Most Common Fetal Arrhythmias and Causes of Intermittent Fetal Bradycardia
The most common fetal arrhythmias causing intermittent bradycardia are blocked atrial ectopic beats (premature atrial contractions with 2:1 atrioventricular block) and ventricular bigeminy, both of which have excellent prognosis and require no treatment 1.
Common Fetal Arrhythmias
The spectrum of fetal arrhythmias includes:
Benign Arrhythmias (Most Common)
- Isolated premature atrial contractions (PACs): Most frequent arrhythmia overall, accounting for the majority of irregular fetal heart rhythms 2, 3
- Blocked atrial extrasystoles with 2:1 block: Creates characteristic rapid alternation between two different heart rate levels on cardiotocography 1
- Ventricular bigeminy: Also presents with alternating heart rate patterns 1
- Intermittent sinus bradycardia: Benign variant with good prognosis 1
Pathologic Arrhythmias Requiring Treatment
- Supraventricular tachycardia (SVT): Most common sustained tachyarrhythmia (>180 bpm) 4, 5
- Atrial flutter: Second most common tachyarrhythmia, typically with 2:1 AV conduction 4, 5
- Complete atrioventricular block: Most common cause of sustained fetal bradycardia (<100 bpm) 5, 2
Specific Causes of Intermittent Fetal Bradycardia
Primary Mechanisms
Blocked ectopic beats are the predominant cause of intermittent bradycardia:
- Atrial extrasystoles with 2:1 or 3:1 block create the appearance of bradycardia on monitoring
- The CTG pattern shows rapid oscillation between two frequency levels 1
- These are benign and self-limited
Intermittent sinus bradycardia occurs transiently and has favorable prognosis 1
Sustained Bradycardia (Different Entity)
When bradycardia is persistent rather than intermittent, consider:
Congenital complete heart block 4:
- Immune-mediated (50% of cases): Transplacental passage of maternal anti-Ro/SSA and anti-La/SSB antibodies in mothers with lupus or other connective tissue diseases
- Structural cardiac defects (50% of cases): Endocardial cushion defects, heterotaxy syndrome, left atrial isomerism 6, 2
Critical Diagnostic Distinction
The CTG pattern differentiates these entities 1:
- Blocked ectopic beats: Characteristic rapid alternation between two heart rate levels
- True sinus bradycardia or complete heart block: No typical distinguishing CTG pattern; requires fetal echocardiography for definitive diagnosis
Clinical Approach
When to Investigate Further
Perform fetal echocardiography when 1:
- Bradycardia is persistent or repetitive
- Duration exceeds what would be expected for benign ectopy
- Any concern for structural heart disease or hydrops
Prognosis Stratification
- Excellent prognosis: Blocked atrial ectopy, ventricular bigeminy, intermittent sinus bradycardia 1
- Poor prognosis: Complete AV block with hydrops, especially when associated with structural heart defects (92% fetal/infant mortality) 4
- Variable prognosis: Complete AV block depends on degree of bradycardia and presence of congenital heart malformation 1
Treatment Considerations
For intermittent bradycardia from blocked ectopy: No treatment required; reassurance only 1, 2
For complete heart block with hydrops: Corticosteroid therapy after third-degree block develops has not shown benefit, and prognosis remains poor 4. In-utero therapy is considered investigational and not recommended outside research settings 4.
Common Pitfall
Do not confuse intermittent bradycardia from blocked ectopic beats (benign, self-limited) with sustained bradycardia from complete heart block (potentially life-threatening). The former requires only observation, while the latter necessitates detailed fetal echocardiography, maternal antibody testing, and consideration for early delivery with neonatal pacing capability 4, 5.