Fast-CT evaluation of the effect of lung volume on upper airway size and function in normal men

The present study was performed to evaluate the regional changes in pharyngeal cross-sectional area (CSA) that occur with changes in lung volume in normal men. Fast-CT and genioglossal electromyogram (EMGgg) were used to study upper airway (UA) size and dilator muscle activity at TLC, lung volume at end-tidal inspiration (VTei), FRC, and residual volume (RV) in 30 men with a mean age of 46 ± 3 yr and no significant sleep-disordered breathing, mean AHI = 4 ± 1 per hour. Compared with values at FRC, minimum CSA (Amin) increased 154 ± 31% at TLC (p = 0.0001), 19 ± 10% at VTei (p = 0.03), and there was a trend toward a decrease of 31 ± 12% at RV (p = 0.07). Similar but smaller changes were observed in mean CSA (Amean), with an increase of 69 ± 14% at TLC (p = 0.0001), 8 ± 5% at VTei (p = 0.01), and a decrease of 17 ± 7% at RV (p = 0.01). Both the velopharyngeal (VP) and orohypopharyngeal (OHP) segments of the UA increased in size with increasing lung volume. Both Amin and Amean of the OHP segment at TLC were larger (55 ± 19 and 38 ± 14%, respectively) than the respective measurements in the VP segment. EMGgg activity doubled from 12 ± 1 μV at FRC to 25 ± 1 μV at TLC (p = 0.006). There was no change in EMGgg with tidal ventilation or with exhalation to RV. Changes in CSA directly paralleled changes in lung volume in this group of normal awake nonobese men. Overall, there were no regional differences in UA response to changes in tidal volume or exhalation to RV. However, the OHP demonstrated proportionately greater enlargement than the VP at TLC. Despite significant changes in CSA with tidal breathing and exhalation to RV, there was no phasic change in EMGgg activity, indicating that changes in genioglossal muscle tone do not correlate with changes in CSA at low lung volumes.