Transfusion medicine (Proceedings)

Transfusion products are commercially available through various commercial vendors, or practitioners can utilize staff or client owned pets as blood donors. Because of the continued increases in demand and repeated shortages, having an internal source is important. Preferred donors are between the ages of 1 and 8 years, not receiving any medications, not received prior transfusions, up to date on vaccinations, and free of appropriate infectious diseases. Donors should have good jugular veins to aide collection; obese animals should be avoided. They should also be on appropriate parasite prophylaxis. Dogs should weigh more than 28 kg and possess a calm demeanor. Cats should weigh more than 4.5 kg and be housed indoors. Dogs can donate 450 ml of whole blood every 4 weeks; prior to donation their PCV should be measured and be > 40%. Most dogs donate without sedation; but if sedation is needed, the author prefers a combination of butorphanol (0.2 mg/kg) and midazolam (0.1 mg/kg). Cats may donate 53 ml of whole blood every 6-8 weeks, and their PCV should be > 35% prior to donation. Cats undergo routine heavy sedation prior to donation; the author utilizes a combination of ketamine (6-8 mg/kg) and midazolam (0.2-0.3 mg/kg). The blood type of blood donors should be known prior to collection.

Blood types

The DEA (dog erythrocyte antigen) system is utilized for dogs. Many types exist (DEA 1.1, 1.2, 1.3, 4, 5, 6, 7, 8), and it is likely additional types will be identified in the future. The author enrolls dogs that are DEA 1.1 and 1.2 negative. Dogs do not possess naturally alloantibodies, and will tolerate a transfusion of a blood type different that their own. If available, cross-matching is preferred with the initial transfusion; but it is required with repeat transfusions that are more than 5 days after an initial transfusion. The recipient will have been exposed, and had time to develop alloantibodies.

The AB blood typing system is utilized in cats; feline blood types include A, B, and AB. Type A is the dominant allele and most common blood type. Type B is recessive, uncommon, although the incidence will vary geographically and with breed. The AB allele is co-dominant with B and is rare. Additional feline antigens exist such as the "Mik" antigen reported in 2007 by Weinstein. Unlike dogs, cats possess naturally occurring, strong alloantibodies that are capable of causing hemolysis and severe, life-threatening transfusion reactions. Because cats posses alloantibodies, cross-matching is recommended prior to any transfusion.

The goal of performing a cross-match is to detect incompatibilities between the donor and recipient. An acceptable cross-match does not ensure safety of the transfusion. Indications to perform a cross-match include any dog that has received a prior transfusion (≥ 5 days), and prior to all feline donations as they posses natural alloantibodies. The major cross match is more important; add 0.2 ml of donor RBC to 4.8 ml 0.9% NaCl. In three separate tubes mix 0.1 ml of RBC mixture and 0.1 ml recipient plasma. Incubate the three tubes for 15 min at 4°C, 25°C, 37°C. Centrifuge and observe for hemolysis in the supernatant, and resuspend the RBC pellet and observe for macro- and microscopic agglutination. The minor cross-match utilizes donor plasma and recipient RBC in the same procedure.

