Bronchoconstriction during alveolar hypocapnia and systemic hypercapnia in dogs with a cardiopulmonary bypass

The roles of the alveolar and systemic CO₂ on the lung mechanics were investigated in dogs subjected to cardiopulmonary bypass. Low-frequency pulmonary impedance data (Z(L)) were collected in open-chest dogs with an alveolar CO₂ level (FA(CO₂)) of 0.2-7% and during systemic hypercapnia before and after elimination of the vagal tone. Airway resistance (R(aw)), inertance (I(aw)), parenchymal damping (G) and elastance (H) were estimated from the Z(L). The highest R(aw) observed at 0.2% FA(CO₂),which decreased markedly up to a FA(CO₂) of 2% (212 ± 24%), and remained unchanged under normo- and hypercapnia (FA(CO₂) 2-7%). These changes were associated with smaller decreases in I(aw) (-16.6 ± 3.7%), mild elevations in G (25.7 ± 4.7%), and no change in H. Significant increases in all mechanical parameters were observed following systemic hypercapnia; atropine counteracted the R(aw) rises. We conclude that severe alveolar hypocapnia may contribute to minimization of the ventilation-perfusion mismatch by constricting the airways in poorly perfused lung regions. The constrictor potential of systemic hypercapnia is mediated by vagal reflexes.