QT interval measurements.

The QT interval, which represents duration of ventricular electrical systole, i.e., the time required for completion of both ventricular depolarization and repolarization, has been a parameter of particular interest in cardiology. However, the relationship between duration of cellular action potentials and the QT interval recorded at 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 the recovery process and its projection on the body surface. Second, significant variation both in the onset of the QRS complex and the end of the T wave among some ECG leads provides different QT values depending on the leads selected for measurement. Third, technical factors such as paper speed and sensitivity influence QT measurements with 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 be less accurate in cardiac patients than in healthy controls. In conclusion, we should accept that QT interval remains merely a gross measure of ventricular electrical systole and/or repolarization and we should not expect significant improvement in accuracy of traditional QT interval measurements. Rather, in clinical research, methods examining the shape or amplitude of the T wave and its changes related to heart rate should be exploited.