QTintervalli

QTintervalli, commonly known as the QT interval, is a measurement on the electrocardiogram (ECG) that spans from the start of the Q wave to the end of the T wave. It represents the total time for ventricular depolarization and repolarization. Because ventricular repolarization varies with heart rate, the QT interval lengthens at slower heart rates and shortens at faster rates. To enable comparison across different heart rates, clinicians use a corrected QT interval (QTc), calculated with formulas such as Bazett’s (QTc = QT / sqrt(RR)) or Fridericia’s (QTc = QT / RR^(1/3)).

Normal values for QTc vary with sex and age, but are generally considered normal up to about 440–460 milliseconds. Prolonged QTc is often defined as greater than about 450–470 ms, with slightly higher thresholds used for women. A shortened QT interval is less common but can be associated with different clinical risks.

Prolongation of the QT interval can predispose to dangerous ventricular arrhythmias, notably torsades de pointes, and may increase the risk of sudden cardiac death in some individuals. Causes include congenital long QT syndromes (such as Romano-Ward and Jervell and Lange-Nielsen), electrolyte disturbances (low potassium, magnesium, or calcium), bradycardia, ischemia, structural heart disease, and a wide range of medications. Drug-induced QT prolongation is a major caution in many therapeutic areas, including antiarrhythmics, certain antibiotics, antipsychotics, and other central nervous system or metabolic drugs.

Measurement challenges include variations in lead selection, T-wave morphology, and the presence of U waves. Accurate assessment often requires careful manual verification when automated readings are uncertain. QTintervalli remains a key parameter in cardiology for risk stratification, drug safety evaluation, and clinical decision-making. See also long QT syndrome and short QT syndrome.