INHOMOGENEITY OF PULMONARY PERFUSION DURING SUSTAINED MICROGRAVITY ONSLS-1

We studied the effects of gravity on the inhomogeneity of pulmonary pe rfusion in humans by performing hyperventilation-breath-hold single-br eath measurements before, during, and after 9 days of continuous expos ure to microgravity during the Spacelab Life Sciences-1 (SLS-1) missio n. In microgravity the indicators of inhomogeneity of perfusion, espec ially the size of cardiogenic oscillations in expired CO2 and the heig ht of phase IV, were markedly reduced. Cardiogenic oscillations were r educed to similar to 60% of their preflight standing size, and the hei ght of phase TV was between 0 and -8% (a terminal fall became a small terminal rise) of the preflight standing value. The terminal change in expired CO2 was nearly abolished in microgravity, indicating more uni formity of blood flow between lung units that close and those that rem ain open at the end of expiration. A possible explanation of this obse rvation is the disappearance of gravity-dependent topographic inequali ty of blood flow. The residual cardiogenic oscillations in expired CO2 imply a persisting inhomogeneity of perfusion in the absence of gravi ty, probably in lung regions that are not within the same acinus.