Pulmonary function in space
J. B. West, A. R. Elliott, H. J. Guy and G. K. Prisk
Department of Medicine, University of California San Diego, La Jolla 92093-0623, USA. jwest@ucsd.edu
The lung is exquisitely sensitive to gravity, and so it is of interest to
know how its function is altered in the weightlessness of space. Studies on
National Aeronautics and Space Administration (NASA) Spacelabs during the
last 4 years have provided the first comprehensive data on the extensive
changes in pulmonary function that occur in sustained microgravity.
Measurements of pulmonary function were made on astronauts during space
shuttle flights lasting 9 and 14 days and were compared with extensive
ground-based measurements before and after the flights. Compared with
preflight measurements, cardiac output increased by 18% during space
flight, and stroke volume increased by 46%. Paradoxically, the increase in
stroke volume occurred in the face of reductions in central venous pressure
and circulating blood volume. Diffusing capacity increased by 28%, and the
increase in the diffusing capacity of the alveolar membrane was
unexpectedly large based on findings in normal gravity. The change in the
alveolar membrane may reflect the effects of uniform filling of the
pulmonary capillary bed. Distributions of blood flow and ventilation
throughout the lung were more uniform in space, but some unevenness
remained, indicating the importance of nongravitational factors. A
surprising finding was that airway closing volume was approximately the
same in microgravity and in normal gravity, emphasizing the importance of
mechanical properties of the airways in determining whether they close.
Residual volume was unexpectedly reduced by 18% in microgravity, possibly
because of uniform alveolar expansion. The findings indicate that pulmonary
function is greatly altered in microgravity, but none of the changes
observed so far will apparently limit long-term space flight. In addition,
the data help to clarify how gravity affects pulmonary function in the
normal gravity environment on Earth.
