Transfusion Medicine

Over the last 25 years, blood transfusions given to ill or injured patients in veterinary medicine have evolved from a relatively rare occurrence to a routine and frequently life-saving practice [1-6]. It is impossible to contemplate a successful surgeon and surgery team not having a strong knowledge base in transfusion medicine and easy access to blood products. The purposes of this chapter are to outline the physiology of blood and plasma, and to describe blood groups, methods of collection, blood components, indications for transfusion, methods of administration and monitoring during transfusions, and transfusion reactions.

Physiology of Blood and Plasma

The purpose of red blood cells is efficiently to carry oxygen to support oxygen delivery to the tissues. Red blood cells are highly evolved to be exceedingly efficient at oxygen transport by saturation of the hemoglobin (Hb) molecule with oxygen absorbed at the interface between the alveolar and pulmonary capillaries. Oxygenated hemoglobin contains approximately 1.34 ml of oxygen per gram of Hb. The oxygen content of the blood is dependant primarily on the oxygen saturation of Hb, but also, to a far lesser extent, on the dissolved partial pressure of oxygen. The oxygen content of blood can be calculated (Table 3-1).

However, the oxygen content alone is only part of the equation of successful oxygen delivery. Oxygen delivery to the tissues is also dependant on cardiac output (CO) which is the product of stroke volume (SV) and heart rate. Thus, in an anemic animal the attempt to improve oxygen delivery by increasing cardiac output is by increasing rate and/or stroke volume. This can result in tachycardia and "bounding pulses". Bounding pulses associated with anemia are a result of a greater pulse pressure (systolic - diastolic), which is usually associated with either increased systolic pressures because of increased stroke volume or from diastolic run-off if concurrent hypovolemia is present. Tachycardia becomes an important branch in decision-making about a transfused pet. In some cases, dogs will have a normal heart rate while resting, but develop a marked tachycardia (e.g., 80 bpm to 180 bpm) upon standing or being encouraged to walk, because oxygen delivery to the tissues is limited and the cardiovascular system attempts to compensate for tissue hypoxemia.

The cellular components of the blood also include white blood cells and platelets. White blood cells have a variety of roles, primarily involving modulation of the immune response and defense against infection. Because of the limited survival times of white blood cells, low white blood cell counts are not treated with transfusion and, in some cases, removal of the white cells (leukoreduction) has been advocated as a method of limiting immune response associated with transfusion [7].

Platelets are involved in primary hemostasis. Thrombocytopenia is common in animals with anemia. Platelet-rich plasma or platelet concentrates may be transfused in dogs. However, if thrombocytopenia is secondary to immune-mediated destruction, the transfused platelets will be destroyed within a matter of minutes [8]. The plasma component of blood acts to reduce overall viscosity in addition to containing plasma proteins (albumin and globulin), antithrombin, electrolytes and glucose, pro- and anticoagulant factors, hormones, and a multitude of other factors.

Blood Groups and Blood Type

Dogs have many different blood groups, which are defined by glycolipid and glycoprotein antigens found on the red blood cell surface [9]. These are named dog erythrocyte antigens (DEA). Up to 9 antigens have been recognized within this system (DEA 1.1, 1.2, 1.3, 3, 4, 5, 6, 7, and 8). Not all antigens are considered equally important in regard to their ability to trigger a transfusion reaction. DEA 1.1 is the most likely to trigger an acute hemolytic transfusion reaction in dogs. Forty to 50 % of dogs are positive for DEA 1.1. A DEA 1.1-negative dog that has been previously sensitized with DEA 1.1 positive red blood cells may develop an acute hemolytic transfusion reaction on subsequent exposure to DEA 1.1-positive red blood cells. DEA 1.2 is also capable of eliciting an acute reaction in sensitized dogs. DEA 4 is the most prevalent of canine blood types, occurring in up to 98% of dogs. While it was previously believed that DEA 4 did not play a role in transfusion reactions, severe hemolytic reactions have been described [10]. Fortunately, transfusion reactions are rare in dogs that have not previously been transfused or that have had puppies. Dogs are unlikely to have preformed antibodies against other determinants. It has been suggested that some DEA 7-negative dogs have preformed antibodies against the DEA 7 antigen, although the antibody is weak. Sensitized DEA 7-negative dogs that are transfused with DEA 7-positive blood experience a delayed transfusion reaction where the transfused red blood cells are lost in 72 hours. Blood typing of both the recipient and the donor dog can be performed at a variety of laboratories and also with commercial blood typing cards (Appendix 3-1). Although blood typing of a recipient dog prior to transfusion is ideal, in an emergency situation it is unlikely that a transfusion will result in a reaction for the first-time recipient, because of the lack of preformed antibodies. Blood typing of both the donors and the recipients helps to maintain a better blood bank by controlling the resources. Blood typing antisera are available for DEA 1.1, 1.2, 3, 4, 5, and 7, although most commercial blood banks and laboratories type for DEA 1.1 only, as it is the most antigenic. In an animal that has been previously transfused or in which the transfusion history is unknown, cross-matching prior to the initial transfusion is prudent to reduce the risk of a reaction.

