Initiation of oral breathing in lambs in response to airway obstruction: mechanisms

Abstract

Our aim was to assess the mechanisms determining the reflex formation of an oral airway in response to nasal obstruction (NO) and tracheal obstruction (TO). In nine conscious lambs (14-37 days old) NO was effected by blockade of nasal tubes; TO was later effected by blockade of an endotracheal tube. We measured arterial O2 saturation, PO2, PCO2, and pH and the depth and duration of inspiratory efforts when mouth opening (MO) occurred. Responses were compared when NO and TO followed breathing of room air, rebreathed air, and 100% O2. After both NO and TO, MO was initiated most rapidly after lambs rebreathed air and least rapidly after they breathed 100% O2. Similar changes in blood gases and pH were measured when MO occurred after air breathing and rebreathing; however, the extent of these changes was greater during TO than during NO. After 100% O2 was breathed, MO occurred when lambs were still hyperoxic, but they were more hypercapnic and acidemic than after breathing air or rebreathed air. There were no differences, related to prebreathed gases or site of airway occlusion, in the depth of inspiratory efforts at the time of MO. We conclude that the formation of an oral airway requires a critical level of inspiratory drive in the presence of airway obstruction. After the prebreathing of different gases, differences in response latency and blood gases at the time of MO can be attributed to the attainment of this threshold level of inspiratory drive. The formation of an oral airway is facilitated by, but not dependent on, receptors in the upper airway.

Postnatal Development of Responses to Airflow Obstruction

“…Thus, the signal received by the animal from airway mechanoreceptors, and the sense of inspiratory effort during obstructed inspiratory efforts, may have been stronger when nasal tubes were blocked. In accordance with our previous studies in conscious lambs (Harding et al 1991), our present findings in sleeping animals indicate that mechanical stimuli (or sense of effort) are of great importance in the initiation of responses to airflow obstruction. It is likely that the asphyxia that accompanies airflow obstruction will eventually cause arousal, but the latency for arousal will be shortened in the presence of mechanical stimuli which increase the intensity of inspiratory efforts.…”

“…Respiratory responses to airflow obstruction during sleep were qualitatively similar to those observed previously in conscious lambs when the nasal airway was obstructed (Harding et al 1987(Harding et al , 1991; that is, there was an augmentation and slowing of inspiratory efforts. However, the magnitude of the increase in inspiratory efforts (in relation to the degree of hypoxaemia or duration of obstruction) was much greater in conscious lambs.…”

“…We found that upper airway stimulation was not necessary for the establishment of an oral airway. The relative roles of chemo-and mechanoreception in the establishment of an oral airway were analysed by having the animals pre-breathe different gas mixtures prior to the onset of nasal obstruction (Harding et al 1991). The onset of oral breathing was advanced by the pre-breathing of a hypoxic gas mixture and delayed by pre-breathing a hyperoxic mixture.…”

“…To investigate the mechanisms whereby oral breathing was initiated in a nose breathing animal we compared the roles of chemoreceptive and mechanoreceptive inputs into the respiratory and upper airway responses to nasal obstruction, including the establishment of an oral airway (Harding et al 1991). By occluding the trachea rather than the nasal airway,…”

Initiating oral breathing in response to nasal loading: asthmaticsversushealthy subjects

“…One study has associated an increased nasal resistance with a tendency to use oral route breathing [21]; however, switching behaviour at rest has only been previously investigated in lambs [22], where complete nasal obstruction was required to induce oral breathing. The only study investigating breathing routes in asthmatic subjects demonstrated that, during an acute exacerbation of asthma, subjects breathed oronasally but switched to nasal breathing post-recovery [1].…”