| The side effects associated with homologous transfusion attracts more and more people's attention. For reducing homologous transfusion, how to save blood has been a new topic studied by doctor. Recent years, with the development of the study on mechanism of Hemodilution (HD), Preoperative HD has become an effective method for blood saving and was frequently used in clinic.HD makes the cell component of the blood decrease comparatively or absolutely and reduces blood concentration to great extent so as to lower the number of cells flowing out. There by, It reduces loss of red blood cell. HD includes two major methods: Acute Normovolemic Hemodilution (ANH)and Acute Hypervolemic Hemodilution(AHH). ANH is administered preoperatively by withdrawing the patient's blood and simultaneously infusing equal volume of crystalloid or colloid solutions. Recent years, some researchers put forth a new concept-ANH. ANH is administered by hemodiluting the patient with crystalloid or colloid solutions preoperatively without removing their autologous blood. As a new technique for blood saving. AHH is a simple as well as time-and cost-saving alternative to ANH.The degree of HD can be expressed by vassel's Heocrit (HCT). Mild degree HD means HCT is 45-30%. Middle-degree HD means HCT is 30%~20%.Severe-degree HD means HCT is 20%~10%. The critical value of HD is HCT 20%, HB 60-70g/L. The ideal fluid for HD should have not only the ability of reducing blood viscosity but also better capacity of carrying oxygen. Until now, no fluid can match these requests. Now crystalloid and intravascular volume substitutes have been used widely. The crystalloid can compensate blood volume immediately and maintain stable hemodynamics. Because of a short half life time(tl/2) of the crystalloid, it may cause cardiac overload and lead to consequent low circulation volume, even worsen brain edema. The intravascular volume substitutes include HES, Gelofusine etc. They can maintain plasma osmotic pressure and have a longer t1/2. It can be infused with crystalloid at acertain ratio.HD is one of the important method of blood protection. It can reduce cell component of blood by infusing intravascular volume substitute. After HD administered, the body's HB concentration decrease, HCT value drop. This will impair the capacity of blood earring O2 and make organs and tissues face with danger of hypoxia. There are several compensatory mechanisms: (1) increasing the cardiac output (CO). The capacity of delivering O2 depends on cardiac output and artery oxygen content. During HD. artery oxygen content decrease while cardiac output increase. There by ,The capacity of delivering O2 of body can maintain unchanged. (2)Reducing the blood viscosity and microcirculation resistance to blood flow and improving the blood perfusion and oxygen supply for tissues. (3) Sympathetic nerve exciting and blood flow being redistributed. Blood flow of vital organs such as heart and Lung increases while that of liver and kidney decreases. This assures vital organ's blood perfusion and oxygen supply. (4)The oxyhaemoglobin dissociation curve shifting right to decrease the affinity between oxygen and haemoglobin so that the tissues can extract more. O2 from blood. The body satisfies the O2 consumption by increasing the ERO2. In a word, during HD, as long as the whole blood volume maintain unchanged, there will be enough oxygen supply even if HCT decreases to20%. Recent years, Some study show that ERDF-NO plays an important role in HD. They point out that during HD blood viscosity decreases; and blood flow fast. Thereby, ion channel is activated and more NO is produced. NO then exerts it's function; dialating vassles, reducing peripheral resistance, relifing heart afterload and improving microcirculation.HD regulates body to a certain state that is abnormal. The most concerned problem of HD is the effects on vital organs and blood coagulation. Some study show that during Mild and Middle degree HD, The increasing extent of CBF is largely over the decreasing extent of artery oxygen content. So that brain oxygen supply can even increase. A healthy heart can tolerant all means of HD. With the age developing this tolerance capacity will be getting worse. Effect of HD on blood coagulation is mainly related with degree of HD. Some research show that propriate degree of HD diluted coagulation factors but won't lead to obvious change in coagulation function. The so-called "diluted coagulation desease" occurs when blood is over-diluted. As long as PLT above 60×1012/L and the other coagutat-ion factors no less than 30% of normal value, the body's coagulation function can be guaranted. In fact, There are a series of physicalcompensation mechanism which help the body enduring HD and prevent vital organs from being damaged.HD is always performed with other blood-saving methods. HD with controlled hypotension(CH) is most frequently used. At one hand, blood flow be come faster in HD so as to compensate vassal dilating in CH. At he other hand, endovassal pressure decreased in CH fectch up tissue fluid overload in HD. But most study show that CH can decrease the security of HD. In CH, blood flow is redistributed to vital organs first and the other organs blood supply is impaired. There fore the contraindication and indication of it must be strictly defined when the two methods are used together. To determine the degree of HD and the time limit of CH, patient's age, vital organ function and the operation must by concerned.HD has been widely used in clinic especially in CPB operation and tumor patients operation. Due to reducing blood loss, HD can prevent many related complications from happening. In the operation which may has a greater loss of blood, HD has good application for ground.Summary: HD has been regarded as one of the effective method for blood saving. But there is still discussions on the tolerance limit of body and whether ANH can really reduce homologous transfusion as well as whether AHH can take place ANH. At present, further study should focus on: (l)Develop and consummate theoretic basis of HD. (2)Choose adequate HD method for different patients. (3) Explore more effective and safety blood saving method.
|
PDF Full Text Request