PaO2 means arterial partial pressure of oxygen, and the normal value of arterial partial pressure of oxygen is between 80-110mmHg. The level of arterial partial pressure of oxygen mainly depends on the partial pressure of oxygen of inhaled gas and the partial pressure of oxygen of exhaled gas. According to the different conditions of arterial partial pressure of oxygen, it can be divided into the following: 1. mild hypoxia, i.e. 80-70mmHg. 2. moderate hypoxia, i.e. 70-60mmHg. 3. severe hypoxia, i.e. below 60mmHg. When the arterial blood oxygen is below 55mmHg, it indicates respiratory failure, and when it is below 30mmHg it will be life threatening. Arterial oxygen values vary by age and can be calculated by the following formula: PO2=102-0.33×age. The clinical significance of arterial partial pressure of oxygen is mainly to reflect the state of lung function and its collective degree of hypoxia, which is commonly found in chronic obstructive pulmonary disease, bronchial asthma, chronic pulmonary heart disease, etc.