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Red blood cells (RBCs) are the cells that remain following separation of plasma from whole blood at any time during storage. RBCs preserved with Adsol contain approximately 30 to 40 mL of CPD plasma, 100 mL of Adsol solution, and 150 - 230 mL of red cells. The hematocrit is 55 to 60%. RBCs produced without Adsol contain approximately 70 mL of CPDA1 plasma and have a hematocrit of 70 to 80%. Treatment of Acute Anemia Transfusion of RBCs is indicated to increase oxygen delivery in patients who are actively bleeding and in those who have symptomatic anemia unresponsive to specific therapy. The primary therapy for acute hemorrhage is volume replacement with crystalloid, because the treatment of or prevention of hypovolemic shock is more important than restoration of oxygen carrying capacity. If symptoms persist after volume repletion, red cell transfusion should be considered. Symptoms include syncope, pallor, dyspnea, postural hypotension, tachycardia, angina, transient ischemic attacks, and others noted below. The presence of these symptoms indicates that the patient was unable to compensate for the reduced oxygen carrying capacity. The normal compensatory mechanisms include increased cardiac output, peripheral vasoconstriction and increased oxygen extraction by peripheral tissues. These symptoms can be used to estimate the percentage of blood loss. Signs & Symptoms of Acute Blood Loss
The loss of 500 mL of blood within 5 minutes is well tolerated by the average adult blood donor. Therefore, it is usually not necessary to transfuse patients with a single unit of RBCs since the recipient probably needs the blood no more than the donor. Patients who have suddenly lost more than 20 to 30% of their blood volume are more critical and develop symptoms in spite of compensatory mechanisms such as peripheral vasoconstriction and fluid shift from the extravascular to the intravascular space. Hemodilution begins almost immediately after the onset of hemorrhage and continues up to 72 h after cessation of bleeding. Although this influx of fluid does not improve oxygen carrying capacity, it does help to maintain blood volume and stabilize circulation. In this situation, transfusion of a single unit of RBCs along with crystalloid solutions is justifiable since the patient has lost the equivalent of three units of blood. Adult patients in hemorrhagic shock have usually lost 35 to 40% of their blood volume, or approximately 2 liters. Blood volume should be immediately replaced with crystalloid solutions such as lactated Ringer's solution or normal saline. Colloids are no more advantageous physiologically than crystalloid during early resuscitation and are more expensive. Furthermore, a recent meta-analysis demonstrated a significant increase in the risk of death with the use of albumin compared to crystalloid. The early administration of fluids allows sufficient time for ABO typing of the recipient, which takes only a few minutes. In this way, ABO type specific blood can be given instead of empirically giving O negative blood, which often is in short supply. Numerous publications have debated the risks and benefits of using hemoglobin levels between 6 and 10 g/dL as a transfusion trigger for patients with acute blood loss. Generally, any isovolemic patient with a hemoglobin < 7 g/dL will require transfusion, while most patients with a hemoglobin > 10 g/dL will not. Patients transfused for a hemoglobin of 7 g/dL and maintained at a hemglobin level between 7 and 9 g/dL have the same or better clinical outcomes as patients transfused at a hemoglobin of 10 g/dL and maintained between 10 to 12 g/dL. The decision to transfuse patients with hemoglobin levels between 7 and 10 g/dL depends on clinical judgment. Patients >65 years old and patients with heart and lung disease should probably be maintained at hemoglobin closer to 10 g/dL. Indications for red cell transfusion to correct acute anemia include:
Reversible causes of chronic anemia such as vitamin B12, folate, and iron deficiency should be ruled out prior to transfusion. Erythropoietin sensitive anemia such as anemia of chronic renal insufficiency and the anemia associated with zidovudine (AZT) treatment of HIV patients should also be ruled out. Red cells should be transfused to alleviate the symptoms of anemia or to reduce morbidity associated with a patient's underlying disease. Hemoglobin trigger levels have been established to guide clinical decision making for transfusions. These trigger levels provide useful guidelines for the protection of a patient's reserve oxygen carrying capacity. Protection of this reserve may be required in certain disease states and age groups where patients are unable to compensate for anemia. The hemoglobin trigger level