Power spectral analysis imperfectly informs changes in sympathetic trafficduring acute simulated microgravity

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 s tudy of immediate neural adjustments to simulated microgravity. We continuo usly recorded the electrocardiogram, noninvasive beat-by-beat arterial pres sure, 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, a nd calculated the ratio of normalized low-to-respiratory frequency RR-inter val spectral power as an index of sympathetic activity. Head-down tilt sign ificantly reduced lower leg volume, MSNA, and MSNA oscillations at the resp iratory frequency (p < 0.05). Head-down tilt did not change RR-interval, ar terial pressure, or their power spectra (p > 0.05). We conclude that non-in vasive frequency-domain estimates do not adequately reveal subtle changes i n sympathetic traffic during acute, simulated microgravity.