Transfusion reactions are defined as any adverse event from a blood product transfusion, and are categorized into immunologic and non-immunologic. Rates of transfusion reactions are 12% or less, and fatal transfusion reactions occur nearly exclusively in cats that did not undergo cross-match or blood typing, or the blood type was misinterpreted. The more common immunologic transfusion reactions include: acute hemolytic transfusion reaction, allergic reaction, febrile nonhemolytic reaction, and delayed hemolytic transfusion reaction. Acute hemolytic transfusion reaction is a type 2 hypersensitivity mediated by preexisting RBC antibodies (IgG or IgM) to transfused blood. Antigen-antibody complexes activate compliment cascade leading to acute intravascular hemolysis. Hemoglobinemia and hemoglobinuria are apparent. Clinical signs are severe and include lethargy, tachycardia, tachypnea, vomiting, diarrhea, hypotension and shock. Treatment involves stopping the transfusion, blood volume expansion with crystalloids or colloids, and supplemental oxygen. Allergic reactions are a type-I hypersensitivity mediated by IgE or IgG. Clinical signs often include cutaneous abnormalities (erythema, urticaria, pruritis). It is important to rule out acute hemolytic transfusion reaction. Treatment involves stopping the transfusion, diphenhydramine (2 mg/kg IM), possibly short acting steroid, epinephrine & oxygen if severe. Febrile-non hemolytic transfusion reactions are most common. The patients body temperature typically elevates 1° C (1.8° F) or more. Leukocyte or platelet antigens from donor react with recipient antibodies leading to pyrogenic cytokines. Treatment involves slowing or stopping the transfusion. Often the transfusion can be continued & completed. Delayed hemolytic transfusion reactions occur 4-14 days following transfusion. Clinically an unexpected decrease in PCV is observed as the recipient develops antibodies for foreign antigen on transfused RBC. The Coombs test may be positive or spherocytes observed. Several non-immunologic transfusion reactions may occur. These include volume overload, citrate toxicity, hypothermia, bacterial contamination, and infectious disease transmission.

Blood Products available include RBC, plasma or platelet preparations. RBC preparations include fresh whole blood (FWB), stored whole blood, and packed red blood cells (PRBC). FWB contains functional platelets as well RBC, coagulation factors and plasma proteins, and it is administered within 6-8 hours of collection. Because platelets become non-functional over time, stored whole blood (>6-8 hours) does not contain functional platelets. To maintain RBC viability a preservation solution is required (e.g. CPDA-1, Biosol or Adsol). PRBC are produced following centrifugation of fresh whole blood at 5000 g for 5 minutes; the plasma is removed and a RBC preservation solution added allowing RBC storage for up to 37days at 1-6° C.

Plasma preparations include fresh frozen plasma (FFP), frozen plasma (FP), cryoprecipitate (CP), or cryosupernatant aka cryo-poor plasma (CPP). FFP is produced simultaneously with a unit of PRBC; the plasma should be frozen within 6-8 hours of collection. FFP contains plasma proteins and all coagulation factors including factor V and VIII. FP is different from FFP in that it does not contain appreciable amounts of factors V or VIII. When collected plasma is not frozen within 6-8 hours, it is FP. When FFP is thawed and refrozen, it becomes FP. When FFP is over one year old, it becomes FP. FP is viable for 4 years, and FFP is viable for 1 year. CP is produced by slowly thawing FFP and a subsequent centrifugation. The precipitant is CP and contains factor VIII, von Willebrand factor, fibrinogen and fibronectin. CPP is the supernatant produced from CP. CPP contains plasma proteins and coagulation factors excluding factors V, VIII, von Willebrand factor, fibrinogen.

Platelet products include platelet rich plasma (PRP), platelet concentrate (CP), and cryopreserved platelet concentrate. Unfortunately transfusion to supply platelets is rarely beneficial as platelet survival and function decrease rapidly during storage and processing. PRP is produced from FWB centrifuged at lower force for a shorter time; functional platelets are in the plasma supernatant. PC can be produced by centrifuging PRP at higher force and longer time; the supernatant will be FFP if frozen in appropriate time. PC is stable for 5 days and requires constant gentile agitation. Cryopreserved platelet concentrate is produced by freezing PC with 6% DMSO at -74° C. Unfortunately platelet numbers and aggregation are significantly decreased by the process.

Transfusion Administration will vary according to the given situation. Also, transfusion policies vary between institutions. Typically most transfusions are given over a 4 hour period to prevent deterioration of the unit. Often the transfusion is initially administered slowly (0.25 ml/kg/hr for 30 minutes) to minimize risk of fatal reactions. The patient should be monitored for vomiting, diarrhea, urticaria, and with a TPR every 15 minutes for the first hour, then hourly. Frozen plasma products should be carefully thawed in a tepid water bath; aggressive thawing or manipulation can result in cracking of the bag at which point it should be discarded. RBC products should not be warmed unless the patient is a neonate or has preexisting hypothermia; inappropriate warming can accelerate product deterioration contributing to hemolysis. Drugs (e.g. heparin) or additives (e.g. 0.9% NaCl) should not be added to blood products; adding substances creates an open system and risks contamination and lacks supportive evidence. Filter sets (160-270 μm) are highly recommended to trap cellular debris or coagulated proteins. Although pretreatment with antihistamines and/or corticosteroids are sometimes advocated to prevent reactions, they are not practiced by the author; pretreatment has the potential to mask transfusion reactions delaying their recognition allowing administration of a larger transfused volume and risk of more severe reaction.

