title: Power spectral analysis imperfectly informs changes in sympathetic traffic during acute simulated microgravity.
abstract: The purpose of this study was to investigate the value of frequency-domain analysis of autonomic rhythms as a simple, non-invasive technique for the study of immediate neural adjustments to simulated microgravity. We continuously recorded the electrocardiogram, non-invasive beat-by-beat arterial pressure, and muscle sympathetic nerve activity (MSNA) during 5-min periods of controlled frequency breathing (15 breaths.min-1) with subjects (n = 10) in supine, and 10 degrees head-down tilt positions. We estimated changes in fluid volume with lower leg circumference measurements. We analyzed data in the frequency domain with fast Fourier-based power spectral analysis, and calculated the ratio of normalized low-to-respiratory frequency RR-interval spectral power as an index of sympathetic activity. Head-down tilt significantly reduced lower leg volume, MSNA, and MSNA oscillations at the respiratory frequency (p < 0.05). Head-down tilt did not change RR-interval, arterial pressure, or their power spectra (p > 0.05). We conclude that non-invasive frequency-domain estimates do not adequately reveal subtle changes in sympathetic traffic during acute, simulated microgravity.
authors: Center for Biomedical Engineering, Michigan Technological University, 1400 Townsend Dr., Houghton, MI 49931, USA. whcooke@mtu.edu
authors: W H
authors: WH
authors: Cooke
authors: M M
authors: MM
authors: Dowlyn
mesh_terms: D000222:Adaptation, Physiological / Q000502:physiology*; D000328:Adult; D000704:Analysis of Variance; D015982:Bias; D001794:Blood Pressure / Q000502:physiology; D004562:Electrocardiography; D005260:Female; D018495:Fluid Shifts / Q000502:physiology; D005583:Fourier Analysis; D018475:Head-Down Tilt / Q000502:physiology*; D006339:Heart Rate / Q000502:physiology; D006801:Humans; D008297:Male; D008991:Monitoring, Physiologic / Q000379:methods / Q000592:standards*; D012680:Sensitivity and Specificity; D012815:Signal Processing, Computer-Assisted; D013564:Sympathetic Nervous System / Q000502:physiology*; D018474:Weightlessness Simulation / Q000379:methods*