QT INTERVAL IN 24-HOUR AMBULATORY ECG RECORDINGS FROM 60 HEALTHY ADULT SUBJECTS

QT and RR intervals were measured in 24-hour electrocardiographic (ECG) recordings from 60 healthy subjects randomly selected among participants in the Copenhagen City Heart Study. Five men and five women of each 10-year age group between the ages of 20 and 79 were included. The mean of three consecutive RR and QT intervals was calculated from ECG strips recorded at 1000, 1400, 1800, 2200, 0200, and 0600 hours in each subject. The RR interval varied between 728 ms at 1400 hours and 984 ms at 0600 hours, and the uncorrected QT interval between 358 ms at 1400 hours and 417 ms at 0200 hours. The QT interval corrected for heart rate by Bazett's formula varied between 418 ms at 0600 hours and 428 ms at 1000 hours, and the QT interval corrected by the regression equation of this study varied between 396 ms at 1400 hours and 422 ms at 0200 hours. Multiple regression analysis of uncorrected data yielded a partial regression coefficient for heart rate influence of 0.14. After correction by Bazett's formula, a statistically significant effect of heart rate remained (partial regression coefficient - 0.08, t = -9,93, p < 0.0001). In addition to the influence of heart rate, the analysis revealed a statistically significant effect of hour of day (F = 11.30; DF 5,286; p < 0.0001) and gender (F = 6.24; DF 1, 53; p < 0.05), whereas age in the range of 20-79 years had no significant effect. The QT intervals of this study differed from the values measured in standard ECG recordings but not from those of other Holter studies. It is recommended that the QT interval be corrected for heart rate, gender, and hour of day before being classified as normal or abnormal.