The role of carotid bodies in the establishment of oral breathing during nasal obstruction in lambs and ewes.

This study was aimed at determining the role of carotid bodies in the initiation and maintenance of oral breathing during nasal obstruction. We have previously shown in sheep that oral breathing during nasal obstruction was less effective in the neonate than in older lambs or adults. We considered it possible that the reduced response of the neonate was due to immaturity of carotid body chemoreceptors. Respiratory and blood gas responses to nasal obstruction were determined in 5 neonatal lambs, 6 older lambs and 6 adult ewes before and after surgically denervating the carotid bodies. Denervations were performed at 4.2 +/- 0.9 days after birth in the neonatal lambs and at 41.5 +/- 2.5 days in the older lambs. Carotid body denervation led to hypoxia, hypercapnia and acidaemia in lambs and ewes during unobstructed nasal breathing, and, in response to nasal obstruction, delayed the onset of oral breathing so that it occurred at a greater degree of hypoxia. Following carotid body denervation the degree of hypercapnia and acidaemia during nasal obstruction was greater in lambs than in ewes. We conclude that the carotid bodies in lambs and ewes play an important role in the regulation of normal blood gases and pH, and in the establishment and effectiveness of oral breathing during nasal obstruction.