The wide range of individual tolerances to the disturbing effects of
vestibular stimulation has emphasized the importance of this factor in
astronaut selection. At the same time, vestibular functions must be
considered jointly with visual task performance, since both have special
significance for such maneuvers as vehicle docking. Vestibular function
in the weightless state remains almost completely unknown. Limited
evidence from animal and manned space flights suggests that head
turning, resulting from vestibular stimulation, may seriously interfere
with visuomotor performance, but that susceptibility to these
disturbances is significantly different between individuals and that
partial adaptation occurs relatively quickly.
NASA is currently collecting extensive baseline electroencephalogram
data under controlled conditions in a form suitable for mathematical
analysis. Data are being taken from about 200 subjects in major national
and overseas centers. It is intended that this study will assist in
astronaut selection and monitoring in space.
Studies on many effects of weightlessness on nervous functions require
monitoring of the autonomic nervous system, including such autonomic
effects as gastrointestinal activity, secretion, lacrimation,
salivation, sweating, and the central control of respiration. Urinary
estimations of catecholamines and 5-hydroxyindoleacetic acid would
provide important data on autonomic system activity if collected in
flight and compared with preflight and postflight controls.
Major areas have been outlined in which prolonged weightlessness may be
expected to interfere with performance, judgment, and, ultimately,
chances of survival. These include cardiovascular, metabolic, central
nervous, psychophysiological, and biorhythmic effects. They have been
dealt with separately and in sequence, but have not been intended to be
viewed as hierarchic. The relative scarcity of data necessarily
precludes such an evaluation.
Soviet experience with zero gravity and weightlessness has increased
their emphasis on this space-flight factor and was an important topic at
the May 1964 COSPAR meeting. Discussion of the postflight medical status
of Bykovsky (5-day flight) and Tereshkova (3-day flight) revealed a
concern for the significance of prolonged weightlessness and the
presence of postflight physical debility and fatigue following Vostok
flights 3 through 6. These changes persisted for several days. Among the
physiological conditions singled out for mention were—
(1) _Body fluids_— Cosmonauts have shown a postflight weight loss of
1.9 to 2.4 kg apparently resulting from a redistribution of body
fluid in response to elimination of the hydrostatic pressure
gradients caused by Earth gravity. There is the suggestion that
this redistribution is complete within the first 24 hours of
flight. Titov is reported to have been dehydrated alter his flight
with early hemoconcentration. These findings directly support
predictions made from ground-based research.
(2) _Cardiovascular_— Postflight orthostatic tachycardia is reported
for Titov as long as 23 hours after landing; at 48 hours there was
significant residual intolerance to the upright posture.
Cosmonauts have demonstrated a 20- to 35-percent increase in
oxygen consumption during the standard postflight exercise test.
In both of these areas there was a return to normal within the
postflight period of study. The Soviets have continued their biological
experiments in space with the Vostok/Voshkod series. Fixing of
histologic specimens in flight by Bykovsky demonstrated a critical role
for man and made possible an expanded experimental program. Biopackages
have become more complex with each succeeding flight.
With the exception of postflight orthostatic intolerance after the third
and fourth Mercury flights, changes as a result of exposure to a
zero-gravity environment have not been noted by U.S. investigations in
space. Ground-based research proceeds here at an advanced pace and is
supported in large measure by both the USAF and NASA. A study of the
relationships among renal and systemic hemodynamics, neurohumoral
cardiovascular regulation, and renal excretory function in differently
positioned subjects is underway, as are studies of acceleration
tolerance.