Changes in inspired gas composition and experimental bronchospasm in the rabbit.

In clinical practice, bronchospasm could be facilitated by hypoxia and by hypercapnia. In this study we assessed the influence of breathing a hypoxic (FIO2 = 0.10) or a hypercapnic (FICO2 = 0.08) gas mixture on the response to nebulized histamine (2% solution for 5 min) in anesthetized, tracheotomized, paralyzed and mechanically ventilated rabbits. Total respiratory resistance (Rrs) and elastance (Ers) were derived by least-square analysis from the relationship between tracheal pressure and flow. Control values of Rrs were larger during hypoxia and hypercapnia than in air while the values of Ers were similar. The absolute change in Rrs after histamine was similar in air and hypoxia, and larger in hypercapnia. The relative change, however, was smaller in hypoxia than in the two other conditions. Ers was also substantially increased by histamine and, contrary to Rrs, remained high 60 min after the aerosol. The results suggest: (1) that both hypoxia and hypercapnia increase airway resistance but do not change tissue properties; (2) that the response to histamine is depressed by hypoxia; (3) that a substantial part of the immediate response, and most, if not all, of the residual response after 60 min is due to changes in lung tissue viscoelastic properties.