A 72-year-old Indian man with chronic hypertension and diabetes presents with progressive dyspnea and syncope. ECG shows second-degree AV block type II (Mobitz II). Which is the most common cause of Mobitz II AV block in the chronic setting?

Explanation

## Causes of Mobitz II AV Block ### Pathophysiology of Mobitz II Mobitz II (second-degree AV block type II) is characterized by **sudden failure of AV conduction** without prior PR prolongation. It occurs at the **infranodal level** (bundle of His or bundle branches) and carries risk of progression to complete heart block. ### Most Common Cause: Degenerative Fibrosis **Key Point:** In the **chronic setting**, degenerative fibrosis of the conduction system (Lenegre disease) is the most common cause of Mobitz II AV block. This is age-related degeneration of the bundle of His and bundle branches, typically seen in elderly patients with long-standing hypertension and diabetes. ### Etiology Comparison by Clinical Context | Cause | Clinical Setting | Mechanism | Prognosis | | --- | --- | --- | --- | | **Degenerative fibrosis (Lenegre)** | Chronic, elderly, HTN/DM | Age-related degeneration of conduction tissue | Progressive; often requires pacing | | **Acute MI (anterior)** | Acute presentation | LAD occlusion → bundle of His ischemia | Variable; may be transient | | **Lyme disease** | Acute, endemic region, tick exposure | Spirochete inflammation of AV node/bundle | Usually reversible with antibiotics | | **Acute rheumatic fever** | Acute, young patients, post-streptococcal | Myocarditis with nodal inflammation | Often reversible | ### High-Yield: **Lenegre disease** = progressive fibrosis of bundle branches in elderly patients → Mobitz II → complete heart block. This is the **chronic degenerative cause** and most common in the outpatient / chronic disease setting. ### Clinical Pearl: Mobitz II at the **infranodal level** (bundle of His or distal) is more dangerous than Mobitz I (nodal level) because it can suddenly progress to complete heart block without warning. Patients with Mobitz II typically require **permanent pacemaker insertion**. ### Warning: Do not confuse **Lenegre disease** (progressive fibrosis of bundle branches) with **Lev disease** (fibrosis of the AV node itself). Both are degenerative, but Lenegre affects the distal conduction system and is more likely to present as Mobitz II.