QT Interval
Definition Time from the start of the Q wave to the end of the T wave Represents time taken for ventricular depolarisation and repolarisation, effectively the period of ventricular systole from ventricular isovolumetric contraction to isovolumetric relaxation The QT interval is inversely proportional to heart rate: The QT interval shortens at faster heart rates The QT interval lengthens at slower heart rates An abnormally prolonged QT is associated with an increased risk of ventricular arrhythmias, especially Torsades de Pointes Congenital short QT syndrome has been found to be associated with an increased risk of paroxysmal atrial and ventricular fibrillation and sudden cardiac death How to measure the QT interval The QT interval is usually measured in either lead II or V5-6, however the lead with the longest measurement should be used Several successive beats should be measured, with the maximum interval taken Large U waves (> 1mm) that are fused to the T wave should be included in the measurement Smaller U waves and those that are separate from the T wave should be excluded The maximum slope intercept method is used to define the end of the T wave (see below) ...