A functional mathematical model to simulate the single-breath nitrogen washout

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

Closing volume (CV) is commonly measured by single-breath nitrogen washout (CV_SBW). A method based on the forced oscillation technique was recently introduced to detect a surrogate CV (CV_FOT). As the two approaches are based on different physiological mechanisms, we aim to investigate CV_FOT and CV_SBW relationship at different degrees and patterns of airway obstruction. A mathematical model was developed to evaluate the CV_SBW and CV_FOT 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_SBW is more sensitive than CV_FOT 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_FOT occurs only when at least ∼65% of lung units are flow limited or closed, regardless of their regional distribution. CV_SBW did not differ between healthy subjects and those with asthma (17 ± 9% VC vs. 22 ± 10% VC), whereas CV_FOT did (16 ± 5% VC vs. 23 ± 6% VC, P < 0.01). In patients with asthma, both CV_SBW and CV_FOT increased post-Mch (33 ± 7% VC P < 0.001 and 43 ± 12% VC P < 0.001, respectively). CV_SBW weakly correlated with CV_FOT (r = 0.45, P < 0.01). The closing capacities (CV + residual volume) were correlated (r = 0.74, P < 0.001), but the changes with Mch in both CVs and closing capacities did not correlate. CV_FOT is easy to measure and provides a reproducible parameter useful for describing airway impairment in obstructive respiratory diseases.NEW & NOTEWORTHY The forced oscillation technique can identify a surrogate of closing volume (CV_FOT). We investigated its relationship with the one measured by single-breath washout (CV_SBW). CV_FOT weakly correlates with CV_SBW. The respective closing capacities were correlated, but their increases after methacholine challenge in asthmatics did not. Our results suggest that CV_FOT is less sensitive than CV_SBW to few flow-limited/closed airways but more specific in detecting increases in flow-limited/closed airways involving the majority of the lung.

<p>Evaluation of the Small Airways in Patients with Chronic Obstructive Pulmonary Disease and Alpha-1 Antitrypsin Deficiency</p>

Introduction

Small airways are not evaluated with traditional pulmonary function tests. The aim of this study was to evaluate the small airways in patients with chronic obstructive pulmonary disease (COPD) with a nitrogen washout test and to verify whether there is a difference between patients with COPD due to smoking and those with COPD due to alpha-1 antitrypsin mutation.

Methods

Sixteen patients with mutation in the SERPINA1 gene and 45 patients with no mutation were included in this cross-sectional study. All pulmonary function tests, including the single breath nitrogen washout test, were performed for all patients and alpha-1 antitrypsin dosage was assessed with immunonephelometry.

Results

A comparison of patients with COPD due to smoking and those with COPD due to smoking and mutation revealed a significant difference in closure volume (%), which was the poorest in the mutation group. In the group with COPD and mutation, there was an inverse correlation between smoking and closure volume (%). We also verified that similar to forced expiratory volume in the first second (FEV1), the phase III slope (%) and ΔN2 750–1250 mL (%) could be used to differentiate the severity of airflow limitation.

Conclusion

Our results suggest that both variables, phase III slope and the ΔN2 750–1250 mL (%), could be related to COPD severity. Therefore, alterations at the distribution of the location of the emphysema could alter the results of closer volume and that the nitrogen washout test is more sensitive when compared to traditional pulmonary function test in evaluating COPD patients.