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 128/82 mmHg, HR 92/min, RR 18/min. Physical examination is unremarkable. His 12-lead ECG shows ST-segment elevation of 3 mm in leads II, III, and aVF, with reciprocal ST depression in leads I and aVL. Troponin I is elevated at 2.4 ng/mL (normal <0.04). What is the most likely diagnosis?

Explanation

## ECG Localization in STEMI **Key Point:** The anatomical location of ST elevation on the ECG directly indicates the coronary artery involved and the wall of the left ventricle affected. ### ST Elevation Distribution and Coronary Anatomy | ECG Leads with ST Elevation | Coronary Artery | Ventricular Wall | Associated Reciprocal Changes | |---|---|---|---| | II, III, aVF | Right coronary artery (RCA) | Inferior | I, aVL | | V1–V4 | Left anterior descending (LAD) | Anterior | II, III, aVF | | V5, V6, I, aVL | Left circumflex (LCx) | Lateral | III, aVF | | II, III, aVF + V4R | RCA with RV involvement | Inferior + RV | I, aVL | ### Analysis of This Case 1. **ST elevation in II, III, aVF** → Inferior wall territory 2. **Reciprocal ST depression in I and aVL** → Confirms inferior STEMI (reciprocal changes are opposite to the infarcted wall) 3. **Elevated troponin I** → Confirms myocardial necrosis 4. **Coronary anatomy:** The inferior wall is supplied by the right coronary artery (RCA) in ~80% of the population; the left circumflex supplies it in ~20% **High-Yield:** Inferior STEMI often presents with bradycardia and hypotension due to RV involvement and increased vagal tone. This patient's HR of 92 is relatively preserved, but RV infarction should be excluded by obtaining a right-sided ECG (V4R). **Clinical Pearl:** Patients with inferior STEMI are at risk for atrioventricular (AV) block and should be monitored closely. Avoid nitrates and diuretics if RV infarction is suspected, as they worsen preload-dependent RV function. **Mnemonic:** **II-III-aVF = Inferior** — the three leads that form the inferior territory. ### Why Reciprocal Changes Matter Reciprocal ST depression (opposite to the site of infarction) is a highly specific marker of true STEMI and helps exclude other causes of ST elevation (e.g., pericarditis, which shows diffuse concave ST elevation without reciprocal depression).