QTIntervall

QTintervall, denoted as the QT interval in English, is the portion of the electrocardiogram (ECG) from the onset of the Q wave to the end of the T wave. It represents the total time for ventricular depolarization and repolarization. The QT interval shortens and lengthens with heart rate, so a heart-rate–corrected value, QTc, is commonly used to compare intervals across different rates. Common correction methods include Bazett’s formula (QTc = QT / sqrt(RR)) and Fridericia’s (QTc = QT / RR^(1/3)); several other formulas exist, each with limitations.

Normal QT and QTc values depend on sex and age. Typical upper limits are around 440 ms for men and 460 ms for women; values above about 480–500 ms are generally considered prolonged. Prolongation of the QT interval increases the risk of torsades de pointes, a potentially life-threatening form of polymorphic ventricular tachycardia that can progress to ventricular fibrillation, particularly at slower heart rates or in the presence of electrolyte disturbances.

Prolonged QT can be congenital (long QT syndromes) or acquired. Congenital forms arise from mutations in cardiac ion channels (examples include KCNQ1, KCNH2, SCN5A). Acquired prolongation is associated with certain medications (such as some antiarrhythmics, macrolide and fluoroquinolone antibiotics, antipsychotics, and antidepressants) and with electrolyte abnormalities (hypokalemia, hypomagnesemia, hypocalcemia). Short QT intervals are less common and may indicate short QT syndrome, which can also predispose to arrhythmias.

Measurement should be performed on a stable rhythm, with careful identification of the Q wave onset and T wave end, ideally using multiple beats for accuracy.