QTintervaliin

QTintervaliin is a term used to describe the QT interval on an electrocardiogram (ECG), the period from the start of the Q wave to the end of the T wave. It reflects the total time for ventricular depolarization and repolarization. Because the QT interval changes with heart rate, clinicians often use a corrected QT interval (QTc) to compare values across different heart rates.

Measurement relies on horizontal time units on the ECG tracings and can vary by lead and interpretation. QT may be underestimated or overestimated if the end of the T wave is not clear, if U waves merge with the T wave, or if the heart rate is very fast or slow. Common correction methods include Bazett’s formula (QTc = QT / sqrt(RR)) and Fridericia’s formula (QTc = QT / (RR)^(1/3)); several alternatives exist for different clinical settings.

Prolongation or shortening of the QT interval has clinical significance. A prolonged QTc raises the risk of torsades de pointes, a potentially life-threatening ventricular arrhythmia, and sudden cardiac death. A short QT interval may be associated with atrial arrhythmias and other cardiac risks. QT prolongation can result from congenital long QT syndromes, electrolyte disturbances, or drugs known to inhibit cardiac repolarization, including certain antiarrhythmics, antibiotics, antipsychotics, and antidepressants.

Normal values vary with age, sex, and heart rate. In general, a QTc above about 450–470 ms is considered prolonged, with higher thresholds (>500 ms) indicating greater risk. Accurate assessment requires careful measurement, appropriate correction, and consideration of clinical context.