The cat blood group system has classically been made up of A, B, and AB types [2,11], however, the existence of a new red blood cell antigen (MIK) has been described in a domestic short hair cat [12]. Cats, unlike dogs, have naturally occurring alloantibodies to blood groups other than their own, even without previous transfusion. Type A is the overwhelmingly prevalent type in domestic mixed-breed cats in the United States. Some purebred cats (notably the Devon Rex and British Shorthair) have a high incidence of type B blood. If a cat with type A is given type B blood, the transfusion will typically be hemolyzed within 48 hours; however, if a cat with type B is given type A blood, a fatal transfusion reaction is likely to occur. Thus, all recipient cats should be typed or cross-matched prior to transfusion. In cats requiring multiple transfusions, cross-matching may be warranted each time, particularly in cats that have had prior transfusion reactions.

Blood Collection and Processing

Although historically in veterinary medicine, fresh whole blood was collected as needed from either clinic animals or pets belonging to the staff of veterinary hospitals, this is considered antiquated and is not advised if it is possible to avoid. Blood banking is commonly performed in the larger veterinary hospitals. Several large commercial blood banks for animals also exist (Appendix 3-1). The purpose of a blood bank is to provide a readily available source of packed red blood cells or plasma for use in hospitalized patients. Most veterinary blood banks rely on client and staff-owned animals. Pre-donation screening is strongly recommended for the donors. Typically, screening involves a physical examination, a complete blood count, and chemistry profile. Species-specific screening for cats includes testing for feline leukemia and feline immunodeficiency viruses, and for dogs, heartworm testing. In some geographic locations, testing for exposure to tick-borne diseases such as Erhlichia is also recommended. The American College of Veterinary Internal Medicine has published a consensus statement concerning the recommended pre-testing of animal donors [13].

The ideal feline donor is a large (more than 8- to 10-pound) well mannered adult cat. Most if not all cats require some sedation for donation. Frequently used sedatives include ketamine, narcotics, and diazepam. Feline blood donors should be carefully ausculted prior to sedation, as occult cardiomyopathies are not rare and could contribute to morbidity or mortality for the donor cat. Approximately 50 ml of whole blood may be collected from a donor cat. Blood may be collected as frequently as every 4 weeks, although with pet cats the donation interval is typically 2 to 3 months. The blood may be anticoagulated with heparin if it is to be used immediately, or with citrate-phosphate-dextrose-adenine (CPDA-1) for storage. Cat blood is often transfused as either fresh or stored whole blood, although interest has been growing in the use of components in cats as well (Fig. 3.1).

Figure 3.1. A cat following collection of one unit of fresh whole blood.

The canine blood donor is typically a large breed (more than than 50 pounds) well mannered adult dog. Unlike cats, typically dogs do not require any sedation (Fig. 3-2). Approximately 1 unit (450 ml) may be collected from each donor. Generally, canine blood is collected into bags containing CPDA-1 designed for human blood donation. Canine blood is almost always divided into components to best utilize the available resources [3]. The different blood products that are available include fresh and stored whole blood, packed red blood cells, platelet-rich plasma, fresh frozen plasma, stored frozen plasma, cryoprecipate, and cryo-poor plasma (Table 3-2). The most widely used components are packed red blood cells and fresh frozen plasma. The blood components are prepared by separating and processing the units after collection.

