## Most Common Site of AV Block in Inferior MI **Key Point:** In the setting of acute inferior myocardial infarction, the most common site of AV block is the **atrioventricular (AV) node**. ### Anatomical Basis The AV node receives its blood supply from the **AV nodal artery**, which arises from the right coronary artery (RCA) in approximately 90% of individuals. Occlusion of the RCA during inferior MI causes ischemia of the AV node, leading to conduction delay and block. ### Clinical Features of AV Nodal Block in Inferior MI | Feature | AV Nodal Block (Inferior MI) | Infranodal Block (Anterior MI) | |---------|------------------------------|--------------------------------| | **Site** | AV node | His bundle or bundle branches | | **QRS width** | Narrow (< 120 ms) | Wide (≥ 120 ms) | | **Escape rhythm rate** | 40–60 bpm (reliable) | 20–40 bpm (unreliable) | | **Prognosis** | Usually reversible | Often permanent | | **Pacing need** | Temporary (if symptomatic) | Permanent | | **Coronary artery** | RCA (90%) | LAD | **Clinical Pearl:** The narrow QRS complex escape rhythm at 45 bpm in this case is a hallmark of AV nodal block. The AV node has inherent pacemaker capability with a reliable escape rate, whereas infranodal blocks produce slow, unreliable escape rhythms (20–40 bpm) with wide QRS complexes. **High-Yield:** In acute inferior MI with complete heart block: - Narrow QRS escape → AV nodal block (usually temporary) - Wide QRS escape → Infranodal block (usually permanent, requires pacing) ### Why Inferior MI Causes AV Nodal Block The RCA supplies the inferior wall and the AV nodal artery in most patients. Acute RCA occlusion causes ischemia of the AV node, resulting in: 1. First-degree AV block (PR prolongation) 2. Second-degree AV block (Mobitz I — Wenckebach) 3. Complete (third-degree) AV block with narrow QRS escape This is usually **reversible** within hours to days as collateral circulation develops.
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