Canine and feline transfusion medicine (Proceedings)

Transfusion of blood products is a frequent necessity in small animal practice. Although potentially life-saving, this procedure does carry some inherent risk. To practice successful transfusion medicine the clinician must understand not only when and how to administer the appropriate blood product; but also how to collect and store these products to minimize the potential for an adverse outcome.

Blood components

The plethora of blood products, fractions, and concentrates now available for transfusion may seem confusing. However, it is rational to use a component wherever possible as this practice not only reduces the element of risk, but is also a more efficient use of valuable resources.

Canine transfusions are commonly provided as components; packed red blood cells (pRBCs) and fresh-frozen plasma (FFP). Each unit (~450 mL) of whole blood, if processed promptly, may be divided into 1 unit of pRBCs and 1 unit of FFP. Fresh frozen plasma represents plasma that has been separated and frozen within 6 hours of collection. It has a shelf-life of 1 year, after which it is stable for an additional 4 years as frozen plasma. Fresh frozen plasma may also be divided into cryoprecipitate and cryo-poor plasma. Cryoprecipitate contains increased concentrations of factor VIII, von Willebrand factor (vWF), and fibrinogen. Fresh whole blood may also be processed into platelet-rich plasma or platelet concentrates and typically has a half-life of 30-40 days.

Collecting blood for feline transfusions is inherently more challenging than for canine blood transfusions. Cats' blood volume is less than dogs' and donation typically requires sedation. Though cat blood transfusions are still commonly administered as whole blood, units can be separated into pRBCs and FFP components and are commercially available.

Blood banking

Blood products may be purchased from commercial blood banks or collected and processed in-house before emergent need. The once common practice of bleeding a staff member's personal dog or an in-house donor on an as-needed basis is not appropriate for most emergency practices because of time constraints and the manpower required.

In dogs, whole blood is collected into bags containing citrate–phosphate-dextrose–adenine (CPDA-1) as the anticoagulant preservative. These bags provide a closed system for collection and separation, which minimizes the opportunity for bacterial contamination. Each bag is designed to collect 450 mL. Sixty-three milliliters of anticoagulant is located in the main collection bag. The storage time for canine packed red blood cells (pRBCs) collected in CPDA-1 has been suggested to be 20 days. This short lifespan is not a problem when the product collected is immediately used, but it does present a significant problem when products are banked for later use. Additive solutions for the preservation of RBCs are available in closed-collection systems used in dogs. Both Adsol and Nutricel have been evaluated as additive solutions for canine pRBC storage. Adsol extended viability from 20 to 37 days, and Nutricel extended viability to 35 days.

Cat blood is most commonly collected into a syringe through a butterfly catheter. The anticoagulant must be added before collection. Anticoagulants used most commonly include acid–citrate–dextrose (ACD) and CPDA-1. The ACD and CPDA-1 are used at a ratio of 1 mL per 6-9 mL of whole blood. Typically, feline whole-blood units contain a total volume of 60 mL (7 mL of anticoagulant and 53 mL of whole blood).

Selection of donors

Donor dogs should weigh at least 50 pounds, be 1-7 years of age, have a good personality, allow restraint, and have no history of heart murmur or seizure. A health screening should be performed on all potential donor dogs that meet the physical requirements. The health screening should include but is not limited to a physical exam, complete blood count, platelet count, chemistry profile, vWF assay, infectious disease screen (Babesia canis, Babesia gibsonii, Ehrlichia spp., Anaplasma phagocytophilum, Neorickettsia risticii, Leishmania donvanii, Bartonella vinsonii), heartworm antigen test, and Brucellosis titer (if intact). All donors should be currently vaccinated for canine distemper, parvovirus, Leptospirosis, and rabies. Potential donors should not be on medications other than those that treat heartworm and prevent fleas and neither endo nor ectoparasites should be present at the time of donation. Before admission to a donor program, blood typing should be sent to a reference laboratory for a full dog erythrocyte antigen (DEA) assay.

