HEMOSTASIS AND THROMBOSIS
I. COMPONENTS INVOLVED IN THE HEMOSTATIC
MECHANISM
II. INITIATION AND LOCALIZATION
III. PROPAGATION
IV TERMINATION
V. ELIMINATION
VI. MAINTAINING THE BALANCE BETWEEN HEMOSTASIS
AND THROMBOSIS
VII. MONITORING THE COAGULANT RESPONSE CLINICALLY
I. NORMAL REVIEW / TESTS:
II. INITIATION/LOCALIZATION: VASCULAR
III. INITIATION/LOCALIZATION: PLATELETS
IV. PROPAGATION: HEREDITARY COAGULATION FACTOR
DEFICIENCIES
V. TERMINATION/ELIMINATION: ARTERIAL AND VENOUS
THROMBOEMBOLIC DISEASE
VI. PROPAGATION/TERMINATION: VITAMIN K
VII. PROPAGATION, TERMINATION AND ELIMINATION:
DISSEMINATED INTRAVASCULAR COAGULATION (DIC)
VIII. PROPAGATION, TERMINATION AND ELIMINATION:
LIVER DISEASE
IX. PROPAGATION/TERMINATION: CIRCULATING ANTICOAGULANTS
X. APPENDIX: DIAGRAMMATIC REVIEW OF NORMAL HEMOSTATIC
MECHANISMS
Normal Biochemistry and Cell Biology
Complex, yet ingenious interrelated systems exist to maintain the fluidity
of the blood in the vascular system while allowing for the rapid formation
of a solid blood clot to prevent excessive blood loss (hemorrhage) subsequent
to blood vessel injury. These interrelated systems are collectively referred
to as hemostasis when they are invoked as part of the body's normal defense
mechanisms to prevent blood loss. Alternatively, these same interrelated
systems are invoked during thrombosis which refers to unwanted, pathological,
and in some instances life-threatening clot formation. Thrombosis is indeed
a pathologic extension of the normal hemostatic mechanism. The blood coagulation
events in either phenomenon are fundamentally identical.
I. COMPONENTS INVOLVED IN THE HEMOSTATIC
MECHANISM
(Figure, "Components Involved in the Hemostatitic Mechanism")
- Vessel Wall- endothelial cells and subendothelial constituents
- Platelets- circulating cellular elements
- Coagulation Proteins (commonly referred to as factors)
- Coagulation Factor Inhibitors
- Fibrinolytic System
The first three components interact to effect a "cascade"
of events which generates the blood clotting enzyme thrombin which then
cleaves the plasma protein fibrinogen to form fibrin. Fibrin is then deposited
at the site of injury in an insoluble form.
Subsequent to vascular injury, highly thrombogenic subendothelial connective
tissue is exposed which supports the adherence and subsequent activation
of platelets. Platelet activation is accompanied by platelet shape change,
release of cytoplasmic granule constituents, and platelet aggregation to
form a platelet plug. This series of events is referred to as primary hemostasis.
Simultaneously, tissue factor, deposited in the subendothelium is exposed
and initiates the activation of the various plasma coagulation factors,
in a series of zymogen to protease reactions, to form thrombin and hence
fibrin which stabilizes the platelet plug. This series of events is referred
to as secondary hemostasis.
Coagulation factor inhibitors, which circulate in the blood, are present
to insure that the clotting activity is restricted to the site of injury
and
by inactivating the blood clotting enzymes which have escaped and are headed
downstream of the injury.
In addition, subsequent to thrombin formation, the fibrinolytic system is
activated which is a critically important mechanism for initially limiting,
and subsequently, eliminating the clot.
These interrelated events can be divided into a series of subprocesses including:
initiation, localization, propagation, termination, and elimination.
Back to Top
II. INITIATION AND LOCALIZATION
Vascular endothelial cell damage leading to platelet plug formation
- Vessel Wall
- Endothelium: The normal vascular endothelium is a thromboresistant
surface since it is not only nonthrombogenic, but also antithrombotic.
However, when injured (either mechanically, chemically or biologically),
it can profoundly promote hemostasis.
- Antithrombotic Mechanisms
(Figure, "Antithrombotic Mechanisms of Vascular Endothelium")
- Inhibition of Platelet Adhesion, Activation and Aggregation
- PGI2- prostacyclin is secreted which is a potent inhibitor of platelet
aggregation and a strong vasodilator.