Figure 3.2. A golden retriever donating a unit of blood.

Fresh whole blood (FWB) contains red blood cells, platelets, leukocytes and plasma. Fresh whole blood is blood that is transfused within 8 hours of collection. The primary indications for transfusion with fresh whole blood include hemorrhage (resulting in loss of both red cells and plasma), thrombocytopenia with active hemorrhage, and lack of other available blood components. If a freshly collected unit of whole blood is not to be used within 8 hours, it should be separated into components or refrigerated at 4°C. Whole blood may be stored for up to 35 days, depending on the anticoagulant-preservative used. Stored whole blood contains viable red blood cells and plasma proteins such as albumin and globulin (although labile coagulation factors V and VIII will lose their activity). The primary indication for the use of stored whole blood is hemorrhage or the lack of other readily available components.

Commonly, individual units of canine whole blood are processed into components. The red cell portion of the unit is called packed red blood cells (pRBC). Packed red blood cells are prepared by spinning a unit of fresh whole blood at 4°C at 5000 rpm for 15 minutes and removing the majority of the plasma. A unit of pRBC may be stored for approximately 35 days at 4°C, depending on the anticoagulant-preservative used. The average hematocrit of a unit of pRBC is 70 to 80%. Typically, a unit of packed red cells is re-suspended in saline (0.9%) prior to transfusion. The ammonia levels will increase in stored red cells and the glucose level (because of the preservatives) will be high (> 500 mg/dl) [14]. However, the clinical significance of this effect is unknown at this time, and packed red blood cells are indicated to provide oxygen-carrying support to anemic animals requiring such support regardless of underlying cause. In a patient with suspected severe hepatic encephalopathy, it may be prudent to avoid packed cells at the end of their shelf life.

Plasma may be classified as fresh frozen plasma, stored plasma, platelet-rich plasma, cryoprecipate, or cryo-poor plasma, depending on how the unit of whole blood is processed. The types of plasma differ in their relative number and efficacy of the clotting factors. Fresh frozen plasma (FFP) is prepared by separating a unit of fresh whole blood within 6 hours of collection. Fresh frozen plasma is a good source of all the clotting factors including V, VIII, and von Willebrand factor. Fresh frozen plasma is stable for up to 1 year if stored at -40°C. After 1 year, the more labile factor activity is diminished and the unit is termed stored plasma. Stored plasma may also be produced at any point from a unit of stored whole blood or if FFP is inadvertently thawed and then refrozen. Stored plasma has adequate amounts of factors II, VII, IX, and X (the vitamin K-dependant factors), and also albumin.

Platelet-rich plasma may be made by centrifugation of fresh whole blood at lower than normal speeds (2000 rpm). Platelet concentrates have also been described in dogs. Practically, transfusions of platelet-rich plasma or platelet concentrates is rarely performed in veterinary medicine because of the technical requirements for preparation of the products [15,16].

Cryoprecipitate may be prepared by thawing and centrifuging partially thawed fresh frozen plasma. Cryoprecipate is rich in clotting factors VIII, von Willebrand factor, and fibrinogen. It also contains clotting factors IX, XI, and XIII. It is stable for 1 year after the initial collection of the plasma. The cryo-poor component may also be used for some clotting factors and albumin. Cryoprecipitate was initially created for use in people with congenital coagulopathies requiring multiple transfusions in whom volume overload with plasma products was a concern.

Indications for Transfusion

Indications for transfusion are plentiful in injured or ill animals. It is helpful to divide the indications for transfusion into absolutely indicated and potentially indicated. Transfusions carry the risk of reaction or possibly even disease transmission and certainly can add extra expense. Transfusions are considered primarily indicated to treat anemia and coagulopathy. In surgical patients, transfusions may be particularly warranted prior to anesthesia to improve the oxygen-carrying capacity and decrease the potential for anesthetic complications.

