Cyanosis refers to the phenomenon of dark purple color of skin and mucous membrane, which is caused by the increase of reduced hemoglobin in the blood and the phenomenon of cyanosis of skin and mucous membrane, also called cyanosis. Usually, cyanosis can be formed when the reduced hemoglobin in capillary blood exceeds 50 g/L. It is most obvious in the lips, nose tip, auricle and nail bed, which is a manifestation of critical condition. Therefore, if a patient has the above-mentioned condition, he/she should go to the hospital in time and improve the examination under the guidance of the doctor to identify the cause of the disease and follow the medical advice for treatment.