Donor cats should weigh at least 10 pounds, be between 1 and 7 years old, have no history of heart murmur or seizure, have good jugular vein access, and have a good personality. Regardless of temperament, most cats will still require some form of sedation or anesthesia during the donation process as any sudden movement during collection can render the product useless. A physical exam, echocardiogram, complete blood count, platelet count, and chemistry profile testing should be performed on all cats. Retroviral testing for feline leukemia virus antigen and feline immunodeficiency virus antibody should be performed on at least 2 separate occasions for each donor to confirm retroviral status. Infectious disease screening for Bartonella spp., Mycoplasma hemofelis (previously Hemobartonella) and Mycoplasma haeminimutum (previously the California strain of Hemobartonella) should be included. In endemic areas, heartworm antigen and/or antibody testing should be considered. Because of infectious disease concerns, donors ideally should be housed exclusively indoors and be kept free of both endo and ectoparasites. Cats should be vaccinated against respiratory complex viruses, panleukopenia and rabies. Blood-typing requirements include a full feline erythrocyte antigen assay.

Blood types

Blood types simply represent proteins present on the surface of the RBC. Similar to any antigenic stimulus, it is thus possible for animals to have or develop antibodies to blood types that are different from their own. Interaction between these antibodies and the red cells form the basis of immunological transfusion reactions. Luckily, dogs do not have preformed antibodies to other blood types therefore they are highly unlikely to have a transfusion reaction on a first transfusion. Dogs have at least 12 blood groups the most important of which are DEA 1.1 and 1.2. The use of an in-house typing card for donors can be used to designate DEA 1.1 status. Remember that the purpose of blood typing a donor–recipient pair is to most closely match the DEA type and to reduce the risk for acute or delayed transfusion reaction.

Cats have two major blood groups (A and B). Cats also have preformed circulating antibodies against the other distinct blood type. If cats are not given the appropriate blood type, there is a significant risk of a serious or even fatal transfusion reaction. Therefore it is imperative that ALL donor cats be blood typed prior to donation and that ALL recipient cats be blood typed prior to transfusion. An in-house typing card is a good screening test; however, a reference laboratory is recommended to confirm all AB-type donors.

Crossmatching

Crossmatch is recommended before any RBC transfusion in any species, and is imperative if there is a history of exposure to blood products. Even if blood type specific components are transfused, crossmatching is recommended because not all RBC antigen groups have been fully characterized. The major crossmatch compares donor RBCs with recipient serum; the minor compares recipient RBCs with donor serum. Incompatibility is seen as hemolysis or agglutination. An apparently compatible crossmatch should be checked microscopically for agglutination. The major crossmatch should always be compatible, although the minor crossmatch is of less importance. Cross matching is done to minimize the chance of transfusion reaction; it also helps decrease the risk of sensitizing patients that might require more than one transfusion. Even when a crossmatch is compatible prior to a transfusion, reactions can still occur. The crossmatch does not include assessment of white blood cells or platelets; cells that are the main source of many immediate transfusion reactions. Cross-match kits and procedures are available from most of the commercial blood banks.

Transfusion reactions

Acute hemolytic transfusion reactions are associated with antibodies directed against RBC antigens and are classified as type II hypersensitivities. The severity and timing of an acute immune-mediated hemolytic transfusion reaction depends on the antibody class involved (IgG, IgM), the temperature at which these antibodies bind to cell-surface antigen, and the degree of complement fixation. Clinical signs include hypotension, tachycardia, tachypnea, pyrexia, vomiting, and even death. Acute intravascular hemolysis results in hemoglobinemia and hemoglobinuria. If an acute hemolytic transfusion reaction is suspected, the transfusion should immediately be stopped and the patient supported with intravenous crystalloid fluid administration and other treatments as indicated by the patient's condition.

Naturally occurring antibodies to erythrocyte antigens 1.1 and 1.2 are extremely rare in dogs, and therefore the likelihood of acute hemolytic transfusion reaction on initial transfusion is unlikely. Dogs that receive a transfusion or serial transfusions with more than 3 days between transfusions require cross-matching to minimize the likelihood of acute immunologic transfusion reaction.

Cats do have preformed, circulating RBC isoantibodies against whatever cat erythrocyte antigen they lack, resulting in blood types A, B, and AB. The administration of type-specific transfusion nearly eliminates the risk of acute hemolytic transfusion reaction for cats receiving their first transfusion.

Acute, febrile nonhemolytic transfusion reactions result from immune-mediated reactions against the donor leukocytes or platelets. Clinically, it is defined when an increase in body temperature of at least 1°C is observed, with no other cause of fever found. When patient antibodies bind to donor platelets, leukocytes, or plasma proteins, an antigen–antibody reaction releases endogenous mediators (e.g., interleukin-1) and causes pyrexia. This acute immunologic reaction occurs within 30 minutes and can continue for up to 20 hours. Vomiting and tachypnea may be observed. The transfusion should be discontinued, and may be restarted at a slower rate.

