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RED BLOOD CELLS

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Red blood cell (RBC) transfusions are indicated for patients who need an increase in oxygen carrying capacity. However, determining which patients need more oxygen carrying capacity can be difficult. It is recommended that the anesthesiologist perform a clinical assessment of tissue perfusion prior to initiating erythrocyte transfusions. In a conscious patient, the signs of inadequate tissue perfusion include:

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  • Respiratory rates above 30 per minute

  • Heart rates above 100 beats per minute

  • Weakness

  • Angina

  • Altered mental status

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The body has several compensatory mechanisms for anemia:

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  1. Blood volume is maintained by increasing plasma volume.

  2. Increased cardiac output: Systemic vascular resistance (SVR) is decreased by decreasing vascular tone and viscosity of blood (from hemodilution). The decrease in SVR results in increased stroke volume and therefore, cardiac output and blood flow to tissues.

  3. Blood flow is redistributed to the brain and heart.

  4. Tissues compensate by increasing the oxygen extraction ratio in multiple tissue beds, leading to an increase in the total body oxygen extraction ratio and a decrease in mixed venous oxygen saturation.

  5. The oxyhemoglobin dissociation curve is shifted to the right. Now hemoglobin has decreased affinity for the oxygen molecule and releases oxygen to the tissues at higher partial pressures. Since this shift occurs only after increased 2,3-DPG, it occurs only with chronic anemia.

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A unit of whole blood or packed red cells will raise the hematocrit by 3% and the hemoglobin by 1 g/dL. However, the American Society of Anesthesiologist recommends not using the hemoglobin or hematocrit as a “trigger” for transfusion.

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In 2006, a Task Force on transfusion practices from the American Society of Anesthesiologists produced the following recommendations:

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  1. A close watch on assessment of blood loss during surgery and assessment of tissue perfusion should be maintained.

  2. Transfusion is rarely indicated when the hemoglobin concentration is greater than 10 g/dL, and is almost always indicated when it is less than 6 g/dL.

  3. For intermediate hemoglobin concentrations (6-10 g/dL), justifying or requiring RBC transfusion should be based on the patient’s risk for complications of inadequate oxygenation.

  4. Use of a single hemoglobin “trigger” for all patients and other approaches that fail to consider all important physiologic and surgical factors affecting oxygenation are not recommended.

  5. When appropriate, preoperative autologous blood donation, intraoperative and postoperative blood recovery, acute normovolemic hemodilution, and measures to decrease blood loss (deliberate hypotension and pharmacologic agents) may be beneficial.

  6. The indications for transfusion of autologous RBCs may be more liberal than for allogeneic RBCs because of the lower risks associated with autologous blood.

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PLATELET TRANSFUSIONS

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Low platelet levels frequently do not lead to clinical signs. Thrombocytopenia is usually found on a routine complete blood count. If clinical signs are seen, they may include bleeding gums, nosebleeds, easy bruising, petechia, and purpura. Significant spontaneous bleeding does not usually occur until the platelet count falls below 5000/μL.

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