From Kreuzer F, Van Lookeren Campagne P. “Resting Pulmonary Diffusing Capacity for CO and O2 at Altitude”. J Appl Physiol 1965; 20: 519
The supine subjects rested for about a half an hour and then breathed, in varying sequence through a low-resistance valve, one of the following gas mixtures listed in Table 1, first without CO for some 10 min, and subsequently with 0.1% CO. In this way similar inspiratory O2 pressures were attained for the three levels of oxygenation at both altitudes (about 80, 150 and 400 mm Hg, respectively, with the following average alveolar PO2 in mm Hg: at sea level 46.5, 1.7.6 and 382; high altitude: 53.6, 117 and 365). After equilibration with the CO-free gas mixture the inspiratory circuit was switched to the corresponding CO-containing muxture (Fig. 1, stopcock A or B). On and one-half minutes were allowed for equilibration with 0.1% CO which is considered sufficient by most authors; aftere simultaneous washing out of spirometer and connections towards the end of this period, stopcocks C and D were turned to collect expired gas in a 9-liter spirometer with continuous recording of breaths during about a a half a minute, bringing total exposure to CO to some min per run.