The T wave on a surface ECG represents which phase of the ventricular action potential, and why is it normally upright in leads with a dominant R wave?

• A Phase 2 plateau; Ca2+ current from endocardium to epicardium dominates
• B Phase 3 repolarisation; endocardium repolarises before epicardium producing positive surface deflection
• C Phase 3 repolarisation; epicardium repolarises before endocardium so current flows outward (epicardium to endocardium) — same direction as depolarisation ✓
• D Phase 4 resting potential; slow diastolic depolarisation at the SA node

Explanation

The T wave reflects ventricular repolarisation (phase 3). Although depolarisation proceeds from endocardium to epicardium, repolarisation occurs in the reverse direction — epicardium repolarises first because epicardial action potentials have a shorter plateau. Since the repolarisation wavefront travels from endocardium (still positive) toward epicardium (returning to negative), the net current vector aligns in the same direction as depolarisation, producing an upright T wave in leads that record a dominant R wave. Inversion of the T wave indicates altered repolarisation gradients (ischaemia, hypertrophy, etc.).

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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