Blood-gas CO2 equilibration in lungs of unanesthetized dogs during hypercapnia

We have reinvestigated the problem of blood-gas equilibration of CO2 in lungs during hypercapnia. Six dogs with chronic tracheostomy and exteriorized carotid artery were subjected in acute experiments to hypercapnic inspired mixtures [CO2 fraction of expired gas (FICO2) = 0.06; 0.08; 0.10]. Expired CO2 partial pressure (PCO2) was continuously measured with a respiratory mass spectrometer and compared with arterial PCO2 determined in blood samples that were collected during apparent steady-state conditions. Particular care was taken in using continuously recorded temperature in the right heart for correction of blood PCO2 measured by CO2 electrodes. In no animal was there a significant difference between arterial and alveolar PCO2. On the average, this difference was -0.1 Torr at FICO2 = 0.06; 0.0 Torr at FICO2 = 0.08, and -0.2 Torr at FICO2 = 0.10. The results are in agreement with the conventional view that PCO2 in pulmonary capillary blood approaches PCO2 in alveolar gas.