QTQTc

QTQTc, commonly written as QTc, refers to the heart-rate corrected QT interval on the electrocardiogram. It measures the duration from the beginning of the Q wave to the end of the T wave of a single heartbeat. Because the QT interval shortens at higher heart rates and lengthens at lower rates, correction formulas are used to enable comparison across different heart rates.

Common correction methods include Bazett's formula (QTc = QT / sqrt(RR)) and Fridericia's formula (QTc = QT / RR^(1/3)), with RR in seconds. Other approaches are Hodges (QTc = QT + 1.75*(HR − 60)) and Framingham (QTc = QT + 0.154*(1 − RR)). Each method has strengths and limitations, particularly at extreme heart rates.

Normal QTc values roughly follow sex differences: about up to 440 ms in men and up to 460 ms in women. Prolonged QTc is generally considered to be above about 450–470 ms, with values exceeding 500 ms associated with a higher risk of torsades de pointes, a potentially life-threatening arrhythmia. Congenital long QT syndromes (LQT1–LQT3, among others) arise from inherited ion channel abnormalities and can cause markedly prolonged QTc.

Clinical significance and risk factors: QTc prolongation can result from electrolyte disturbances (low potassium, magnesium, or calcium), bradycardia, ischemia, or drugs that prolong repolarization. Many medications—such as certain antiarrhythmics, macrolide and fluoroquinolone antibiotics, antipsychotics, antidepressants, and antiemetics—are known to lengthen QTc and require monitoring.

Measurement considerations include choosing a lead with a clear T wave, accounting for U waves or conduction abnormalities, and interpreting QTc in the broader clinical context, including electrolytes and comorbidities.