Ventilation limitation has a significant adverse effects on cardiovascular function and cerebral oxygenation during exercise in patients with COPD. High flow nasal cannula (HFNC) has been shown to improve ventilation by washing out the anatomical dead space and permitting a better gas exchanges. Moreover, it is able to ensure the desired inspired oxygen fraction (FiO2) even at high level of patient's minute ventilation by minimizing the room air entrainment. The effects of HFNC on exercise performance in terms of hemodynamic changes and exercise endurance in COPD patients remain unclear. The primary purpose of this study is to examine the effects of HFNC on the exercise endurance in COPD patients. The investigator's secondary purpose is to investigate whether HFNC could improve efficiency of ventilation, leading to an improvement of hemodynamic and cerebral oxygenation response.

This is a randomized crossover study. Patients who have been diagnosed as COPD will be recruited from outpatient department. After signing informed consent form, patients undertake an incremental exercise test and two constant load exercise at the 70% of maximum workload achieved at a previous incremental exercise test on arm ergometer in two separate days. The constant load tests will be performed with HFNC and with nasal cannula at the same inhaled oxygen fraction in random order. The hemodynamics CO, SV, EF of all subjects during exercises will be measured by a bioelectrical impedance device (Physioflow), and cerebral oxygenation status (oxygenated hemoglobin (O2Hb), deoxygenated hemoglobin (HHb), total hemoglobin (tHb) was measured by a near infrared spectrophotometer (NIRS).

Inclusion Criteria:

1. with diagnosis of COPD
2. OPD patients
3. has no AE (within 1 month)
4. not receiving any O2 therapy at home
5. no smoking or quick

Exclusion Criteria:

1. fever
2. unstable hemodynamics at resting
3. orthopadeic or neurologic problems that limited exercise