When, why & what to transfuse?

The major indications for transfusion include anemia and coagulopathy. Anemia is the most common. Etiology of underlying anemia may be blood loss (e.g. trauma, neoplasia, etc), RBC destruction (e.g. – IMHA, parasite, etc), or decreased production (e.g. renal disease, bone marrow disorders). The decision of when to transfuse a patient will vary according to the patient's clinical signs (e.g. tachycardia) and rate of PCV decrease. Acute blood loss from trauma may lead to a PCV of 30%, and the patient may have marked tachycardia despite fluid resuscitation and analgesics. Alternatively a patient with advanced chronic renal failure has had time to adapt and may have a normal heart rate with a PCV of 22%. Regardless of the underlying cause or rate of PCV decrease, when the PCV approaches 12% tissue hypoxia will be present. Patients with a PCV ≤ 12% should always be transfused. Continued tissue hypoxia can result in organ dysfunction. Dosages of FWB or stored whole blood are 10-20 ml/kg, and PRBC is 10-15 ml/kg. Approximately 2 ml/kg of FWB will increase the patients PCV by 1%; and 1 ml/kg PRBC will increase a patient's PCV by 1%.

When coagulopathy is present, the clinician should determine if the defect is affecting primary hemostasis (platelets, endothelial cells, vWf) or secondary hemostasis (intrinsic or extrinsic coagulation systems). Classically, thrombocytopenic patients can have hemorrhage from epithelial surfaces (e.g. mucous membranes, gastrointestinal or urinary tract, skin, etc). Transfusions to affect thrombocytopenia are often ineffective. Platelets are easily activated and do not store well. Case situations that warrant attempts include those with evidence of central nervous system hemorrhage, or those in which surgery is planned. Fresh whole blood, platelet rich plasma, platelet concentrate, and cryopreserved platelet concentration can be considered.

When coagulopathy affecting secondary hemostasis is identified, the supportive treatment options are far more appropriate and effective. Disorders of secondary hemostasis typically result in cavitary hemorrhage or prolonged coagulation times (e.g. – PT, PTT, ACT) are identified during the patient's work up. Differentials include anticoagulant rodenticide toxicity, hepatic synthetic failure (cirrhosis, hepatic lipidosis, bile duct obstruction), inherited coagulopathy (hemophilia A & B), miscellaneous (trauma, sepsis, dilutional, surgical hemorrhage), and disseminated intravascular coagulation (DIC). Dosages of FFP and FP are 6-10 ml/kg; PC, PRP, and CP are dosed at 1 U per 10 kg.

Other considerations for plasma transfusions include DIC or colloidal support. DIC is always a secondary process and treatment efforts should be directed towards the underlying disease process. The author prefers to treat DIC in dogs with plasma when they have clinical hemorrhage, and heparin therapy when clinical hemorrhage is not present and coagulation times are normal.

Colloidal therapy is often considered in patients with increased losses (e.g. GI, renal, trauma, vasculitis), or decreased production (hepatic insufficiency, acute phase diseases). Albumin provides approximately 70% of the colloidal osmotic pressure. One 220 ml unit of FFP or FP has an average of 7.5 grams albumin. Because only 30-40 percent of albumin is in the intravascular space, approximately 4 ½ units of FFP would be needed in a 10 kg dog to result in an increase in the plasma albumin concentration of 1.0 g/dl. Volume overload is a major concern when FFP is used in this fashion. Alternatives include human albumin or synthetic colloids (Hespan®, Hexten®, Voluven®). Human albumin should be used with extreme caution as the amino acid homology is 79.3% and severe and life-threatening transfusion reactions can occur.

References available upon request.