Practical transfusion therapy (Proceedings)
The transfusion of blood products to treat acute blood losses, coagulopathies, and severe anemia has become indispensable in the care of critically ill veterinary patients. As with any therapy, the risks, cost and potential benefits associated with the use of blood products must be carefully considered and every effort should be made to minimize the occurrence of adverse effects. The critical care clinician should be familiar with the various forms of blood products available, their indications, proper use, and their potential side effects. As the importance of blood products continues to grow as an essential part of the therapeutic regimen available to treat critically ill patients, clinicians should strive to incorporate the various transfusion modalities in their practice environment.
An important advancement in transfusion medicine was the recognition that not all patients required whole blood to treat their condition. As a matter of practicality, administration of whole blood to a patient that only requires only plasma or packed red blood cells (pRBC) is probably not harmful but could be considered wasteful. As techniques designed to extend the viability of blood products were developed further, the concept of blood component therapy was born. This not only resulted in improving resource management practices, but also diminished unnecessary risks to patients receiving blood products. By administering only the desired portion of blood, patients are not subjected to the possible complications of receiving the other components. Additionally, each unit collected may help more than one animal.The common blood products available for transfusion to veterinary patients include packed red blood cells (pRBC), fresh frozen plasma (FFP), frozen plasma (FP), cryoprecipitate, fresh whole blood, and synthetic blood substitutes such as Oxyglobin. The following discussion describes the different components and their indications for use.
Packed red blood cells
Commercial veterinary blood banks routinely supply units of pRBCs that are prepared by extracting most of the plasma and its associated clotting factors. The resultant product contains red blood cells and a small amount of plasma and anticoagulant. A full unit of canine pRBC is approximately 200-250 ml, and contains the same oxygen carrying capacity as 1 unit of whole blood (450 ml). Because pRBC contains only a small amount of plasma proteins, its colloid osmotic pressure (COP) is approximately 5 mm Hg. This relatively low COP, as compared to whole blood (20 mm Hg), makes pRBC a reasonable choice for transfusing anemic, normovolemic animals (e.g., dogs with hemolytic anemia, non-regenerative anemia). The recommended dosages for transfusions using pRBC range from 6 -10 ml/kg.
Some commercial veterinary blood banks are now supplying feline units of pRBC. The typical feline unit of pRBC contains 30 – 35 ml. Cats with a limited capacity to tolerate large volumes of fluids (e.g., cats with heart disease) but in need of transfusion may benefit from the administration of pRBC rather than whole blood.
Fresh frozen plasma
From a unit of whole blood, plasma is extracted after refrigerated centrifugation. Fresh plasma contains all clotting factors, and albumin. If immediately frozen at -30°C, all clotting factors retain their activity for 1 year. The main indications for use of FFP include inherited and acquired coagulopathies. Animals demonstrating prolonged clotting times and expected to undergo invasive diagnostics i.e., liver biopsies, should receive particular consideration for FFP transfusion. The use of FFP to specifically treat hypoalbuminemia is impractical for several reasons. Increasing a patient's serum albumin concentration by 1 g/dl may require the administration of as much as 45 ml/kg of FFP. With the exception of very small patients, this would be cost prohibitive. While FFP has a COP of approximately 20 mm Hg, increasing a patient's COP would also require large volumes and the efficacy of such therapy is unknown. The use of FFP for resuscitation of hypovolemic patients is controversial and also currently not recommended. For the treatment of coagulopathies, FFP is administered at a starting dose of 10 ml/kg. This however should serve only as a guideline and some patients may require greater amounts of FFP.
Another proposed use of FFP is to replace depleted concentrations of antithrombin (AT) to patients with conditions such as pancreatitis and disseminated intravascular coagulation (DIC). Unfortunately, studies have not supported the efficacy of such measures in improving outcomes in human patients. Even when AT concentrates are used, administering supraphysiological amounts of AT have not significantly improved outcome. In light of these findings, there is little support for using FFP to treat these conditions in veterinary patients.
On occasion, a unit of FFP is inadvertently thawed for a patient but not used. If this unit is refrozen it is referred to as "frozen plasma." The activity of all clotting factors except for factors V and VIII are preserved in frozen plasma. Plasma obtained from centrifuging stored whole blood also loses the labile clotting factors and therefore used for making frozen plasma. The use of frozen plasma for treating patients with anticoagulant rodenticide toxicity is adequate and effective. Patients with hemophilia or von Willebrand's disease may not be adequately treated using frozen plasma. Some commercial veterinary blood banks are now supplying feline units of plasma. If frozen, these units can be stored for up to a year and are also referred to as frozen plasma. The typical feline plasma unit is approximately 20 ml.