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Ted Hall
The short answer to what happens to an astronaut's heart rate in zero-g seems to be that heart rate goes up, to compensate for reduced cardiac volume, which results from net fluid loss. Then it may go down again. The long answer is more complicated.
Lower diastolic blood pressure and a tendency to spontaneous syncope (fainting) have been noted consistently among space crews; these are attributed to fluid loss. On Salyut and Skylab, the heart's stroke volume and cardiac output were generally elevated during flight but fell to subnormal levels upon return to Earth. Radiographic measurements indicated a progressive decrease in cardiac size.
A more elaborate study was conducted during a flight of the shuttle Discovery in 1985 (mission 51-D, April 12-19, 1985), with data collected from four crew members. Discovery was better equipped for cardiovascular research, but its 7-day flight was significantly shorter than any of the Skylab missions. In this study, both systolic and diastolic pressures rose, with a mean increase of 20 percent - contrary to earlier findings. In comparing Discovery and Skylab, one expert speculated that the reason for the discrepancy might be that the Discovery blood pressure readings were taken earlier in the flight.
Other Discovery measurements confirmed previous findings. Echocardiography revealed that the volume of the right ventricle decreased by 35 percent during the first day of flight; the left ventricle increased by 20 percent during the first day, then decreased to 85 percent of its preflight volume during the second day. Stroke volume varied with the left ventricular volume, while heart rate increased by 20 percent. As a result, cardiac output (heart rate times stroke volume) increased substantially during the first day, then decreased to preflight level. Increased blood pressure and near-normal cardiac output indicated an increase in peripheral vascular resistance (pressure divided by output).
To summarize: in weightless, an increase of fluid in the thoracic area leads initially to increases in left ventricular volume and cardiac output; as the body seeks a new equilibrium, fluid is excreted, the left ventricle shrinks, and cardiac output decreases; upon return to gravity, fluid is pulled back into the lower extremities, and cardiac output falls to subnormal levels. It may take several weeks for fluid volume, peripheral resistance, cardiac size, and cardiac output to return to normal.
Beverly Merz. "The Body Pays a Penalty for Defying the Law of Gravity." Journal of the American Medical Association, vol. 256, no. 15, pages 2042-2043, October 17, 1986. American Medical Association.
She states that heart output and heart rate typically increase during weightlessness, but that output decreases to subnormal levels after return to Earth. But see the next two notes (and note the relative publication dates) ...
Mary M. Connors, Albert A. Harrison, and Faren R. Akins. Living Aloft: Human Requirements for Extended Spaceflight, page 20. NASA Scientific and Technical Information Branch, 1985. Special Publication 483.
They summarize the cardiovascular changes as "reduced output of the heart, decreased heart rate, decreased heart size, and diminished blood volume regulation." [Emphasis mine.] They cite: Charles A. Berry. "Weightlessness." Bioastronautics Data Book, second edition, pages 349-416. NASA SP-3006, 1973.
Charles Marwick. "Physicians Called Upon to Help Chart Future Space Effort." Journal of the American Medical Association, vol. 256, no. 15, page 2020, October 17, 1986. American Medical Association.
He quotes Robert H. Moser, the chairman of NASA's Life Sciences Advisory Committee: "the decrease seen in the left ventricular output is normal." [Emphasis mine.]
Various psychological factors may have contributed to the elevated blood pressure readings obtained on the 51-D Discovery mission. The failed deployment of an $85-million satellite significantly disrupted the mission and led to the most intensive orbital repair attempt since Skylab. Also, Discovery's crew of seven was crowded into a smaller volume than Skylab's crew of three.
Craig Covault. "Astronauts, Controllers Mobilize for Leasat Rescue Attempt." Aviation Week and Space Technology, vol. 122, no. 16, pages 18-21, April 22, 1985. McGraw-Hill.
Anonymous. "Shuttle Crew Returns to Leasat, Conducts Space Medical Tests." Aviation Week and Space Technology, vol. 122, no. 17, page 39, April 29, 1985. McGraw-Hill.
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