- EDRF- endothelial- derived relaxing factor [also known as NO (nitrous
oxide) provides functions similar to PGI2 .
- ADPase- enzymes elaborated at their surface which convert the strongly
proaggregating ADP released from platelets to adenine nucleotide platelet
inhibitors.
- Binding and Inhibition of Thrombin
- Heparin sulfate proteoglycans- carbohydrate moieties expressed at
their membrane surface which act as cofactors for the plasma inhibitor
antithrombin III, which neutralizes thrombin and several of the other coagulation
enzymes.
- Thrombomodulin- a surface-expressed, integral membrane protein which
binds and inactivates thrombin with respect to its procoagulant properties.
A thrombin/thrombomodulin complex is a potent anticoagulant complex since
it activates protein C to activated protein C. Activated protein C down-regulates
coagulation by inactivating important coagulant proteins, factors Va and
VIIIa. These reactions will be discussed in detail later.
- Initiation of Fibrinolysis- vascular endothelial cells synthesize
and release plasminogen activators which promote fibrinolytic activity
to lyse and clear clots.
SUBENDOTHELIAL CONNECTIVE TISSUES INITIATE THE HEMOSTATIC RESPONSE- These
tissues not only provide support for the endothelial monolayer, but also
are extremely thrombogenic since they consist of various proteins such as
vonWillebrand factor (vWF) and fibrillar collagen which are important players
in the hemostatic response to injury. Exposure of these proteins initiates
hemostasis through the following mechanism.
- PLATELET ADHESION AND AGGREGATION
- A platelet is a discoid cell fragment derived from a megakaryocyte.
They circulate in blood in a quiescent state in numbers of 200,000 to 400,000/~1.
As shown below, they constitutively express at their membrane surface glycoprotein
(GP) receptors- two of which are always poised to interact specifically
with proteins (vWF and collagen) in the extracellular matrix leading to
platelet adhesion and aggregation at the site of vascular injury.
- Through a metabolically passive process, platelets interact with
vWF through GP Ib-IX resulting in platelet adherence. GP Ia-IIa forms the
collagen receptor and when occupied leads not only to platelet adherence
at the site of injury, but to platelet activation, as well. Platelet activation
leads to the "functional" expression of GP IIb-IIIa which can
now bind fibrinogen at one of two sites. Hence a single fibrinogen molecule
can "bridge" two platelets together leading to platelet aggregation.
Through these series of reactions, a platelet plug is formed to "plug"
the leaky blood vessel temporarily and to "localize" subsequent
reactions to the site of injury. (Figure, "Site of Injury")
- Individuals with vonWillebrand's disease have reduced or abnormal
synthesis of vonWillebrand's factor protein and display a bleeding problem
since this important ligand is not present in the subendothelium to adhere
platelets to the site of injury. Likewise, individuals who express defective
GP Ib-IX complexes and thus do not possess the receptor for vonWillebrand
factor, also display a bleeding diathesis which is called Bernard-Soulier
Syndrome. Individuals defective in GP IIb-IIIa also manifest bleeding problems;
this syndrome is termed Glanzmann's thrombasthenia.
Back to Top
III. PROPAGATION
The process of platelet activation leads also to the secretion of
a variety of intraplatelet contents from their a- and dense- granules, some
of which lead to further activation of platelets (e.g., ADP) and hence additional
recruitment of platelets to the site of injury. Activation also results
in the expression of previously inaccessible membrane receptors which bind
the various coagulation factors thereby supporting coagulation complex enzyme
assembly to ultimately yield thrombin. The thrombin so formed is a potent
platelet agonist (activator) and will amplify the clotting response by recruiting
more platelets to the site of injury. Thrombin will also cleave fibrinogen
to its insoluble form, fibrin, which will stabilize the platelet plug resulting
in formation of a clot.
- Coagulation Complex Assembly Leading to Thrombin Formation- All
these enzymes express their activities as surface-bound multicomponent
complexes which include:
- a serine-type proteolytic enzyme
- one or more cofactor proteins
- a surface or membrane component
- Ca2+.
"Intrinsic" vs "Extrinsic" pathway of blood coagulation
(Figure "Intrinsic vs Extrinsic")
Intrinsic- All clotting proteins required for thrombin formation are "intrinsic"
to plasma.
Extrinsic- An extravascular protein source is required to initiate thrombin
formation.
