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.

Blood products

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.

Cryoprecipitate

A commonly held misconception is that a unit of cryoprecipitate contains more clotting factors than a unit of FFP. In reality, a unit of cryoprecipitate is actually prepared from a unit of FFP and therefore contains approximately the same amount (or less) of clotting factors (there is some loss of clotting factors during processing). The advantage of cryoprecipitate is that it is a concentrated source of von Willebrand's factor, fibrinogen, factors VIII and XIII. Units of cryoprecipitate are prepared by thawing FFP unit to temperatures between 0-6 °C, allowing a white precipitate to form, centrifuging the unit and extracting the plasma (referred to as cryo-poor plasma). The remaining portion is considered a unit of cryoprecipitate. The administration of vasopressin (DDAVP) to blood donors before blood collection may enhance the yield of von Willebrand's factor in the blood collected and therefore in the unit of cryoprecipitate made from that blood. By concentrating these factors, a patient can receive multiple units of cryoprecipitate without the risk of becoming volume overloaded. Cryoprecipitate is commonly used in dogs with von Willebrand's disease that require surgical interventions or are undergoing treatment of acute hemorrhage. Recommended dosage for cryoprecipitate is 1 unit per 10 kg body weight.

Platelet-containing blood products

Platelet-containing blood products include fresh whole blood, platelet-rich plasma, and platelet concentrates. By centrifuging whole blood at lower speeds, platelets can be extracted into the plasma with greater efficiency, yielding platelet-rich plasma. While the administration of platelet-containing blood products to animals undergoing hemorrhage as a result of severe thrombocytopenia would seem ideal, there are several logistical and practical factors which make such practice difficult. To maximize the viability of platelets, fresh whole blood must be collected and administered within 8 hours without refrigeration. This requires a practice to have blood donors readily available and the ability to collect and process blood expediently. While administration of platelet-containing blood products may control bleeding in some patients, it is unlikely to significantly increase platelet numbers. Actively hemorrhaging patients will require several units of platelet-containing blood products that make such practice cost-prohibitive. Patients that may benefit from fresh whole blood transfusions include severely thrombocytopenic patients requiring surgery (e.g., patients with splenic hemangiosarcoma), and patients developing dilutional coagulopathy as a result from massive fluid therapy. Administration of platelet-rich products to dogs with immune-mediated thrombocytopenia is likely unrewarding as it has been shown in people that platelets are destroyed within minutes in such patients. Platelet-containing blood products have also been administered to patients with neoplastic bone marrow diseases while undergoing either chemotherapy or radiation therapy. The efficacy of platelet transfusion in veterinary patients is unknown.

Whole blood

In many practice environments, whole blood is the only blood product available. It contains red blood cells, all of the clotting factors, plasma proteins, platelets, and anticoagulants. A typical unit of canine whole blood contains approximately 450-500 ml of blood + 63 ml of anticoagulant (citrate-phosphate-dextrose-adenine or CPDA-1). Dosage of whole blood to dogs range from 10 – 22 ml/kg.

For cats, a typical unit of whole blood contains 54 ml of blood and 6 ml of CPDA-1 or another anticoagulant such as acid-citrate-dextrose (ACD). Most blood transfusions administered to cats are whole blood transfusions, although some commercial veterinary blood banks are providing feline component blood products. Cats are commonly transfused a single unit, although some cats may require additional units. If multiple units of blood are administered to cats, careful monitoring for volume overload is required.

Synthetic blood substitutes

The introduction of a synthetic hemoglobin-based oxygen-carrying fluid (Oxyglobin) to the veterinary market has dramatically changed transfusion medicine. The use of Oxyglobin does not require blood typing or cross-matching, has a long shelf-life, and can be used in dogs or cats (not licensed). Disadvantages are high cost, inconsistent availability (currently), interference with biochemical analyzers, and a short in vivo half-life. Once removed from its light-protecting pouch, Oxyglobin should be administered within 24 hours. The dosages of Oxyglobin for dogs range from 10 – 30 ml/kg and 3 – 5 ml/kg for cats. The potent colloidal effects of Oxyglobin (COP 42 mm Hg) merit close monitoring of volume status in both dogs and especially cats being administered this blood substitute.

Blood types

A crucial aspect of transfusion medicine revolves around the safety of the recipient. It is useful to think of blood transfusion as a form of organ transplantation, where a tissue (blood) is infused from a donor into a new host and is subjected to the similar processes akin to tissue rejection. This is particularly important in administering blood to cats, where use of the incorrect blood type can result in death. Red blood cells of dogs and cats express specific markers or antigens, which define their blood type. In dogs, many different Dog Erythrocyte Antigens (DEA) have been identified. Of these, DEA 1.1 is considered to most important in respect to transfusion medicine. In cats, the recognized blood types include type A, type B, and type AB. The importance of these red cell antigens is that animals are usually tolerant of transfused cells expressing the same antigens they possess. Transfusion of cells expressing different red cell antigens can result in shortened life-span of red cells, acute hemolysis and even death. The degree of reaction is dependent on whether the recipient was primed against the transfused blood type previously or the presence of natural antibodies specific to the other blood type. The latter is particularly important in cats. Type A cats possess anti-type B antibodies, type B cats possess large quantities of anti-type A antibodies, and type AB cats, possess no antibodies against either red cell antigen. For this reason, all cats either receiving or donating blood should be typed and only receive type-specific blood, with the exception of AB-type cats, which tolerate transfusions from type A cats as well.

