A 68-year-old man with a history of acute inferior wall myocardial infarction presents to the emergency department with syncope. On examination, blood pressure is 85/55 mmHg, heart rate 42 bpm, and JVP is elevated with prominent 'a' waves. ECG shows complete dissociation between atrial and ventricular activity, with atrial rate 100 bpm and ventricular rate 42 bpm. The QRS complex is narrow (< 0.12 s). What is the most likely diagnosis?

Explanation

## Clinical Presentation Analysis **Key Point:** Complete AV dissociation with independent atrial and ventricular rates, narrow QRS escape rhythm, and acute inferior MI context defines third-degree AV block with junctional escape. ### Diagnostic Features | Feature | Finding | Significance | | --- | --- | --- | | **AV dissociation** | Complete (atrial 100, ventricular 42) | Pathognomonic for 3° block | | **QRS width** | Narrow (< 0.12 s) | Escape pacemaker is junctional | | **Clinical context** | Inferior MI | Nodal ischemia → AV nodal dysfunction | | **Hemodynamics** | Hypotension, syncope | Loss of AV synchrony + bradycardia | | **JVP 'a' waves** | Prominent | Atrial contraction against closed tricuspid valve | ### Pathophysiology in Inferior MI 1. RCA occlusion → AV nodal ischemia 2. Complete block at AV node level 3. Junctional pacemaker emerges (intrinsic rate 40–60 bpm) 4. Narrow QRS (pacemaker above bundle branches) 5. Loss of AV synchrony → reduced cardiac output **High-Yield:** In **inferior MI**, third-degree AV block is typically **nodal** (narrow QRS, junctional escape) and often **transient** (resolves in 3–7 days). Contrast with **anterior MI**, where block is **infranodal** (wide QRS, unreliable escape, permanent). ### Management Implications - **Immediate:** Atropine 0.5–1 mg IV (nodal block often responsive) - **If hypotensive:** Temporary pacing (transcutaneous or transvenous) - **Prognosis:** Usually recovers with reperfusion; permanent pacemaker rarely needed **Clinical Pearl:** The prominent 'a' waves on JVP (cannon waves) occur because the atrium contracts against a closed tricuspid valve when ventricular systole coincides with atrial systole — a hallmark of complete AV dissociation.