The extent of clinical bleeding associated with various coagulation factor
deficiencies is not satisfactorily explained by the organization of enzymatic
complexes depicted on the previous page. Individuals whose blood lacks prekallikrein,
HMWK, or factor XII display no requirement for therapeutic plasma replacement
therapy. Factor XI deficiency is generally characterized as a mild form
of hemophilia. Factor VII deficiency leads to a variable bleeding response.
In contrast, deficiencies of factors VIII, IX, X, or V present serious bleeding
defects that require clinical management. Factor VIII deficiency (a sex-linked
disease) is termed hemophilia a. Factor IX deficiency is termed hemophilia
b.
- Current Concepts of Enzymes Involved in the Coagulant Response in
Vivo (Figure, "Clot Formation")
Once a small amount of thrombin is generated, the coagulant response
literally "explodes" due to thrombin-induced effects. As mentioned
previously, thrombin is a potent platelet activator. It also activates factors
V and VIII to the required cofactors, factors Va and VIIIa, and activates
factor VII to factor VIIa.
- The serine proteases are derived from Vitamin K- dependent zymogens.
(Figure, "Serine Protease")
Each of the vitamin K-dependent zymogens circulates as an inactive
precursor and must be proteolytically cleaved to give rise to the product
vitamin K dependent enzyme. As either zymogens or active serine proteases,
the proteins must be able to associate with a membrane surface to effect
their function (thrombin is the exception). Their membrane-binding properties
are imparted, in part, by a post-translational modification which involves
the addition of carbon dioxide to 9-11 glutamic acid residue side chains
near the amino termini of these proteins. This reaction forms g-carboxy-glutamic
acid (GLA). Vitamin K (which is oxidized during the reaction) is an absolutely
required cofactor. Warfarin and other dicoumarol analogues which block reformation
of reduced vitamin K essentially stop the post-translational modification
from occurring and hence result in the synthesis of inactive procoagulant
proteins.
- Relevant Cofactors
- Plasma-derived: Factors Va and VIIIa are derived from their procofactors,
factors V and VIII by limited proteolysis resulting in activation. The
most potent physiological activator of both factors V and VIII is thrombin.
These cofactors have no enzymatic activity, but rather are essential binding
components of their respective enzyme complexes. (Figure, "Cofactors")
- Integral membrane protein: Tissue factor is irreversibly anchored
on the surface of cells which, under normal circumstances, do not come
in contact with flowing blood (i.e. extravascular cells).
- Complex Formation on an Appropriate Membrane Surface (Figure, "Complex
Formation")
Each of these vitamin K-dependent complexes of coagulation consists
of a 1:1 protein cofactor/vitamin K-dependent enzyme on a membrane surface.
Each of these complexes exhibits discrete substrate and proteolytic specificity,
but are functionally analogous. Three key regulatory events required for
their assembly: 1) proteolysis- conversion of a vitamin K-dependent zymogen
to a serine protease; 2) cofactor activation- proteolytic activation of
factors V and VIII, or expression of integral membrane cofactor protein;
and, 3) presentation of an appropriate membrane surface to accommodate the
protein binding interactions.
- Relevance of Complex Formation
- Localization - enzyme activity is restricted to the site of injury
- Amplification - increase the concentration of substrates and products
by restricting them to a small volume
- Modulation - when in their respective complexes, the proteases and
cofactors are protected, for the most part, from their inhibitors
- Fibrinogen Cleavage - Fibrinogen is a six polypeptide chain plasma
protein composed of two Aa, two Bb, and two g chains (Aa2Bb2g2). Thrombin
cleaves off the A and B peptides from the Aa and Bb chains, respectively,
to produce fibrin monomers. Thrombin cleavage e~poses binding sites on
the E domain that can interact with complementary sites on the D domains
leading to fibrin polymerization in a half-staggered array. (Figure, "Half-staggered
Array")
Thrombin also activates factor XIII to factor XIIIa. Factor XIII circulates
in plasma and when activated functions as a transglutaminase by catalyzing
the addition of a glutamine side chain residue to a lysyl side chain residue
(called a g-glutamyl-e-aminolysyl peptide bond). This reaction results in
cross-linking the fibrin monomers to each other between g-chains and a chains
and thereby stabilizing the fibrin clot.
Back to Top
IV. TERMINATION
Termination reactions involve both constitutive inhibition processes
and clotting initiated or regulated reactions.
- Constitutive Inhibition Processes:
- Antithrombin III - is the principle protein inhibitor of coagulation.
