A 68-year-old man with a history of inferior wall myocardial infarction 2 days ago presents to the cardiac care unit with complaints of dizziness and syncope. On examination, blood pressure is 88/56 mmHg, heart rate is 38 bpm and regular. Jugular venous pressure shows cannon waves. ECG reveals complete dissociation between P waves and QRS complexes, with P waves at a rate of 90 bpm and QRS complexes at 38 bpm with a wide morphology (>120 ms). What is the most likely diagnosis?

Explanation

## Diagnosis: Third-Degree AV Block with Ventricular Escape Rhythm ### Key ECG Findings **Key Point:** Complete AV dissociation with independent P and QRS rates is the hallmark of third-degree (complete) AV block. | Feature | Finding | Significance | |---------|---------|---------------| | P-QRS relationship | Complete dissociation | No conducted beats | | P wave rate | 90 bpm | Atrial rate | | QRS rate | 38 bpm | Escape rhythm rate | | QRS width | >120 ms (wide) | Ventricular origin | | Cannon waves | Present in JVP | AV dissociation sign | ### Escape Rhythm Classification **High-Yield:** The width of the escape QRS complex determines the escape site: - **Junctional escape**: QRS <120 ms, rate 40–60 bpm - **Ventricular escape**: QRS >120 ms, rate 20–40 bpm This patient has a wide QRS (>120 ms) at 38 bpm, indicating a **ventricular escape rhythm** — the most distal safety pacemaker. ### Clinical Context: Inferior MI **Clinical Pearl:** Third-degree AV block complicating inferior MI is typically due to: 1. Ischemia of the AV nodal artery (branch of RCA) 2. Increased vagal tone 3. Transient and often reversible within 3–7 days However, when the escape rhythm is **ventricular** (not junctional), it suggests: - Infranodal block (below the AV node) - Higher risk of sudden asystole - **Requires temporary pacing** (transvenous or transcutaneous) ### Why Ventricular Escape Here? The QRS width >120 ms and bradycardia (38 bpm) indicate the escape pacemaker is in the **ventricular myocardium**, not the AV junction. This is a more ominous rhythm than junctional escape and requires urgent intervention. **Mnemonic:** **VEIN** — Ventricular Escape Is Narrow-escape (i.e., needs pacing NOW) ### Management 1. **Immediate:** Transcutaneous pacing (external pacing pads) 2. **Definitive:** Transvenous pacing if persistent 3. **Atropine:** May be tried (0.5–1 mg IV) but often ineffective in infranodal block 4. **Isoproterenol:** Not recommended in acute MI (increases ischemia) [cite:Harrison 21e Ch 235]