On Earth, tissue weight generates compressive forces that press on body structures and act on the walls of vessels throughout the body. In microgravity tissues no longer have any weight and tissue compressive forces are lost, suggesting individuals who weigh more may show greater effects from microgravity exposure. One unique effect of long-duration microgravity exposure is the spaceflight-associated neuro-ocular syndrome (SANS) which can present with globe flattening, choroidal folds, optic disc edema, and a hyperopic visual shift.

They analyzed data from 45 individual long-duration astronauts (mean age 47, 36 male, nine female, mean mission duration 165 days) who had pre- and post-flight measures of disc edema, choroidal folds, and manifest ocular refraction.  The mean pre-flight weights of astronauts who developed new choroidal folds (78.6 kg no new folds vs. 88.6 kg new folds, F=6.2, p=0.02) and disc edema (79.1 kg no edema vs. 95 kg edema, F=9.6, p=0.003) were significantly greater than those that did not.

Chest and waist circumference were also significantly greater in those who developed folds or edema. The odds of developing either disc edema or new choroidal folds was 55% in the highest weight quartile, and 9% in the lowest. In this cohort, no women developed either disc edema or choroidal folds, although women also weighed significantly less than men (62.9 kg female vs. 85.2 kg male, F=53.2, p<0.0001).

Preflight body weight and anthropometric factors may predict microgravity-induced ocular changes. The study introduces a new method for analyzing respiratory (vagally) mediated Heart Rate Variability, to better assess subtle variations in sympathovagal balance, using ECG information.

They evaluated the sensitivity of our new method comparatively to classic HRV analyses with the cognitive workload. The method aims at enhancing classical frequency analysis of HRV by focusing the quantification of respiratory sinus arrhythmia (RSA) gain on the respiratory frequency. Instantaneous respiratory frequency was obtained using ECG-derived respiration and was used for Variable Frequency Complex Demodulation of RR intervals to extract RSA.

They concluded that our method adds significant value to the usual frequency-based HRV methods by being the most responsive to cognitive workload, particular thanks to its robustness to transient changes in respiratory frequency.

Thanks to the new analysis method presented here, the study also provided clear evidence of the way short episodes of spontaneous low breathing rate in some individuals affect classic frequency-based analyses and lead to wrong conclusions about the effects of a mental task.