VARIABILITY OF QT DISPERSION MEASUREMENTS IN THE SURFACE ELECTROCARDIOGRAM IN PATIENTS WITH ACUTE MYOCARDIAL-INFARCTION AND IN NORMAL SUBJECTS

QT dispersion (defined as maximal QT interval minus minimal QT interva l) as assessed on the surface electrocardiogram has been demonstrated to reflect regional inhomogeneity of ventricular repolarization. Howev er, the variability of repeated QT dispersion measurements has not bee n validated in a prospective study. Thats, the present study is based on the analysis of standard 12-lead surface electrocardiographic (ECG) tracings obtained in 127 persons including 50 subjects without struct ural heart disease and 77 patients presenting with acute myocardial in farction. RR and QT intervals were measured by means of a digitizer ta blet and QT/QTc dispersion was subsequently calculated automatically b y PC-based analysis software. Measurements were obtained on 2 separate occasions by the same observer to assess the intraobserver variabilit y. In addition, all tracings were evaluated by a second investigator t o determine the interobserver variability. QT dispersion in persons wi thout heart disease averaged 30 +/- 10 ms compared with 56 +/- 24 ms i n patients with acute myocardial infarction (p < 0.0001). Patients wit h infarction who developed ventricular fibrillation within the first 2 4 hours after admission (11 of 77) had an even larger QT dispersion of 88 +/- 30 ms (p < 0.0001). Repeated measurements of QT dispersion in all 127 subjects revealed a correlation coefficient of 0.91 for both i ntra- and interobserver variability. similar results were obtained for repeated determination of QTc dispersion (r = 0.93 and r = 0.90, resp ectively). When only patients with infarction were considered, correla tion coefficients between 0.84 and 0.88 were obtained. QT dispersion d etermined in ECG tracings recorded at a paper speed of 50 or 100 mm/s was less variable than that obtained from recordings taken at a speed of 25 mm/s. Thus, the intra- and interobserver variability of QT dispe rsion measurements in the standard 12-lead electrocardiogram permits t he use of this ECG technique to assess changes in ventricular recovery times due td therapeutic interventions or changes in the underlying d isease process.