Pathophysiology of Congenital Complete Heart Block (CHB)
Congenital complete heart block is primarily caused by either maternal autoimmune factors or structural cardiac abnormalities, resulting in complete dissociation between atrial and ventricular electrical activity with significant implications for morbidity and mortality.
Primary Etiologies
1. Immune-Mediated CHB
- Maternal Autoantibodies: The most common cause of isolated congenital CHB is maternal autoantibodies (anti-Ro/SSA and anti-La/SSB) that cross the placenta 1
- Mechanism: These autoantibodies attack and destroy the developing atrioventricular (AV) node in susceptible fetuses 2
- Risk Period: The highest risk period occurs between 16-28 weeks of gestational age 2
- Maternal Status: Many mothers are asymptomatic carriers, with less than one-third having a pre-existing diagnosis of autoimmune disease such as lupus erythematosus or other connective tissue diseases 3, 2
- Incidence: Occurs in approximately 2-5% of pregnancies with positive maternal autoantibodies, with a recurrence rate of 12-25% in subsequent pregnancies 2
2. Structural CHB
- Congenital Heart Malformations: CHB is frequently associated with complex congenital heart defects 3
- Common Associations: Particularly common in corrected transposition of the great arteries (ccTGA), ostium primum atrial septal defects, and ventricular septal defects 3
- Developmental Abnormality: Results from abnormal embryonic development of the AV node 3
Electrophysiological Characteristics
- Complete AV Dissociation: Characterized by complete absence of conduction from atrium to ventricle 3, 1
- ECG Findings: Normal atrial activation with slower, dissociated regular QRS complexes 3, 1
- Escape Rhythm: Usually a stable narrow QRS-complex escape rhythm in isolated CHB 3
- QRS Duration: Varies by age - >90 ms in children <4 years, >100 ms in children 4-16 years, and ≥120 ms in adults 1
Epidemiology and Natural History
- Incidence: Occurs in approximately 1 in 15,000 to 20,000 live births 3, 1
- Mortality Risk: Highest during the first 3 months of life 3
- Long-term Risk: Significant risk of late sudden death at any age, even in previously asymptomatic patients 3
- Mechanism of Sudden Death: Either due to pauses without an escape pacemaker or pause-mediated ventricular tachyarrhythmias 3
Risk Factors for Poor Outcomes
- Heart Rate: Ventricular rate <55 bpm in infants or <70 bpm when associated with structural heart disease 3, 1
- QRS Morphology: Wide QRS escape rhythm 3, 1
- Ventricular Function: Presence of ventricular dysfunction 1
- QT Interval: Prolonged QT interval (particularly concerning due to risk of torsades de pointes) 3, 1
- Structural Disease: Presence of associated congenital heart defects 1
Pathophysiological Complications
- Ventricular Dysfunction: May occur as a consequence of myocardial autoimmune disease at a young age 3
- Pacemaker-Associated Issues: Long-term right ventricular apical pacing may lead to pacing-induced cardiomyopathy years or decades after pacemaker implantation 3, 2
- Bradycardia-Related Arrhythmias: Patients with CHB and prolonged QT interval are at risk for early afterdepolarizations and dispersion of ventricular refractoriness due to pauses or long-short RR sequences 3
Clinical Implications
- Pacemaker Indications: Permanent pacing is indicated in symptomatic patients, infants with heart rates <55 bpm (or <70 bpm with structural heart disease), and those with wide QRS, complex ventricular ectopy, or ventricular dysfunction 3
- Asymptomatic Patients: Even asymptomatic patients may benefit from pacemaker implantation due to the unpredictable risk of sudden death 3, 2
- Pacing Considerations: Alternative site pacing or biventricular pacing may be necessary to prevent pacing-induced cardiomyopathy 2
Understanding the pathophysiology of congenital CHB is essential for appropriate risk stratification, monitoring, and intervention to reduce morbidity and mortality in affected patients.