Pulmonary diffusing capacity and capillary blood flow during forward acceleration

We studied possible causes of the decreased arterial oxygen saturation seen when a subject is accelerated in a centrifuge by measuring simultaneously the pulmonary diffusing capacity, DlCO, and the effective pulmonary capillary blood flow, Qc, using breath-holding techniques with carbon monoxide and acetylene. After 1 min of forward ("eyeballs in") acceleration at eight times normal gravity, 8 G, average Dl decreased 35% from an initial control of 33.7 to 21.5 ml/(min x mm Hg) in four subjects. Although this decrease was statistically significant, the values observed were not low enough to indicate that impaired diffusion was a prime cause of arterial unsaturation. Average Qc decreased 35% during acceleration from an initial control value of 12.9 to 8.2 liters/min, also a significant change. These values may have indicated that total pulmonary blood flow was reduced, but a more likely explanation is that a large portion of pulmonary flow perfused nonventilated regions. Dl and Qc returned toward initial control levels within 8 min after acceleration in most instances.