NRAN 80413 Physiology of Hemostasis1.pptx

Physiolog y of Hemostas is NRAN 80413 Casey Crow DNP, CRNA Happy Valentine’s Day Objectives Define hemostasis Understand the role of platelets in hemostasis Describe the normal vessel wall Understand the initial response to blood vessel damage Understand the process of formation of a platelet plug U...

Physiolog y of Hemostas is NRAN 80413 Casey Crow DNP, CRNA Happy Valentine’s Day Objectives Define hemostasis Understand the role of platelets in hemostasis Describe the normal vessel wall Understand the initial response to blood vessel damage Understand the process of formation of a platelet plug Understand blood coagulation and clot formation Describe/Diagram the following pathways: Extrinsic, Intrinsic, Common, Contemporary Understand and describe the fibrinolytic system Describe coagulation tests and laboratory values Describe drugs that effect coagulation Decrease bleeding Increase bleeding Overview Hemostasis Primary Hemostasis Role of platelets Vascular spasms and platelet plug Secondary Hemostasis Coagulation cascade (classical) Extrinsic Intrinsic Final common pathway Coagulation cascade (contemporary) Fibrinolysis Tests of Coagulation Drugs that affect bleeding Hemostasis “The process by which the body maintains the delicate balance bleeding and clotting.” Notbetween homeostasis Nagelhout, p. 823 Hemostatic mechanisms most effective in dealing with bleeding in small vessels Arterioles Capillaries Venules Vascular Injury No injury: Blood remains a liquid Coagulation proteins circulate in inactive form Undamaged endothelium Repels clotting factors/platelets Does not express tissue factor (TF) or collagen Prevents activation of platelets and coagulation cascade Vascular injury: body attempts to prevent hemorrhage Vascular spasm: “Initialization” Primary hemostasis: Platelet plug Secondary hemostasis: Coagulation and fibrin formation Fibrinolysis: breakdown of clot Balance exists between factors that create clots and those that prevent clots A disruption in either direction can lead to pathology Platelets Inactive until vascular trauma occurs Work interdependently with plasma proteins of the coagulation cascade to build a clot Round, disc-like, small Formed in bone marrow Lifespan 8-12 days 150,000 – 300,000/mm3 Cleared by macrophages in reticular activating system (RAS) and spleen Platelet Components Platelet Receptors Receptors are critical for formation of platelet plug GPIIb-IIIa Glycoproteins: links platelets together to form platelet plugs Thrombin Dual roles in coagulation and anticoagulation Thromboxane A2 (TxA2) Potent vasoconstrictor Platelet aggregation ADP Platelet aggregation GPIb Attaches the activated platelets to vWF Key targets for antiplatelet therapy Interdependent Process Occurs in rapid succession in 4 stages Vascular spasm/initialization Vessel injury and exposed subendothelium Primary hemostasis Involves binding of platelets to exposed collagen Secondary hemostasis Process of activation of coagulation factors Leads to production of thrombin Fibrinolysis Occurs when the clot is no longer needed Normal Vessel Wall 3 Layers Tunica Intima: Endothelium Tunica Media: Smooth Muscle Layer Tunica Externa: Connective Tissue Adventitia Normal Vessel Wall Tunica Intima: Endothelium Lines the lumen, separates the flowing blood from the vessel Endothelial cell primarily Tissue factor vWF Prostacyclin Nitric Oxide TxA2 ADP Normal Vessel Wall Tunica media: Smooth Muscle Layer Very thrombogenic and active Active in forming a clot Collagen, Fibronectin Normal Vessel Wall Tunica Externa: Adventitia Anchors the vessels and provides protection to the vessels Controls blood flow by influencing the vessel’s degree of contraction Nitric Oxide: Inhibits platelet adhesion, aggregation and binding of fibrinogen Prostacyclin: Powerful vasodilator, interferes with platelet formation and aggregation Vascular Spasm/Initialization 1st Stage Occurs immediately following vessel injury Tunica media: undergoes contraction, tamponade, reduces blood flow Contraction due to ANS reflexes and TxA2 and ADP