Fluid extravasation is a typical symptom of capillary leak syndrome, which refers to a group of clinical syndromes such as tissue interstitial edema, hypoproteinemia, hypovolemic shock, acute renal ischemia, etc., caused by capillary endothelial cell damage and increased vascular permeability due to various pathogenic factors. There are hundreds of causes of CLS, including severe trauma, sepsis, DIC, post-operative extracorporeal circulation (especially in infants and children) and reperfusion injury, snake bite, acute lung injury or acute respiratory distress syndrome (ARDS), burns, severe pancreatitis, and toxic effects of many drugs such as recombinant interleukin-2 and doxorubicin. The most common clinical condition is sepsis. In the ICU, the majority of CLS occurs as a result of sepsis, with extensive damage to capillary endothelial cells in response to endotoxin and inflammatory mediators. The extracorporeal circulation activates the coagulation system, fibrinolytic system, complement system, and monocyte-macrophage system, as well as polymorphonuclear leukocytes and endothelial cells to release large amounts of inflammatory mediators and cause CLS.