The grade of hypoxemia is divided into 3 criteria: 1. The partial pressure of arterial oxygen ranges from 50-80mmHg and the saturation of blood oxygen is about 80% is generally mild; 2. If the partial pressure of arterial oxygen is in the range of 30-50mmHg and the saturation of blood oxygen is 60%-80%, it is called moderate hypoxemia; 3. If the partial pressure of arterial oxygen is lower than 30mmHg and the saturation is below 60% it is more severe hypoxemia. Among these 3 levels, mild hypoxemia will not have cyanosis, moderate will have cyanosis, and severe will have especially obvious cyanosis, which is caused by insufficient oxygen in the patient’s body, resulting in insufficient saturation and partial pressure of blood oxygen.