T wave

The T wave is the rounded hump that follows the tall QRS on each heartbeat, and it marks the heart's ventricles recovering, or resetting, after they have fired. If the QRS is the muscle switching on, the T wave is it switching back off and recharging, ready for the next beat.

In electrical terms the T wave reflects ventricular repolarization: heart cells restoring their resting charge after the wave of activation. In most leads a normal T wave is upright, smooth, and slightly asymmetric, pointing in the same general direction as the QRS. Its appearance depends on healthy, evenly timed recovery across the ventricular wall.

Because recovery is sensitive to many disturbances, the T wave is a frequent early warning sign. Tall, peaked T waves can appear with high blood potassium; flattened or inverted T waves may follow ischemia, strain, or a range of other conditions. T-wave changes are rarely specific on their own, so they are interpreted alongside the patient's symptoms, the rest of the ECG, and other tests.