Blood-gas equilibration of CO2 and O2 in lungs of awake dogs during prolonged rebreathing

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Abstract
To reinvestigte the blood-gas CO2 equilibrium in lungs, rebreathing experiments were performed in 5 unanesthetized dogs prepared with a chronic tracheostomy and an exteriorized carotid loop. The rebreathing bag was initially filled with a gas mixture containing 6-8% CO2, 12, 21, or 39% O2, and 1% He in N2. During 4-6 min of rebreathing PO2 in the bag was kept constant by a controlled supply of O2 while PCO2 [partial pressure of CO2] rose steadily from .apprx. 40 to 75 Torr. Spot samples of arterial blood were taken from the carotid loop; their PCO2 and PO2 were measured by electrodes and compared with the simultaneous values of end-tidal gas read from a mass spectrometer record. The mean end-tidal-to-arterial PO2 differences averaging 16, 4 and 0 Torr with bag PO2 .apprx. 260, 130 and 75 Torr, respectively, were in accordance with a venous admixture of .apprx. 1%. No substantial PCO2 differences between arterial blood and end-tidal gas (PaCO2 [partial arterial pressure CO2]-PE''CO2 [end-tidal pressure CO2]) were found. The mean PaCO2-PE''CO2 of 266 measurements in 70 rebreathing periods was -0.4 .+-. 1.4 (SD) Torr. There was no correlation between PaCO2-PE''CO2 and the level of arterial PCO2 or PO2. The mean PaCO2-PE''CO2 became +0.1 Torr when the blood transit time from lungs to carotid artery (estimated at 6 s) and the rate of rise of bag PCO2 (4.5 Torr/min) were taken into account. These experimental results do not confirm the presence of significant PCO2 differences between arterial blood and alveolar gas in rebreathing equilibrium.