Why is the T wave from beginning to end considered repolarization, rather than depolarization, during active ventricular (heart muscle) contraction?

Understanding the T Wave as Repolarization, Not Depolarization

The T wave represents ventricular repolarization—not depolarization—because it reflects the electrical process of myocardial cells returning to their resting state (from +10 mV to -85 mV), which occurs after mechanical contraction has already begun during the plateau phase. 1

The Key Conceptual Distinction: Electrical vs. Mechanical Events

The confusion arises from conflating electrical and mechanical cardiac events, which do not occur simultaneously:

• Depolarization (QRS complex) represents the rapid electrical activation of ventricular myocardium, triggering the initiation of mechanical contraction 1
• The ST segment corresponds to the plateau phase (phase 2) of the action potential when cells maintain a stable voltage around +10 mV with minimal voltage gradients—this is when active ventricular contraction is occurring mechanically 1
• The T wave represents phase 3 (rapid repolarization) when the transmembrane potential returns from approximately +10 mV to -85 mV, creating the voltage gradients that generate the T wave on the surface ECG 1

Why the T Wave Cannot Be Depolarization

Depolarization has already occurred and is complete by the end of the QRS complex. 1 The electrical sequence is:

1. QRS complex = ventricular depolarization (phase 0 of action potential) 1
2. ST segment = plateau phase with stable transmembrane voltage and minimal gradients 1
3. T wave = rapid repolarization (phase 3) as cells return to resting potential 1

The voltage gradients that create the T wave are generated by the sequential repolarization of myocardial cells, not by depolarization. 1 Importantly, repolarization proceeds from epicardium to endocardium—opposite to the direction of depolarization—which explains why T waves are typically concordant with the QRS despite the reversed sequence. 1

The Timing of Mechanical Contraction

Active ventricular contraction occurs primarily during the ST segment (plateau phase), not during the T wave. 1 By the time the T wave is inscribed:

• Mechanical systole is already well underway or completing 1
• The electrical process is one of recovery (repolarization), allowing cells to prepare for the next depolarization 1
• The transmembrane action potential is rapidly returning to its resting state of approximately -85 mV 1

Clinical Implications of This Understanding

Understanding that the T wave represents repolarization is critical because:

• Primary repolarization abnormalities (changes in T wave shape/duration without QRS changes) indicate problems with the repolarization phases themselves—from ischemia, electrolyte abnormalities, drugs, or toxins 1
• Secondary repolarization abnormalities (T wave changes accompanying QRS changes) result from altered depolarization sequence affecting subsequent repolarization, as seen in bundle branch blocks 1
• The R-on-T phenomenon is dangerous precisely because a premature depolarization occurs during the vulnerable repolarization phase (T wave), potentially triggering ventricular fibrillation 2

Common Pitfall to Avoid

Do not assume that electrical and mechanical events are synchronous. 1 The electrical repolarization (T wave) occurs while mechanical relaxation is beginning, but the active contraction phase corresponds to the ST segment plateau, not to the T wave. 1 This temporal dissociation between electrical repolarization and mechanical events is fundamental to understanding cardiac electrophysiology.