It inhibits the activity of the serine proteases, thrombin, factors IXa
and Xa when they are free in solution and factor VIIa when it is in complex
with tissue factor. Factors IXa and Xa are protected from antithrombin
III inhibition when complexed with factors VIIIa and Va, respectively.
Formation of an antithrombin III/protease complex is irreversible. Antithrombin
III interaction with the serine proteases is significantly enhanced by
its binding to cell-surface heparin sulfate.
- TFPI (Tissue factor pathway inhibitor) - Reversibly inhibits both
factor Xa and the factor VIIa/tissue factor complex through binding interactions
requiring Ca2+. Factor Xa must bind first leading to its inhibition followed
by factor VIIa.
- Clotting Initiated Inhibition Processes:
- Activated Protein C - Proteolytically inactivates factors Va and
VIIIa, which are not in complex with factors Xa and IXa, respectively.
Protein S participates in this reaction as well although its role is less
clear. Formation of activated protein C is directly linked to thrombin
formation. (Figure, "PC to APC)
Thrombin (a serine protease) binds to its cofactor, thrombomodulin
(an integral membrane protein expressed by vascular endothelial cells) in
a Ca2+dependent manner and activates the vitamin K-dependent protein, protein
C to activated protein C.
- THROMBIN BOUND TO THROMBOMODULIN BECOMES A POTENT ANTICOAGULANT
SINCE IT LOSES ITS ABILITY TO PERFORM ANY OF ITS PROCOAGULANT FUNCTIONS.
THROMBIN CAN NO LONGER ACTIVATE PLATELETS, FACTOR V, FACTOR VIII OR FACTOR
XIII AND IS NOT INHIBITED BY ANTITHROMBIN III.
Since antithrombin III, TFPI, protein C, protein S and thrombomodulin
are important in regulating clotting activity, deficiencies of any of these
proteins may lead to thrombosis.
Back to Top
V. ELIMINATION
(Figure, "Fibrinolysis")
- The process by which blood clots are dissolved is called fibrinolysis.
The plasma protein, plasminogen, must first be activated to the serine
protease, plasmin. (Figure, "Plasmin")
- Plasminogen activators (urokinase and tissue plasminogen activator)
are released from vascular endothelial cells in an active form. Whereas,
urokinase is an effective activator of circulating plasminogen, tissue
plasminogen activator is an inefficient activator of soluble plasma plasminogen,
but an efficient activator of plasminogen when both proteins are bound
to insoluble fibrin. For this reason, tissue plasminogen activator is said
to be clot-specific.
- Plasmin begins to break down the fibrin clot at the same time that
wound repair processes are occurring at the damaged site. Eventually the
clot is dissolved and new endothelial cells have grown where the clot once
was. (Figure, "Fibrin Breakdown")
- Plasmin also regulates the clotting mechanism by inactivating some
of the clotting factors required for thrombin generation (factors V and
VIII and fibrinogen), which, if they become too low, can lead to bleeding
problems. (Figure, "Inactivation")
- Plasmin activity is neutralized by its plasma inhibitor, a2-antiplasmin.
- Plasminogen activators are also produced by bacteria which are of
special interest to clinicians- most notably, streptokinase, produced by
streptococci. Streptokinase is not an enzyme and exerts its activator function
by binding to plasminogen leading to a conformational change in the plasminogen
molecule by exposing the active site capable of activating unbound plasminogen.
- Plasmin cleaves fibrinogen and fibrin in the connecting regions
between the D and E domains to produce soluble fragments corresponding
to free D and E domains that can circulate in blood. These proteolytic
products are referred to as fibrinogen- and fibrin- degradation products.
(Figure, "Proteolytic Products")
Back to Top
VI. MAINTAINING THE BALANCE BETWEEN
HEMOSTASIS AND THROMBOSIS
Bleeding disorders can span the spectrum from weeping blood vessels to full
fledged internal and external hemorrhage.
The balance can be offset in the direction of hemorrhage by: (Figure, "Balance,
Hemorrhage")
- Genetic Defects:
- platelet abnormalities
- vessel wall abnormalities
- clotting protein deficiencies (hemophilias)
- excess fibrinolytic activity
- Acquired Defects:
- liver disease
- vitamin K deficiency
- drug-induced deficiencies
- autoimmune disease
- trauma
VII. MONITORING THE COAGULANT RESPONSE
CLINICALLY
(Figure, "Monitoring the Response") This series of events
is referred to as primary hemostasis.
