Impulse Oscillometry Detects Airway Impairment in All COPD Severity Levels

Airway impairment assessed with impulse oscillometry (IOS) was present in patients with all severity levels of chronic obstructive pulmonary disease (COPD), especially in those with Global Initiative for Chronic Obstructive Lung Disease (GOLD) stages 3 to 4, according to a study in BMC Pulmonary Medicine.

Researchers in China analyzed the proportion and clinical characteristics of airway impairment detectable by IOS in patients with varying levels of COPD severity. They also investigated the possibility that airway impairment could be a subtype of COPD.

The study analyzed data from the ECOPD cohort, a prospective, observational, population-based study that recruited patients with COPD between July 2019 and August 2021 in China. Participants aged 40 to 80 years completed questionnaires and underwent pre-bronchodilator IOS, pre-bronchodilator lung function tests, post-bronchodilator lung function tests, and computed tomography.

COPD was defined as a postbronchodilator forced expiratory volume in 1 second (FEV₁)/forced vital capacity (FVC) less than the lower limit of normal (LLN). Respiratory resistance and reactance were measured with use of IOS. Patients with COPD were grouped into 6 IOS parameter abnormalities groups based on the following criteria: (1) resistance at 5 Hz (R₅) > upper limit of normal (ULN); (2) R₂₀ > ULN; (3) R₅-R₂₀ > ULN; (4) reactance at 5 Hz (X₅) < LLN; (5) area of low-frequency resistance (AX) > ULN; (6) and resonant frequency (F_res) > ULN.

A total of 768 patients were included in the analysis. The proportion of airway impairment corresponding with R₅, R₂₀, R₅-R₂₀, X₅, AX, and F_res in patients with COPD was 59.8%, 29.7%, 62.5%, 52.9%, 60.9%, and 67.3%, respectively. Patients in IOS parameter abnormalities groups had more respiratory symptoms (cough, phlegm, wheeze, and dyspnea) and were more likely to have asthma, emphysema, and air trapping, compared with those who had normal IOS parameters.

Airway impairment was evident among patients with all severity levels of COPD, particularly in those with GOLD stages 3 to 4. The proportion of airway impairment (R₅, R₅-R₂₀, X₅, AX, and F_res) progressively increased with an increase in COPD severity. The proportion of airway impairment assessed with R₂₀ did not increase significantly with COPD severity (GOLD 1, 18.8%; GOLD 2, 36.5%; GOLD 3, 33.3%; and GOLD 4, 35.3%).

Patients with IOS parameter abnormalities assessed with R₅ (risk ratio [RR] 1.58; 95% CI, 1.13-2.19; P =.007), R₅-R₂₀ (RR 1.73; 95% CI, 1.22-2.45; P =.002), X₅ (RR 2.11; 95% CI, 1.51-2.95; P <.001), AX (RR 2.20, 95% CI, 1.53-3.16; P <.001), and F_res (RR 2.13; 95% CI, 1.44-3.15; P <.001) had a significantly increased risk of acute exacerbations of COPD within the previous year, compared with patients with normal IOS parameters.

Logistic regression analysis demonstrated that patients with IOS parameter abnormalities (R₅, R₅-R₂₀, X₅, AX, and F_res) had more emphysema and air trapping vs those with normal IOS parameters, after adjustment for age, sex, body mass index, smoking status, smoking index, family history of respiratory diseases, occupational exposures, biomass exposure, and history of asthma.

Study limitations include the number of patients with COPD with GOLD stages 1 to 2, which may affect the proportion of airway impairment. In addition, FEV₁ measured with body plethysmograph instead of spirometry was more accurate in reflecting COPD severity grade, and the forced oscillation technique may be more sensitive than IOS for measuring reactance in patients with airflow obstruction.

“Our study revealed that airway impairment may potentially be a phenotype of COPD,” stated the study authors. “Further studies are warranted to identify underlying mechanisms and longitudinal progression of airway impairment.”