CME: Bleeding Disorders - Applied Physiology

Applied Physiology of Applied Physiology of Bleeding DisordersBleeding Disorders

Dr.B.Gowri Shankar MD pg

Prof.Dr.G.Sundaramurthy’s unit(M 7).

Hemostasis

• Blood must be fluid

• Must coagulate (clot) at appropriate time– Rapid– Localized– Reversible

Thrombosis…inappropriate coagulation

3 Major systems involved

• Vessel wall– Endothelium

– ECM = BM (type 4 collagen + other proteins)

• Platelets• Coagulation cascade

– Coagulation factors (proteins)• Names and numbers

• Active and inactive forms (zymogens)

Antithrombogenic Thrombogenic

Vessel injury

(Favors fluid blood) (Favors clotting)

Antithrombotic Properties of the Endothelium

• Anti-platelet properties– Covers highly thrombogenic basement

membrane– Uninjured endothelium does not bind platelets– PGI2 (prostacyclin) and NO from uninjured

endothelium inhibit platelet binding– ADPase counters the platelet aggregating

effects of ADP

Antithrombotic Properties of the Endothelium

Anticoagulant properties

*HEPARIN-LIKE MOLECULES: activate anti-thrombin III (inactivates active proteases)

*THROMBOMODULIN: changes specificity of thrombin (activates protein C , which

inactivates factors Va and VIIIa

*Endothelial cells produce tPA which activates fibrinolysis via plasminogen to plasmin

Prothrombotic Properties of the Endothelium•Synthesis of von Willebrand factor

•Release of tissue factor

•Production of plasminogen activator inhibitors (PAI)

•Membrane phospholipids bind and facilitate activation of clotting factors via Ca bridges

Platelet Response to Agonists

Platelets - unstimulated Addition of ADP

(mild stimulation)

Addition of thrombin

(strong stimulation )Characteristic discoid shape

Shape change (elongation and crescents) and Fila form process formation (arrows)

Increased spreading, Fila form process extension (arrows) and aggregate formation (stars)

PLATELET

Coagulation Cascade

• Enzymatic cascade (amplification)• Several serine proteases

– Produced by liver (most)

– Require Vit K (several)

• 3 protein cofactors (not enzymes)• Requires Ca 2+

• Localized to site of injury • Reversible (via production of plasmin)

Fibrinogen FibrinThrombin

Prothrombin

XaVa

VIIa

TF

Extrinsic Pathway

IXa

VIIIa

XIa

XIIa

Intrinsic pathway

XIIIa

Soft clot

FibrinHard clot

VVIII

Fibrinogen Fibrin

Fibrinogen FibrinThrombin

Fibrinogen FibrinThrombin

Prothrombin

XaVa

Fibrinogen FibrinThrombin

Prothrombin

XaVa

VIIa

TF

Extrinsic Pathway

Fibrinogen FibrinThrombin

Prothrombin

XaVa

VIIa

TF

Extrinsic Pathway

IXa

VIIIa

XIa

XIIa

Intrinsic pathway

Fibrinogen FibrinThrombin

Prothrombin

XaVa

VIIa

TF

Extrinsic Pathway

IXa

VIIIa

XIa

XIIa

Intrinsic pathway

XIIIa

Soft clot

FibrinHard clot

Fibrinogen FibrinThrombin

Prothrombin

XaVa

VIIa

TF

Extrinsic Pathway

IXa

VIIIa

XIa

XIIa

Intrinsic pathway

XIIIa

Soft clot

FibrinHard clot

VVIII

Hemophilia A

Deficiency of/nonfunctional VIII

Hemophilia BDeficiency of /nonfunctional IX

Why do they bleed?

Hemophilia C

Deficiency of/nonfunctional XI

Thrombin (IIa)

Prothrombin (II)

Xa

VIIa

TF

IXa

Revised Coagulation Pathway (Tissue Factor Pathway)

IX

NB: production of IXaInteraction of intrinsic and extrinsic pathways

Tissue Factor Pathway Inhibitor

• Kunitz-type protease inhibitor (kringles)• 34 and 41 KD forms in plasma (C-term

truncation)• Directly inhibits Xa• Inhibits VIIa-TF complex in a [Xa]-dependent

manner• Bound to LDL, HDL and Lp (a)• ~10% present in platelets (endothelium also)

Thrombin (IIa)

Prothrombin (II)

Xa

VIIa

TF

IXa

Tissue Factor Pathway Inhibitor

IX

TFITFPI

NB: Inhibition of Xa and VIIa

Proposed Mechanism of Tissue Factor Pathway Inhibitor (TFPI) Activity

F-Xa

Endothelium

Tissue factorF-VIIa

TFPI

F-Xa

TFPITFPI

F-Xa

Physiologic Inhibitors of coagulation

• Antithrombin III (serpin)• Activated Protein C + protein S

– Inactivates Va and VIIIa (via proteolysis)– NB: Factor V Leiden

• Thrombomodulin (EC glycoprotein)– Binds to thrombin– Decreases ability to produce fibrin– Increases ability to activate Protein C