Thrombosis can be manifested as a transient, short-term or episodic event
individuals with chromic or recurring clotting. It is the major cause of
both strokes and heart attacks.
The balance can be offset in the direction of thrombosis by: (Figure, "Balance,
Thrombosis")
- Genetic Defects:
- plasma inhibitor deficiencies
- protein C and protein S deficiencies
- low fibrinolytic activity
- Acquired Defects:
Back to Top
Pathology of Coagulation and Hemostasis
- Introduction
- Normal review: Laboratory tests
- Initiation and localization of hemostasis: Abnormalities of vessels
and platelets
- Propagation of hemostasis: Hereditary bleeding diatheses: Hemophilia/
von Willebrand's disease and other coagulation factor deficiencies.
- Termination and elimination of hemostasis: Arterial and venous thromboembolic
disease: atherosclerosis and hereditary thrombotic diatheses like protein
C deficiency, antithrombin III deficiency etc.
- Propagation, termination and elimination of hemostasis, Acquired
bleeding diatheses: Liver disease, Vitamin K deficiency/ coumadin and Disseminated
Intravascular Coagulation
- Propagation and termination of hemostasis: Acquired non-immune and
immune inhibitors of hemostatic mechanisms
- Case studies
OBJECTIVES:
- To acquire a working knowledge of hemostatic mechanisms and the
common laboratory tests of hemostasis.
- To understand the basic mechanisms and clinical characteristics
of the most common acquired and hereditary bleeding and thrombotic diatheses.
- To gain an initial appreciation of the therapeutic implications
of these pathological conditions.
Back to Top
I. NORMAL REVIEW / TESTS:
- Normal mechanisms of hemostasis: Complex, yet ingenious interrelated
systems exist to maintain the fluidity of the blood in the vascular system
while allowing for the rapid formation of a solid blood clot to prevent
excessive blood loss (hemorrhage) subsequent to blood vessel injury. These
interrelated systems are collectively referred to as hemostasis when they
are invoked as part of the body's normal defense mechanisms to prevent
blood loss. Alternatively, these same interrelated systems are invoked
during thrombosis which refers to unwanted, pathological and in some instances
life-threatening clot formation. Thrombosis is indeed a pathologic extension
of the normal hemostatic mechanism. The blood coagulation events in either
phenomenon are fundamentally identical and involve the following components
of the hemostatic mechanism:
- Vessel wall- endothelial cells and subendothelial constituents
- Platelets- circulating cellular elements
- Coagulation proteins- (commonly referred to as factors)
- Coagulation factor inhibitors
- Fibrinolytic system
These interrelated processes can be divided into a series of subprocesses
including: initiation, localization, propagation, termination and elimination.
- Screening tests of hemostatic function:
- Specimen Requirements: EDTA and citrate act as anticoagulants because
of their abilities to chelate calcium in the plasma, and therefore inhibit
the clotting process. EDTA (purple top tube) is the anticoagulant of choice
to enumerate all blood cells, including platelets. The vast majority of
the remaining coagulation tests require citrate (blue top tube) as an anticoagulant.
These latter assays are based on mixing 9 parts of whole blood with one
part of citrate, either 3.2% or 3.8%. The vacuum in the collection tube
is calibrated to provide the exact mix of blood:citrate. In performing
a clotting assay, the exact amount of calcium (calcium chloride) is added
back to over-ride the citrate.
- Platelet Count
- Bleeding Time: The test screens for abnormalities in the reactions
of primary hemostasis (formation of the platelet plug). This is mostly
independent of the plasma coagulation reactions (secondary hemostasis).
While a blood pressure cuff is maintained at 40 mmHg, a standardization
incision is made in the forearm. The excess blood is gently blotted away,
and the time to cessation of blood flow is recorded. This is a very crude
test, which is subject to many variables- contributed by both the technologist
and the patient. This is also one of the most misused (abused) and misunderstood
tests that the laboratory offers. In particular, it is frequently (mis)used
to predict the potential for bleeding in a patient about to undergo surgery
or an invasive procedure. There is a large body of literature which does
not support the use of the bleeding time in this clinical setting. The
best predictor of whether a patient will suffer excessive bleeding during
a procedure is an adequate patient HISTORY! In a patient with no personal
or family history of excessive bleeding, the BT is close to worthless as
a predictor of such an event. This is a classic example of applying a SCREENING
test to the GENERAL population, as opposed to a SELECTED population, such
as those with a positive bleeding history. In the former, this test loses
its predictive value, while for the latter, the test has real clinical
utility.
