A 58-year-old diabetic man presents with 2 hours of chest pain. ECG shows horizontal ST depression of 3 mm in V1 to V4 with prominent R waves and upright T waves in those leads; there is no ST elevation anywhere on the standard 12-lead tracing. What is the next best step to establish the diagnosis?
- A Record right-sided leads V3R and V4R
- B Record posterior leads V7 to V9 ✓
- C Repeat the 12-lead ECG after 30 minutes
- D Proceed directly to echocardiography
Correct answer: B. Record posterior leads V7 to V9
Explanation
ST depression in V1 to V4 with dominant R waves and upright T waves suggests acute posterior STEMI, usually from a left circumflex or RCA occlusion. Confirmation requires posterior chest leads V7 to V9, where ST elevation of 0.5 mm or more establishes the diagnosis and makes the patient eligible for reperfusion therapy as a STEMI equivalent. Right-sided leads detect RV infarction instead, so option A addresses a different territory.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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Written and medically reviewed by the StethoPrep medical team.