Group and individual characteristics of orthostatic tolerance in participants of the one-year SIRIUS-23 isolation experiment

Introduction. Prolonged isolation and hypokinesia are known to reduce orthostatic tolerance, posing a significant safety risk for space crews, particularly in the context of prospective interplanetary missions. Despite extensive research, the physiological mechanisms driving vascular dysfunction in these conditions remain poorly understood, and no reliable individual predictors of orthostatic intolerance have been identified to date.

Objective. To analyze changes in the regulatory mechanisms of the cardiovascular system based on heart rate variability and hemodynamic parameters in response to an orthostatic challenge during the one‑year SIRIUS‑23 isolation experiment.

Materials and methods. The study was conducted as part of the SIRIUS‑23 isolation experiment, during a 12‑month isolation period, in a ground‑based experimental facility. Six healthy volunteers participated: two men (age 34 ± 3 years, BMI 24.9 ± 2.8 kg/m²) and four women (age 30 ± 5 years, BMI 23.1 ± 2.2 kg/m²). Five assessment sessions were scheduled at 2–3-month intervals throughout the isolation period, beginning one month after confinement. Additionally, a baseline session was carried out one to three days prior to the start of isolation, and a follow‑up (post‑exposure) session was conducted during the recovery period. In each session, an active orthostatic test was performed with recording of heart rate variability (HRV) and hemodynamic parameters. The data were processed using the Statgraphics 18 software package.

Results. Exposure to isolation conditions resulted in a statistically significant reduction in resting heart rate (F = 3.43; p = 0.011) and a marked decline in autonomic reactivity during the active stand test from Month 3 onward. Specifically, the relative increase in the LF component was significantly blunted (F = 3.29; p = 0.015), whereas the HF component exhibited a pronounced negative increase (F = 3.64; p = 0.009). Individual analysis revealed considerable heterogeneity in response patterns. In most participants, the observed “economization” of autonomic regulation appeared to be adaptive, as the rise in mean arterial pressure during orthostasis remained consistently positive. However, one female participant exhibited signs of decompensation by the end of the isolation period, manifested as exhaustion of sympathetic vasomotor activation: the LF component during orthostasis decreased to 197 ms² compared with 480 ms² in the supine position, and HF dropped to 16 ms². This resulted in a diminished rise in mean arterial pressure, which reached negative values.

Conclusions. The key determinant of orthostatic tolerance is the state of the autonomic nervous system, rather than anthropometric parameters or aerobic capacity. The obtained data substantiate the need for individual monitoring of autonomic regulation to predict the risks of maladaptation in extreme conditions.

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