A 52-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 102/min, RR 18/min. His 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, QRS duration 0.08 seconds, and QT interval 0.38 seconds. What is the primary abnormality indicated by the ST-segment changes?

Explanation

## Clinical Presentation and ECG Interpretation **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). ### Anatomical Localization The inferior wall of the left ventricle is primarily supplied by the right coronary artery (RCA) in approximately 80% of the population. The ECG pattern of inferior STEMI is: | Lead Group | Finding | Interpretation | |---|---|---| | II, III, aVF | ST elevation | Inferior wall infarction | | I, aVL | ST depression | Reciprocal changes | | V1–V3 | Normal or depression | Excludes anterior STEMI | ### Normal ECG Intervals in This Patient **High-Yield:** The PR interval (0.16 s), QRS duration (0.08 s), and QT interval (0.38 s) are all within normal limits: - PR interval: 0.12–0.20 seconds ✓ - QRS duration: 0.06–0.10 seconds ✓ - QT interval: corrected QT (QTc) = 0.38 s (normal for HR ~102) ✓ These normal intervals rule out conduction abnormalities or prolongation that might suggest alternative diagnoses. ### Pathophysiology of STEMI 1. Acute coronary occlusion → transmural ischemia 2. Loss of repolarization voltage in infarcted zone 3. Current of injury flows from viable to infarcted tissue 4. **Epicardial leads over infarct zone** → ST elevation 5. **Reciprocal leads** → ST depression (opposite vector) **Clinical Pearl:** Inferior STEMI carries a lower in-hospital mortality (3–5%) compared to anterior STEMI (5–10%), but right ventricular involvement (present in ~30% of inferior STEMI) can cause hemodynamic collapse if not recognized early. ### Management Implications **Mnemonic: MONA** — Morphine, Oxygen (if SpO₂ <90%), Nitrates, Aspirin (for STEMI management) This patient requires: - Immediate 12-lead ECG confirmation ✓ (already done) - Troponin and other cardiac biomarkers - **Primary percutaneous coronary intervention (PCI) within 90 minutes** (door-to-balloon time) - Dual antiplatelet therapy (aspirin + P2Y₁₂ inhibitor) - Beta-blocker and ACE inhibitor (post-reperfusion) **Warning:** Do NOT delay reperfusion waiting for biomarker results; ST-elevation STEMI is a clinical diagnosis.