Coagulation and Dissolution of a Blood Clot Flashcards
normal hemostasis
balance between generation of hemostatic clots and uncontrolled thrombus formation; anticoagulants dominate
extrinsic pathway
- plasma mediated, initiation of hemostasis
- primary hemostasis
- key –> tissue factor
- activated when blood contacts cells outside the vascular endothelium
- nonvascular cells express a membrane protein called tissue factor III which initiates this pathway
intrinsic pathway
- amplifies and propagates hemostasis
- secondary hemostasis
- key –> thrombin
- triggered when blood contacts a negatively charged surface (exposed sub-endothelial collagen)
common pathway
- results in an insoluble fibrin clot; starts at Xa
- where intrinsic and extrinsic pathways converge with activation of factor X
How do you decide whether or not to get preoperative coagulation testing?
- based on patient’s history and planned surgery
- balance between risk of surgical bleeding and risk of developing postoperative thromboembolism
mechanism of normal hemostasis
- vasoconstriction
- platelet plug
- clot formation
- clot dissolution
vasoconstriction in normal hemostasis
- vascular endothelium provides nonthrombotic or antiplatelet surface (basically makes it so the blood doesn’t stick to the surface and clot)
- damage to the endothelium exposes the underlying extracellular matrix and elicits contraction (vasoconstriction)
what other things can induce prothrombotic endothelial changes?
- thrombin
- hypoxia
- high fluid sheer stress
formation of platelet plug in normal hemostasis
when platelets are exposed to the extracellular matrix in damaged endothelium they undergo a series of biochemical and physical alterations
3 major phases of platelet plug
- adhesion
- activation
- aggregation
platelet normal concentration
150,000-400,000 per microliter
spontaneous bleeding can occur at <50,000
lethal is <10,000
what is the life of a platelet?
8-12 days
platelet adhesion
exposure to the subendothelial matrix proteins allows platelets to undergo a conformational change to adhere to the vascular wall; basically conformational change makes them more sticky
platelet activation
- after platelets adhere to damaged endothelial cell wall, several intracellular signaling pathways are activated when ligands bind to platelet receptors and a series of physical and biochemical changes occur
- platelets develop pseudopod-like membrane extensions to increase platelet surface area
platelet aggregation
- plt recruitment
- release granular contents resulting in recruitment and activation of additional platelets
- completes the formation of a platelet plug
- activators released during the activation phase recruit and amplify the response of additional platelets to the site of injury
- newly activated GPIIb/IIIa receptors on the platelet surface bind fibrinogen to provide for cross-linking with adjacent platelets
Von Willebrand Factor (vWF)
- produced in endothelium and platelets
- released by endothelial cell and by activated plts
- primary function is to bind other proteins
vWF primary function
- important bridging molecule between subendothelial matrix and platelets forming cross links
- Glycoprotein IIb/IIIa –> platelet to platelet
- Glycoprotein Ib/factor IX/factor V receptor complex –> plt to endothelium
glycoprotein Ib-factor V-factor IX complex (GPIb-V-IX)
- binds vWF allowing platelet adhesion and platelet plug formation at sites of vascular injury
- absence of this complex = Bernard-Soulier syndrome
Von Willebrand Disease (vWD)
- mainly activated in conditions of high blood flow and shear stress
- 1 in 100 individuals, but clinically significant cases are 1 in 10,000
- deficiency of vWF therefore show primarily in organs with small vessels such as skin, GI, and uterus
vWD common presentation
woman with heavy periods, bleeding when flossing or brushing teeth
vWD diagnosis
measure amount of vWF in a vWF antigen assay and the functionality of vWF with biding assays
type 1 vWD
- 60-80% of cases
- failure to secrete vWF into circulation or vWF being cleared more quickly than normal
- mild, often goes undiagnosed until bleeding following surgery, easy bruising, or menorrhagia
type 2 vWD
- 15-30%
- qualitative defect and bleeding varies (4 subtypes)
- decreased ability to bind to GPIb
- decreased ability to bind to VIII
type 3 vWD
- most severe, homozygous defective gene, complete absence of production of vWF
- leads to extremely low levels of factor VIII since it does not exist to protect VIII from proteolytic degradation
platelet type or pseudo-vWD
- defect of the platelets GPIb receptor
- vWF is normal but the platelet receptor GPIb is abnormal
what medications do we use that are a GPIIb/IIIa inhibitor?
