Reciprocal changes in ECG are most indicative of which acute event?

• A. STEMI.
• B. Stable angina.
• C. Pericarditis.
• D. Hyperkalemia.

Answer: (A)

Reciprocal changes on an ECG occur when the ST-segment elevations caused by injury to one region of the heart are mirrored by ST-segment depressions in the opposite leads. This pattern signals transmural ischemia from an acute infarct, i.e., a STEMI, because the electrical vector of injury points toward the leads showing elevation and away from the leads showing depression. The presence of these opposite changes helps confirm a genuine acute STEMI rather than other conditions.

In contrast, pericarditis typically shows diffuse ST elevations that are not accompanied by regional reciprocal depressions, and hyperkalemia presents with tall peaked T waves and widened QRS rather than reciprocal ST changes. Stable angina does not usually produce a new, organized pattern of ST elevations with reciprocal depressions.