Differential effects of CO2 and hypoxia on bronchomotor tone in the newborn dog.

We compared the effects of hypoxia and hypercapnia on pulmonary mechanics in the newborn dog. Animals were anesthetized with a mixture of chloralose/urethane, paralyzed and ventilated with the chest open. Following an inflation to control volume history, mean inspiratory resistance (RLi) and dynamic compliance (CLdyn) were measured on a breath-by-breath basis during ventilation with control (FIO2 = 0.4), hypoxic (FIO2 = 0.1) and hypercapnic (FICO2 = 0.05) gas mixtures. Hypercapnia increased RLi 63% (n = 9) while hypoxia increased RLi in only 1/9 animals. Neither gas mixture changed CLdyn compared to control. The response to hypercapnia and the lone hypoxic response were eliminated by denervation of the airways by atropine administration or vagotomy. Following airway denervation hypoxia caused a small but statistically significant fall in CLdyn compared to the denervated control. These findings demonstrate that the newborn dog is capable of reflexly increasing bronchomotor tone and that vagal efferent innervation to the airways is functional at birth. Our data also suggest that in the newborn, central chemoreceptors are more effective than peripheral chemoreceptors in altering vagal tone to airway smooth muscle. Increased bronchomotor tone with hypercapnia may help to prevent dynamic compression of the airways.