Gender-related Differences in Nonlinear Indices of Heart Rate Variability

The effect of day-night variations was evaluated in both a female and a male population of healthy subjects. A total of 239 subjects were included in this study, of which 119 male and 120 female. 24 hour Holter recordings were used to analyze heart rate variability. Power spectral indices (total power, low and high frequency power) were calculated together with the nonlinear indices fractal dimension, 1/f slope, the largest Lyapunov exponent and approximate entropy. The female population presented overall lower heart rate variability compared to the male population as expressed in the spectral components of heart rate variability. There was a significant difference between values in the male and female population for approximate entropy and the largest Lyapunov exponent, both during the day and during the night. Fractal dimension showed only a gender difference during sleeping hours. All frequency domain indices and nonlinear measures presented a daynight difference, with exception of low frequency power. Fractal dimension and approximate entropy were highly correlated with high frequency power. The 1/f slope was correlated with low frequency power. Our data indicate an increase in heart rate dynamic complexity during the night. Also women seem to have a more complex regulatory mechanism of heart rate as expressed in differences of nonlinear indices.