Anemia

Anemia should be divided into normovolemic anemia and hypovolemic anemia. Animals that are anemic with a normal volume status typically have either nonregenerative or hemolytic anemias. In these animals the anemia may have been long-standing and/or accompanied by a relative increase in plasma volume. Normovolemic anemic patients appear relatively bright, have normal to slightly elevated total solids, and have pale rather than white mucous membranes.

Normovolemic anemic patients require slow administration of packed red cells to avoid volume overload. In comparison, hypovolemic anemic patients have typically suffered from catastrophic blood loss within the last 12 to 24 hours. These patients are very weak, have rapid and faint pulses, and mucous membranes that appear more pale than their hematocrit might suggest. Their total solids are almost invariably low. These patients require aggressive volume resuscitation as well as blood transfusion. It is not uncommon to find that the post transfusion hematocrit may be lower that the starting point as a result of dilution of the circulating volume with crystalloids or colloids. It is crucial for the clinician to remember that oxygen delivery may be improved dramatically by increasing blood volume (and thereby letting cardiac output improve) even in the face of a lower number for the hematocrit. Massive transfusion, defined as one entire blood volume (90 ml/kg over 24 hours or half a blood volume over 3 hours) has been described in dogs [17].

Packed red cells are usually transfused at either a dose of 1 ml per pound of body weight x the desired increase in hematocrit (Table 3-3) or simply in increments of 1/4, 1/2, or 1 unit. Each unit of packed red cells contains approximately 225 to 250 ml.

Coagulopathy

Blood or plasma transfusions are also considered indicated to treat coagulopathy from decreased coagulation factors. (Table 3-4 and Table 3-5). This may be inherited or acquired. Coagulopathy may be a difficult term to completely define. The normal coagulation system has a relatively large pool of circulating inactive coagulation factors, which are available if massive hemorrhage ensues. The methods of routinely assessing coagulation (prothrombin time, activated partial thromboplastin time, activated clotting time) typically rely on the detection of the formation of a fibrin clot. By the time these assays are abnormal, much of the coagulation ability is lost. Inherited coagulopathies that may benefit from plasma transfusion include hemophilia A or B, von Willebrand disease, and other specific coagulation factor deficiencies. Acquired coagulopathies are common in critically ill animals and include anticoagulant rodenticide toxicities, liver failure, and possibly disseminated intravascular coagulation (DIC). Coagulopathy is markedly worsened by hypothermia and acidosis. Plasma is usually given at a dose of 8 to 15 ml/kg, repeated every 12 hours as needed until normalization of the coagulation times. Plasma is also commonly transfused at 1/4, 1/2, or full unit volumes. A full unit of plasma is approximately 225 to 250 ml.

Other Indications

Transfusions are also potentially indicated with therapy of sepsis/multiple organ dysfunction syndrome, pancreatitis, hypoalbuminemia, and DIC without associated laboratory-proven coagulopathy. It appears commonplace among clinicians to want to give plasma when animals appear critically ill. However, it is prudent to try to determine what the end-point of transfusion is (i.e., normalization of a specific laboratory parameter versus cardiovascular parameter versus other). Correction of an albumin deficit is usually impractical in all but very small patients as much of the patient's albumin is actually extravascular.

Thrombocytopenia

In patients with severe thrombocytopenia that require surgical intervention, attention to meticulous hemostasis is mandatory. In these specific cases, transfusion with platelets in the form of fresh whole blood, platelet-rich plasma, or platelet concentrates may be appropriate. Practically, it is essential that the surgical team recognizes the time constraints (45 - 60 minutes) of either collection of fresh whole blood and/or processing into platelet-rich plasma or platelet concentrates.

Thus, while transfusions may be beneficial for a variety of reasons, it is wise to carefully consider the predicted benefit in each patient and post-transfusion to try to assess whether that benefit was reached.

Transfusion Administration and Reactions

Transfusions of both plasma and pRBC should be completed within 4 hours. Transfusions should be administered through filters and may be allow to either flow in by gravity or through the use of specific fluid pumps. If concurrent fluid therapy is warranted, the transfusate should not be allowed to mix with a crystalloid solution that contains calcium (e.g., lactated Ringer's solution) as the calcium may interfere with the action of the anticoagulant citrate.

