FORCED EXPIRATIONS AND MAXIMUM EXPIRATORY FLOW-VOLUME CURVES DURING SUSTAINED MICROGRAVITY ON SLS-1

Gravity is known to influence the mechanical behavior of the lung and chest wall. However, the effect of sustained microgravity (mu G) on fo rced expirations has not previously been reported. Tests were carried out by four subjects in both the standing and supine postures during e ach of seven preflight and four postflight data-collection sessions an d four times during the 9 days of mu G exposure on Spacelab Life Scien ces-1. Compared with preflight standing values, peak expiratory flow r ate (PEFR) was significantly reduced by 12.5% on flight day 2 (FD2), 1 1.6% on FD4, and 5.0% on FD5 but returned to standing values by FD9. T he supine posture caused a 9% reduction in PEFR. Forced vital capacity and forced expired volume in 1 s were slightly reduced (similar to 3- 4%) on FD2 but returned to preflight standing values on FD4 and FD5, a nd by FD9 both values were slightly but significantly greater than sta nding values. Forced vital capacity and forced expiratory volume in 1 s were both reduced in the supine posture (similar to 8-10%). Forced e xpiratory flows at 50% and between 25 and 75% of vital capacity did no t change during mu G but were reduced in the supine posture. Analysis of the maximum expiratory flow-volume curve showed that mu G caused no consistent change in the curve configuration when individual in-fligh t days were compared with preflight standing curves, although two subj ects did show a slight reduction in flows at low lung volumes from FD2 to FD9. The interpretation of the lack of change in curve configurati on must be made cautiously because the lung volumes varied from day to day in flight. Therefore, the flows at absolute lung volumes in mu G and preflight standing are not being compared. The supine curves showe d a subtle but consistent reduction in flows at low lung volumes. The mechanism responsible for the reduction in PEFR is not clear. It could be due to a lack of physical stabilization when performing the maneuv er in the absence of gravity or a transient reduction in respiratory m uscle strength.