QT interval
The QT interval is the span of the ECG from the start of the QRS complex to the end of the T wave. It captures the entire electrical life of a single ventricular beat — from the moment the ventricles fire to the moment they have fully reset — so it represents depolarization and repolarization combined.
Think of it as the length of time the ventricles are electrically busy, the window during which the heart cells cannot yet be cleanly restarted. The faster the heart beats, the shorter this window naturally becomes, which is why the raw QT is usually adjusted for heart rate before it is judged abnormal.
The QT interval matters chiefly because a prolonged one signals a longer vulnerable period that can predispose to a dangerous ventricular rhythm. Many things can lengthen it, including certain medicines, low potassium or magnesium, and inherited channel disorders. Because of these risks, the QT is checked routinely and is one reason clinicians scrutinise the ECG before and after starting some drugs.
Because the QT shortens as heart rate rises, it is usually reported as the rate-corrected value, the QTc.