The color of the skin and mucous membranes varies with the color of the blood stream. The red color of blood is due to the hemoglobin contained in red blood cells. When hemoglobin is sufficiently combined with oxygen to become oxyhemoglobin, it is bright red in color; when it has released oxygen and becomes reduced hemoglobin, the color changes to dark red. The blood in the arteries and capillaries contains more oxyhemoglobin and less reduced hemoglobin Facial cyanoglobin, so its color is bright red, and the red color is still obvious through the thin mucous membrane and translucent nails. The skin is thicker and contains pigment, so it is white with red or slightly brown with red. Venous blood contains more reduced hemoglobin and less oxygenated hemoglobin, so it is dark red, and through the skin, it appears greenish-purple. The veins are what are commonly called “veins” on the arms. Chemicals such as aniline, nitrobenzene and nitrite can change hemoglobin to denatured hemoglobin, which is itself purple. Therefore, cyanosis occurs when there is a decrease in oxygenated hemoglobin and an increase in reduced or denatured hemoglobin in the blood in capillaries and small arteries in the mucous membranes, nails and skin. Cyanosis of the skin and mucous membranes is caused by an increase in reduced hemoglobin in the blood. Cyanosis usually develops when the reduced hemoglobin in capillary blood exceeds 50 g/L. Cyanosis can be classified as central, peripheral or mixed. In addition, cyanosis can also develop from the presence of abnormal hemoglobin derivatives in the blood due to drug and chemical poisoning. This type of cyanosis is characterized as generalized, involving the skin of the trunk and mucous membranes in addition to the extremities and face, but the skin of the affected area is warm. The causes of cyanosis are mostly due to respiratory failure caused by cardiac and pulmonary diseases, ventilation and ventilation dysfunction, and lower Sa02 due to insufficient pulmonary oxygenation. Generally, it can be divided into: 1. Pulmonary cyanosis: that is, due to respiratory insufficiency and insufficient pulmonary oxygenation. It is common in various severe respiratory diseases, such as obstruction of larynx, trachea and bronchus, pneumonia, obstructive emphysema, diffuse interstitial pulmonary fibrosis, pulmonary stasis, pulmonary edema, acute respiratory distress syndrome, pulmonary embolism, primary pulmonary hypertension, etc.; 2. Cardiac mixed cyanosis: due to abnormal channel shunt, so that part of the venous blood does not pass through the pulmonary circulation for oxygenation and human circulating arteries, such as shunt flow exceeds 1/3 of the cardiac output, cyanosis can occur. It is common in cyanotic congenital heart disease, such as Fallot tetralogy, Eisenmenge syndrome, etc.