Accurate diagnosis and treatment of the bleeding patient requires a basic understanding of the pathophysiology of hemostasis.
Abnormal bleeding or bruising is frequently encountered in veterinary clinical practice. Accurate diagnosis and treatment of the bleeding patient requires a basic understanding of the pathophysiology of hemostasis. A working knowledge of the hemostatic mechanism along with a complete medical history, thorough physical examination, and specific laboratory tests are integral steps in the diagnostic process. The most appropriate treatment options can only be identified after the cause of bleeding has been determined.
Alterations in primary and/or secondary hemostasis can interrupt normal blood coagulation to varying degrees, putting the patient at risk for hemorrhage. Platelet and coagulation factor replacement is achieved by administering specific whole blood derived components, including platelet-rich plasma, fresh frozen plasma, frozen plasma and cryoprecipitate. Prior to receiving blood transfusion support, each patient must be evaluated by a thorough history, physical exam and specific laboratory tests to determine whether they require isolated or multiple component replacement. As with red cell products, use of plasma products involves a careful balance of anticipated benefit versus risk to the patient (e.g., allergic reaction, infectious disease transmission).
Platelet-rich plasma (PRP) is harvested from a unit of fresh whole blood (FWB) that is less than 8 hours old and has not been cooled below 20° C; refrigerated platelets do not maintain function or viability as well as platelets stored at room temperature. In human medicine, platelets are routinely stored at room temperature under constant agitation for up to 5 days; however, studies have shown that, even at room temperature, function begins to be compromised after 24 hours. Attempts have been made to freeze platelets to achieve a longer shelf life, but these techniques have not been validated for animals.
PRP may be administered following centrifugation, or the platelets may be concentrated by further centrifugation and removal of most of the supernatant plasma. Under optimal conditions, platelets prepared from a single unit of FWB administered to a 30 kg dog would be expected to result in an increase in the patient's platelet count of approximately 10,000/μl.
The major indication for platelet transfusion is to stop severe, uncontrolled or life-threatening bleeding in patients with significantly decreased platelet number and/or function. Patients experiencing massive hemorrhage may require platelet support to compensate for excessive consumption during hemostasis and the dilution factor associated with volume replacement therapy. In veterinary medicine, platelet preparation is difficult in regard to the volume needed in order to measurably increase platelet numbers in larger breed dogs. In some patients, however, cessation of bleeding following platelet transfusion has been achieved without a measurable increase in platelet number. Due to the impracticality associated with production of this component in the required volume necessary for significant impact, specific storage requirements, and the short shelf life, in veterinary medicine we routinely treat thrombocytopenia/thromobpathia with active bleeding with FWB through which the patient will receive both platelets and red blood cells. In situations of platelet destruction, such as immune-mediated thrombocytopenia (IMT), the survival of transfused platelets is a matter of minutes rather than days; however, platelet transfusion may still be warranted if the patient is acutely bleeding into a vital structure (i.e., brain, heart, lung).
In addition to water and electrolytes, plasma contains albumin, globulins, coagulation factors and other proteins. Plasma is primarily used for its coagulation factor value; it does not contain functional platelets. Most coagulation proteins are stable at 1-6°C, with the exception of factors V and VIII. In order to maintain adequate levels of all factors, plasma must be harvested from a unit of FWB and frozen at -18° C or below within 8 hours from the time of initial collection. This product is referred to as fresh frozen plasma (FFP). FFP will retain its coagulation factor efficacy for a period of 12 months provided it is maintained at the appropriate temperature. FFP can be used to treat most coagulation factor deficiencies (e.g. DIC, liver disease, anticoagulant rodenticide toxicity, hereditary coagulopathies) and potentially other conditions (e.g., acute pancreatitis). FFP is not recommended for use as a blood volume expander or for protein replacement in animals with chronic hypoproteinemia.
If FFP is not used within 12 months, it can be relabeled as frozen plasma (FP) and stored for an additional 4 years. Also, plasma may be separated from a unit of whole blood at anytime during its recommended storage time. When stored at -18°C or less, this component is also called FP and may be kept for up to 5 years. If not frozen, it is called liquid plasma (LP) and has a shelf life not exceeding 5 days following the expiration date of the WB from which it was harvested. Plasma prepared from outdated WB may have higher levels of ammonia than FFP as a result of longer contact with red cells prior to its preparation.
