Lauzon A M, Prisk G K, Elliott A R, Verbanck S, Paiva M, West J B
Department of Medicine, University of California, San Diego, La Jolla 92093-0931, USA.
J Appl Physiol (1985). 1997 Mar;82(3):859-65. doi: 10.1152/jappl.1997.82.3.859.
During single-breath washouts in normal gravity (1 G), the phase III slope of sulfur hexafluoride (SF6) is steeper than that of helium (He). Two mechanisms can account for this: 1) the higher diffusivity of He enhances its homogeneous distribution; and 2) the lower diffusivity of SF6 results in a more peripheral location of the diffusion front, where airway asymmetry is larger. These mechanisms were thought to be gravity independent. However, we showed during the Spacelab Life Sciences-2 spaceflight that in sustained microgravity (microG) the SF6-to-He slope difference is abolished. We repeated the protocol during short periods (27 s) of microG (parabolic flights). The subjects performed a vital-capacity inspiration and expiration of a gas containing 5% He-1.25% SF6-balance O2. As in sustained microG, the phase III slopes of He and SF6 decreased. However, during short-term microG, the SF6-to-He slope difference increased from 0.17 +/- 0.03%/l in 1 G to 0.29 +/- 0.06%/l in microG, respectively. This is contrary to sustained microG, in which the SF6-to-He slope difference decreased from 0.25 +/- 0.03%/l in 1 G to -0.01 +/- 0.06%/l in microG. The increase in phase III slope difference in short-term microG was caused by a larger decrease of He phase III slope compared with that in sustained microG. This suggests that changes in peripheral gas mixing seen in sustained microG are mainly due to alterations in the diffusive-convective inhomogeneity of He that require > 27 s of microG to occur. Changes in pulmonary blood volume distribution or cardiogenic mixing may explain the differences between the results found in short-term and sustained microG.
在正常重力（1G）条件下的单次呼吸冲洗过程中，六氟化硫（SF6）的第三阶段斜率比氦气（He）的更陡。有两种机制可以解释这一现象：1）He较高的扩散率增强了其均匀分布；2）SF6较低的扩散率导致扩散前沿更靠近外周，而在外周气道不对称性更大。这些机制被认为与重力无关。然而，我们在太空实验室生命科学-2号航天飞行期间发现，在持续微重力（ microg ）条件下，SF6与He的斜率差异消失了。我们在短时间（27秒）的微重力（抛物线飞行）期间重复了该实验方案。受试者对含有5%He - 1.25%SF6 - 其余为O2的气体进行肺活量吸气和呼气。与持续微重力条件下一样，He和SF6的第三阶段斜率均下降。然而，在短期微重力条件下，SF6与He的斜率差异分别从1G时的0.17±0.03%/升增加到微重力条件下的0.29±0.06%/升。这与持续微重力条件相反，在持续微重力条件下，SF6与He的斜率差异从1G时的0.25±0.03%/升降至微重力条件下的-0.01±0.06%/升。短期微重力条件下第三阶段斜率差异的增加是由于与持续微重力相比，He第三阶段斜率下降幅度更大。这表明在持续微重力条件下观察到的外周气体混合变化主要是由于He的扩散 - 对流不均匀性变化所致，这种变化需要>27秒的微重力时间才会出现。肺血容量分布的变化或心源性混合可能解释了短期和持续微重力条件下结果的差异。
