Earlobe oximetry, indirect measurements of blood flow and of blood
pressure by finger plethysmography or impedance plethysmography, and
ballistocardiographic techniques have potential application to manned
space flight.
Adaptation to prolonged exposure to weightlessness or to lunar gravity
may cause difficulties when the astronaut is exposed again to reentry
forces and terrestrial gravity. It is possible that these adaptive
changes may thus produce unacceptable effects on performance or cause
risk to life. It is important to obtain experimental evidence on this
subject.
It is common knowledge that following a stay in bed, dizziness,
faintness, and weakness characterize arising, and that a feeling of
general weakness may persist for several days. The phenomenon has been
investigated in a number of laboratories. One approach has been to put
healthy young subjects to bed, and even in extensive casts for periods
of 2 or 3 weeks or more. Two major findings have emerged from these
studies. First, a substantial adjustment in the blood circulatory system
occurs, which is termed the "hypodynamic state." Second, there is a
large decrease in the skeletal and muscle mass of the body.
There are two kinds of evidence for the hypodynamic state: measurement
of parameters of circulatory function, and measurement of the response
of the individuals to a quantitatively imposed mild gravitational load.
After 3 weeks in bed, otherwise healthy persons exhibit an increase of
more than 20 percent in heart rate; a reduction of 10 to 20 percent in
total blood volume, primarily as a result of reduction of plasma volume;
and a decrease in heart size of about 8 percent. Coupled with these
cardiovascular changes is a reduction of 10 percent in the basal
metabolic rate. It appears as though the circulation and metabolism are
reset to a lower functional level commensurate with the reduced demands
placed on the whole organism.
After 3 weeks of bed rest, all of the subjects tested showed pronounced
orthostatic hypotension. After tilting, the average heart rate increased
by 37 beats per minute, the systolic blood pressure fell some 12-mm Hg,
and some of the subjects fainted. The measurements were continued for 16
days after the bed-rest period, and it was round that recovery was not
quite complete when the experiment was terminated.
There is little question that in prolonged exposures to the weightless
state, there is a fair probability of extensive circulatory adjustments,
the seriousness of which cannot yet be foretold. While it is likely that
the astronauts will adapt successfully to long periods of weightlessness
at some new circulatory functional level, the remote possibility exists
that the circulatory changes may be progressive to the point of ultimate
failure.