Closing volume detection by single-breath gas washout and forced oscillation technique.

Closing volume (CV) is commonly measured by single-breath nitrogen washout (CV). A method based on the forced oscillation technique was recently introduced to detect a surrogate CV (CV). As the two approaches are based on different physiological mechanisms, we aim to investigate CV and CV relationship at different degrees and patterns of airway obstruction. A mathematical model was developed to evaluate the CV and CV sensitivity to different patterns of airway obstruction, either located in a specific lung region or equally distributed throughout the lung. The two CVs were also assessed during slow vital capacity (VC) maneuvers in triplicate in 13 healthy subjects and pre- and postmethacholine challenge (Mch) in 12 subjects with mild-moderate asthma. Model simulations suggest that CV is more sensitive than CV to the presence of few flow-limited or closed airways that modify the contribution of tracer-poor and tracer-rich lung regions to the overall exhaled gas. Conversely, CV occurs only when at least ∼65% of lung units are flow limited or closed, regardless of their regional distribution. CV did not differ between healthy subjects and those with asthma (17 ± 9% VC vs. 22 ± 10% VC), whereas CV did (16 ± 5% VC vs. 23 ± 6% VC, < 0.01). In patients with asthma, both CV and CV increased post-Mch (33 ± 7% VC < 0.001 and 43 ± 12% VC < 0.001, respectively). CV weakly correlated with CV ( = 0.45, < 0.01). The closing capacities (CV + residual volume) were correlated ( = 0.74, < 0.001), but the changes with Mch in both CVs and closing capacities did not correlate. CV is easy to measure and provides a reproducible parameter useful for describing airway impairment in obstructive respiratory diseases. The forced oscillation technique can identify a surrogate of closing volume (CV). We investigated its relationship with the one measured by single-breath washout (CV). CV weakly correlates with CV. The respective closing capacities were correlated, but their increases after methacholine challenge in asthmatics did not. Our results suggest that CV is less sensitive than CV to few flow-limited/closed airways but more specific in detecting increases in flow-limited/closed airways involving the majority of the lung.