Besides the improved symptoms, X-rays and sufficient spontaneous breathing efforts, the eligibility criteria for extubation were peak inspiratory pressure (PIP) of 15-20 cm H2O, positive end expiratory pressure (PEEP) of 4-6 cm H2O, respiratory rate of 15 to 30, FiO2 from 0.21 to 0.30, hematocrit more than 35%. and arterialized capillary blood gas analysis need to meet the following criteria: pH>7.20, PaCO2<=55 mmHg.