Nine months in space: effects on human autonomic cardiovascular regulation

1 Center for Biomedical Engineering, Michigan Technological University, Houghton, Michigan 49931; 2 Departments of Medicine, Physiology, and Mathematical Sciences, Medical College of Virginia at Virginia Commonwealth University, and Hunter Holmes McGuire Department of Veterans Affairs Medical Center, Richmond, Virginia 23249; 3 Department of Applied Physics, University of Turku, Finland 20014; 4 Department of Clinical Physiology, Kuopio University Hospital, Kuopio, Finland 33521; 5 University of Texas Southwestern Medical Center, Dallas, Texas 75235; 6 Deutsche Forschungsanstalt für Luft- und Raumfahrt, Institute of Aerospace Medicine, Köln, Germany 51147; 7 Department of Autonomic Neuroscience, Research Institute of Environmental Medicine, Nagoya, Japan 504-8601; and 8 Institute for Exercise and Environmental Medicine and Presbyterian Hospital, Dallas, Texas 75231 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. baroreflex; cardiac control; space station Mir