Sex Differences in QT Interval Variability Computed over 24-Hour Holter Recordings

Many studies analyzed sex differences in heart period (HP) variability without reaching definite conclusions for the limited ability of HP variability in exploring sympathetic control. The study of the QT interval (QT) variability could provide more insights. The aim of the study was to compare nocturnal HP and QT variability in 37 healthy females (F, age $36.35 \pm 6.58 \mathrm{yrs}$) and 44 healthy males (M, age $34.94 \pm 4.90 \mathrm{yrs}$). Mean and variance of the HP and QT series were calculated. Parametric power spectral analysis was applied. HP power in the high frequency and QT power in the low frequency band ($\mathrm{LF}_{\mathrm{QT}}$) were calculated. None of the HP indexes were different between F and M. QT mean, variance and LF<inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">QT</inf> were higher in F. We conclude that QT variability was more sensible than the HP one in revealing sex differences, suggesting an increased QT variability in F in presence of similar HP variability indexes.