Effect of the slope of the alveolar plateau on determination of closing volume.

Repeated closing volume (CV) determinations were obtained simultaneously with the nitrogen method (CV-N2) and with a helium bolus method (CV-He) in 19 selected subjects with normal routine spirometry. Ten subjects had normal slopes of phase III and nine had abnormally steep slopes of phase III obtained with the nitrogen method. No systematic difference was found between CV-N2 and CV-He irrespective of the steepness of phase III, which seems to indicate that at least in spirometrically normal subjects airway closure is adequately assessed by the nitrogen method - even in the presence of a steep slope of phase III.