More light on QT interval measurement.

Aprolonged QT interval signifies a delay in the ventricular repolarisation phase, which renders the heart vulnerable to malignant arrhythmias such as torsade de pointes ventricular tachycardia. This sign of disease is sometimes subtle and may avoid recognition unless meticulously sought. In this issue of Heart, Malik and colleagues draw attention to methodological difficulties in the measurement of the QT interval. The action potential duration of the myocardium, reflected as the QT interval on the surface ECG, depends on heart rate which hampers estimation of the intrinsic myocardial repolarisation time. The relation of the QT interval and heart rate, or cardiac cycle length, is modified by a number of physiologic processes. The mechanism causing the change in heart rate may variably influence ventricular repolarisation. For example, an equal increase in heart rate shortens the QT interval less during sympathetic stimulation by cold pressor test and during cardiac pacing compared to physical exercise. Furthermore, the response of the QT interval to a change in rate is not instantaneous, full adaptation taking 1–3 minutes.