Diseases Associated With Blood Loss or Hemostatic Dysfunction
Johanna L. Watson • Debra Deem Morris
Blood loss may be acute or chronic, and the clinical and laboratory manifestations are widely different because physiologic adaptation occurs in the chronic state.
Acute Blood Loss
TREATMENT. Treatment of acute blood loss should initially be aimed at stopping the hemorrhage. External hemorrhage may be managed by pressure wraps or appropriately placed ligatures, but attempting to control internal hemorrhage when the patient is a poor risk for general anesthesia may be inadvisable and the source of hemorrhage may not be found. Hypovolemic shock should be treated by prompt intravenous (IV) administration of 40 to 80 mL/kg of sodium-containing crystalloid solutions. Studies indicate that a small volume of hypertonic saline (2 to 4 mL/kg 7.2% sodium chloride) may temporarily reverse the pathophysiologic sequelae of severe hemorrhagic shock.1-3 The total volume of crystalloid solution required is usually much greater than the volume of blood lost because crystalloid solutions distribute throughout the extracellular space. The clinical response to fluid administration should be evaluated in light of ongoing losses to determine the necessary replacement volume.
If anemia becomes life threatening, whole blood transfusion must be considered. A PCV less than 20% in an animal with acute blood loss suggests depletion of erythrocyte reserves, and persistent reduction of the PCV over a period of 24 to 48 hours to 12% or less indicates the need for blood transfusion. A low but stable PCV (12% to 20%) does not necessitate transfusion, because transfusion should be reserved for instances in which oxygen delivery to the tissues is inadequate to support life. Blood transfusion can only be viewed as a temporary therapeutic procedure because even crossmatch-compatible allogeneic erythrocytes are removed from the circulation by the mononuclear phagocyte system (MPS) within 2 to 4 days of transfusion.* Horses and cattle display a high degree of blood type polymorphism, and minor antigenic incompatibilities are only delineated by blood typing.
Serum antibodies against nonhost erythrocyte antigens (erythrocyte alloantibodies) probably mediate the short life span of transfused erythrocytes. Compatibility testing is used to avert life-threatening antigenantibody reactions caused by major blood group mismatching.The routine crossmatch involves incubating washed erythrocytes from donor (major) and recipient (minor) with serum from the other. Gross and microscopic examination for clumping demonstrates serum agglutinins in horses. Sensitized cattle erythrocytes do not become clumped in saline solution but do lyse in the presence of rabbit complement, so only a hemolytic crossmatch can be performed in this species. Not all equine erythrocyte alloantibodies act as agglutinins, and hemolysins must be detected by adding complement to the reaction mixture. Pooled rabbit serum must first be absorbed with equine erythrocytes to remove naturally occurring antibodies. The necessity for special handling and storage of rabbit serum makes hemolytic crossmatch procedures impractical for most veterinarians. These tests are best performed by veterinary hematology laboratories (e.g., Veterinary Genetics Laboratory, School of Veterinary Medicine, University of California, Davis), which usually require serum and whole blood in sodium citrate or acid-citrate-dextrose (ACD) to crossmatch.
The first transfusion of whole blood to a horse or ruminant not previously transfused or sensitized by immunization or pregnancy is usually well tolerated because natural alloantibodies are of low concentration and weak activity. After incompatible transfusions, alloantibodies develop rapidly, making subsequent transfusions more hazardous.
Blood for transfusion should be collected aseptically into sterile containers of sodium citrate (2.5% to 4%) or ACD solution and used immediately. The necessary dosage can only be estimated, but in most instances replacing 20% to 40% of the calculated blood loss is sufficient to maintain life until the bone marrow can respond.
A drop in PCV from 36% to 12% in a 500-kg animal (8% body weight blood) represents a loss of 27 L of blood. In this case, 6 to 8 L of whole blood would be therapeutic and easily donated by another 500-kg individual. Blood warmed to 37° C (98.6° F) should be administered through an in-line filter to remove clots. After pretransfusion vital parameters have been recorded, 0.1 mL of blood/kg body weight is given over 5 to 10 minutes and the evaluation is repeated. If parameters and attitude are unchanged, the transfusion can be continued at a rate not to exceed 20 mL/ kg/h. The recipient should be continuously monitored so that the transfusion can be stopped if adverse reactions like tachypnea, dyspnea, restlessness, defecation, tachycardia, piloerection, muscle fasciculations, or sudden collapse occur. Although these signs may not indicate anaphylaxis, severe reactions should be treated with epinephrine (0.01 to 0.02 mg/kg). Mild signs may respond to a slowed transfusion rate or administration of corticosteroids or flunixin meglumine. Because it is often impossible to delineate the cause of transfusion reactions, the safest approach is to discontinue the blood and administer isotonic crystalloid solutions.The prognosis is good for most cases of acute blood loss if hypovolemic shock is quickly treated and bleeding stops. Myocardial damage as indicated by elevated cardiac troponin I (cTnI) and arrhythmia has been reported with acute hemorrhage in the horse.6 Normal bone marrow begins to replace lost cells within 5 days. Sequential PCV analysis will be necessary to determine whether blood loss is controlled. Examples of specific disorders follow.
Guttural Pouch Mycosis. See Chapter 31 for an in-depth discussion of this and other respiratory diseases.