- Prothrombin Time (PT) (Summary figure for PT, PTT, TT)
- citrated plasma + Thromboplastin + CaCl2 ---> time to clot.
- Thromboplastin (a lipoprotein) = phospholipid + tissue factor (activates
FVII)
- evaluates the Extrinsic system (VII, X, V, II and fibrinogen)
- Prolonged PT:
- mild to severe vitamin K deficiency
- Warfarin therapy
- liver disease
- deficiencies of extrinsic system components (congenital and acquired)
- consumptive states (DIC)
- excessive heparin
- Activated Partial Thromboplastin Time (aPTT)
- citrated plasma + Partial Thromboplastin + Activator + CaCl2 --->
time to clot
- Partial thromboplastin = a phospholipid - does not contain tissue
factor
- Activator = a negatively charged surface (koalin, glass, elagic
acid) - activates FXII
- evaluates the Intrinsic system (XII, HMWK, PK, XI, IX, VIII, X,
V, II and fibrinogen)
- Prolonged aPTT:
- heparin: most common cause of a long aPTT in an inpatient
- lupus-like inhibitor - probably the second most common cause of
a long aPTT
- deficiencies of intrinsic factors (congenital and acquired)
- liver disease
- consumptive states (DIC)
- profound vitamin K deficiency
- excessive Warfarin therapy
- Thrombin Time
- citrated plasma + dilute thrombin ---> time to clot
- Prolonged TT:
- heparin (much more sensitive to heparin than aPTT)
- hypofibrinogenemia
- dysfibrinogenemia (congenital and acquired)
- interference with fibrin polymerization
- FDP's
- paraproteinemia
- uremia
- Fibronogen
- Several different methodologies - most common is the Clauss method
- citrated plasma + excess thrombin ---> time to clot ---> standard
curve
- an acute phase reactant - may be elevated in many disease states
- May be decreased in various clinical settings including:
- liver disease
- dysfibrinogenemia
- DIC
- FDP's
- D-Dimer (This is a cross-linked fibrin degradation product which
results from plasmin proteolysis)
- More specific for DIC than the FDP assay (requires BOTH thrombin
and plasmin)
- May be elevated in:
- DIC
- Resorption of extensive clots/bleeds (surgery)
Back to Top
II. INITIATION/LOCALIZATION: VASCULAR
(Rare, usually diagnosis by exclusion)
- Hereditary
- Hereditary Hemorrhagic Telangiectasia (HHT)
- Collagen (eg., Ehlers-Danlos, Ostogenesis Imperfecta, Marfans)
- Acquired
III. INITIATION/LOCALIZATION: PLATELETS
- Quantitative defects (thrombocytopenia)
- Decreased production
- Leukemias
- Myeloproliferative disorders
- Myelophthisic processes
- Drugs (e.g. ETOH, chemotherapy)
- Increased destruction
- Splenic
- Autoimmune
- Disseminated Intravascular Coagulation (DIC)
- Thrombotic Thrombocytopenic Purpura
- Viral infections
- Neonatal alloimmune purpura
- Post transfusion purpura
- Splenic sequestration (e.g. hypersplenism)
- Qualitative defects (definition: normal platelet count with abnormalities
of adhesiveness and aggregation. Diagnose with bleeding time, platelet
aggregometry and more sophisticated studies).
- Hereditary
- Thrombopathies (e.g. storage pool disease)
- Glanzmans Thrombasthenia (abnormal or absent GP IIb-IIIa)
- Bernard Soulier Syndrome (abnormal or absent GP Ib-IX)
- Von Willebrand's Disease (really a plasma protein deficiency)
- Acquired
- Uremia
- DIC
- Drugs (e.g. Aspirin, non-steroidal anti-inflammatory drugs)
- Fibrin degradation products
- Cardiopulmonary bypass
Back to Top
IV. PROPAGATION: HEREDITARY COAGULATION
FACTOR DEFICIENCIES
General Rule: Hereditary deficiencies tend to be single, while acquired
deficiencies are multiple.
- Hemophilia A vs. von Willebrand's Disease
- Factor IX (PTC, Christmas disease) Same hereditary and clinical
pattern as hemophilia A; incidence = 1:30,000; Dx: prolonged PTT, normal
PT, normal bleeding time Rx: Factor IX - concentrate available.
