A 58-year-old man presents with acute chest pain. ECG shows horizontal ST depression of 3 mm in leads V1 to V3 with dominant R waves in V2 and upright T waves in those leads. Which additional investigation will best confirm the diagnosis?
- A Right-sided chest leads V3R and V4R
- B Leads placed one intercostal space higher on the left
- C Posterior chest leads V7 to V9 ✓
- D Signal-averaged ECG
Correct answer: C. Posterior chest leads V7 to V9
Explanation
Isolated ST depression in V1-V3 with prominent R waves represents reciprocal change of ST elevation from a posterior wall infarction, usually due to occlusion of the left circumflex or a dominant RCA. ST elevation of 0.5 mm or more in posterior leads V7-V9 confirms the diagnosis and upgrades the patient to STEMI eligible for reperfusion. Right-sided leads detect right ventricular involvement, which is a different extension of inferior infarction.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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