FP and LP may have varying levels of the more stable coagulation factors, as well as albumin, but they do not contain functional platelets or the labile coagulation factors V and VIII. FP and LP can be used to treat stable clotting factor deficiencies and certain cases of acute hypoproteinemia (i.e. parvoviral enteritis). If animals are severely or chronically protein deficient, plasma must be administered in large volumes in order to have a measurable impact in managing the acute effects of hypoproteinemia (i.e., pulmonary edema, pleural effusion). In this case, synthetic colloid solutions should be considered in that they are readily available and more effective in increasing oncotic pressure. As with FFP, FP and LP are not recommended for use as a blood volume expander.
Cryoprecipitate (CRYO) is the cold-insoluble portion of plasma that precipitates after FFP has been slowly thawed at 1-6° C (refrigerator). The precipitated material contains concentrated amounts of von Willebrand factor (vWF), factor VIII, fibrinogen, and fibronectin. Following production, cryoprecipitate can be frozen at -18° C or colder and has a shelf life of one year from the original date of whole blood collection. CRYO can be used in patients with suspected or diagnosed von Willebrand disease (vWD), hemophilia A (factor VIII deficiency), or fibrinogen deficiency. Fresh frozen plasma or fresh whole blood may also be used based on product availability.
Problems with limited access to blood transfusion resources still exist in veterinary medicine today. Blood component therapy also carries potential risks for the patient; therefore, an awareness of alternatives to allogeneic blood transfusion must exist. Pharmacologic hemostatic agents can aid in the management of certain hemostatic disorders, and may decrease or preclude the use of blood components. The relevant hemostatic agents discussed in this paper are corticosteroids, vincristine, desmopressin acetate, vitamin K1, and human recombinant factor VIIa.
Corticosteroids and Vincristine
Of all the primary hemostatic defects, thrombocytopenia is the most common and can result from an increased loss of platelets (i.e., hemorrhage), increased destruction of platelets, increased consumption of platelets (i.e., excessive clotting), or decreased production of platelets (i.e., bone marrow disease). The cause of severe thrombocytopenia in dogs is most often IMT, in which bleeding is typically associated with a platelet count below 40,000/μl. Treatment recommendations for IMT vary; however, corticosteroids (Prednisone @ 1-2 mg/kg PO BID or Dexamethasone @ 0.2 mg/kg IV BID) are the mainstay of treatment and act primarily to impair clearance of antibody-coated platelets by macrophages.
Vincristine (0.02 mg/kg IV once) is also frequently administered as an initial therapy in addition to corticosteroids. Vincristine, a vinca alkaloid, may increase platelet counts in dogs with IMT through several proposed mechanisms, including stimulation of thrombopoiesis, increased fragmentation of megakaryocyte cytoplasm to release new platelets, and impairment of macrophage activity which leads to decreased phagocytosis of platelets. Other drug options do exist if corticosteroids and vincristine are not successful, but discussion of them is beyond the scope of this paper.
von Willebrand factor is a large plasma protein produced by and stored in endothelial cells. It facilitates platelet adhesion and aggregation, as well as acts as a carrier for factor VIII. vWF multimers vary in size, with the larger multimers being more hemostatically active. Assays are available to determine the amount of vWF in the circulation; however, they do not determine the multimeric distribution.
vWD, a deficiency of vWF, is the most common hereditary bleeding defect in the dog and is recognized in more than 70 breeds. There are 3 major types of vWD differentiated by the amount of vWF and multimer pattern. The bleeding tendency in vWD is characterized by mucosal surface bleeding and hemorrhage following surgery or trauma. Severity of bleeding is variable in dogs with type 1 vWD and may not correlate with plasma vWF concentration in individual dogs. Dogs with type 2 and 3 vWD generally experience the most severe bleeding episodes.
