QT interval measurements

The QT interval, which represents duration of ventricular electrical systole, i.e., the time required forcompletion of both ventricular depolarization and repolarization, has been a parameter of particular interest incardiology. However, the relationship between duration of cellular action potentials and the QT interval recordedat the body surface is very complex. As a result, the QT interval is difficult to measure with precision. First,there is inherent imprecision in identifying the end of the T wave because of incomplete understanding of therecovery process and its projection on the body surface. Second, significant variation both in the onset of theQRS complex and the end of the T wave among some ECG leads provides different QT values depending on the leadsselected for measurement. Third, technical factors such as paper speed and sensitivity influence QT measurementswith higher paper speed leading to shorter interval values and higher sensitivity resulting in QT prolongation.The above problems do not appear to be solved by automatic QT measurement techniques, which have been found to beless accurate in cardiac patients than in healthy controls.In conclusion, we should accept that QT interval remains merely a gross measure of ventricular electricalsystole and/or repolarization and we should not expect significant improvement in accuracy of traditional QTinterval measurements. Rather, in clinical research, methods examining the shape or amplitude of the T wave andits changes related to heart rate should be exploited.