Accelerated idioventricular tachycardia: Generally benign; rate near normal; usually no therapy unless hydrops sources/arrhythmia-pregnancy-hrs-2023
• Sustained VT in fetus: associated with high morbidity and mortality

Management

• First-line: Maternal IV magnesium + lidocaine; limit magnesium to <48 hours (maternal toxicity); oral propranolol and mexiletine are alternatives sources/arrhythmia-pregnancy-hrs-2023
• Refractory: Sotalol, flecainide, amiodarone — no single drug proven superior; drug choice guided by arrhythmia aetiology and maternal substrate sources/arrhythmia-pregnancy-hrs-2023
• If suspected LQTS/IAS-related VT: avoid QT-prolonging antiarrhythmics (all can worsen LQTS) sources/arrhythmia-pregnancy-hrs-2023
• Close-to-term fetus with sustained VT: delivery and direct treatment (cardioversion/defibrillation + antiarrhythmics) superior to in utero transplacental therapy sources/arrhythmia-pregnancy-hrs-2023
• Myocarditis-associated VT: dexamethasone + IVIG (limited, conflicting data) sources/arrhythmia-pregnancy-hrs-2023
• Inefficacy of transplacental drugs is magnified in hydrops (impaired placental transfer) sources/arrhythmia-pregnancy-hrs-2023

Fetal Bradycardia and Conduction Disorders

Congenital Heart Block (CHB)

• Most commonly caused by transplacental passage of maternal anti-Ro/SSA and anti-La/SSB antibodies (neonatal lupus) sources/arrhythmia-pregnancy-hrs-2023
• Complete (third-degree) CHB: high risk of fetal hydrops and death; requires multidisciplinary management
• Dexamethasone: May prevent progression from second to third-degree heart block; benefit for established third-degree CHB is controversial sources/arrhythmia-pregnancy-hrs-2023
• IVIG: Used in immune-mediated CHB; limited data
• After delivery: pacing indicated for symptomatic neonatal complete heart block
• Isolated sinus bradycardia: may reflect hypoxia, vagal response, or medication effect; evaluate underlying cause

Fetal Bradycardia Monitoring

• Serial fetal wellbeing assessment with doppler/echo mandatory when CHB detected
• Cardio-obstetrics team involvement essential for antenatal management and delivery planning sources/arrhythmia-pregnancy-hrs-2023

Inherited Arrhythmia Syndromes in the Fetus

• 50% risk of inheriting IAS if one parent affected (autosomal dominant); double mutations possible for SCN5A (LQT3) sources/arrhythmia-pregnancy-hrs-2023
• LQT1 (KCNQ1), LQT2 (KCNH2), LQT3 (SCN5A) account for 75–85% of fetal IAS presentations sources/arrhythmia-pregnancy-hrs-2023
• Fetal manifestations of LQTS: bradycardia, second-degree heart block, TdP (polymorphic VT); reliably diagnosed after 27 weeks gestation on fetal echo sources/arrhythmia-pregnancy-hrs-2023
• Genetic testing of parents informs fetal risk stratification; post-mortem genetic testing in unexplained stillbirth (5–10% may harbour IAS) sources/arrhythmia-pregnancy-hrs-2023
• Fetal LQTS-VT management: maternal IV magnesium first-line; propranolol/mexiletine second-line; avoid QT-prolonging drugs sources/arrhythmia-pregnancy-hrs-2023
• See concepts/LQTS-Pregnancy-Management for LQTS fetal management

Contradictions / Open Questions

• No prospective RCT exists comparing flecainide vs digoxin vs sotalol for fetal SVT — all evidence is from retrospective non-randomised studies; superiority of flecainide is based on meta-analysis of heterogeneous observational data sources/arrhythmia-pregnancy-hrs-2023
• AFL: no clear first-line agent — systematic review/meta-analysis of AFL found no significant difference between sotalol and digoxin; single multicentre study (Jaeggi) showed sotalol advantage, but overall data conflicting sources/arrhythmia-pregnancy-hrs-2023
• Dexamethasone for CHB: Studies show it may prevent progression (2nd → 3rd degree) but does not reverse established complete heart block; risk-benefit debate ongoing given steroid adverse effects sources/arrhythmia-pregnancy-hrs-2023
• Verapamil contraindication: Based on case reports of unexpected fetal death; confounders possible; safer alternatives now available make further study in humans unlikely sources/arrhythmia-pregnancy-hrs-2023
• Direct fetal drug injection: Role is empirical; risk-benefit assessment for intramuscular/intraperitoneal vs preterm delivery is case-by-case and not protocol-driven sources/arrhythmia-pregnancy-hrs-2023

Connections

• Related to concepts/Arrhythmia-in-Pregnancy — parent concept
• Related to concepts/LQTS-Pregnancy-Management — LQTS as primary cause of fetal VT
• Related to concepts/Cardio-Obstetrics — multidisciplinary team framework
• Related to entities/Flecainide — first-line for fetal SVT
• Related to entities/Long-QT-Syndrome — primary IAS cause of fetal VT
• Related to entities/Atrial-Flutter — fetal AFL management
• Related to entities/Amiodarone — last resort for refractory fetal SVT

Sources

• sources/arrhythmia-pregnancy-hrs-2023