Effects of continuous negative airway pressure on lung volume and respiratory resistance

This study was designed to determine the responses of lung volume and respi ratory resistance (Rrs) to decreasing levels of continuous negative airway pressure (CNAP). Twenty normal subjects were studied in the basal state and under CNAP levels of -5, -10, and -15 hPa. Rrs was measured by the forced oscillation technique (4-32 He). End-expiratory lung volume (EELV) and tida l volume (VT) were measured by whole body plethysmography Rrs was extrapola ted to 0 Hz (R-0) and estimated at 16 Hz (R-16) by linear regression analys is of Rrs vs. frequency. Specific Rrs, SR0 and SR16, were then calculated a s R-0 (EELV + VT/2) and R-16 (EELV + VT/2), respectively. EELV significantl y decreased, whereas R-0, R-16, SR0, and SR16 significantly increased, as t he CNAP level decreased (P < 0.0001 for all). At the lowest CNAP level, R-0 and R-16 reached 198 +/- 13 and 175 +/- 9% of their respective basal value s. The CNAP induced increase in R-0 was significantly higher than that in R 16 (P < 0.004). Our results demonstrate that the CNAP-induced increase in R rs does not result from a direct lung volume effect only and strongly sugge st the involvement of other factors affecting both intrathoracic and extrat horacic airway caliber.