Week 12 - Transfusion, Coagulation, and Anticoagulants Flashcards
What are the normal regulators of hemostasis?
- Vascular extracellular matrix and endothelial reactivity
- Platelets
- Coagulation proteins
- Inhibitors of coagulation
- Fibrinolysis
What does clot formation involve?
- Tissue factor release
- Factor VIIa generation
- Platelet activation
- Cellular and humoral activation pathways
What are the endogenous anticoagulants?
- PGI2
- tPA
- Heparin sulfate
- Antithrombin III
- Protein C
- Endothelium derived relaxing factor (EDRF)
-Inhibit platelet activation, fibrin formation, and provide for vascular patency
What are the steps of platelet activation?
- Receptors on platelets bind to the damaged blood vessel wall – form bridges with vWF to initiate platelet adhesion
- Once adhered, platelet surface receptor changes – causes platelet aggregation
- Once they aggregate, expose surface factors that activate the coagulation cascade – form an early hemostatic plug
*abnormalities in platelet number or function are an issue
What are the four stages of hemostasis?
- Initiation
- Amplification
- Propagation
- Stabilization
What causes the initiation of clotting during vascular injury? What does it bind to?
Tissue Factor expression on the subendothelial vascular basement of the blood vessel
-Tissue Factor binds factor VIIa to form factor VIIa/TF complex
What converts factor X to factor Xa and what is factor Xa function?
factor VIIa/TF complex converts factor X to factor Xa
factor Xa generates trace amounts of thrombin
What factors from the intrinsic cascade generate Thrombin?
factors XI, IX, and VIII dependent activites
also factor VIIa/TF complex which forms Xa that generates thrombin
What enzymatically cleaves prothrombin to produce thrombin?
Prothrombinase Complex (factors Xa, Va, and prothrombin) assembly
What are common clotting factors targeted in anticoagulation therapy?
- factor Xa
- prothrombinase complex (factors Xa, Va, and prothrombin)
What regulates thrombin generation to localize the clot at the site of vascular injury?
(regulators of TF-trigger procoagulant response)
Tissue factor pathway inhibitor – neutralizes factor Xa when it is in a complex with TF-factor VIIa
Antithrombin III – neutralizes the initially formed factor Xa and thrombin
What is the generalization of the initiation stage of hemostasis?
factor VIIa in circulation searches for exposed TF at sites of vascular injury and trace quantities of factor Xa and thrombin initiate a procoagulant response
What is the generalization of the amplification stage of hemostasis?
- vWF binds to the damaged blood vessel
- Platelets then adhere to subendothelial collagen-vWF via their glycoprotein Ib receptor are are activated
- Thrombin activation of platelets further amplify clot formation
*factors XI, VIII, and V further amplify and sustain procoagulant responses after thrombin-mediated activation
What is the function of fibrinogen? What does it bind to?
Critical protein for clot formation – creates a dense lattice structure
Binds to platelet glycoprotein IIb/IIIa receptors to facilitate clot formation
When is it recommended to treat Fibrinogen levels?
at 1.5-2 g/L
normal = 2-3 g/L
What role does Factor XIII have in the clotting cascade? How is it converted to be active?
Plays a major role in the terminal phase – promotes formation of cross-linked fibrin polymers and generation of a stable hemostatic plug
-protects fibrin from fibrinolysis
*converted to factor XIIIa by thrombin and calcium
What does t-PA do?
converts plasminogen to active plasmin which promotes fibrinolysis
*endothelial activation releases t-PA from endothelial stores of Weibel-Palade bodies
What is the function of Protein C and Protein S?
Exert anticoagulant and anti-inflammatory activities
Protein C is activated by thrombin – activated protein C binds protein S
Together they function as a critical anticoagulant by inhibiting factor Va and factor VIIIa
What is disseminated intravascular coagulation (DIC)?
Coagulation disorder that occurs when pathologic activation of the hemostatic system
End result is an imbalance resulting in either a hemorrhagic coagulopathy or procoagulant state
What are characteristics of DIC?
- Thrombocytopenia (platelets <100,000 in 50-60% and <50,000 in 10-15%)
- Coagulopathy – decreased coagulation factors (prolonged PT or aPTT, bleeding)
- Fibrinogen consumption
- Reduced antithrombin and Protein C levels
What increases tissue factor expression?
Surgery
Trauma
Inflammation
What are inherited (genetic) risk factors for hypercoagulability?
- Inherited Antithrombin Deficiency – decreased levels of antithrombin
- Prothrombin G20210A Mutation – increases prothrombin levels (2% population; Heterozygous - 3x greater risk of thrombus, Homozygous is very rare)
- Factor V Leiden – gene mutation so that procoagulant factor V is not inactivated by activated protein C (5% population; Heter = 3x increase; Homo = 18x increase)
- Dysfibrinogenemia – high levels or structurally defective fibrinogen (2x increase in thrombus – >450 mg/Dl = poor outcome for CVA pts)
- Protein C Deficiency (vitamin K dependent - decrease = 5x increased risk of VTE)
- Antithrombin III Deficiency (hetero = 50% population, homo = fatal)
- High levels of Lipoprotein(a) – inhibits fibrinolysis
What are acquired coagulation risk factors?
- Antiphospholipid Antibodies: causes by a disease state (Lupus anticoagulants, anticardiolipin antibody, anti-B2-GP-1 antibody) – secondary to decreased thrombomodulin expression, increased TF expression, or impaired protein C
- Liver or Renal Failure – decreased production of anticoagulant factors (ATIII, Protein C & S, plasminogen), increases platelet activation
- Blood Stasis – immobility, low CO, A-Fib, Virchow’s triad, metabolic syndrome, cancer, age
What is heparin induced thrombocytopenia (HIT)?
- Heparin binds to platelet factor 4 – antibodies recognize the complex, release microparticles that stimulate thrombin aggregation, and thrombi are formed
- Excessive thrombus formation leads to endothelial damage and tissue factor production and more prothrombosis
- 0.5-5% incidence in pts treated with heparin in past 5 days
- unexplained drop in platelets by 50% or more
How is HIT diagnosed and treated?
Diagnosis:
- platelet drop by 50%
- new thrombus in pts on heparin for 5-14 days
- rule out sepsis, IABP destruction, drug induced thrombocytopenia
- severe HoTN, pulmonary HTN or tachy 20-30 minutes after heparin bolus may indicate HIT
Treatment:
- STOP heparin – eliminate ALL sources (flushes)
- initiate anticoagulation with a direct thrombin inhibitor
What increases risk for bleeding?
- Advanced age
- Low preop RBC volume
- Preop antiplatelet or antithrombotic medication
- Redo surgery
- Emergency surgery
- Comorbidities
- Thrombocytopenia (platelet <50,000)
- Hemophilia
- von Willebrands disease
Hemophilia A vs Hemophilia B
Hemophilia A = factor VIII deficiency
-1:5000-10,000 males
Hemophilia B = factor IX deficiency
-1:20,000-34,000 males
What are the types of von Willebrands disease?
Genetic:
- Partial quantitative deficiency (type I) – may respond to desmopressin (DDAVP)
- Qualitative deficiency (type II) - 4 variants
- Total Deficiency (type III)
Acquired vW disease
Treatment: measure vWF activity, administer factor VIII/vWF concentrate; IGG; cryoppt; plasma pheresis