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. Blood pressure is 145/92 mmHg, heart rate 102/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 evident on this ECG?

Explanation

## ECG Interpretation in Acute MI **Key Point:** ST-segment elevation in the inferior leads (II, III, aVF) with reciprocal changes in lateral leads is the hallmark of acute inferior wall myocardial infarction (IWMI). ### Normal ECG Intervals and Segments | Parameter | Normal Range | Clinical Significance | |-----------|--------------|----------------------| | PR interval | 0.12–0.20 sec | AV conduction time | | QRS duration | 0.06–0.10 sec | Ventricular depolarization | | QT interval | 0.36–0.44 sec (varies with HR) | Ventricular repolarization | | ST segment | Isoelectric (±1 mm) | Baseline between QRS and T wave | **High-Yield:** In this case: - PR interval = 0.16 sec → **normal** - QRS duration = 0.08 sec → **normal** - QT interval = 0.38 sec → **normal** - **ST elevation in II, III, aVF** → **abnormal and diagnostic** ### Localization of MI by ECG ```mermaid flowchart TD A[ST Elevation on ECG]:::outcome --> B{Which leads?}:::decision B -->|II, III, aVF| C[Inferior Wall MI]:::action B -->|V1-V4| D[Anterior Wall MI]:::action B -->|I, aVL, V5-V6| E[Lateral Wall MI]:::action B -->|V4R| F[Right Ventricular MI]:::action C --> G[RCA occlusion most common]:::outcome D --> H[LAD occlusion]:::outcome ``` **Clinical Pearl:** Inferior wall MI (IWMI) is most commonly caused by right coronary artery (RCA) occlusion (80% of cases). Always check for right ventricular involvement (V4R lead) as it affects fluid management and hemodynamics. **Mnemonic:** **STEMI Leads** — - **Anterior:** V1–V4 (LAD territory) - **Inferior:** II, III, aVF (RCA/LCx territory) - **Lateral:** I, aVL, V5–V6 (LCx territory) - **Posterior:** V7–V9 (RCA/LCx territory) ### Why the Other Parameters Are Normal The PR interval (0.16 sec), QRS (0.08 sec), and QT (0.38 sec) are all within normal limits in this patient. The **primary abnormality is the ST elevation**, which is the diagnostic finding for acute transmural myocardial injury.