A 32-year-old man presents with sharp chest pain relieved by sitting forward. ECG shows diffuse ST elevation and PR segment depression in lead II. The PR segment depression reflects abnormal current flow during which part of the cardiac cycle?

• A Ventricular depolarization (QRS complex)
• B Atrial repolarization (Ta wave) during the PR segment ✓
• C Early ventricular repolarization (J point)
• D Late ventricular repolarization (T wave)

Explanation

In acute pericarditis, epicardial inflammation generates an atrial injury current that shifts the PR segment. Atrial repolarization (Ta wave) normally occurs during the PR segment and is usually buried and invisible. In pericarditis, the enlarged and prolonged Ta wave produces PR segment depression, most visible in lead II and reciprocal PR elevation in aVR. The ST elevation represents ventricular epicardial injury current during early repolarization.

Reference: Harrison's Principles of Internal Medicine, 21st ed.

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