Assessment of the QT Interval in WPW: A New Approach to Calculating the QT Interval in Pre-excitation

Abstract

Background:The initial slurring of the QRS complex, in WPW increases the QT interval rendering it a less accurate assessment of repolarization.Objective: The objective was to develop a method to calculate the underlying QT interval in WPW from the 12 lead ECG. Methods:Patients with a diagnosis of intermittent WPW on a single ECG had measurements of QT interval, delta wave duration, QRS duration and JT interval. Seven different equations were evaluated in pre-excited compared to non-pre excited conduction with the latter considered the 'true' or underlying (intrinsic) QT interval. The best formulae were tested in a separate group of 100 patients with WPW. Results:The QT interval of 'pre excited' QRS complexes (QTwpw) were significantly longer than the non-pre excited QT interval (QTi), in each of four different heart rate correction formulae. Three equations showed highly significant (p < 0.0001) correlations with QTi (r = 0.918 to 0.977) using either parametric or non-parametric testing and were better than subtracting delta wave duration from QTwpw an equations with the strongest correlation, highest F value, was selected. In a separate sample of 100 persons with pre excitation, QT heart rate dependency was not completely corrected by the Bazett formula and this formula produced unacceptably high estimates for QT prolongation. Three other heart rate correlation formulae showed a consistent and low prevalence of QT prolongation. Conclusion:An accurate assessment of the underlying QT interval in pre-excitation is attained from the formula QTwpw -0.462 (QRSwpw) + 18.26 followed by a heart rate correction formula other than the Bazett approach.