Inclusion Criteria:  AECOPD patients. The patients with hypercapnia respiratory failure were admitted to ICU and needed noninvasive ventilation. The results of blood gas analysis showed pH <7.30, PaCO2> 50 mmHg. There were high risk factors for failure of noninvasive ventilation: after receiving continuous 2 hours of noninvasive ventilation, the arterial blood pressure showed the pH value was ≤ 7.30, while PaCO2 was 20% higher than the baseline value, and any one or more of the following conditions were combined: respiratory rate ≥ 30 times / min, assisted respiratory muscle breathing; contradictory breathing. Informed consents were sighed.  Exclusion Criteria:  The mean arterial pressure was less than 60 mmHg after volume and vasoactive drug treatment. There were anticoagulant contraindications. Weight over 120kg. Patients with malignant tumor or other complications, the expected survival time was less than 30 days. It could not cooperate with noninvasive ventilation or has contraindication of noninvasive ventilation.