Reproducibility of QTc-measurements in healty volunteers

To assess the potential time-dependency and the reproducibility of the QTc-interval, ECG's from 84 healthy volunteers, recorded between 8 a.m. and 4 p.m., were analyzed. QT and RR intervals of all ECG's were measured by the same observer and Bazet's formula was used to correct QT intervals. The QTc-value showed a diurnal rhythm; its duration being minimal at 10 a.m., after which is gradually returned to the 8 a.m. value by 4 p.m. The 10 and 12 a.m. QTc's were statistically significantly shorter than the 8 a.m. QTc-interval (p < 0.05). There was not circannual rhythm. There was marked QTc-variability within one day, irrespective of the time at which the ECG had been obtained. Similar QTc-variability was also observed after longer intervals (week(s) or month(s)).