The effects of pentobarbitone, diazepam and alcohol on oral breathing in neonatal and mature sheep.

Our aim was to determine the effects of pharmacological sedation on oral breathing induced by nasal obstruction in chronically prepared newborn and mature sheep. Nasal obstruction (5 min) was achieved by blocking tubes temporarily fixed into the nostrils. We continuously recorded EMG activity of the diaphragm, genioglossus and digastric muscles, intrapleural pressure and percent O2 saturation in arterial blood (SaO2). Blood samples were taken intermittently and analysed for SaO2, P02, PCO2 and pH. As previously reported, nasal obstruction in lambs and ewes led to asphyxial changes in blood gases and pH; lambs were affected more than ewes. Respiratory responses to nasal obstruction were retested after administration of pentobarbitone Na (5 and 10 mg/kg), diazepam (0.2 and 0.4 mg/kg) and alcohol (nominally 0.075 and 0.11% of blood w/v). Pentobarbitone and diazepam, both of which produced sedation, delayed the onset of oral breathing and led to a greater degree of asphyxia during nasal obstruction. EMG activities in the genioglossus and digastric muscles were inhibited by the drugs, whereas the depth of inspiratory efforts was not. Alcohol had no apparent sedative effect, nor did it significantly affect responses to nasal occlusion. We conclude that sedating doses of pentobarbitone and diazepam depress the effectiveness of oral breathing when the nose is blocked, probably owing to their inhibitory effects on activation of muscles maintaining patency of an oral airway.