A 58-year-old man with acute inferior wall myocardial infarction presents with symptomatic bradycardia (HR 38/min) and hypotension (BP 85/50 mmHg). Physical examination reveals cannon 'a' waves in the jugular venous pulse. A 12-lead ECG shows complete dissociation between atrial and ventricular activity with a ventricular rate of 38/min and atrial rate of 100/min. Which investigation is most appropriate to assess the site and nature of the conduction block?

Explanation

## Investigation of Choice for Localizing AV Block ### Clinical Context This patient has **complete heart block (CHB)** with: - Atrial rate 100/min, ventricular rate 38/min (complete dissociation) - Cannon 'a' waves (hallmark of AV dissociation) - Acute inferior MI (RCA territory) - Hemodynamic compromise requiring urgent intervention ### Why His Bundle Electrogram (HBE) is Correct **Key Point:** HBE is the **gold standard invasive investigation** for precisely localizing the site of AV block within the conduction system. **High-Yield:** HBE can differentiate: 1. **Infranodal block** (His-Purkinje system) — narrow QRS, escape rate 30–40/min, poor prognosis 2. **Nodal block** (AV node) — narrow QRS, escape rate 40–60/min, often transient in inferior MI 3. **Supranodal block** (rare) — rare in acute MI ### HBE Findings in This Case - **Expected:** Infranodal block (inferior MI → RV branch block) - **A-H interval:** Normal (atrial to His) - **H-V interval:** Prolonged or absent (block below His) - **Escape rhythm:** Ventricular (wide QRS, rate 30–40/min) ### Clinical Pearl In **acute inferior MI with CHB**: - Block is usually **nodal** (AV node ischemia) → transient, responds to atropine - However, if **infranodal** (as suggested by low escape rate 38/min) → permanent, requires pacing - HBE determines which → guides management (pacing vs. observation) ### Diagnostic Algorithm ```mermaid flowchart TD A[Complete Heart Block in Inferior MI]:::outcome --> B{Escape Rate?}:::decision B -->|40-60/min, narrow QRS| C[Nodal Block]:::outcome B -->|30-40/min, wide QRS| D[Infranodal Block]:::outcome C --> E[HBE: Normal H-V]:::action D --> F[HBE: Prolonged/Absent H-V]:::action E --> G[Transient, Atropine trial]:::action F --> H[Permanent, Pacing required]:::action ``` ### Timing & Indications for HBE - **Acute CHB with hemodynamic instability** → Temporary pacing first, then HBE - **Diagnostic uncertainty** (nodal vs. infranodal) - **Prognostic assessment** (predict need for permanent pacemaker) [cite:Harrison 21e Ch 276] ---