Unit 3

von willebrand factor

required for platelets to stick to each other and the blood vessel endothelium. It serves to stabilize platelet plug. aggregation of platelets also cause a release of initiating factor for the chemical phase of hemostasis. most common inherited coaguation disoreder of domestic animals. large glycoprotein that circulates with Factor VIII. Very common in Dobermans, and has been reported in dozens of other breeds along with Rabbits, and swine.

coagulation cascade

includes intrinsic/induced pathway and extrinsic/tissue. the end result is formation of mesh of fibrin strands that forms the clot. final phase of hemostasis involves the degradation of the fibrin clot

intrinsic/ induced mechanical pathway

starts with cell damage. initial mechanical phase is initiated by interactions of negatively charged phospholipids surfaces of cells and platelets or microparticles. Tissue factor binds to Factor VIII (antihemophilic factor) in the plasma to initiate the coagulation reaction, and Factor I (Fibrinogen) through Factor XI (plasma thromboplastin antecedent) serve to amplify the cascade. the extrinsic pathway actually serves to help initiate the intrinsic pathway. A small amount of thrombin is generated during the initial phase phase, and this also recruits and activates platelets and inhibits fibrinolysis. When platelets are activated, phosphatidylserine (PS) is exposed on the outer surface of the membrane. Platelets also release small vesicles from their surface during activation. These microparticles are enriched in PS. PS acts as a binding site for the complexes of the coagulation cascade, which activate Factor X (Stuart Factor) and prothrombin (Factor II), respectively.

Extrinsic/ Tissue Factor Pathway

Often starts with endotoxemia, neoplasia, etc. The extrinsic pathway involves initiation by Factor III (Tissue Factor) and its interaction with Factor VII (Proconvertin). The pathway then continues down the common pathway.

Common Pathway

The activation of Factor X (stuart factor) results in the generation of a large amount of thrombin (Fig. 14.2). That thrombin continues to recruit and activate more platelets and triggers the conversion of fibrinogen to fibrin. The generation of fibrin proceeds through two phases, with a soluble form being generated initially, followed by the creation of an insoluble form that consists of cross-linked fibrin strands. The coagulation process is also modulated by and its resulting clots broken down through a series of interrelated reactions. In the presence of tissue plasminogen activator (tPA) and plasmin, the soluble fibrin is also broken down into fibrin degradation products (FDPs). Plasmin and tPA also act on the insoluble fibrin to produce a cross-linked version of FDPs and D-dimers.

Thromboelastograph

Some make use of fresh whole blood, and others involve the use of citrated samples. In general, the analyzers contain the sample in a cup to which reagent is added. The analyzer then evaluates the entire clotting process, from the formation of the initial clot through fibrinolysis. Test results are then recorded that provide a measure of the time required for clot formation, the evaluation of the strength of the clot, and the time required for the breakdown of the clot. The results are usually provided graphically and used to identify whether a patient is hypercoagulable or hypocoagulable

Platelet Function Analyzers

assessment of platelet adhesion and aggregation. The PFA-100 analyzer (Siemens USA, Palo Alto, CA) has been validated for use with canine samples.  The analyzer makes use of a disposable cartridge that contains a collagen-coated membrane with a small aperture. Blood is drawn through the aperture, and platelets adhere to the membrane. When a sufficient number of platelets have adhered and aggregated, blood can no longer flow through the aperture. The time required is then recorded.

Point-of-Care Analyzers

hey are frequently used in human emergency departments and physician offices, Some are also available for human patients who are receiving anticoagulant therapy to monitor their coagulation status

PIVKA

refers to proteins that are induced (invoked) by the absence of vitamin K. Vitamin K is required to activate coagulation Factors II, VII, IX, and X. When vitamin K is deficient, precursor proteins of Factors II, VII, IX, and X build up and can be detected by PIVKA testing or by the Thrombotest (Axis-Shield PoC, Oslo, Norway). The test may help to differentiate rodenticide toxicity from primary hemophilia when activated clotting time is prolonged. It is a slightly more sensitive test than the prothrombin test when the vitamin K-dependent factors are depleted. may become prolonged within 6 hours of the ingestion of anticoagulant rodenticides, whereas prothrombin time is prolonged within 24 hours, and activated partial thromboplastin time is prolonged within 48 hours.

D-DIMER AND FIBRIN DEGRADATION PRODUCTS

both are used to evaluate tertiary hemostasis (i.e., fibrinolysis). are formed as a clot is degraded. These tests are therefore useful aids in identifying the presence of DIC and to provide diagnostic information in cases of liver failure, trauma, and hemangiosarcoma

COAGULATION FACTOR ASSAYS

Assays that can be used to identify specific factor deficiencies are performed in reference laboratories, and they are generally performed to identify specific hereditary factor deficiencies. The majority of these assays involve the use of photometric principles.

