Nonlinear heart rate dynamics: circadian profile and influence of age and gender.

Heart rate variability (HRV) is used as a marker of autonomic modulation of heart rate. Nonlinear HRV parameters providing information about the scaling behaviour or the complexity of the cardiac system were included. In addition, the chaotic behaviour was quantified by means of the recently developed numerical noise titration technique. 24h Holter recordings of a large healthy population (N=276, 141 males, 18-71 years of age) were available. The goal was to investigate the influence of gender, age and day-night variation on these nonlinear HRV parameters. Numerical titration yielded similar information as other nonlinear HRV parameters do. However, it does not require long and cleaned data and therefore applicable on short (5min) noisy time series. A higher nonlinear behaviour was observed during the night (NLdr; day: 50.8±19.6%, night: 59.1±19.5%; P<0.001) while nonlinear heart rate fluctuations decline with increasing age (NLdr; Pearson correlation coefficient r between -0.260 and -0.319 dependent on gender and day or night, all P<0.01). A clear circadian profile could be found for almost every parameter, showing in particular which changes occur during the transition phases of waking up and going to sleep. Our results support the involvement of the autonomic nervous system in the generation of nonlinear and complex heart rate dynamics.