Spectral characteristics of heart rate fluctuations during parabolic flight

Parabolic flight is used to create short successive periods of changing gravity in a range between 0 and 1.8 Gz (1 Gz: 9.81 m/s(2)). The purpose of the present study was to evaluate whether cyclic variations in heart rate during +/-20 s periods of stable gravity in parabolic flight reflect autonomic modulation of cardiac chronotropy. During the 29th and 32nd ESA parabolic flight campaign ECG and respiration were recorded in 13 healthy volunteers in both standing and supine postures. We developed and validated a spectral algorithm especially adapted to study frequency components of heart rate among ultrashort (+/-20 s) stable gravity periods of parabolic flight. A low frequency (LF) component, starting from the lowest measurable frequency (+/-0.05 Hz) up to 0.15 Hz was distinguished from a high frequency (HF) component, ranging from 0.16 Hz up to 0.4 Hz. Powers were calculated by integration between corresponding limits and represented in normalized units (nu). With our method, we were able to reproduce normal findings in the upright posture at 1 Gz, i.e., less power in the HF component compared to supine (HFnu: 0.18+/-0.09 vs. 0.40+/-0.16). These postural related differences are shown to be eliminated at 0 Gz (HFnu: 0.30+/-0.12 vs. 0.32+/-0.13) and amplified at 1.8 Gz phases (HFnu: 0.15+/-0.10 vs. 0.39+/-0.16) of parabolic flight. In the supine position no coherent differences were shown in the measured variables among different gravity phases. Our observations strongly indicate that spectral characteristics of heart rate fluctuations among stable gravity periods of parabolic flight reflect parasympathetic nervous system control of cardiac chronotropy. At 1 Gz, there is a normal upright situation with less parasympathetic modulation of heart rate compared to supine. This effect is augmented during 1.8 Gz-conditions due to a suppressed parasympathetic control of heart rate in the upright posture. Alternatively, at 0 Gz, increased parasympathetic control in standing position eliminates differences in cardiac chronotropy compared to supine.