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TABLE 8.1 Changes in Pulmonary Function Observed and the Number of Subjects Studied During Spacelab Missions SLS-1 and D-2
Physiological Response to Microgravity
Number of Subjects
Changes in Microgravity (In-flight vs. Preflight
Standing Measurements)
Pulmonary blood flow
Total pulmonary blood flow (cardiac output)
4
18% increase
Cardiac stroke volume
4
4% increase
Diffusing capacity (carbon monoxide)
4
28% increase
Pulmonary capillary blood volume
4
28% increase
Diffusing capacity of alveolar membrane
4
27% increase
Pulmonary blood flow distribution
7
More uniform but some inequality remained
Pulmonary ventilation
Respiration frequency
8
9% increase
Tidal volume
8
15% decrease
Alveolar ventilation
8
Unchanged
Total ventilation
8
Small decrease
Ventilatory distribution
7
More uniform but some inequality remained
Maximal peak expiratory flow rate
7
Decreased by ≤12.5% early in-flight but then returned to normal
Pulmonary gas exchange
O2 uptake
8
Unchanged
CO2 output
8
Unchanged
End-tidal pO2
8
Unchanged
End-tidal pCO2
8
Small increase when CO2 concentration in spacecraft increased
Lung volumes
Functional residual capacity
4
15% decrease
Residual volume
4
18% decrease
Closing volume
7
Unchanged as measured by argon bolus
NOTE: Pulmonary blood flow in normal subjects is the same as cardiac output, the amount of blood pumped by the heart per minute. Stroke volume is the volume of blood pumped per beat. The ability of carbon monoxide to diffuse into the blood is a standard clinical test of the integrity of the alveolar membrane and its surrounding capillary blood supply. Tidal volume is the air breathed in a single breath. Alveolar ventilation is the volume of air moving into and out of the alveoli of the lung per minute. The data indicate that more alveoli are expanded and ventilated in space than on Earth. Peak expiratory flow rate is the maximal flow of air that can be forcefully exhaled. Oxygen uptake is the oxygen consumed by the subject per minute, while CO2 output is the carbon dioxide produced per minute. End-tidal gas partial pressures are the respective partial pressures at the end of an expired breath and reflect the concentration of those gases in the alveoli. Functional residual capacity is the volume of gas in the lung that can still be exhaled at the end of a normal breath. Residual volume is the remaining gas in the lung after the subject has made a maximal effort to exhale. Closing volume refers to the volume in the lung where the alveoli close in significant numbers.
SOURCE: West, J.B., Elliott A.R., Guy, H.J.B., and Prisk, G.K. 1997. Pulmonary function in space. J. Am. Med. Assoc. 277:1957-1961. References for various experiments are included in the text of this report and in West's review article.
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