QTC

QTc stands for the corrected QT interval, a measurement used on the electrocardiogram (ECG) to assess the time of ventricular electrical repolarization. The QT interval runs from the start of the Q wave to the end of the T wave, expressed in milliseconds, and typically shortens as heart rate increases.

Because the QT interval changes with heart rate, clinicians use a corrected QT interval (QTc) that aims to reflect the duration at a standard heart rate. The most common correction is Bazett's formula: QTc = QT / sqrt(RR), where QT is in milliseconds and RR is the interval between successive R waves in seconds. Other corrections, such as Fridericia's (QTc = QT / RR^(1/3)), are also used because Bazett's can overcorrect at high rates or undercorrect at low rates. No method is perfect, and the choice of formula can affect classification, especially at extreme heart rates.

Normal QTc values are approximate and may differ by sex. A normal upper limit is about 440 ms in men and 460 ms in women; prolonged QTc is commonly defined as greater than about 450 ms in men or 470 ms in women, with QTc above 500 ms carried by a higher risk of torsades de pointes.

Prolongation can result from drugs, electrolyte disturbances, or congenital syndromes, and it may be worsened by multiple risk factors. Clinicians monitor QTc when starting or adjusting QT-prolonging medications and in patients with known risk factors, recognizing measurement limitations such as waveform morphology and overlapping U waves.