Blood Transfusions in Animals
Frequently, the need for blood transfusions is acute, as in acute hemolysis or hemorrhage; transfusions are also appropriate in treatment of acute or chronic anemias. Animals with hemostatic disorders often require repeated transfusions of whole blood, red cells, plasma, or platelets. Blood transfusions must be given with care, because they have the potential to further compromise the recipient.
Whole blood frequently is not the ideal product to be administered. If the need is to replace the oxygen-carrying capability of the blood, then packed RBCs are more appropriate; if replacement of circulatory volume is needed, crystalloid or colloid solutions may be used, with packed RBCs added as needed. Platelet numbers rise rapidly after hemorrhage, so replacement is rarely needed. Plasma proteins equilibrate from the interstitial space, so plasma is not needed except in massive hemorrhage (>1 blood volume in 24 hours). Animals that require coagulation factors benefit most from administration of fresh-frozen plasma or cryoprecipitate if the need is specifically for factor VIII, von Willebrand factor, or fibrinogen. Platelet-rich plasma or platelet concentrates may be of value in thrombocytopenia, although immune-mediated thrombocytopenia usually does not respond to administration of platelets because they are removed rapidly by the spleen. If equipment needed to prepare blood components is not available on site, most components can be provided by commercial blood banks.
The decision to transfuse RBCs is determined by clinical signs, not by any pre-selected PCV. Animals with acute anemia show signs of weakness, tachycardia, and tachypnea at a higher PCV than animals with chronic anemia. The amount of RBCs required to relieve clinical signs will generally increase the PCV above 20%. Domestic animals have blood volumes of 7%–9% of their body weight; cats have a slightly lower volume of ~6.5%. By determining the recipient’s blood volume and knowing the animal’s PCV, the required replacement RBC volume can be calculated. For example, a 25-kg dog has a total blood volume of ~2,000 mL; with a PCV of 15%, the RBC volume is 300 mL; if the PCV is to be increased to 20%, that equals an RBC volume of 400 mL. Therefore, 100 mL of RBCs or 200 mL of whole blood (with PCV of 50%) would be required to increase the recipient’s PCV to the desired level. A simple rule of thumb is that a transfusion of 20 mL/kg of whole blood or 10 mg/kg of packed RBCs will raise the Hct of the recipient by 10 points. These calculations assume no ongoing losses of RBCs through hemorrhage or hemolysis. The post-transfusion PCV is the most important measure of adequacy of red cell dose.
Collection, storage, and transfusion of blood must be done aseptically. The anticoagulant of choice is citrate phosphate dextrose adenine (CPDA-1). Commercial blood bags contain the appropriate amount of anticoagulant for a “unit” (500 mL). Heparin should not be used as an anticoagulant, because it has a longer half-life in the recipient and causes platelet activation; also, heparinized blood cannot be stored.
Blood collected in CPDA-1 with added RBC preservation or nutrient solutions may be safely stored at 4°C for 4 weeks. If the blood will not be used immediately, the plasma can be removed and stored frozen for later use as a source of coagulation factors or albumin for acute reversible hypoalbuminemia. Plasma must be frozen at –20° to –30°C within 6 hours of collection to assure that levels of factor VIII are adequate and will remain so for 1 year. Chronic hypoproteinemia is not helped by plasma, because the total body deficit of albumin is so large that it could not be improved by the small amount contained in plasma. Colloid solutions such as hetastarch are more effective for treatment of hypoalbuminemia. Human albumin has been used in dogs; however, the risk of sensitization and allergic reactions is significant.
Patients should be closely supervised during transfusion. In general, RBC transfusions are administered over a 3- to 4-hour period except when acute volume replacement is needed, as in acute hemorrhage. If venous access is not possible, as in very small or neonatal animals, blood may be given via catheter inserted into the femur through the trochanteric fossa. The intraperitoneal route is not advised because many transfused cells never reach the circulation.
The most serious risk of transfusion is acute hemolysis. Fortunately, this is rare in domestic animals. Dogs rarely have clinically significant preformed antibodies, so only those that have received repeated transfusions are at risk. The most common hemolytic reaction in dogs that have received multiple transfusions is delayed hemolysis, seen clinically as shortened survival of transfused RBCs and a positive Coombs’ test. Even crossmatch-compatible RBCs given to horses or cattle survive only 2–4 days. Nonimmune causes of hemolysis include improper collection or separation of blood, freezing or overwarming of RBCs, and infusing under pressure through a small needle.
Other complications include sepsis from contaminated blood, hypocalcemia from too much citrate, and hypervolemia (especially in animals with preexisting heart disease or in very small animals). Urticaria, fever, or vomiting are seen occasionally. Transfusions can also spread disease from donor to recipient. Even when donors are properly screened, other infections such as those caused by viruses, rickettsiae, or bacteria, can also be spread if the donor is currently infected.
In species where reagents exist, donors and recipients should be typed to assure compatibility and reduce the risk for sensitization.
A major crossmatch should be done before transfusion to any dog that has been previously transfused, and in any cat, even for the first transfusion.
The decision to transfuse RBCs should be made based on clinical signs in the patient rather than at a specific Hct.