Background: Upon return from space many astronauts experience symptoms of orthostatic intolerance. Research has implicated altered autonomic cardiovascular regulation due to spaceflight with further evidence to suggest that there might be pre-flight autonomic indicators of post-flight orthostatic intolerance. We used heart rate variability (HRV) to determine whether autonomic regulation of the heart in astronauts who did or did not experience post-flight orthostatic intolerance was different pre-flight and/or was differentially affected by short duration (8 -16 days) spaceflight. HRV data from ten-minute stand tests collected from the 29 astronauts 10 days preflight, on landing day and three days post-flight were analysed using coarse graining spectral analysis. From the total power (P TOT ), the harmonic component was extracted and divided into high (P HI : >0.15 Hz) and low (P LO : = 0.15 Hz) frequency power regions. Given the distribution of autonomic nervous system activity with frequency at the sinus node, P HI /P TOT was used as an indicator of parasympathetic activity; P LO /P TOT as an indicator of sympathetic activity; and, P LO /P HI as an estimate of sympathovagal balance.
Results: Twenty-one astronauts were classified as finishers, and eight as non-finishers, based on their ability to remain standing for 10 minutes on landing day. Pre-flight, non-finishers had a higher supine P HI /P TOT than finishers. Supine P HI /P TOT was the same pre-flight and on landing day in the finishers; whereas, in the non-finishers it was reduced. The ratio P LO /P HI was lower in non-finishers compared to finishers and was unaffected by spaceflight. Pre-flight, both finishers and non-finishers had similar supine values of P LO /P TOT , which increased from supine to stand. Following spaceflight, only the finishers had an increase in P LO /P TOT from supine to stand.
Conclusions:
Both finishers and non-finishers had an increase in sympathetic activity with stand on pre-flight, yet only finishers retained this response on landing day. Non-finishers also had lower sympathovagal balance and higher pre-flight supine parasympathetic activity than finishers. These results suggest pre-flight autonomic status and post-flight impairment in autonomic control of the heart may contribute to orthostatic intolerance. The mechanism by which higher pre-flight parasympathetic activity might contribute to post-flight orthostatic intolerance is not understood and requires further investigation.