Proposed Mechanism of AT III-Heparin System

HeparinThrombin Antithrombin

Lysine sites

Serine site

Argininesite

H

Th

H

AT III

AT III

Th

Proposed Mechanism of Thrombomodulin, Protein C and Protein S (TM-PC-PS) System

Thrombin

Prothrombin

Protein C

Thrombomodulin

Thrombin

F-Xa

Activatedplatelet

PS

F-Va

x

Ca++

Ca++

ActivatedProtein C

Role of vitamin K

Some clotting factors require a post-translational modification before they are active in clotting

These factors are II, VII, IX, X, proteins C and S

This PTM involves the addition of a COO- to certain Glu residues in the clotting factors

This PTM results in the formation of several -carboxy glutamates = Gla

This PTM requires vitamin K

Role of vitamin K

• Vit K is altered in carboxylation Rx and must be regenerated before reuse

• Regeneration involves 2 reductases which convert vit K epoxide to vit K

• Vit K antagonists inhibit these reductases

• Vit K-dependent PTM provides sites on modified coag factors for Ca bridging

Non-physiologic inhibitors of coagulation

• Vitamin K antagonists (in vivo only)

• Ca chelators (in vitro only)– EDTA– Citrate– Oxalate

* Heparin (in vivo and in vitro)

Clot removal

Fibrin Fibrin Split Products (FSP)Plasmin

Fibrinolysis

Fibrin Fibrin Split Products (FSP)Plasmin

Plasminogen

tPA

Fibrinolysis

Inhibitors of fibrinolysis

• Plasminogen activator inhibitors (PAIs)

2-antiplasmin (serpin)

Pathophysiology of DIC loss of localization of coagulation process caused by abnormalities in any of four

major components of hemostasis1. vascular integrity2. platelets3. coagulation cascade4. clot lysis

Pathophysiology of DIC

Disruption of Vascular Integrity vascular endothelial

damage can initiate coagulation and DIC causes platelet

adhesion and aggregation

vasculitis (infectious, systemic lupus)

burns dissecting aortic

aneurysms

Pathophysiology of DIC

Abnormal Platelet Function

Pathophysiology of DIC

Abnormal Platelet Function altered platelet aggregation causes DIC

platelets coated with immune complexes or bacteria aggregate without vascular injury

immune complex disease pregnancy toxemia (immune complexes) viraemia sepsis or bacteremia

diffuse platelet consumption and destruction

Pathophysiology of DIC

Clotting Factor Abnormalities

Pathophysiology of DIC

I. Abnormal Factor Activation intravascular tissue phospholipid causes

abnormal activation of clotting factors provides surface for factor activation

massive trauma necrotic tumor red cell hemolysis of any cause obstetric complications: retained dead fetus,

placental abruption, amniotic fluid embolus

Pathophysiology of DIC

I. Abnormal Factor Activation widespread activation of coagulation leads

to formation of fibrin coagulation factors consumed

concentrations of fibrinogen and labile cofactors (V and VIII) decline first

used once and destroyed serine proteases capable of catalyzing multiple reactions

Pathophysiology of DIC

II. Abnormal Factor Control hemostasis controlled by antithrombin (AT)

and protein C antithrombin inhibits active serine proteases protein C regulates thrombin formation

deficiencies lead to excess clot formation and factor consumption precipitate or exacerbate DIC

Pathophysiology of DIC

Accelerated Fibrinolysis

fibrinolysis → fibrin split products (FSPs) also called fibrin degradation products (FDPs)

clearance mechanisms may be overwhelmed FSPs normally cleared by Kupffer cells

excess FSPs cause decreased clot formation coat platelets and inhibit aggregation inhibit fibrin cross-linking

Pathophysiology of DIC

Accelerated Fibrinolysis urokinase lyses

fibrin clots catalyzes synthesis

of plasmin from plasminogen

leads to excess FSP production

promotes DIC prostate disease,

urinary tract surgery

Heparin Induced Thrombocytopenia

HIT(heparin-induced thrombocytopenia)

HAT(heparin-associated thrombocytopenia)

White- clot syndrome

HIT An immunoglobulin-mediated adverse

drug reaction characterized by: platelet activation thrombocytopenia thrombotic complications

Pathogenesis of HIT Most commonly caused by IgG antibodies

(designated HIT-IgG) that activate platelets through their Fc receptors

Antigenic Heparin/PF4 Complex

antigen in HIT is a complex of “-” charged heparin polysaccharide and “+” charged protein tetramer (platelet factor 4 or PF4)

PF4 - released from platelet storage granules during platelet activation

unfractionated heparin wraps around PF4 to a greater extent than LMWH

Effects on the coagulation system

Binding of heparin to PF4 neutralizes the anti-coagulant effect of heparin

Immune complexes composed of heparin, PF4, and IgG binds to platelet Fc receptors, resulting in strong platelet activation, and ultimate increase in thrombin generation

10/98 medslides.com 55

Iceberg Model

Multiplethrombosis

(white clot syndrome)0.01-0.1%

Isolated thrombosis30-80% of below groups

Asymptomatic thrombocytopenia30-50% of below group

HIT - IgG seroconversion 0-10%

Warkentin TE, et al. 1994;75-127

Pathophysiology of Drug-induced thrombocytopenia Certain drugs(quinine, quinidine, sulfa

antibiotics) link non-covalently to platelet membrane glycoproteins

very rarely, IgG antibodies are produced that recognize these drug-glycoprotein complexes

macrophages remove the complexes causing severe thrombocytopenia