A 55-year-old woman with acute inferior wall myocardial infarction develops progressive AV block on continuous monitoring. ECG shows PR interval prolongation followed by dropped beats with a regular atrial rate. All of the following are characteristic features of Mobitz type II AV block EXCEPT:

Explanation

## Mobitz Type II AV Block: Distinguishing Features **Key Point:** Mobitz type II is an infranodal block with a poor prognosis — it is fundamentally different from Mobitz type I in location, mechanism, and clinical significance. ### Mobitz Type I vs Mobitz Type II: Critical Comparison | Feature | Mobitz Type I (Wenckebach) | Mobitz Type II | |---------|---------------------------|----------------| | **Site of block** | AV node (nodal) | Bundle of His or below (infranodal) | | **PR interval pattern** | Progressive lengthening → dropped beat | Constant PR, then sudden dropped beat | | **QRS width** | Usually narrow (<120 ms) | Usually wide (>120 ms) | | **Atrial rate** | Regular | Regular | | **Ventricular rate** | Irregular (grouped beats) | Irregular | | **Progression to CHB** | Rare (5–10%) | Common (30–50%) | | **Prognosis** | Benign; often resolves | Serious; often requires pacing | | **Associated with** | Inferior MI, digitalis toxicity, athletes | Anterior MI, conduction disease | **High-Yield:** The hallmark of Mobitz type II is a **constant PR interval** before the dropped beat — there is NO progressive lengthening. This distinguishes it immediately from Mobitz type I (Wenckebach). ### Why Option 1 Is Wrong Option 1 states: "It occurs most commonly at the level of the **AV node**." This is **FALSE**. Mobitz type II occurs at the level of the **bundle of His or below** (infranodal block), NOT at the AV node. Mobitz type I (Wenckebach) occurs at the AV node. This is a critical distinction that students frequently confuse. ### Why the Other Options Are Correct - **Option 0:** The defining ECG feature of Mobitz type II is a **constant PR interval** before the dropped beat. Unlike Mobitz type I, there is no progressive lengthening. ✓ - **Option 2:** Because Mobitz type II is infranodal (bundle of His or below), the escape rhythm originates from the bundle branches or ventricle, resulting in a **wide QRS complex** (>120 ms). ✓ - **Option 3:** Mobitz type II has a **high risk of progression to complete heart block** (30–50%), whereas Mobitz type I rarely progresses (5–10%). This is a major prognostic difference. ✓ **Clinical Pearl:** A patient with Mobitz type II and symptoms (syncope, hypotension) requires **urgent temporary pacing** and almost always needs **permanent pacemaker implantation**. Do not delay — this is a medical emergency. **Mnemonic:** **"Mobitz II = Infranodal, Wide QRS, Dangerous"** — remember that Mobitz II is the "serious" one with infranodal block, wide QRS, and high risk of progression. ```mermaid flowchart TD A["AV Block with Dropped Beats"]:::outcome --> B{"PR interval pattern?"}:::decision B -->|"Progressive lengthening → drop"| C["Mobitz Type I<br/>Wenckebach"]:::outcome B -->|"Constant PR → sudden drop"| D["Mobitz Type II"]:::outcome C --> E["Site: AV Node"]:::outcome C --> F["QRS: Usually narrow"]:::outcome C --> G["Prognosis: Benign"]:::outcome D --> H["Site: Bundle of His<br/>or below"]:::outcome D --> I["QRS: Usually wide"]:::outcome D --> J["Prognosis: Serious<br/>→ Pacing"]:::urgent ``` [cite:Harrison 21e Ch 297]