- Factor XI - "Hemophilia C" Autosomal recessive, increased
incidence in Ashkenazi Jews, variable bleeding tendency; incidence 1:100,000.
Dx: prolonged PTT, normal PT Rx: FFP.
- Contact Factors: XII - rare (patients do not bleed); paradoxical
increased incidence of thrombosis; Prekallikrein and HMWK deficiencies
- no bleeding; autosomal recessive. Prolonged PTT.
- Congenital Factor VII Deficiency - prolonged PT and normal PTT;
variable bleeding tendency; incidence 1:500,000; autosomal recessive. Rx:
FFP and factor VII concentrate.
- Congenital Factors II, V, X and fibrinogen Deficiencies - prolonged
PT and/ or PTT (common pathway); rare; autosomal recessive; bleeding tendency
proportionate to severity of biochemical lesion. Rx: FFP and specific concentrates
(e.g. for fibrinogen = cryoprecipitate).
- Factor XIII - fibrin stabilizing factor with abnormal fibrin urea
solubility test. Not tested in usual screening tests. Severe cases have
delayed bleeding and form keloid scars. Rare; autosomal recessive; Rx:
cryoprecipitate.
- Fibrinogen - rare; autosomal recessive; bleeding tendency; Rx: Cryoprecipitate
Back to Top
V. TERMINATION/ELIMINATION: ARTERIAL
AND VENOUS THROMBOEMBOLIC DISEASE
- Pathogenesis of arterial thrombosis.
- Arterial thrombi result from disruption of underlying vessel wall
(eg most commonly due to atherosclerosis, but also vasculitides and other
causes of vessel wall damage) which exposes subendothelial adhesion molecules
like von willebrand factor and procoagulants like tissue factor. Clots
forming in the high shear rate, arterial environment are initiated by a
platelet nidus forming at the site of damage and grow as platelet rich
"white thrombi" knit together with a fibrin meshwork. Over 50%
of deaths in the western world result from thrombi forming as a result
of atherosclerosis.
- Hemostatic mechanisms play an important role in atherogenesis as
well as the eventual thrombus formation involved in the end stage manifestations
of the disease such as coronary artery occlusion. This pathogenic role
is best understood in the context of the "response to injury"
hypothesis which postulates that the initiation and progression of the
atherosclerotic lesion is driven by the multifaceted response to the initial
injury by, for example, lipid damage to the vessel wall. Part of the response
to injury is stimulation of the hemostatic system. Epidemiologic evidence
over the past decade have firmly linked hemostatic mechanisms with atherogenesis.
For example, high fibrinogen levels have are associated with an elevated
risk of developing atherosclerosis which is about equal to the risk attributed
to high cholesterol. The precise mechanistic links between the hemostatic
response and atherogenesis remain to be fully elucidated and are a focus
of intense investigation.
- Pathogenesis of venous thromboembolic disease
- The vessel wall is usually intact in venous thrombosis. Thrombi
form as a result of stasis and an imbalance between propagation and termination
of hemostasis. Thrombi formed in the low flow venous system tend to be
fibrin rich "red clots" with numerous trapped red blood cells
and islands of platelet aggregates. When these thrombi embolize they are
the cause of more than 50,000 deaths a year in the United States.
- Clinical risk factors are usually present even in hereditary thrombotic
diatheses attributed to underlying biochemical abnormalities. These risk
factors include:
- Surgical and non-surgical trauma
- Age > 40 years
- Previous venous thromboembolism
- Immobilization
- Malignant disease
- Heart Failure
- Obesity
- Estrogens
- Leg paralysis
- Myocardial infarction
- Varicose veins
- Pregnancy
- Most venous thromboembolic disease occurs in older individuals with
the well defined risk factors listed above. However, a significant subset
of patients develops disease at a younger age and have a syndrome referred
to as Juvenile Thrombophilia defined as having the following 3 characteristics:
young age at onset (<40-50 years), recurrent disease sometimes in unusual
vascular beds (eg mesenteric) and a positive family history for venous
thromboembolic disease. Over half the individuals with the clinical picture
of Juvenile Thrombophilia will have demonstrable biochemical abnormalities
of hemostasis involving primarily mechanisms of termination of thrombus
formation and elimination of formed thrombus. Although Juvenile Thrombophilia
can be attributed to a heterogeneous group of underlying abnormalities
they are all autosomal dominant and have the same clinical manifestations
of venous thromboembolic disease. As a result, clinicians usually look
at a panel of laboratory tests in this clinical setting since they can
not discriminate clinically. These abnormalities include:
- Protein C deficiency
- Protein S deficiency
- Factor V Leiden mutation (confers resistance to PC inactivation
of activated factor V)
- Antithrombin III deficiency
- Dysfibrinogenemia (mostly mutations conferring resistance to fibrinolysis)
- Homocysteinemia (also a risk factor for arterial disease)
- Treatment of venous thromboembolism is the same for acquired and
hereditary disease: initial heparin infusion to inhibit the acute process
for 5-7 days followed by 3-6 months of oral anticoagulant therapy with
warfarin. In patients with severe recurrent disease, and especially those
with Juvenile Thrombophilia, lifelong oral anticoagulant therapy may be
indicated.
