Hypoxemia is a common complication after aortic surgery. As this complication has an adverse effect on the postoperative course of the patient, early treatment is important; however, the mechanism of hypoxemia after surgery for acute aortic dissection remains unclear. Recently, the investigators found that inhaled Nitric Oxide can improve the oxygenation in some of these patients. The investigators are trying to evaluate the effectiveness and safety of inhaled Nitric Oxide in patients with refractory hypoxemia after aortic surgery.

Several risk factors for severe hypoxemia after aortic surgery have been reported including advanced age, obesity, smoking history, previous heart surgery,emergency surgery,reduced cardiac function, advanced chronic obstructive pulmonary disease, excessive volume of blood transfusion,and prolonged CPB time. The routine treatment includes lung protective mechanical ventilation, recruitment maneuvers and glucocorticoids.

No previous clinical studies have reported the effectiveness and safety of inhaled Nitric Oxide in patients with refractory hypoxemia after aortic surgery.

Inclusion Criteria:

1. Adult patients with refractory hypoxemia after aortic surgery;
2. Accepting invasive mechanical ventilation;
3. Chest X-ray and lung ultrasound to exclude the respiratory factors (eg. pulmonary edema, obstructive atelectasis, pleural effusion, pneumothorax) and hemodynamic factors (pericardial tamponade, acute pulmonary hypertension, intracardiac shunt);
4. The ventilator parameters: PEEP>10cmH2O, VT 6-8ml/kg;
5. The PaO2/FiO2 <= 100mmHg.

Exclusion Criteria:

1. Age <18 years old;
2. Pregnant women;
3. Past medical history included COPD or mental illness;
4. The serious infection or sepsis patients;
5. Patients with pulmonary hypertension and right ventricular dysfunction