Fast-computed tomographic evaluation of the effect of route of breathing on upper airway size and function in normal men.

Nasal obstruction is associated with increased sleep disordered breathing (SDB), even in normal subjects. This increase in SDB may result from narrowing of the orohypopharyngeal (OHP) or retroglossal segment of the upper airway (UA) due to retropositioning of the jaw and tongue base as the mouth is opened and route of breathing changed from nasal to oral. It is postulated that significant narrowing of the OHP occurs with oral breathing even in the awake state. To ascertain the effect of route of breathing on the UA, fast-CT was used to study the UA response to the route of breathing in 30 normal, awake men, with each subject breathing via the nasal and oral routes under the following conditions: end-inspiration during tidal breathing (VTei) and functional residual capacity (FRC). In the velopharyngeal (VP) or retropalatal segment of the UA, minimum (Amin) and mean (Amean) cross-sectional areas (CSA) decreased 49 +/- 11 percent and 16 +/- 6 percent, respectively, with oral compared with nasal breathing at FRC. In the OHP, Amin at FRC increased by 26 +/- 15 percent with oral compared with nasal breathing with no significant change in Amean. Similar changes in CSA of both the VP and OHP were observed at VTei. Genioglossal electromyographic (EMGgg) activity increased from 12 +/- 1 microV breathing nasally to 27 +/- 4 microV breathing orally at FRC. Although the CSA of the VP segment decreased with conversion from nasal to oral breathing, Amin of the OHP segment was unexpectedly observed to increase with oral breathing. The doubling of EMGgg activity with oral breathing suggests that active contraction of the genioglossus may function to increase the patency of the OHP segment during oral breathing in supine, awake, normal subjects.