Nine months in space: effects on human autonomic cardiovascular regulation

We studied three Russian cosmonauts to better understand how long-term expo sure to microgravity affects autonomic cardiovascular control. We recorded the electrocardiogram, finger photoplethysmographic pressure, and respirato ry flow before, during, and after two 9-mo missions to the Russian space st ation Mir. Measurements were made during four modes of breathing: 1) uncont rolled spontaneous breathing; 2) stepwise breathing at six different freque ncies; 3) fixed-frequency breathing; and 4) random-frequency breathing. R w ave-to-R wave (R-R) interval standard deviations decreased in all and respi ratory frequency R-R interval spectral power decreased in two cosmonauts in space. Two weeks after the cosmonauts returned to Earth, R-R interval spec tral power was decreased, and systolic pressure spectral power was increase d in all. The transfer function between systolic pressures and R-R interval s 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 sug gest 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.