Effects of airway versus arterial CO2 changes on lung mechanics in dogs

The effects of changes in airway CO2 partial pressure (PAco2) and arterial CO2 partial pressure (Paco2) on lung mechanics were studied in dogs by utilizing unilateral pulmonary artery occlusion and a tracheal divider which allowed separate variation of PAco2 and Paco2. When Paco2 was held at a reasonably normal level, lower than normal PAco2 levels resulted in large compliance decreases, alteration of the complete static pressure-volume curves, and increases in resistance. Invreases in PAco2 to hypercapnic levels did not produce changes. When PAco2 was held at a reasonably normal level, changes in Paco2 levels were positively and directly related to resistance with small and inconsistent effects on compliance and on complete static pressure-volume curves. A combination of low PAco2 and high Paco2 produced large increases in resistance, alterations of the static pressure-volume curve, and decreases in compliance. Vagotomy during the combined stimulus resulted in only a decrease in resistance without change in lung elastic properties. The results suggest that the mechanical effects of airway hypocapnia and systemic hypercapnia are additive. However, small airways effects of low PAco2 appear to be maximal and uninfluenced by the vagally mediated response to Paco2 increases.