Study of long-term HFNC for COPD patients with HOT

INTERVENTION: Arm A (myAIRVO2+ HOT): Subjects receive following protocol treatment; HOT plus nocturnal high‐flow nasal cannula therapy with the myAIRVO2 within 52weeks. Arm B (HOT): Subjects receive following protocol treatment; HOT only within 52weeks. CONDITION: Chronic Obstructive Pulmonary Disease(COPD) ; COPD PRIMARY OUTCOME: Frequency (the number of occurrences per year) of COPD exacerbation (Moderate or Severe) SECONDARY OUTCOME: 1. Term from enrollment to the date of first COPD exacerbation (Moderate or Severe) ; 2. Term from enrollment to death from any cause ; 3. Frequency (the number of occurrences per year) of COPD exacerbation (All Severity and Severe only) ; 4. Total St. George's respiratory questionnaire (SGRQ‐C) score, and each components score (symptom score, activity score, and impact score) ; 5. Quality‐adjusted life year (QALY) by mapping the EQ‐5D‐5L utility scores ; 6. Total SRI (Severe Respiratory Insufficiency Questionnaire) score ; 7. Total PSQI‐J(Japanese version of the Pittsburgh Sleep Quality Index) score ; 8. Dyspnea intensity: the modified medical research council (mMRC) score ; 9. Arterial blood gas analysis (ABG): pH, PaO2, PaCO2, HCO3‐, BE ; 10. Oxygen Saturation (SpO2) ; 11. Pulmonary functions: FVC, FEV1, FEV1% ; 12. 6‐minute walk test (6MWT): 6‐minute walk test distance, SpO2, modified borg scale (mBS) ; 13. Term from enrollment to the date of long‐term (more than 1month) NPPV(Noninvasive Positive Pressure Ventilation) usage ; 14. Oxygen flow rate, and Total flow rate (Arm A only) ; 15. Total hours of myAIRVO2‐use (Arm A only) ; 16. Adverse events INCLUSION CRITERIA: 1. Patients who are diagnosed with the global initiative on obstructive lung disease (GOLD) stage 2‐4 COPD. 2. Patients who have received nocturnal HOT 16 hours or more per day for 1 month or more at the time of the informed consent. 3. Patients with PaCO2 >= 45 Torr and pH >= 7.35 at screening. 4. Patients with COPD exacerbation (Moderate or Severe; judged by the investigators) within the past 1 year prior to the informed consent. 5. Patients who are more than 40 years old at the time of the informed consent. 6. Patients who agree to participate in the study with the written informed consent.