Hemodilution and Blood Conservation

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.