Nine months in space: effects on human autonomic cardiovascular regulation

TLDR

The study examined how prolonged microgravity exposure alters autonomic cardiovascular regulation in three Russian cosmonauts. Electrocardiogram, finger photoplethysmographic pressure, and respiratory flow were recorded before, during, and after two 9‑month Mir missions, with measurements taken during spontaneous, stepwise, fixed‑frequency, and random‑frequency breathing. During flight, R‑R interval variability and respiratory‑frequency R‑R spectral power decreased, and after return, R‑R spectral power remained low while systolic pressure spectral power increased; the systolic pressure–R‑R transfer function was reduced in‑flight, further diminished post‑landing, and had not returned to baseline by 14 days, indicating persistent reductions in vagal‑cardiac traffic and baroreflex gain up to two weeks after return.

Abstract

We studied three Russian cosmonauts to better understand how long-term exposure to microgravity affects autonomic cardiovascular control. We recorded the electrocardiogram, finger photoplethysmographic pressure, and respiratory flow before, during, and after two 9-mo missions to the Russian space station Mir. Measurements were made during four modes of breathing: 1) uncontrolled spontaneous breathing; 2) stepwise breathing at six different frequencies; 3) fixed-frequency breathing; and 4) random-frequency breathing. R wave-to-R wave (R-R) interval standard deviations decreased in all and respiratory frequency R-R interval spectral power decreased in two cosmonauts in space. Two weeks after the cosmonauts returned to Earth, R-R interval spectral power was decreased, and systolic pressure spectral power was increased in all. The transfer function between systolic pressures and R-R intervals was reduced in-flight, was reduced further the day after landing, and had not returned to preflight levels by 14 days after landing. Our results suggest that long-duration spaceflight reduces vagal-cardiac nerve traffic and decreases vagal baroreflex gain and that these changes may persist as long as 2 wk after return to Earth.