Third-Degree AV Block

The clinician managing complete heart block (CHB) must determine etiology, level of block, likelihood of reversibility, and appropriate pacing strategy. CHB can be classified by level (AV nodal, His bundle, infra-Hisian), etiology (degenerative, ischemic, inflammatory, drug-induced, congenital, infiltrative), and reversibility (transient vs permanent). Degenerative conduction system disease (Lev/Lenegre disease) is the most common cause in elderly and produces progressive fibrosis of the conduction system over years. Diagnostic approaches include electrophysiology study for level determination (AH vs HV prolongation), cardiac MRI for infiltrative disease (sarcoidosis shows patchy late gadolinium enhancement; amyloidosis shows diffuse subendocardial enhancement), and genetic testing for familial conduction disease. Congenital CHB associated with maternal anti-Ro/SSA and anti-La/SSB antibodies causes immune-mediated destruction of the AV node in utero; these children may present in infancy or remain asymptomatic into adulthood with reliable junctional escape rhythms. Pacing strategy for CHB has evolved: traditional RV apical pacing causes ventricular dyssynchrony that can lead to pacing-induced cardiomyopathy (PICM) in 10-20% of patients over 5+ years. Conduction system pacing (His bundle pacing or left bundle branch area pacing) maintains physiologic ventricular activation and avoids...