Gender and the relationship between ventricular repolarization and cardiac cycle length during 24-h Holter recordings

There are gender-related differences in the QT interval measured from standard ECG tracings. However, these observations are based on a limited number of beats recorded in resting conditions. Computerized Holter techniques enable ventricular repolarization and its relationship with cardiac cycle length to be analysed long term. Previous studies used only the initial portion of the QT interval to the T wave apex (QTa) to measure ventricular repolarization; however, QTa may underestimate the total QT duration (QTe). The aims of this study were to verify whether QTa and QTe had similar rate-dependence in normal subjects and whether gender-related QTe differences observed in the resting ECG were also present in the long-term QT intervalcycle length relationship.

Twenty-four hour Holter recordings were obtained in 40 healthy young subjects. 20 females and 20 males (mean age 28 +/- 9 and 26 +/- 5 years, respectively ns). Two-channel ECG digitized signals were processed using new automatic QT analysis software (Ela Medical), which converted the 24-h recordings into 2880 30-s templates. It also measured the QT apex (QTa) QT end (QTe) and the RR interval (ms) of each template, and computed the slopes of the linear regressions of QTe and QTa values plotted against the corresponding RR interval (QTe/RR and QTa/RR). Females had a shorter RR interval than males (803 +/- 129 vs 877 +/- 86 ms. P = 0.037), with longer mean QTc (420 +/- 17 vs 400 +/- 200 ms. P = 0.0005). In both genders. QTa/RR slopes were steeper than QTe/RR slopes (P = 0.0001). Both QTa/RR and QTe/RR slopes were steeper in females than in males (QTa/RR 0.20 +/- 0.04 vs 0.16 +/- 0.03, P = 0.001; QTe/RR 0.16 +/- 0.04 vs 0.13 +/- 0.03, P = 0.027). Of note, QTa and QTe at fixed long cycle lengths (1000 ms) were longer in women than in men (QTa1000 330 +/- 20 vs 309 +/- 18 ms: P = 0.002; QTe1000 410 +/- 17 vs 389 +/- 19 ms: P = 0.002), while they did not differ at fixed short cycle lengths (600 ms).

This study demonstrates that both the initial portion of the QT interval (QTa) and the entire QT interval (QTe) are useful since QTa is more prolonged than QTe at increasing cycle lengths, and thus includes most of the heart rate dependency of ventricular repolarization. In normal subjects, both the QTc and the long-term relationship between ventricular repolarization and heart rate are affected by gender. The differences in QTa and QTe duration between males and females are more marked at long cycle lengths and disappear at short cycle lengths. Finally, this study also proves the clinical feasibility of assessing the long-term relationship between ventricular repolarization and heart rate by utilizing the automatic measurement of the QT interval from 24-h Holter recordings.