Changes in respiratory control after 5 days at altitude

These experiments examined changes in the chemoreflex control of breathing and acid–base balance after 5 days at altitude (3480 m) in six healthy males. The partial pressures of carbon dioxide ( P C O 2 ) at which ventilation increased during isoxic hypoxic and hyperoxic modified rebreathing tests at sea level fell significantly at altitude by mean ± S.E.M. of 12.8 ± 2.51 mmHg and 9.5 ± 1.77 mmHg, respectively, but response slopes above threshold were unchanged. Altitude exposure produced a respiratory alkalosis evidenced by a decrease in mean resting end-tidal P C O 2 from 41 ± 0.84 mmHg at sea level to 32 ± 2.04 mmHg at altitude, but pH did not increase significantly from its sea level value. Blood samples were analyzed to discover acid–base changes, using a modification of the equations for acid–base balance proposed by [Stewart, P.A., 1983. Modern quantitative acid–base chemistry. Can. J. Physiol. Pharmacol. 61, 1444–1461]. While strong ion difference at altitude was not significantly different from its sea level value, albumin concentration was increased significantly from 38.6 ± 0.30 g L −1 to 49.8 ± 0.76 g L −1. We suggest that the respiratory alkalosis was produced by a fall in the chemoreflex threshold and pH was corrected by an elevation in the concentration of weakly dissociated protein anions.