In dogs, administration of type-specific blood is more important from a resource management point-of-view than medical necessity. Of the many different recognized canine blood types, DEA 1.1, 1.3 and 7 are the only ones believed to play any significant roles, with DEA 1.1 being the most important. Most dogs either receiving or donating blood are screened for DEA 1.1 antigens. Blood units from dogs not expressing DEA 1.1 are highly sought because they can be administered to almost any dog, regardless of their blood type. Dogs that are DEA 1.1 positive are typically administered blood of the same type, but could also receive blood that is DEA 1.1 negative. Therefore, from a practical stance, DEA 1.1 negative blood is usually reserved for patients that are also DEA 1.1 negative.

Blood typing can be done by most diagnostic laboratories, but should also be available in most emergency and critical care settings, as blood administration may be required urgently. Commercially available blood-typing kits for dogs and cats have facilitated expedient on-site blood typing and enable emergency and critical care clinicians to use blood products expediently.

Crossmatching

While the administration of type-specific blood cells minimizes immediate and acute transfusion reactions, it does not prevent the sensitization of the recipient immune system to subsequent transfusion of cells expressing similar antigens. Within ten days of being transfused, the recipient's immune system will be programmed to react if ever exposed to similar antigens. For this reason, once a patient receives a blood transfusion, a cross-match is required for all subsequent transfusions after approximately 10 days following the first transfusion. Cross-matching allows the in vitro demonstration of gross incompatibility between donor and recipient. However, lack of gross agglutination on a cross-match does not guarantee that no transfusion reaction will occur.

Administration

Before the administration of any blood product, careful inspection of the product is absolutely critical. Unusual discoloration of the product should prompt close inspection and as a precautionary measure, discarded. The observation of a white or pink layer within whole blood or pRBC is indicative of lipemia and is safe for administration. The presence of blood clots does not necessarily preclude it from being administered as they are removed by the filter. Frozen units of plasma should be inspected for cracks in the packaging which may occur during thawing.

The most common route of administration of blood products is via an intravenous catheter. In neonates, transfusion of blood products can be safely administered through an intraosseous catheter or even directly into the peritoneal cavity. Preferably, blood should be administered through a short catheter because of decreased resistance. The use of intravenous sets designed for blood transfusions is necessary as the embedded filters remove macro- and microscopic blood clots and debris. The filter size typically used in veterinary medicine is 170 μm in size.

The recommended initial rate of blood delivery ranges from 1-4 ml/kg/hr and should be completed within 4 hours. Plasma transfusions may be administered at 4-6 ml/kg/hr. Blood products can be administered through some infusion pumps which have been approved specifically for blood transfusions.

Animals experiencing life threatening hemorrhage and believed to be in imminent danger of undergoing cardiac arrest, can be bolused several units of blood rapidly. This is considered a salvage procedure whereby the consequences of delaying the transfusion outweigh the risks of transfusion reactions. Oxyglobin can also be administered in similar fashion in such situations. The practice of auto-transfusion, whereby hemorrhaged blood from the patient is salvaged from a cavity (thorax or abdomen) utilizing special auto-transfusion equipment should be reserved for catastrophic situations, where no other alternative is available. This has been described in open-chest CPR situations and in hemorrhaging intra-operative patients.

Monitoring and complications

During the administration of blood products, close patient monitoring is essential. Minimally, heart rate, respiration rate and temperature should be measured at the onset of the transfusion, every fifteen minutes for the first hour and then on an hourly basis until complete. The occurrence of vomiting, diarrhea, urticaria, tachycardia, respiratory distress, or acute alterations in mentation is indicative of a transfusion reaction. Transfusion reactions occur as a result of both immunological and non-immunological processes. There are four classes of transfusion reactions described; these include acute and delayed forms of both immunologic and nonimmunological reactions. Acute transfusion reactions occur soon after commencing or soon after completion of the transfusion, while delayed transfusion reactions are noted hours after completion of the transfusion.

Of all transfusion reactions, acute immunologic reactions are the most devastating and may result in hemolysis, shock, and death. Less severe manifestations include fever, urticaria, restlessness, and vomiting. Efforts to reduce the risks of life threatening acute immunological reactions include administration of type specific blood and performing crossmatching procedures when indicated. Fortunately, the most common forms of transfusion reactions are not life threatening, especially if identified early. With frequent and adequate monitoring these complications can be dealt with and minimized. Any increase in body temperature prompt immediate evaluation of vital parameters including heart rate, pulse quality, respiration rate and effort, blood pressure, and neurological status. If all other parameters are normal, the rate of infusion can be slowed and the patient should be closely monitored. In the event of other abnormalities or the progression of fever, the transfusion should be discontinued and the unit evaluated for possible contamination (bacterial culture, gram stain, etc). The patient is also evaluated for evidence of hemolysis and treated aggressively with supportive measures, including resuscitative efforts and short-acting corticosteroids. The use of corticosteroids is controversial because the lack of efficacy information, however, its use is widely recommended. The development of urticaria is usually treated with short-acting corticosteroids and anti-histamines. The practice of administering anti-histamines before blood transfusions has not been shown to effectively prevent transfusion reactions, but could exacerbate hypotension in critically ill patients. Therefore, this practice is not recommended.