Acute hypersensitivity reactions that are anaphylactic (response of patient antibodies against donor immunoglobulin A) or allergic (mediated by immunoglobulin E antibodies) are classified as type I hypersensitivities. These stimulate mast cells to produce vasoactive substances that cause urticaria (response of antibody to donor plasma proteins) and pruritus. These reactions are most commonly associated with the transfusion of plasma products. They usually occur within the first 45 minutes, and are treated by discontinuing the transfusion and administering antihistamines and or steroids.

Transfusion related acute lung injury (TRALI) has been associated with the presence of antibodies in the donor plasma reactive to recipient leukocyte antigens or with the production of inflammatory mediators during storage of cellular blood components. It is characterized by abrupt onset of noncardiogenic pulmonary edema. Severe cases may require assisted ventilation with a high fraction of inspired oxygen.

Delayed immunologic transfusion reactions occur in patients who have developed antibodies from previous transfusion. Delayed hemolysis may occur when at the time of pretransfusion testing; the antibody in question is too weak to be detected. Subsequent transfusion with red cells having the corresponding antigen results in an anamnestic antibody response and hemolysis of the donor's cells. Neonatal isoerythrolysis may occur when antibodies generated by the bitch (queen) are transferred via colostrums to puppies (kittens) positive for the corresponding antigen; the end result is neonatal hemolytic anemia (isoerythrolysis), occurring after the initial days of nursing colostrums. Another delayed immunologic reaction is post transfusion purpura caused by antibodies from previous transfusions against recipient platelets. This reaction occurs approximately 1 week after the transfusion, can persist for as long as 2 months, and is usually self-limiting.

Acute nonimmunologic reactions can also occur following transfusion. Hypocalcemia may occur resulting from the citrate used in the anticoagulant. Embolism risk from clots in the transfusion product and can be minimized with the appropriate use of an in-line filter during every transfusion. Circulatory overload may occur with rapid administration of fluid in a euvolemic patient. Others include bacterial infection from contaminated blood and hemolysis from physical or thermal damage to the red cells.

Delayed nonimmunologic reactions such as disease transmission can occur, especially in immuno-compromised patients.

Administration

Prior to administration of any blood product check to be certain the correct component and type is being transfused and that the product is a normal color and consistency. All blood products should be administered through a 270 µm filter. The filter facilitates removal of small clots and other debris that may be present. It can be an integral part of the blood administration set or attached separately, as when using a syringe for the transfusion (common with cats). Peristaltic type infusion pumps may be used safely however some forms of fluid pump may damage the red cells; if in doubt the manufacturer should be contacted to check the pump's compatibility with blood products. With stable patients, an initial infusion rate of 0.25-0.5 ml/kg/hr should be used for the first 15-30 minutes. During this time the patient should be monitored for any evidence of a transfusion reaction. As long as no problems are identified the rest of the unit should be delivered over no more than 4 hours dependent on the animal's intravascular volume status. In an emergency (e.g. severe acute hemorrhage), blood products can be given as fast as necessary.

Clinical use of blood products

Anemia is one of the most common reasons for transfusion in veterinary medicine. Patients with anemia resulting from many causes including hemorrhage, intravascular hemolysis and bone marrow disease will benefit from additional red cells. In patients with a normal intravascular volume (commonly those with intravascular hemolysis, and non regenerative anemia), packed red cells are the best choice, whereas in patients with blood loss (especially those that are also coagulopathic) fresh whole blood is optimal. It is not possible to give a precise PCV where transfusion becomes necessary. Factors to take into consideration other than simply the PCV include the rapidity of onset of the anemia, the clinical signs of anemia that the patient is displaying and whether the red cell loss is ongoing. With that being said, a PCV less than 15% is nearly always an indication for transfusion of RBCs. When administering whole blood, 20 mL/kg will raise the PCV by ~10%. When administering packed red blood cells, 10 mL/kg will raise the PCV by ~10%.

