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1 Biomedical Physics Laboratory, Université Libre de Bruxelles, 1070 Brussels, Belgium; and 2 Department of Medecine, University of California, San Diego, La Jolla, California 92093-0931
We studied heart rate (HR), heart rate
variability (HRV), and respiratory sinus arrhythmia (RSA) in four male
subjects before, during, and after 16 days of spaceflight. The
electrocardiogram and respiration were recorded during two periods of 4 min controlled breathing at 7.5 and 15 breaths/min in standing and
supine postures on the ground and in microgravity. Low (LF)- and high
(HF)-frequency components of the short-term HRV (
3 min) were computed
through Fourier spectral analysis of the R-R intervals. Early in
microgravity, HR was decreased compared with both standing and supine
positions and had returned to the supine value by the end of the
flight. In microgravity, overall variability, the LF-to-HF ratio, and RSA amplitude and phase were similar to preflight supine values. Immediately postflight, HR increased by ~15% and remained elevated 15 days after landing. LF/HF was increased, suggesting an increased sympathetic control of HR standing. The overall variability and RSA
amplitude in supine decreased postflight, suggesting that vagal tone
decreased, which coupled with the decrease in RSA phase shift suggests
that this was the result of an adaptation of autonomic control of HR to
microgravity. In addition, these alterations persisted for at least 15 days after return to normal gravity (1G).
heart rate; heart rate variability; respiratory sinus arrhythmia; controlled breathing; microgravity; spaceflight; gravity; autonomic control
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