1. Main Objective: To compare the concentration of exhaled nitric oxide in patients with different severity of respiratory failure in ICU.

2. Secondary Purpose:

   1. To observe the change of nitric oxide concentration in exhaled breath of patients with mechanical ventilation over time;
   2. The relationship and differences between pulmonary ventilation blood flow levels in patients with different exhaled NO concentrations;
   3. Reactivity of patients with different exhaled NO concentrations to inhaled nitric oxide therapy and the relationship between them.

Inclusion Criteria:

1. Adult patients (≥18 years of age) with mechanical ventilation or advanced respiratory support in the ICU, regardless of gender;
2. The expected mechanical ventilation time of tracheal intubation is more than 24 hours;
3. The initial ventilator Settings of clinical routine were adopted: oxygen fraction (FIO2) = 50%, respiratory rate = 15/min, positive end-expiratory pressure (PEEP) = 5 cmH2O, tidal volume = 6-8 ml/kg ideal body weight (IBW), inspiration-exhalation ratio =1:2;
4. Agree to participate in this experiment and sign the informed consent.

Exclusion Criteria:

• 1.Use of any NO-containing drugs (such as nitroglycerin), anti-inflammatory drugs, or drugs or means that affect the airway diameter within 6 hours before the first FeNO measurement; 2. Participating in other clinical research projects; 3. Patients deemed unsuitable by the investigators.

ICU patients with advanced respiratory support