A 72-year-old woman with chronic hypertension and diabetes mellitus type 2 presents with progressive dyspnea and fatigue over 3 weeks. On examination, heart rate is 58 bpm, blood pressure 138/82 mmHg, and there are signs of pulmonary congestion. ECG shows a PR interval of 0.28 s with each P wave followed by a QRS complex; however, every third P wave is not followed by a QRS complex. Echocardiography reveals an ejection fraction of 38%. What is the most likely diagnosis?

Explanation

## ECG Pattern Recognition **Key Point:** A regular pattern of blocked conduction (e.g., every 3rd P wave drops) without progressive PR prolongation before the block is **Mobitz type II**, not Wenckebach. ### Differential Features of Second-Degree AV Blocks | Feature | Mobitz I (Wenckebach) | Mobitz II | Clinical Significance | | --- | --- | --- | --- | | **PR interval** | Progressive lengthening before dropped beat | Constant (fixed) | Mobitz II = infranodal block | | **Pattern** | Irregular; PR gets longer until one drops | Regular pattern (e.g., 3:2, 4:3) | Mobitz II is more dangerous | | **Block location** | AV node (nodal) | Below AV node (infranodal) | Infranodal → unreliable escape | | **QRS width** | Normal (< 0.12 s) | Often wide (≥ 0.12 s) | Infranodal block → wide QRS | | **Prognosis** | Usually benign, rarely progresses | Can progress to complete block | Mobitz II requires pacing | | **Response to atropine** | Often improves | No response | Nodal vs. infranodal distinction | ### Why This Is Mobitz Type II 1. **Fixed PR interval** (0.28 s) — does NOT lengthen before the dropped beat 2. **Regular pattern** — every 3rd P wave fails to conduct (3:2 AV conduction) 3. **Infranodal location** — chronic hypertension and diabetes cause fibrosis below the AV node 4. **Reduced ejection fraction** — suggests structural/conduction disease 5. **Symptomatic** — dyspnea and fatigue indicate inadequate cardiac output from bradycardia **High-Yield:** **Mobitz I** shows a **"footprint" pattern** on ECG — PR progressively lengthens, then a P wave drops, then the PR resets short and lengthens again. **Mobitz II** shows a **"regular skip" pattern** — PR stays the same, and beats drop at regular intervals (2:1, 3:2, etc.). **Mnemonic:** **"Wenckebach Walks"** — PR interval takes progressive steps (gets longer) before stumbling (dropping a beat). **Mobitz II** is **"Sudden Stops"** — no warning, just regular blocks. ### Clinical Implications - **Mobitz II** is a sign of **infranodal disease** (fibrosis, infiltration, ischemia) - **Risk of progression** to complete AV block - **Pacemaker indicated** (unlike Mobitz I, which is usually benign) - Atropine and beta-blockers are ineffective **Clinical Pearl:** Chronic hypertension and diabetes cause myocardial fibrosis and conduction system degeneration, predisposing to Mobitz II. The reduced ejection fraction (38%) reflects both the bradycardia-induced cardiomyopathy and underlying structural disease.