Definition, Recognition, and Risk Assessment

Complications arising from the transfusion of homologous (also called allogeneic) blood products have been recognized since the beginning of modern transfusion therapy. Bacterial blood contamination was fairly common before the introduction of refrigerated storage and sterile plastic bags. Subsequently, contamination with viruses (e.g., cytomegalovirus, hepatitis B and C, HIV, and human T-cell lymphotropic virus) became a source of greater morbidity. Now, West Nile virus and possibly variant Creutzfeldt-Jakob disease have been added to the list of viral diseases transmissible by blood transfusion. Fortunately, improvements in donor screening and blood component testing have reduced the risk of both HIV and hepatitis C transmission to less than 1 per 1 million units, and that for hepatitis B to about 1 per 137,000 units. Cytomegalovirus remains prevalent in the blood pool, but its transmission is generally not a problem in the absence of clinical immunosuppression. Nevertheless, many blood banks now routinely apply leukoreduction techniques to all cellular blood components before dispensing them, which has greatly reduced the risk of cytomegalovirus transmission. Thus viral transmission by blood transfusion is now so rare that bacterial contamination once again poses the highest risk for infectious complications, which is 1 in 30,000 red blood cell (RBC) units and 1 in 2000 to 3000 platelet units, although transmission of actual clinical infection rates are substantially lower than that (approximately 1 in 5 million for RBC units and 1 in 100,000 platelet units). Blood group incompatibility and anaphylactic reactions remain rare.

Implications

Considerable evidence supports immunosuppression as a significant consequence of blood transfusion. This increases the risk of cancer recurrence and of bacterial infection among transfusion recipients.

Large blood loss and hemodilution also raise the question of what constitutes a reasonable minimum hemoglobin level in an anesthetized patient with acceptable intravascular volume and vital signs. This is a surprisingly complex issue, but in general, healthy patients safely tolerate hemoglobin concentrations as low as 6 g/dL. Sicker patients may require hemoglobin concentrations as high as 10 g/dL.

Assuming that the hypothetical patient described in the case synopsis is otherwise healthy, the limiting factor may be the rate and predictability of blood loss, because some margin of safety is desirable if sudden additional blood loss should occur. Also, one must consider the possibility of significant postoperative bleeding. Consequently, the patient’s hemoglobin concentration of 7.5 g/dL signals the possible need for homologous transfusion, unless shed blood is being effectively salvaged.

Autologous Predonation

Patients can donate blood up to 42 days before operation, which constitutes the maximum storage period for modern anticoagulant and storage solutions. The frequency and amount of donation depend on the patient’s ability to tolerate serial phlebotomy while maintaining an adequate hemoglobin level. Typically, a patient donates 2 units of blood per week starting 2 to 4 weeks before surgery. The minimum recommended hemoglobin level for donation is 11 g/dL. To maintain this level, patients are routinely given iron supplementation. Erythropoietin can be used to increase hemoglobin levels during predonation, which enables patients to donate more units; this is expensive, however, costing approximately $800 per unit of erythropoietin “manufactured.” Erythropoietin augmentation of autologous predonation may be justified if some combination of the following factors exists:

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  The preoperative timeline is short (e.g., cancer resection).

  
  
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  Homologous transfusion is not possible (e.g., Jehovah’s Witness).

  
  
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  The patient is anemic.

  
  
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  The anticipated surgical blood loss is large (>2000 mL).

Autologous predonation is most effective at avoiding homologous transfusion when used in combination with other autotransfusion techniques, such as intraoperative blood salvage. The cost-effectiveness of autologous donation varies widely, but it often fails to meet the usual standards of efficacy. For this reason, its popularity has waned. The donation itself carries a hospitalization risk of approximately 1 in 17,000, which is 12 times that for community donations by healthy individuals. Even though the blood is autologous, its use still incurs some of the usual homologous transfusion risks, including bacterial contamination or clerical errors leading to incompatible blood transfusions. Compared with allogeneic blood units, autologous units typically require the same testing procedures but more complex storage and identification procedures, so the cost for each unit is higher.

Acute Normovolemic Hemodilution

Acute normovolemic hemodilution (ANH) involves the removal of blood just before or after the induction of anesthesia, combined with volume replacement using crystalloid or colloid. The technique requires standard anesthesia monitors (electrocardiogram, blood pressure, pulse oximetry, and temperature) and large-bore intravenous access with a 14- or 16-gauge peripheral or central venous catheter. Blood is collected into standard citrate-phosphate-dextrose bags. Collected blood can then safely be stored at room temperature for up to 8 hours or frozen if not transfused. Whole blood stored at room temperature should be constantly agitated (placed on shaker) to ensure platelet function.

The rationale for ANH is that the patient will be losing fewer RBCs into the surgical field because shed blood has a lower hematocrit due to hemodilution. Assuming that the lowest hematocrit remains acceptable (>20%) and that intravascular volume also remains intact, tissue perfusion will be maintained (and perhaps enhanced). Also, oxygen delivery will be sufficient owing to reduced blood viscosity. Additional clinical advantages include low cost, simple storage, and ease of transportation and record keeping.

Acute normovolemic hemodilution risks hypovolemia if volume replacement is inadequate. Further, the obligatory drop in hemoglobin concentration could induce unanticipated end-organ ischemia if there is an undiagnosed condition such as critical stenosis of a coronary artery or carotid artery. Mathematical analyses strongly suggest that the blood loss savings are fairly minor unless this technique is used quite aggressively—for example, hemodilution from a starting hematocrit of 40% to one of 20% or lower. Typically, this would require withdrawing 6 to 10 500-mL bags of blood. One study found no difference in allogeneic transfusion exposure when 3 units of acute normovolemic hemodilution were compared with a similar volume of autologous predonation in patients undergoing total hip arthroplasty.

Postoperative Blood Salvage

This technique involves the collection and reinfusion of blood shed postoperatively. The blood is collected through a relatively large filter and reinfused through a small-pore filter. This blood can be reinfused unmodified (“unwashed”), or it can be washed and concentrated in the same way as for intraoperative blood salvage.

Reinfused blood typically contains very low concentrations of plasma coagulation factors and platelets. It also contains elevated levels of fibrin degradation products, free hemoglobin, and inflammatory products such as cytokines. With total hip arthroplasty, it might also contain fat and bone spicules. As a result, many clinicians elect to administer salvaged blood only after it has been washed. This somewhat controversial technique reduces the need for allogeneic blood only when postoperative blood losses are large (e.g., >1000 mL), because postoperatively shed blood typically has a hematocrit of 15% to 20%.

Intraoperative Blood Salvage

This method involves using a suction apparatus to collect the patient’s blood as it is shed intraoperatively into the surgical field. An anticoagulant solution is added to the shed blood, and it is then stored in a filtered reservoir. Once an adequate amount of blood has been collected (typically >700 mL), it is washed and concentrated so that the final product usually has a hematocrit between 55% and 70%.

Because intraoperative blood salvage conserves RBCs but not plasma or platelets, a dilutional coagulopathy should be anticipated if blood losses approach or exceed one blood volume. Otherwise, the risks of this technique are low if appropriate procedures and standards are followed and the blood is not contaminated with bacteria. The ability to conserve RBCs with this technique depends largely on the surgeon’s ability to capture shed blood using suction. In this regard, total hip arthroplasty is in an intermediate category between laparotomy for aortic aneurysm repair, where blood pools in a body cavity and is easily captured, and a more superficial procedure such as reduction mammoplasty, where blood typically runs off the surgical field onto the drapes or is absorbed by sponges.