Lung recoil and gas trapping during oxygen breathing at low lung volumes

If airways are closed at lung volumes less than the closing volume (CV), there should be correlations among 1) the volume of trapped N2 (VTN) during N2-washout performed below CV, 2) the increase in static lung recoil pressure (delta P) while breathing below CV after denitrogenation compared with breathing air (due to absorption atelectasis distal to closed airways), and 3) the CV. Static inspiratory pressure-volume (PV) curves and CV were measured in 18 seated normal volunteers (ages 24-48 yr). Subjects then breathed air for 30 s and O2 for 2.5 min at RV + 0.6 liter (LVB-air), and an inspiratory PV curve and VTN were determined. While still breathing O2, the subjects repeated the 3 min of low-volume breathing (LVB-O2). There was a significant (P less than 0.001) delta P with LVB but no difference between delta P (LVB-air) and delta P (LVB-O2). CV was not related to VTN or to either delta P. VTN was not related to delta P (LVB-O2)--delta P (LVB-air) nor to delta P (LVB-air), but was related to delta P (LVB-O2). Evidence of airway closure could not be demonstrated in all subjects by LVB and when present showed no correlation with CV.