Complications are possible with blood transfusions and deserve mention (Table 3-6). Transfusion reactions occur from time to time, but the actual incidence is not well established in dogs and cats. Studies in dogs have reported transfusion reactions occurring in 2.9 to 13% of cases, with the majority of reactions being mild [6,18]. Transfusion reactions are usually classified as immune- or nonimmune-mediated and acute (within 48 hours) or delayed. Immunologic reactions may include hemolysis or hypersensitivity reactions (such as urticaria or facial swelling). Nonimmunologic reactions may include circulatory overload, bacterial contamination, hyperammonemia, or infectious disease transmission. It is important to monitor the patient carefully for any signs of problems (vomiting, fever, hemoglobinuria/hemoglobinemia). Delayed transfusion reactions, when they occur, are likely due to the development of alloantibodies [19]. Rarely, transfusion-related acute lung injury (TRALI) may occur, although the syndrome has been much better described in people [20].

The standard monitoring during a transfusion should include measurement of temperature, pulse, and respiration every 15 minutes for the first hour, then hourly, and monitoring for increased respiratory effort, vomiting, pigmenturia or signs of facial swelling. The most life-threatening type of transfusion reaction is the acute hemolytic transfusion reaction. This occurs most commonly when a previously sensitized DEA 1.1-or 1.2-negative recipient is given DEA 1.1- or 1.2-positive blood. The severity of these reactions varies, but the reactions are most severe when complement is activated by IgM antibody. The release of vasoactive mediators during an acute hemolytic transfusion reaction can lead to shock, DIC, and multiple organ failure. Diagnosis of an acute hemolytic transfusion reaction is usually based on suspicious clinical findings such as development of hemoglobinuria, hemoglobinemia, and fever shortly after blood transfusion in an animal that may have been transfused 4 or more days earlier. If an acute hemolytic transfusion reaction is suspected, the transfusion should be stopped immediately and supportive care to maintain vital organ perfusion should begin. Supportive care may include IV fluid administration, furosemide, dopamine, or mannitol to prevent or treat acute renal failure. It may include fresh frozen plasma administration if DIC is suspected. Monitoring should include careful measurement of systemic arterial pressure, central venous pressure, and urine output.

Delayed hemolytic transfusion reactions may occur 3 to 21 days after a blood transfusion. These reactions usually involve extravascular hemolysis and are mediated by IgG antibody. These reactions are typically mild. Fever is common, and anorexia or icterus has been reported [18]. A positive Coombs test provides strong support for the suspicion of a delayed hemolytic reaction in a patient that was previously Coombs negative. Specific treatment is not usually required. The lifespan of transfused red blood cells may be shortened, resulting in the need for further transfusion.

Conclusions

Transfusion medicine represents a vital adjuvant to clinical practice. Basic tenets of transfusion medicine include knowledge of benefits and risks surrounding transfusion and concepts associated with practical transfusion. For surgical patients in particular, careful planning is advisable to avoid emergent crisis.

Appendix 3-1

Resources for Transfusion Medicine (Current as of 8/2008)

Collection Bags

BAXTER/FENWAL Product 4R-34-20NM
3 bag system for pRBCS, and plasma- Contains ADSOL as a preservative

Blood Typing Cards

RapidVet-H
DMS Laboratories, Inc.
2 Darts Mill Road
Flemington, NJ 08822
Tel: (800) 567-4DMS

Animal Blood Banks

Eastern Veterinary Blood Bank
808 Bestgate Rd. Suite 111
Annapolis, MD 21401
1-800-949-EVBB
www.evbb.com

Animal Blood Bank

1-800-243-5759
www.animalbloodbank.com
California

The Pet Blood Bank

1-800-906-7059/1-877-212-4134
www.PetsHelpingPets.com
3610 Lohman Ford Road Suite 100
LagoVista, TX 78645

Midwest Animal Blood Services Inc.

4983 Bird Drive, Stockbridge, Michigan 49285
Toll Free: 877-517-MABS
Office: 517-851-8244
Fax: 517-851-7762
www.midwestabs.com

Hemopet - California

hemopet.free.fr/index.html