Altered Lung Volumes in Relationship to Airflow Obstruction and Small Airways Dysfunction in Asthma

Background:

Small airways dysfunction (SAD) and airflow obstruction (AO) are prevalent in asthma and lung volumes are also commonly affected due to gas trapping. This study assessed relationships between SAD and AO on lung volumes in patients with asthma.

Methods:

Adults with asthma who had a set of full lung function measurements between (January 2016 to April 2021) were included from a tertiary hospital in the West Midlands, United Kingdom. Predicted values were calculated using the 2012 Global Lung Initiative equations. AO was defined as a standardized residuals (SR) of < -1.645 in FEV₁/FVC. (1). SAD was defined as a SR of < -1.645 in MMEF. Rstudio software was used for statistical analysis. Wilcoxon rank test was used to analyze the differences between the groups. The study was approved by the Research Ethics Committee and the Health Research Authority of the NHS (20/WM/0024).

Results:

208 patients were included, of which 36 (17.3%) and 46 (22.1%) had physiological evidence of AO and SAD, respectively. Residual volume (RV) % predicted was higher in asthma patients with AO (n = 36) compared to patients without AO (n = 172), (median [IQR]: 102 [95.8-108] vs. 92.3 [84.6-101], (P < .001). The total lung capacity (TLC) was also higher in those with AO compared to without AO (median [IQR]: 102 [95.8-108] vs. 92.3 [84.6-101], P < .001). RV was higher in patients with SAD compared to those without SAD, (median [IQR]: 109 [88.6-125] vs. 85.7 [74.6-98.0], (P < .001). TLC was not different between groups (SAD: 97.7 [87.8-103] vs. non-SAD 93.9 [86.3-103], P = .35), see Figure 1. In a univariate analysis of SAD on the total group (n = 208), RV was associated with SAD (OR 1.05, 95% CI: 1.03-1.06, P < .001).

Conclusions:

In patients with asthma, SAD and AO are both associated with a higher RV and RV/TLC, which are suggestive of gas trapping. This may be contributing to the symptomatic burden in these patients and MMEF may be a useful parameter to measure.

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Figure 1. % predicted of the lung volumes in SAD and AO in the whole population. It shows that there was an increase of RV in those with SAD similar to those with AO, although TLC was maintained in the SAD group. All values are median (IQR).