There are many reasons for a sudden decrease in blood oxygen, including diseases such as acute respiratory distress syndrome, lung infections, airway obstruction, and cardiac insufficiency. Decreased blood oxygen refers to a decrease in the oxygen content of the arterial blood, a dysfunction of pulmonary ventilation and lung gas exchange, and a phenomenon in which the entry of oxygen into the alveoli is blocked, resulting in an oxygen partial pressure below the lower limit of normal. 1. Acute respiratory distress syndrome, respiratory failure, acute pulmonary embolism and other causes of acute respiratory distress, the emergence of airway alveolar surface type II cell destruction, the formation of oxygen diffusion obstacles, intrapulmonary shunt leading to a significant decrease in blood oxygen saturation or even hypoxemia. 2. severe infection, can reduce the oxygen diffusion area, thicken the oxygen diffusion interval, reduce the blood oxygen molecules or appear oxygen molecules diffusion to the blood difficulties, resulting in decreased oxygen saturation or even hypoxemia. 3. Airway obstruction, such as airway obstruction, gastrointestinal vomitus, foreign body inhalation leading to airway obstruction, resulting in hypoxia and lower oxygen saturation. 4. Cardiac insufficiency, such as rheumatic heart disease, pulmonary heart disease, etc., cardiovascular system pathology reduces arterial blood oxygen, cardiopulmonary blood oxygen supply is insufficient, resulting in low blood oxygen in the body. In short, there is a sudden decrease in blood oxygen, it is recommended that patients seek timely medical treatment, clear diagnosis for treatment.