expression Common source of injury: surgery, trauma, plaque dislodgement, microinjury (bruises, abrasions) 2 Functions: Reduce blood loss Localize procoagulants to remain in affected area Primary Hemostasis 2nd Stage Three steps Adhesion Activation Aggregation Platelets adhere to exposed collagen Platelets contract Platelet plug is formed Help of vonWillebrand Factor (vWF) Releases compounds that attract other platelets Clotting is promoted Small injuries- process complete Larger injuriescoagulation process is required to strengthen the clot Formation of Platelet Plug Adhesion Collagen is exposed Platelets adhere to collagen Via vWF at the GPIb and GPIIb-IIIa receptors at site of injury vWF binds to the GPIb receptor on the platelet to anchor platelet to subendothelium Formation of Platelet Plug Activation Triggers platelets to degranulate Release of mediators Induce changes in metabolism, shape, and surface of platelets ADP: amplifies platelet adherence and degranulation Serotonin: constriction Thromboplastin: coagulation Formation of Platelet Plug Aggregation Newly activated platelets aggregate onto growing platelet plug Release more platelet-attracting chemicals Positive feedback mechanism Thromboxane A2 (TxA2) is synthesized and released into ECF to further simulate platelet aggregation ADP and TxA2: platelet activation and aggregation End-Result of Platelet Plug Seals small breaks in vessel Contracts via actin and myosin Interaction in the aggregated platelets strengthens the plug Vascular smooth muscle contracts simultaneously Decrease in blood flow TxA2 mediated Inhibition of Platelet Plug Prostacyclin I2 and Nitric Oxide work to inhibit further platelet aggregation Vasodilators Inhibitors of platelet aggregation Otherwise, platelet plug would continuously expand 3rd Stage Secondary Hemostasis Broad overview of the clotting pathway Simplified diagram of the clotting pathway, but much more complex The intrinsic and extrinsic pathways produce THROMBIN Then meet at the common pathway to produce FIBRIN OVERALL: The primary purpose of the clotting Clotting Pathway Overview Thrombin has 3 effects: Generation of fibrin Thrombin (IIa) converts fibrinogen (I) to fibrin (Ia) at site of vessel damage Mesh forms Activation of Factor XIII Thrombin activates Factor XIII Factor XIII is a plasma protein- stabilizes the fibrin meshwork Positive feedback on the cascade Thrombin formation is end result of a cascade of reactions, it then exerts a positive feedback by activating platelets and clotting factors (XIa, VIIIa, and Va) Activated platelets are essential for the cascade Clot Formation Fibrin meshwork- the end result Essential component of clot is fibrin Platelet activation causes platelets to display specific receptors that bind clotting factors Clotting can occur in absence of all cellular elements except platelets Activated platelets are essential because several of the cascade reactions take place on the Classical vs Contemporary Clotting Cascade Classical Contemporary Cell-based theory: newer concept that hypothesizes why platelets and extrinsic/intrinsic pathways work interdependently Classical Clotting Cascade 2 distinct pathways, arrive at same endpoint Extrinsic Activated when coagulation is initiated outside of the intravascular space Cellular element outside the blood is needed Thrombin generated from extrinsic pathway is insufficient to provide adequate, sustained coagulation but is enough to trigger thrombin’s role of activating the intrinsic pathway (positive feedback) Intrinsic Activated when coagulation is initiated inside of the intravascular space Everything necessary for clotting is within the blood Final common pathway The intrinsic and extrinsic pathways merge at Xa to produce thrombin Thrombin is activated Thrombin converts fibrinogen to fibrin Extrinsic Pathway Tissue Factor (TF), factor III NOT a plasma protein Located on the outer plasma membrane Various tissue cells: fibroblasts, cells in the walls of blood vessels outside the endothelium Blood is exposed to the subendothelial cells when vessel damage occurs disrupting the