A 58-year-old man from Delhi presents to the emergency department with acute onset chest pain radiating to the left arm for 2 hours. He is diaphoretic and anxious. Vital signs: BP 145/92 mmHg, HR 102/min, RR 18/min. An ECG is performed immediately. The ECG shows ST-segment elevation in leads II, III, and aVF, with reciprocal ST depression in leads I and aVL. The PR interval measures 0.16 seconds, and the QRS duration is 0.08 seconds. What is the most likely diagnosis based on the ECG findings?

Explanation

## ECG Interpretation: Acute Inferior Wall MI ### ST-Segment Elevation Pattern **Key Point:** ST-segment elevation in the inferior leads (II, III, aVF) with reciprocal ST depression in the lateral leads (I, aVL) is pathognomonic for acute inferior wall myocardial infarction (IWMI). ### Lead-Based Localization | Wall/Region | Diagnostic Leads | Reciprocal Leads | Coronary Artery | |---|---|---|---| | **Inferior** | II, III, aVF | I, aVL | Right coronary artery (RCA) or left circumflex | | **Anterior** | V1–V4 | II, III, aVF | Left anterior descending (LAD) | | **Lateral** | I, aVL, V5, V6 | II, III, aVF | Left circumflex | | **Posterior** | V1–V2 (tall R wave) | V5–V6 (ST depression) | RCA or circumflex | ### Clinical Significance **High-Yield:** The combination of: 1. ST elevation in **II, III, aVF** (inferior leads) 2. Reciprocal ST depression in **I, aVL** (lateral leads) 3. Normal PR interval (0.16 s, normal range 0.12–0.20 s) 4. Normal QRS duration (0.08 s, normal < 0.12 s) This pattern is diagnostic of acute inferior wall MI, most commonly from right coronary artery (RCA) occlusion in ~80% of cases. ### Pathophysiology **Clinical Pearl:** Inferior wall MI often involves the RCA, which supplies the sinoatrial (SA) node in 60% of people. This explains why inferior MI frequently presents with bradycardia, AV block, or nodal rhythm — features to watch for on serial ECGs. ### Normal Intervals in This Case - **PR interval 0.16 s** → Normal (0.12–0.20 s). Rules out first-degree AV block or PR prolongation. - **QRS 0.08 s** → Normal (< 0.12 s). Rules out bundle branch block or ventricular ectopy. These normal intervals support acute transmural ischemia (ST elevation) rather than chronic conduction disease.