Retrograde autologous priming (RAP) is a means to effectively and safely restrict the hemodilution caused by the direct homologous blood transfusion and reduce the blood transfusion requirements during cardiac surgery. It is also generally considered a blood conservation method used in most patients during the cardiopulmonary bypass (CPB). The processing of RAP includes three main steps, and the entire procedure of RAP (about 1L CPB prime volume) could be completed within 5 to 8 minutes. This technique is proposed by Panico in 1960 for the first time and restated by Rosengart in 1998 to eliminate or reduce the risk of hemodilution during CPB. Moreover, to precisely determine the clinical efficacy of RAP, many related studies were conducted. Most results of researches indicate that RAP is available to provide some benefits to reducing the requirements for red blood cell transfusion. However, there are still some studies showing a failure of RAP to limit the hemodilution after the open heart operation.
Background
Cardiopulmonary bypass (CPB)
The requirement of blood transfusion during open-heart surgical operations is almost inevitable because the metabolism must be continuously kept working through the peripheral blood flow. Cardiopulmonary bypass (CPB) is a medical technique to oxygenate the blood and remove the carbon dioxide during the cardiac operation. It can be seen as a "pump" to serve as a heart-lung machine whose function is sustaining blood circulatory and transporting oxygen to red blood cells before blood is flowing backwards the arterial circulation. It can also be considered a kind of extracorporeal circulation.
Homologous blood transfusion
There are two primary methods, homologous blood transfusion and autologous blood transfusion, to reduce the massive blood loss resulted from different surgeries. Homologous blood transfusion refers to using blood from other compatible donors to improve the oxygen-carrying capacity for blood by raising the number and concentration of blood red cells. Autologous transfusion uses patients' own blood which is stored previously. Compared to autologous blood transfusion, the homologous blood transfusion has more side effects, such as postoperative Infection, immunosuppression and viral transmission. These adverse effects facilitate the development of approaches to conserve the blood during cardiac surgery. These methods contain preoperative autologous blood donation (PABD), acute normovolemic hemodilution (ANH) and cell salvage. There are two main ways to salvage red blood cells. Cell processing and direct injection. Cell processors are red cell washing devices like the Cell Saver that collect anticoagulated shed or recovered blood, wash and separate the red blood cells (RBC) by centrifugation. Filtration devices like HemoClear microfilter, constitute the second major type of blood salvage in operating rooms. In general, ultrafiltration devices filter the patient's anticoagulated whole blood. Direct transfusion is a blood salvaging method associated with cardiopulmonary bypass (CPB) circuits or other extracorporeal circuits (ECC) that are used in surgery such as coronary artery bypass grafts (CABG), valve replacement, or surgical repair of the great vessels. Following bypass surgery, the ECC circuit contains a significant volume of diluted whole blood that can be harvested in transfer bags and re-infused into patients.
Furthermore, the number of patients requiring red blood transfusions could be anticipated by observing a variety of variables before cardiac surgery. The red cell mass, as a critical factor, it matches the assumption that the blood transfusion is commonly demanded by patients who have a low red cell mass during CPB. At the same time, these patients are more likely to have low hematocrit values. Patients expect to reduce the blood product used during surgery because the red blood transfusion generally has a negative physiologic impact and highly related cost. Especially when CPB is applied in surgical operations, there will be a large number of blood transfusions required due to blood loss and hemodilution.
Hemodilution Furthermore, a significant risk caused by the homologous blood transfusion in open-heart surgeries is hemodilution. The hemodilution is the primary hematologic interference which results in coagulopathy and decreased capability of carrying oxygen in the blood. The root cause of this situation is the decreased blood concentration and viscosity in the body. During the cardiopulmonary bypass, blood is mixed directly through the homologous blood transfusion, but homologous blood may be not compatible, acceptable or available. Thus, there is a high incidence of distinct degrees of hemodilution with CPB.
Medical principles RAP is a blood conservation technique to decrease the priming volume of cardiopulmonary bypass and then reduce the level of the hemodilution. To be more specific, some cardiopulmonary bypass circuit prime are replaced by autologous circulating blood at the beginning of bypass in both venous cannula and arterial cannula. The final objective of RAP is reducing the number of patients requiring blood transfusion during CPB.
There is a type of glycoprotein growth factor - erythropoietin that can be used to mitigate the hemodilution by increasing the total number of red blood cells. Compared with erythropoietin, RAP is more rapid and inexpensive. Because RAP aims to directly influence the CPB prime volume instead of red cell masses, but for patients who have extremely low red cell masses, like anaemic patients and some particular groups, such as women and elderly, they would have greatly decreased hematocrit values and need a larger amount of blood transfusions. As a consequence, these patients are thought to be more suitable to use erythropoietin rather than RAP.
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