U.S. manned space flight: the first twenty years, a biomedical status report.

In the last 20 years, the biomedical problems facing man in space have been brought into sharper focus. Space motion sickness is presently our most serious problem. Its etiology remains obscure, but the "sensory conflict" theory appears most plausible. No valid predictive tests of susceptibility exist and presently we must rely on medication for prevention or mitigation of symptoms. Adaptation/biofeedback techniques may prove useful. Cardiovascular "deconditioning" may be effectively attenuated by use of anti-g suits or plasma expanding techniques. Recent bedrest simulation studies would seem to indicate that concerns about chronically elevated central venous pressure during space flight are unfounded. The loss of red cell mass in space flight appears to be self-limited, independent of mission duration, and not of clinical concern, based on recent Soviet experiences. And finally, clodronate, a new diphosphonate effective in preventing hypercalciuria and negative calcium balance in normal human bedrested subjects, may prove effective in preventing or lessening skeletal mineral loss in space.