endothelial lining Extrinsic Pathway TF is located on the outer membrane of cells Injury outside the intravascular space Extrinsic Pathway is started TF (Factor III) forms a complex with VIIa (plasma protein) TF/VIIa complex activates X to Xa Xa converts Prothrombin to Thrombin with the help of Va and Activated Platelets Additionally, TF (Factor III) forms a complex with VIIa TF/VIIa complex activates IX to IXa With VIIIa and Activated Extrinsic Pathway The extrinsic and intrinsic pathways merge at Xa to produce thrombin Thrombin is generated Thrombin triggers positive feedback to activate clotting factors and the intrinsic pathway Liver requires vitamin K for normal production of prothrombin and other clotting factors Clotting Factors Extrinsic Pathway Tissue Factor Pathway Activated when coagulation is initiated outside of the intravascular space Vascular injury activates it Fast – required – 15 seconds Measured by PT and INR Inhibited by warfarin Specific factors: III, VII Extrinsic Pathway Prothrombinase: collection of coagulation factors that lead to thrombin activation (IIa) Intrinsic Pathway Factor XII is the first plasma protein in the intrinsic pathway Becomes activated to XIIa when contacts collagen fibers underlying damaged endothelium Intrinsic Pathway is started Factor XIIa activates XI to XIa XIa activates IX to IXa With VIIIa and activated platelets, IXa activates X to Xa Xa with the help of Va and activated platelets converts prothrombin to thrombin Additionally, Thrombin also contributes to the activation of Factors XIa and VIIIa and Va in the intrinsic pathway Intrinsic Pathway Activated by blood trauma – collagen exposure Slower – 6 min to form a clot Measured by PTT and ACT Inhibited by heparin Specific factors: XII, XI, IX, VIII Intrinsic Pathway Final Common Pathway Begins at prothrombin activator (prothrombinase) Prothrombinase: Xa, with Va and activated platelets Converts prothrombin (II) to thrombin (IIa) Thrombin (IIa): Proteolytic enzyme, changes fibrinogen to fibrinogen monomer, known as fibrin After platelets aggregate, fibrin is incorporated into the platelet plug Clot remains until the underlying vascular tissue has repaired itself Final Common Pathway All Together Coagulation Cascade Animation Coagulation Cascade Ani mation - Physiology of He mostasis (youtube.com) Contemporary Coagulation Cascade Hypothesizes why platelets, extrinsic and intrinsic pathways of coagulation cascade do not work independently of one another More accurately represents interaction between cellular activity and coagulation proteins leading to thrombus formation Hypothesis: Coagulation takes place on different cell surfaces that have TF Theses surfaces play a pivotal role in factor expression leading to hemostasis in 3 phases: initiation, amplification and propagation Cell-based Theory of Coagulation Three overlapping phases Initiation Exposure of TF to blood leads to activation of factor VII Amplification Small amounts of thrombin (from the initiation phase) activate platelets Binds factors Va, VIIIa, and IXa at their surface Propagation Bound to the activated platelets’ surface, coagulation factors (activated in amplification phase) are able to convert large amounts of prothrombin into thrombin The thrombin burst accounts for conversion of fibrinogen to fibrin and activation of factor XIII Cell-based Theory of Coagulation Initiation Phase Injury to endothelial surface of blood vessel TF is exposed Endothelial surface of blood vessels becomes acidic Phospholipid is less repellent to platelets TF recruits platelets, activates factor VII TF/VIIa reaction results in activation of factors X (common pathway) and IX (intrinsic pathway) Factor Xa forms complex with factor Va, generates small amount of thrombin for clot formation Cell-based Theory of Coagulation Amplification Phase Thrombin generation continues Factor Xa is inactivated by TF pathway inhibitor TFPI: Can only form small amounts of thrombin (IIa) Small amounts of thrombin activate platelets Platelets bind factors Va, VIIIa, and IXa at their surface Cell-based Theory of