is 7 g/dL (hematocrit of 21%) in individuals with chronic anemia without symptoms. Symptoms in normovolemic patients that indicate the need for transfusion include dyspnea, syncope, transient ischemic attacks, postural hypotension, tachycardia, tachypnea, and angina. Chronic anemia patients undergo compensatory changes that acclimate them to lower hemoglobin levels. A point of fundamental importance is that blood volume is decreased only slightly in patients with chronic anemia due to compensatory increases in plasma volume. Thus, transfusion of chronically anemic patients regularly causes hypervolemia which has the potential for precipitating cardiac decompensation, particularly in elderly patients or in patients with known heart failure. Physicians must not be overly aggressive in the transfusion of patients with severe anemia. Transfusion will improve functional status in symptomatic patients up to a hemoglobin level of 10 g/dL. Transfusions beyond this level provide no further improvement in functional status in most patients. This is especially true for patients with impaired cardiac output because their inability to compensate for increased blood viscosity can actually decrease tissue oxygenation. The major exception is patients with severe chronic obstructive pulmonary disease (COPD) who may still be symptomatic at hemoglobin levels of 10 g/dL and require a hemoglobin level between 10 and 12g/dL to alleviate symptoms. Patients taking medications, such as beta-blockers, are not able to mount an adequate sympathetic response to blood loss. Transfusion to a hemoglobin level of 10 g/dLmay be necessary to relieve symptoms. Transfusion in the Setting of Angina Angina may be indicative of an impending myocardial infarction. Indications for transfusion of patients with myocardial infarction are unclear. Transfusion may improve myocardial oxygen delivery, but may also increase myocardial oxygen consumption secondary to increased blood volume and blood viscosity. The decision to transfuse should be based on critical patient evaluation and internal hemodynamic pressure monitoring. Hemoglobin of 8 g/dL is usually tolerable in surgical patients without risk factors for ischemia, while hemoglobin of 10 g/dL may be better for patients with increased risk. Perioperative myocardial infarction is more likely when the hemoglobin is < 9 g/dL, especially if the patient has tachycardia. Transfusion in Surgery Red cell transfusion is indicated during surgery when active bleeding causes a blood loss of 10-15% or more of the patient's blood volume. Blood volume may be estimated as 70 mL per kg; thus, a 70 kg person's blood volume will be 4900 mL and an estimated blood loss of 500-750 mL will represent a 10-15% loss. The transfusion of colloid and/or crystalloid solutions is also indicated in bleeding patients to maintain adequate blood volume. Indeed, as long as normovolemia is maintained with colloid and/or crystalloid solutions and the patient's hemoglobin level is adequate, it is not necessary to replace all losses of red cells. Fresh frozen plasma should not be used for volume replacement since this incurs the unnecessary risk of infectious disease transmission. Hemodynamic instability may also occur during acute blood loss, usually after loss of 15% or more of blood volume. Accordingly, red cell transfusion is indicated if acute blood loss causes the blood pressure to drop by 20% or to a level of < 100 mm Hg, or if the pulse increases to >100/min. Red cell transfusion has often been used empirically prior to general anesthesia when the hemoglobin is less than 9 or 10 g/dL. There are no data that strongly support this practice. Rather than use a formula approach, proper preoperative assessment should correlate the adequacy of the hemoglobin level with the cause and duration of the anemia, the patient's cardiac and pulmonary status and the type and probable duration of the surgery. The key to tolerance of anemia is the maintenance of normovolemia and compensatory mechanisms that increase cardiac output and improve oxygen transport. Each patient should be evaluated based upon the anticipated ability of his/ her cardiovascular system to compensate. Postoperative hemoglobin in the range of 8-9 g/dL appears to be safe for patients free of cardiovascular disease, and justification should be provided if blood is transfused other than to replace losses at this level. On the other hand, the compensation for acute anemia is through increased cardiovascular performance. Moderate hemodilution (below a hemoglobin of 8-10 g/dL) does affect cardiac workload, and its risks should be weighed carefully in patients with extensive cardiovascular disease. The indications for red cell transfusion perioperatively include:
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