On a 12-lead ECG, an acute posterior myocardial infarction manifests in the standard leads as:
- A ST elevation in V1–V4 and reciprocal ST depression in II, III, aVF
- B Wide Q waves in V5–V6 with ST elevation in lateral leads
- C Tall R waves and ST depression in V1–V2 with ST elevation in V7–V9 ✓
- D PR depression and saddle-shaped ST elevation diffusely in all leads
Correct answer: C. Tall R waves and ST depression in V1–V2 with ST elevation in V7–V9
Explanation
The posterior wall of the left ventricle is not directly viewed by standard precordial leads; posterior MI shows reciprocal changes in anterior leads — tall R waves (mirror image of posterior Q waves) and horizontal ST depression in V1–V2. Posterior leads V7–V9 (placed on the back) show the expected ST elevation and confirm the diagnosis. Anterior MI causes ST elevation in V1–V4. Lateral MI affects V5–V6 with I and aVL. Diffuse saddle-shaped elevation is characteristic of pericarditis.
Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.
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