PULMONARY GAS-EXCHANGE AND ITS DETERMINANTS DURING SUSTAINED MICROGRAVITY ON SPACELABS SLS-1 AND SLS-2

We measured resting pulmonary gas exchange in eight subjects exposed t o 9 or 14 days ofmicrogravity (mu G) during two Spacelab flights. Comp ared with preAight standing measurements, mu G resulted in a significa nt reduction in tidal volume (15%) but an increase in respiratory freq uency (9%). The increased frequency was caused chiefly by a reduction in expiratory time (10%), with a smaller decrease in inspiratory time (4%). Anatomic dead space (VDa) in mu G was between preflight standing and supine values, consistent with the known changes in functional re sidual capacity. Physiological dead space (VDB) decreased in mu G, and alveolar dead space (VDB - VDa) was significantly less in mu G than i n preflight standing (-30%) or supine (-15%), consistent prith a more uniform topographic distribution of blood flow. The net result was tha t, although total ventilation fell, alveolar ventilation was unchanged in mu G compared with standing in normal gravity (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 O-2 uptake, CO2 output, or end-tidal PO2 in mu G compared with sta nding in 1 G, End-tidal PCO2 was unchanged on the 9-day flight but inc reased by 4.5 Torr on the 14-day flight where the PCO2 of the spacecra ft atmosphere increased by 1-3 Torr. Cardiogenic oscillations in expir ed O-2 and CO2 demonstrated the presence of residual ventilationperfus ion ratio (VA/Q) inequality. In addition, the change in intrabreath VA /Q during phase III of a long expiration was the same in mu G as in pr eflight standing, indicating persisting VA/Q inequality and suggesting that during this portion of a prolonged exhalation the inequality in 1 G was not predominantly on a gravitationally induced topographic bas is. However, the changes in PCO2 and VA/Q at the end of expiration aft er airway closure were consistent with a more uniform topographic dist ribution of gas exchange.