- GPIIb/IIIa inhibitors (class of antiplatelet)
- abcizimab (ReoPro)
- eptifibatide (Integrilin)
- tirofiban (Aggrastat)
- blocks ability of fibrinogen to form around aggregated platelets
which medications do we use that are a thromboxane A2 inhibitor?
- aspirin - inhibits the ability of COX enzyme to synthesize the precursors of thromboxane within platelets
- naproxen - nonselective COX inhibitor
P2Y12 receptor
further amplify the response to ADP and draw the forth of the completion of aggregation
formation of blood clot
- follows platelet plug
- fibrinogen breaks down to produce fibrin, which becomes cross-linked into a stable mesh
- coagulation factors activated and initiate coagulation cascade
- soluble fibrinogen converted to insoluble fibrin
- converted by thrombin (IIa)
what is the key step in blood clotting?
conversion of fibrinogen (I) to fibrin (Ia) by thrombin (IIa)
coagulation factors
- identified with roman numerals
- most are glycoproteins
- most synthesized in liver
- circulate in an inactive state
- lower case “a” indicates active enzyme
which coagulation factors are not enzymes?
- vWF
- Tissue factor (III)
- glycoprotein
which coagulation factors are not synthesized in the liver?
- calcium (from diet)
- vWF (synthesized in endothelial cells)
Which factors are dependent on vitamin K for utilization?
factors II, VII, IX and X
intrinsic pathway of coagulation
- contact activation system
- begins with damage to blood vessels
- formation of primary complex on collagen and thrombin generation by way of factor XII and ultimately merges to the common pathway to activate factor X
extrinsic pathway of coagulation
- tissue factor pathway
- initial step in plasma-mediated hemostasis
- following damage to the blood vessel, factor VII comes into contact with tissue factor (which is prevalent in the sub-endothelial tissues surrounding vasculature) and forms an activated TF-VIIa complex
- the TF-VIIa circulating the plasma activates factor X to promote the conversation of X to Xa
common pathway of coagulation
- common to both extrinsic and intrinsic
- prothrombin (II) is cleaved by activated factor X to produce thrombin (IIa)
- signal amplification
blood clot
- prothrombin gets activated to thrombin
- thrombin activates fibrinogen to form fibrin
- fibrin –> covalent bonds and cross-linking of fibers create a meshwork in all directions of blood cells, platelets and plasma which adhere to the surface of damaged blood vessel
- after clot is formed, actin/myosin of platelets trapped in fibrin mesh and interact in a manner like that in muscle contraction
dissolution of blood clot
- clot lysis occurs when plasminogen is activated to plasmin
- activation occurs by tissue plasminogen activator (t-PA) released from the tissue, vascular endothelium, plasma, and urine
- plasmin is an enzyme which digests fibrin fibers, fibrinogen, Factor V, Factor VIII, prothrombin, and Factor XII
prothrombin time (PT)
- evaluation of extrinsic pathway
- sample blood plasma incubated with tissue factor in the presence of excess Ca2+
- particularly sensitive to three of the four vitamin K factors (II, VII, and X)
- commercial prothrombin reagents vary markedly in their responsiveness to warfarin-induced decreases in clotting factors and are not interchangeable between laboratories
partial thromboplastin time (PTT)
- indicates performance of intrinsic pathway
- a sample of blood triggered by adding an activator surface plus phospholipid and Ca2+
ACT
- performed by mixing whole blood with an activated substance to initiate activation of the clotting cascade
- widely used and is reliable for high heparin concentrations
- influenced by hypothermia, thrombocytopenia, coagulation deficiencies
viscoelastic testing
- thromboelastometry (TEG)
- rotational thromboelastometry (ROTEM)
- global assay for whole blood clotting including coagulation factors, inhibitors, anticoagulant drugs, platelets, and fibrinolysis
bleeding time
- sensitive test of platelet function
- small incision made in underside of forearm and the amount of time it takes for bleeding to stop is recorded
heparin concentration measurements
- increasing concentrations of protamine added to samples of heparin containing blood
- time to clot measured by the sample in which heparin and protamine are most closely matched will clot first
platelet function tests
-classic method involves centrifugation of patient blood to obtain platelet rich plasma, which then analyzed in a cuvette at 37 degrees placed between light source and photocell
ACT
80-150 seconds
