Measurement of the QT interval.

Ventricular repolarization is reflected by the T-wave of the surface electrocardiogram, the QT interval being the time taken for ventricular recovery. QT measurement has assumed clinical importance as prolongation is associated with a variety of conditions includingthe Romano-Ward' and JervellLange-Neilson syndromes, drug toxirity' and an adverse prognosis following acute myocardial infarction.. The QT interval is also modified by physiological factors, including heart rate"" and catecholamine release'"*. The QT interval is commonly corrected (QTc) using one of a variety of formulae to normalize the measurement to a specific heart rate, usually 60 beats per minute'. Surprisingly little attention has been paid to the methods of QT measurement. Over how many cardiac cycles should it be measured? Should the cycles be consecutive or be chosen at random? Which ECG leads should be used? If in the context of acute myocardial infarction, is the QT in an ECG lead reflecting infarction different from those leads without changes of infarction? In most electrocardiographic leads, theonset of the QRS complex is easily defined, but identifying the end of the T-wave may be less reliable particularly in the presence of low amplitude T-waves, bifid T-waves and U-waves. Despite these problems and despite a lack of cellular electrophysiological correlates, theQTinterval remains an accessible and potentially important electrocardiographic measurement. Re-evaluation of measurement methodology is essential to provide a standard for the many studies investigating the relevance and clinical association of QT interval abnormalities.