QT and QT-peak interval measurements. A methodological study in patients with subarachnoid haemorrhage compared to a reference group.

To study the properties of QT and QT-peak intervals, ECGs were compared between 56 consecutive patients who were suffering from subarachnoid haemorrhage (SAH) and 50 reference subjects. The routine QTc interval was compared to the mean QTc from all of the 12 leads with identifiable U waves and to the mean QT-peakc. The interval between peak and end of T(Tp-Te) was subsequently calculated. In the reference group the mean QT-peak correlated with the mean QT (r = 0.925). The rate-dependence of the mean QT-peak was not different from that of the mean QT and showed the same correlation (r = -0.607 and -0.630, respectively). No rate-dependence for the Tp-Te interval could be demonstrated. Following SAH, ECG abnormalities were most pronounced after 8-9 days, and increased with age and the degree of cerebral dysfunction. Two patient groups, where the mean QTc of each particular patient was either below (n = 27) or above (n = 29) the reference limit, were analysed. For the group without an abnormally prolonged mean QTc, the average of the individual mean QTc was significantly longer than in the reference group. Both groups had longer mean QT-peakc intervals than the reference group. About 70% of the patients with an abnormally prolonged mean QTc also had a prolonged mean QT-peakc interval, while the rest had a prolonged Tp-Tc interval; simultaneous prolongation of these two intervals also occurred. Prolongation of the Tp-Te interval did not occur in the group without an abnormally prolonged mean QTc. Routine QTc and mean QT-peakc had sensitivities of 96% and 67% respectively, specificities of 76% and 96% and predictive values of 81% and 95%. In conclusion, the routine QTc measurements, without reference to an identified U wave, may result in falsely prolonged estimates of cardiac repolarization time. In this respect the mean QT-peakc may provide additional information. In the majority of patients the prolonged mean QTc was dependent on a disturbed rate-dependent function (prolonged mean QT-peakc) while some patients had an increased asymmetry of the repolarization process within the myocardium (prolonged Tp-Tc).