A 68-year-old woman with chronic hypertension and diabetes mellitus is found to have second-degree AV block (Mobitz II) on routine ECG. What is the most common site of conduction block in Mobitz II AV block?

Explanation

## Most Common Site of Block in Mobitz II AV Block **Key Point:** The most common site of conduction block in **Mobitz II AV block is infranodal** — specifically at the level of the **His bundle or bundle branches**. ### Distinction Between Mobitz I and Mobitz II | Feature | Mobitz I (Wenckebach) | Mobitz II | |---------|----------------------|----------| | **Site of block** | AV node (nodal) | His bundle or bundle branches (infranodal) | | **PR interval pattern** | Progressive lengthening, then dropped beat | Fixed PR interval, then sudden dropped beat | | **QRS width** | Usually narrow (< 120 ms) | Usually wide (≥ 120 ms) | | **Prognosis** | Usually benign, often reversible | Serious; may progress to complete heart block | | **Pacing** | Rarely needed | Often required | | **Cause** | Increased vagal tone, drugs, ischemia | Organic conduction disease, anterior MI | ### Why Mobitz II is Infranodal **High-Yield:** In Mobitz II block: - The **PR interval remains constant** until a beat is suddenly dropped (no warning) - This pattern indicates a **fixed anatomical lesion** below the AV node - The AV node does not have the capacity for progressive conduction delay; it either conducts or it doesn't - A fixed lesion in the His bundle or bundle branches causes the sudden, unpredictable dropped beat **Clinical Pearl:** The presence of a **wide QRS complex (≥ 120 ms)** in Mobitz II strongly suggests infranodal block, as the His bundle and bundle branches are responsible for ventricular depolarization. ### Pathophysiology Infranodal block typically results from: 1. **Organic conduction system disease** — fibrosis, degeneration (Lev disease, Lenegre disease) 2. **Acute anterior wall MI** — LAD occlusion damages the bundle branches 3. **Chronic hypertension and diabetes** — as in this patient — cause degenerative changes in the conduction system ### Clinical Significance **Warning:** Mobitz II is a serious arrhythmia because: - It can abruptly progress to **complete heart block** without warning - The escape rhythm is **unreliable and slow** (20–40 bpm) - **Permanent pacemaker implantation is indicated** in symptomatic patients or those at high risk of progression ### ECG Recognition ``` Mobitz II pattern: P—R—P—R—P—(dropped QRS)—P—R—P—R—P—(dropped QRS) ↑ PR interval stays the same ↑ sudden drop, no warning ```