What is the medical etiology of heart block?

Medical Etiology of Heart Block

Heart block results from disruption of the cardiac conduction system at various anatomical levels, with etiologies broadly categorized into congenital abnormalities, acquired cardiac diseases, ischemic injury, iatrogenic causes, infiltrative processes, and medication effects.

Congenital Causes

Congenital atrioventricular block arises from two primary mechanisms 1:

• Abnormal embryonic development of the AV node represents the structural developmental pathway 1
• Maternal lupus erythematosus causes immune-mediated destruction, with maternal SSA antibodies crossing the placenta and causing autoimmune destruction and fibrosis of the fetal AV node 2
• Associated congenital heart diseases including corrected transposition of the great arteries, ostium primum atrial septal defects, and ventricular septal defects frequently present with third-degree AV block 1
• Endocardial cushion defects are inherently associated with a fragile atrioventricular conduction system requiring rigorous monitoring 1

Ischemic Causes

Acute myocardial infarction represents the most common acquired etiology 3, 4:

• Up to 20% of MI patients develop some form of conduction disturbance, with complete heart block occurring in 8% of post-MI patients 3
• Anterior infarctions with persistent intraventricular conduction disturbances carry an unfavorable prognosis and increased sudden cardiac death risk 1
• Inferior MI-associated AV block typically occurs at the AV node level and has a more favorable prognosis, with permanent pacing rarely necessary unless block persists beyond 14-16 days 1
• Patients with ischemic complete heart block are younger and have lower ejection fractions (mean 49.6% vs 57.42%) compared to non-ischemic causes 4

Cardiac Surgery and Iatrogenic Causes

Post-surgical heart block occurs in specific clinical contexts 1:

• Congenital heart disease surgery results in AV block in 1-3% of operations 1
• Persistent post-operative block lasting 7 days warrants pacemaker implantation, though late recovery occurs in a significant percentage 1
• High-risk procedures include atrial manipulation and suturing (Fontan, Mustard, and Senning procedures) 1
• Valve surgery produces variable natural history, with permanent pacing decisions at physician discretion 1
• AV nodal ablation carries a 2-3% sudden cardiac death risk, particularly in the first 24 hours post-procedure due to bradycardia-dependent prolongation of repolarization 1

Infiltrative and Systemic Diseases

Non-ischemic systemic conditions affecting the conduction system 1:

• Sarcoidosis and amyloidosis may warrant pacemaker implantation due to disease progression potential even if AV block transiently reverses 1
• Kearns-Sayre syndrome commonly presents with heart block and cardiomyopathy 1
• Neuromuscular diseases including muscular dystrophy and Friedreich ataxia can cause progressive conduction system disease 1
• Infectious causes include myocarditis, infective endocarditis, and Lyme disease (which may follow a natural history to resolution) 1, 3

Medication and Metabolic Causes

Reversible etiologies requiring correction before permanent pacing 1:

• AV nodal blocking agents including beta-blockers, calcium channel blockers, and digitalis can precipitate or worsen heart block 1
• Antiarrhythmic drugs such as amiodarone or sotalol often worsen bradycardia episodes, necessitating backup ventricular pacing 1
• Electrolyte abnormalities represent reversible causes that should be corrected before considering permanent pacing 1, 3

Autonomic and Functional Causes

Vagally-mediated heart block occurs in specific circumstances 1:

• Sleep apnea can cause long sinus pauses and AV block during sleep; in the absence of symptoms, these are reversible and do not require pacing 1
• Hypervagotonia due to recognizable and avoidable physiological factors represents a reversible cause 1
• Neurocardiogenic causes in young patients should be assessed before proceeding with permanent pacing 1

Critical Clinical Pitfalls

Common diagnostic and management errors to avoid:

• Do not implant permanent pacemakers prematurely in post-surgical patients; waiting <72 hours should generally be avoided, though waiting 7-9 days is likely unnecessary 1
• Exercise-induced AV block not secondary to myocardial ischemia usually indicates His-Purkinje disease with poor prognosis and warrants pacing 1
• Patients with ischemic complete heart block are significantly less likely to receive permanent pacemakers (42.83% vs 93.75%) compared to non-ischemic causes, despite similar mortality 4
• Congenital complete heart block carries high risk for late sudden death at any age, with unpredictable disease progression warranting consideration of permanent pacing even in asymptomatic individuals 1