repolarization

Repolarization is the heart cell's recovery — the reset that returns its voltage to the negative resting baseline after it has fired. If depolarization is the cell switching on, repolarization is it switching back off and recharging, getting ready for the next beat. Without it, the cell could never respond again.

Mechanistically, repolarization happens as potassium ions flow out of the cell while the inward currents that drove depolarization shut off. In working ventricular cells this comes only after the long calcium plateau, so repolarization is relatively slow and orderly. Restoring the original ion gradients afterward is the job of pumps such as the sodium-potassium pump, which run quietly in the background.

Repolarization matters clinically because it governs the refractory period and because disturbances in it are dangerous. On the ECG, ventricular repolarization writes the T wave, and the total time from the start of depolarization to the end of repolarization is the QT interval. When repolarization is abnormally prolonged — by genetic long QT syndrome, by certain drugs, or by low potassium — the heart becomes vulnerable to a life-threatening arrhythmia called torsades de pointes.