Coagulation Propagation Phase All coagulation factors are active, promoting coagulation Protease reactions: convert soluble protein fibrinogen to insoluble fibrin strands by thrombin Thrombus formation Factor VIIIa binds to IXa Generates large amounts of factor Xa Factor Xa combines with Va to form prothrombinase complex Converts prothrombin (II) to thrombin (IIa) in large amounts End result: Large “burst” of thrombin (IIa) Thrombin converts fibrinogen (I) to fibrin (Ia), resulting in fibrin mesh at site of vessel injury Cell-based Theory Summary Initiation: Exposure of TF to blood Activation of factor VII Amplification: Small amounts of thrombin activate platelets Bind factors Va, VIIIa, and IXa at their surface Propagation Coagulation factors convert large amounts of prothrombin to thrombin Factor VIIIa/IXa complex acts a cofactor for generation of large amounts of factor Xa Va-Xa complex results in thrombin burst Thrombin bursts accounts for conversion of fibrinogen to fibrin 4th Stage Fibrinolysis Limits the size of the clot Clotting is counterbalanced Vasodilation and washout of ADP and TxA2 Tissue Factor Pathway Inhibitor (TFPI) neutralizes TF Protein C and S inhibit factors Va and VIIIa Antithrombin III inactivates thrombin (IIa) and factors IXa, Xa, XIa, and XIIa Fibrinolysis Fibrinolytic System Mechanisms that break down the clot Mechanisms that turn off fibrinolysis when no longer needed Fibrinolysis Regulated by plasma proteins Clot composition Plasminogen Fibrin Fibrin degradation products (FDPs) Plasminogen (enzyme) Stored in the liver in the inactive form Incorporated during clot formation, activated to plasmin (with help of body’s tissue plasminogen activator (tPA) and urokinase) Then causes degradation of fibrin to FDPs Fibrinolysis Mechanism of clot break down Plasminogen- a proenzyme, synthesized in the liver Inactive until needed to digest fibrin Proteolytic- degrades fibrin into FDPs tPA is released by injured tissue over a period of days Plasmin activators are used therapeutically to dissolve thrombi and restore blood flow Mechanisms Turning off Fibrinolysis Clot is degraded, fibrinolytic process needs to stop tPA inhibitor (tPAI): Inhibits conversion of plasminogen to plasmin Alpha-2 antiplasmin: Inhibits the action of plasmin on fibrin Stop plasmin from breaking down fibrin Entire Process Platelet plug, primary and secondary hemostasis, clotting cascade, thrombin to fibrin, then break down of fibrin with plasmin and then stopping plasmin from breaking down all the fibrin Coagulation Tests Activated Partial Thromboplastin Time (aPTT) Assesses: Intrinsic and final common pathway Measures: Time to form a clot using phospholipid, calcium and an activator Monitors: Therapeutic response to unfractionated heparin Not LMWH Normal value: 25-32 seconds Factors must be reduced by >30% before a change in a PTT is observed Remember: Intrinsic pathway is slower than extrinsic, normal aPTT will be longer than the normal PT Coagulation Tests Prothrombin Time (PT) Assesses: Extrinsic and final common pathway Measures: Time to form a clot using tissue factor and calcium Monitors: Therapeutic response to warfarin Normal value: 12-14 seconds Factors must be reduced by >30% before a change in PT is observed Coagulation Tests International Normalization Ratio (INR) Calculation that standardizes PT results Based on the ratio between the patient’s PT and standard mean PT Different labs might report different PT results without standardization, making it impossible to compare results from different labs Normal value: approx 1 Target for patients on warfarin = approx 2-3 times the control Coagulation Tests Activated Clotting Time (ACT) Guides heparin dosing Normal value: 90-120 seconds ACT should be >400 seconds before going on CPB ACT is measured before heparin administration, 3 minutes after its given and every 30 minutes thereafter More accurate than the PTT when large doses of heparin are administered Coagulation Tests Platelet Count Measures: Number of platelets Does not measure the function of the platelets Normal value: 150,000 – 300,000 mm3