Although blood component therapy is generally indicated preoperatively in dogs with clinically severe forms of vWD and a history of bleeding, desmopressin (DDAVP), a synthetic analog of vasopressin (antidiuretic hormone), may be sufficient to control minor bleeding in some (but not all) dogs with type 1 vWD. In humans, DDAVP causes a significant increase in plasma vWF concentration; however, the plasma vWF concentration increases only marginally in dogs, despite improvement in hemostasis (demonstrated by improved buccal mucosal bleeding time). The beneficial hemostatic effects of DDAVP are evident within 30 minutes following administration of 1 μg/kg subcutaneously (intranasal spray preparation), and may last for up to 4 hours. DDAVP is generally given once, but may be repeated in 24 hours. DDAVP is a potent antidiuretic agent, and there is a slight risk of fluid retention and hyponatremia, especially with repeated use.
Vitamin K is required for the synthesis of certain coagulation factors (II, VII, IX and X) and anticoagulation proteins C and S. These coagulation factors are produced in the liver as nonfunctional precursors and, without vitamin K, the liver is incapable of completing their synthesis. The inactive precursors in the circulation are known as proteins induced by vitamin K antagonism (PIVKA).
Adequate amounts of Vitamin K are found in commercially prepared pet foods, but this fat-soluble vitamin may also be synthesized by bacteria in the large intestine. Both the dietary and microbial form of vitamin K is readily absorbed in the intestine. A dietary deficiency of vitamin K is rarely encountered in small animals; most often, the deficiency is the result of decreased intestinal production, intestinal malabsorption (secondary to underlying gastrointestinal, hepatic, or pancreatic disorders), or anticoagulant rodenticide toxicity. Anticoagulant rodenticides act as antagonists to vitamin K rejuvenation; therefore, biologically active vitamin K-dependent coagulation factors cannot be produced.
The body does not contain an excessive store of vitamin K, so oral or parenteral vitamin K must be administered to allow these factors to function. Vitamin K1 (phylloquinone) can be administered subcutaneously or orally (1-5 mg/kg BID) depending on the origin of the deficiency. Intramuscular administration of vitamin K1 should be avoided given these patients are at risk for excessive bleeding.
Serial coagulation screening is necessary to assess the effectiveness and duration of vitamin K therapy. The extrinsic and common pathways are assessed by the prothrombin time (PT). The PIVKA test also detects any coagulation factor deficiency of the extrinsic and common pathways, but is not specific for the detection of anticoagulant rodenticide poisoning. The intrinsic and common pathways are assessed by activated partial thromboplastin time (aPTT) or the activated clotting time (ACT). Rodenticide poisoned animals that are bleeding have severally prolonged clotting times including the cuticle bleeding time, PT and aPTT. In patients that are actively bleeding, vitamin K administration may be insufficient by itself and transfusion support may be necessary to stop hemorrhage and correct a resulting severe anemia.
Human recombinant factor VIIa
Recombinant factor FVIIa (rFVIIa) is a human protein that has a proven role in the treatment of hemophiliac patients that have inhibitors (antibodies to factors VIII and IX) and factor VII deficiency. It is increasingly being used in a number of "off-label" settings in human medicine, including catastrophic trauma, obstetric hemorrhage, and cardiac surgery.
rFVIIa has its effect at the site of initial injury. With the help of tissue factor, rFVIIa initiates the extrinsic and common cascade with subsequent sustained activation of the intrinsic cascade, ultimately resulting is an increase in the amount of thrombin generation.
Although this hemostatic agent has not been approved for use in small animals, one experimental study in FVII-deficient dogs did show a shortening of the cuticle bleeding time and PT following infusion of a very small volume. Antibodies do develop approximately 2 weeks following administration and immediate hypersensitivity reactions have been documented in dogs. In humans, an increased risk of thrombosis is associated with rFVIIa therapy as well, but this has not been documented in the dog.
Other disadvantages of rFVIIa are its short half-life and cost. It is thought to have a dose-dependent effect and, because of its short half-life, repeat administration is recommended in bleeding patients. Unfortunately, rFVIIa is extremely expensive, and the current cost may be prohibitive for use in our veterinary patients. One study showed that the direct cost for a single 90 μg/kg dose (recommended dose in humans) in a 25 kg dog would cost approximately $3300.
Blood components will most likely be the mainstay of therapy in most animals that are actively bleeding as a result of a primary or secondary hemostatic defect. However, as blood products are often in short supply and adverse events continue to unfold regarding adverse reactions to blood transfusion, alternative therapies should be utilized when appropriate.
References available upon request