Day-Time and night-Time HRV ultradian rhythms in normal and pathological Subjects

Heart rate variability (HRV) is a powerful marker of the autonomic control of the heart. While HRV circadian rhythm has been well characterized, ultradian oscillations have been studied to a lesser extent. In this study, ultradian autonomic regulation is assessed by evaluating the temporal evolution of HRV fractal dimension (FD). In order to highlight possible differences in the wake-sleep states, FD is evaluated in the day-time (15.00-20.00) and in the night-time (23.30-5.30) separately. The RR series extracted from 24-h Holter recordings of 50 subjects in different physiopathological conditions were analyzed. All subjects showed a main common rhythm (0.09mHz) that persists with generally higher amplitude during night-time. A further rhythm becomes predominant in normal subjects in the day-to-night transition (0.18mHz), probably under the influence of the REM/non- REM ultradian sleep cycle. Alterations in this rhythm characterize the HRV fractal dimension of all patients according to their particular condition. In conclusion, the fractal dimension could be used as a marker of ultradian cardiac autonomic regulation and could provide new insights into autonomic physiology of normal and patients.