Pulmonary gas exchange and its determinants during sustained
microgravity on spacelabs sls-1 and sls-2.
Prisk, G. Kim, Ann R. Elliott, Harold J. B. Guy, Janelle M. Kosonen,
and John B. West.
Department of Medicine, University of California, San Diego, La
Jolla, CA 92093-0931.
APStracts 2:0206A, 1995.
We measured resting pulmonary gas exchange in 8 subjects exposed to 9
or 14 days of microgravity ([mu]G) during two Spacelab flights.
Compared with preflight standing measurements, [mu]G resulted in a
significant reduction of tidal volume (15%) but an increase in
respiratory frequency (9%). The increased frequency was chiefly
caused by a reduction in expiratory time (10%) with a smaller
decrease in inspiratory time (4%). Anatomic deadspace (VDA) in [mu]G
was between preflight standing and supine values, consistent with the
known changes in FRC. Physiologic deadspace (VDB) decreased in [mu]G,
and alveolar deadspace (VDB -VDA ) was significantly less in [mu]G
than preflight standing (-30%) or supine (-15%), consistent with a
more uniform topographical distribution of blood flow. The net result
was that although total ventilation fell, alveolar ventilation was
unchanged in [mu]G compared with standing 1-G. Expired vital capacity
was increased (6%) compared with standing but only after the first
few days of exposure to [mu]G. There were no significant changes in
O2 uptake, CO2 output, or end-tidal PO2 in [mu]G compared with
standing 1-G. End-tidal PCO2 was unchanged on the 9 day flight, but
increased by 4.5 Torr on the 14 day flight where the PCO2 of the
spacecraft atmosphere increased by 1 to 3 Torr. Cardiogenic
oscillations in expired O2 and CO2 demonstrated the presence of
residual ventilation-perfusion ratio (V.A/Q. ) inequality . In
addition, the change in intra-breath V.A/Q. during phase 3 of a long
expiration was the same in [mu]G as preflight standing indicating
persisting V.A/Q. inequality, and suggesting that during this portion
of a prolonged exhalation the inequality in 1-G was not predominately
on a gravitationally-induced topographical basis. However the changes
in PCO2 and V.A/Q. at the end of expiration following airway closure
were consistent with a more uniform topographical distribution of gas
exchange.
Received 19 December 1994; accepted in final form 12 May 1995.
APS Manuscript Number A1283-4.
Article publication pending Journal of Applied Physiology.
ISSN 1080-4757 Copyright 1995 The American Physiological Society.
Published in APStracts on 30 May 1995.