Coagulopathy is another common reason for transfusion. It is necessary to have some understanding of the mechanism of the coagulopathy to determine the optimal transfusion product. Fresh whole blood and fresh frozen plasma contain all the clotting factors and thus are suitable for most coagulopathy. Fresh whole blood is preferred if the patient is anemic from concurrent hemorrhage whereas plasma is preferred if the patient is not anemic. The use of FFP allows delivery of large amounts of clotting factors with less risk of volume overloading the animal with red cells that it does not need. It should be remembered that in patients with hemorrhage from severe thrombocytopenia, transfusions are principally used to support the red cell count as the number of functional platelets delivered even in fresh whole blood is very low. Examples of congenital factor deficiencies include vWF, hemophilia A (factor VIII), and hemophilia B (factor IX). In patients bleeding from vWF deficiency or hemophilia A, large volumes of vWF and factor VIII may be needed. Cryoprecipitate (12 to 20 mL/kg every 10 to 12 hours or 1 unit per 10 kg of body weight until the active bleeding stops) is the most appropriate product to allow delivery of large amounts of vWF and factor VIII in a small volume of fluid. Hemophilia B can be similarly treated with cryopoor plasma. Common causes of acquired factor deficiencies include liver failure, warfarin toxicity, and disseminated intravascular coagulopathy. For liver failure FFP is ideal whereas rodenticide toxicity can be treated efficiently with frozen plasma as the vitamin K dependent factors (II, VII, IX, and X) are maintained.

Fresh frozen plasma may be useful in treating patients with sepsis or systemic inflammatory response syndrome. The plasma may contain a number of beneficial substances including the clotting factors, antithrombotic proteins, inhibitors of circulating proteases and albumin. The use of plasma as a specific source of albumin has been abandoned in human medicine. This is because of the availability of other supplements (i.e. hydroxyethyl starch, dextran, and human albumin solution). The dose of plasma recommended for albumin supplementation is considerably higher than that for coagulation factor replacement (45 mL/kg vs. 10-20 mL/kg). This dose should increase the serum albumin concentration by ~1 g/dL. Given the limited availability of plasma, other colloidal solutions should probably be the first line of treatment in hypoalbuminemic patients with no evidence of coagulation abnormalities.

Oxyglobin (Biopure) increases plasma and total hemoglobin concentration, which in turn increases arterial oxygen content. Because it is made only from hemoglobin there is less risk of infectious disease transmission and no need for blood-type determination or cross-matching. The shelf life is up to 3 years at room temperature and it is readily available. Oxyglobin has potent colloid and vasoactive properties. Because the colloid oncotic pressure (37 torr) is higher than hetastarch, it causes greater retention of fluid within the vascular space. The plasma half-life is 30 to 40 hours. The initial dose recommended for dogs is 10 to 15 mL/kg (with a maximum rate of 5 mL/kg/h). Because of an increased risk of volume overload in cats the lower initial dose used is 5 mL/kg. It can be repeated with careful monitoring. Possible adverse effects include antigenicity and volume overload. Unfortunately, many colorimetric and optical tests are invalid, as well as urine dipsticks, after Oxyglobin administration.

Autotransfusion

Autotransfusion is the process of collecting autologous blood after a bleeding episode, filtering it and infusing it back into the donor. It has a number of advantages and disadvantages that need to be taken into consideration before administration. The biggest advantage is a ready source of compatible blood that can be given quickly and inexpensively, without the need for typing, cross-matching, or infectious disease screening. Disadvantages include the potential for hemolysis, coagulopathy, contamination, and acute lung injury. Autotransfused blood (ATB) is subject to hemolysis resulting from prolonged contact with serosal surfaces and from mechanical trauma incurred during the collection process. Coagulopathy may also be a concern because pleural or peritoneal contact results in reduced platelet and clotting-factor levels that may lead to dilutional coagulopathy when large volumes are reinfused. Additionally, because ATB may contain large amounts of fibrin degradation products, RBC fragments, activated leukocytes, platelets, and inflammatory mediators; it may initiate coagulation, exacerbate consumptive coagulopathy and lead to DIC. Acute lung injury (ALI) is another potential complication of autotransfusion. ALI is believed to result from microembolization of cellular aggregates, fat, and protein to the pulmonary vasculature. For this reason, the use of micropore filters (20–40 µm) has been recommended when ATB is administered. Additionally ATB has the potential to spread neoplasia if the shed blood contains tumor tissue or cells, and for contamination with bacteria if gastrointestinal perforation has occurred. For these reasons, ATB should not be used in these situations. Autotransfusion is indicated only when there is active bleeding into a major body cavity AND no other source of RBCs is available.