Sign of congential or acquired deficiencies in coagualtion proteins

usually involve delayed deep tissue hemorrhage and hematoma formation. clinical signs include superficial petechial and ecchymotic hemorrhages, epistaxis, melena, and prolonged bleeding at injection and incision sites. Usually show before the animal reaches 6 months of age. disorders can result form decreased production or the increased destruction of platelets as well as from nutritional deficiencies, liver disease, and the ingestion of certain medications or toxic substances. liver is primary site for production fo coagulation factors.

Factor II prothrombin deficiency

cocker spaniel, beagle

Factor VII deficiency (proconvertin)

Beagle, malamute

Factor VIII (antihemophilic factor)

Hemophilia A. Most common inherited coagualtion deficiency in dogs. x linked recessive traits

Factor IX (plasma thromboplatin)

Hemophilia B, christmas disease. x linked recessive traits

Factor X (stuart factor)

cocker spaniel

Factor XI (plasma thromboplastin antecedent)

Great pyrenees, english springer spaniel

Factor XII (Hageman factor)

Poodle, Shar Pei

type 1 vWD

autosomal dominant inheritance pattern with incomplete penetrance, it is characterized by low levels of circulationg vWF with normal structure.

Type 2 vWD

low circulating levels of of vWF and their vWF is abnormal in both strucutre and function. inherited as dominant trait. experience severe bleeding during estrus, after venipuncture, and after surgery. buccal mucosa bleeding time is prolonged.

Type 3 vWD

characterized by the near absence of any vWF. autosomal recessive. experience severe bleeding during estrus, after venipuncture, and after surgery. buccal mucosa bleeding time is prolonged.

canine von willebrand disease type I

variably reduced levels; all multimer sizes proportionatly reduced; most common and recognized in >70 breeds; hemorrhage tendency variable. often with surgery or trauma. Doberman pinscher, German shepherd, golden retriever, rottweiler, Manchester terrier, Cairn terrier, Pembroke Welsh corgi, Bernese mountain dog, Kerry blue terrier, poodle, papillon

canine von willebrand disease type II

Disproportionately low activity; deficiency of high molecular weight multimers; larger and more effective multimers absent; bleeding can be severe. German shorthair pointer, German wirehair pointer

canine von willebrand disease type III

complete deficiency (<1% plasma von Williebrand Factor); most severe is that all multimers absent. scottish terrier, shetland sheepdog, chesapeake Bay retriever, Kooiker

Thrombocytopenia

refers to a decreased number of platelets, and it is the most common coagulation disorder seen in small animal veterinary practice. The causes of platelet deficiencies are often unknown. However, infection with certain bacterial, viral, and parasitic agents can result in thrombocytopenia. Thrombocytopenia can also occur as a result of bone marrow depression that reduces the production of platelets, or it may result from autoimmune disease that increases the rate of platelet destruction. Aspirin and acetaminophen are common toxins encountered in small animal patients. These medications may destroy or permanently inhibit the circulating platelets, so clinical signs may not resolve until undamaged platelets begin to be released from the bone marrow

vitamin K deficiency

required for the synthesis and activation of some coagulation factors. The vitamin-K-dependent factors include Factors II, VII, IX, and X. Vitamin K deficiency can occur as a result of dietary insufficiency or bile duct obstruction. Any disease that alters vitamin K activity can lead to bleeding disorders. The ingestion of toxic substances (e.g., warfarin, moldy sweet clover) can also create bleeding disorders. Anticoagulant rodenticide toxicity is a significant cause of secondary hemostasis in small animal veterinary practice. Warfarin is one of several compounds that are found in rodenticides that have anticoagulant properties. Clinical signs may not appear for several days after ingestion and include lethargy, anorexia, and dyspnea as a result of bleeding into the thoracic cavity. Ecchymosis, petechiae, and hemarthroses may also occur. Bleeding into the brain or spinal cord may result in neurologic signs. The prothrombin time is generally the first coagulation test to increase, followed by activated partial thromboplastin time and activated clotting time. The PIVKA test has also been suggested as a diagnostic aid. Patients are usually decontaminated when ingestion is known to be recent. Vitamin K therapy is sometimes initiated and may require several weeks for successful treatment

DISSEMINATED INTRAVASCULAR COAGULATION

Although it is not a disease entity on its own, disseminated intravascular coagulation (DIC) is associated with many pathologic conditions. DIC is often seen in trauma cases as well as with many infectious diseases. A large number of events can trigger DIC. Box 18.2 contains a summary of some common conditions associated with DIC. The resulting hemostatic disorder may manifest as systemic hemorrhage or microvascular thrombosis. Microthrombi can result in tissue hypoxia, and the formation of thrombi consumes platelets and coagulation factors, which leads to an increased tendency for hemorrhage. Fibrinolysis of the microthrombi leads to the formation of excess fibrin degradation products and D-dimers. Shock can also occur. There is no single test that can be used for diagnosis, not all tests exhibit abnormal results. patients have prolonged activated thromboplatin time and prothrombin time as well as significant thrombocytopenia. Schistocytes are often present on the blood film. Fibrinogen may be normal or decreased. Buccal mucosal bleeding time is prolonged, and fibrin degradation products and D-dimers are generally increased.