In addition to pharmacological treatment, nonpharmacological treatment with high-flow nasal cannula (HFNC) may play a vital role in the treatment of patients with chronic obstructive pulmonary disease (COPD). The impulse oscillation system (IOS) is new noninvasive technique to measure the impedance of different portions of the airway with higher sensitivity than that of the conventional pulmonary function test (PFT). However, whether IOS is an appropriate technique to evaluate the efficacy of HFNC in improving the impedance of peripheral small airways in patients with COPD is unclear.

Participants with stable COPD from the chest medicine outpatient department will be randomized into two groups receiving HFNC or nasal cannula (NC) for 10 min followed by a 4-week washout period and crossover alternatively. We used a novel analytical measurement technique, IOS, to detect the difference in airway impedance in the participants after the HFNC or NC interventions. All data, namely IOS parameters, transcutaneous partial pressure of carbon dioxide, peripheral oxygen saturation, body temperature, respiratory rate, pulse rate, blood pressure, and PFT results at the time of pre-HFNC, post-HFNC, pre-NC, and post-NC, were analysed using SPSS (version 25.0, IBM, Armonk, NY, USA).

Inclusion Criteria:

• age of 20 to 75 years old
• diagnosis of COPD made by pulmonologist (Kuo YL) if the patient had a long-term smoking or noxious gas exposure history, typical clinical manifestations, and airflow limitation with a postbronchodilator FEV1/FVC ratio of < 0.7 in spirometry,4 and
• provision of written informed consent.

Exclusion Criteria:

• severe and unstable comorbidities or active malignancy
• history of obstructive sleep apnoea syndrome
• COPD exacerbation within the 4 weeks prior
• current use of long-term oxygen therapy or noninvasive ventilation or use within the 6 weeks prior
• cognitive impairment or a psychiatric disorder
• pregnancy