Back to Top
VI. PROPAGATION/TERMINATION: VITAMIN
K
- Fat Soluble
- Phylloquinone (K1) Diet
- Menaquinone (K2) Intestinal Flora
- Mode of Action
- Procoagulant Factors: II, VII, IX, X
- Inhibitors: Protein C, Protein S
- Post Translational Vitamin K-dependent modification leading to formation
of gamma carboxyglutamic acid residues at the aminoterminus of the vitamin
K dependent factors. This modification is necessary for the factors to
bind negatively charged phospholipid membranes.
- Surface Binding Model
- Pathologic Processes
- Decreased Vitamin K Intake (typical case presents with isolated
increased PT and bleeding)
- Malnutrition
- Antibiotic therapy
- Fat malabsorption
- Usually treated with 10mg of vitamin K1 injection
- Hemorrhagic Disease of Newborn
- Due to immature hepatic carboxylase & decreased Vitamin K in
maternal milk
- Bleeding diathesis in first week to 10 days of life; therefore,
in U.S., all infants get vitamin K prophylaxis at birth (1mg injection).
- Warfarin (see mechanism above)
- Warfarin (Coumadin) is used as an anticoagulant. This drug inhibits
the normal cycling of Vitamin K, such that it accumulates in an inactive
form (epoxide) as opposed to the functional (reduced) form. The administration
of Warfarin renders the patient "functionally" Vitamin K deficient.
The magnitude of the prolonged PT is used to monitor and adjust the dose
of Warfarin. Since Warfarin is administered orally, it is also referred
to as "oral anticoagulation". Once started on Warfarin, the average
patient requires 3-6 days to reach the desired therapeutic range. The half-life
of Warfarin is approximately 35-40 hours. Patients are frequently placed
on Warfarin for an extended period of time after a thrombotic event - usually
3-6 months. This drug can cross the placenta and induce severe fetal defects
- mainly abnormal bone formation. Many drugs either potentiate or antagonize
the anticoagulant actions of Warfarin.
Back to Top
VII. PROPAGATION, TERMINATION AND ELIMINATION:
DISSEMINATED INTRAVASCULAR COAGULATION (DIC)
- DIC is an intermediary mechanism of disease that is caused by a
large number of different conditions which lead to the generation of intravascular
thrombin with consequent consumption of the cellular and humoral components
of the hemostatic system:
- Shock
- Metastatic adenocarcinoma (Trousseau's syndrome when thrombosais
occurs)
- Sepsis
- Viremia
- Major intravascular hemolysis
- Obstetric complications (eg dead fetus, amniotic fluid embolism...)
- Snake bites
- Massive trauma
- Acute brain injury
- Acute myelogenous leukemia
- The dynamic nature of the consumptive process in DIC is demonstrated.
Depending on the magnitude of the stimulus the consumptive process may
be decompensated, compensated or overcompensated.
- The pattern of laboratory abnormalities in DIC
- The treatment of DIC
- First and foremost TREAT THE UNDERLYING CAUSE
- Replace consumed factors and platelets with blood component therapy
- Heparin - ~ 5% of cases (eg., acute promyelocytic leukemia, organ
ischemia, amniotic fluid embolism, etc.)
- Future possibilities: antithrombin III & Protein C concentrates
Back to Top
VIII. PROPAGATION, TERMINATION AND ELIMINATION:
LIVER DISEASE
- Acute hepatic necrosis due to infection, shock or toxin all lead
to acute hemostatic insufficiency with inability to produce most coagulation
factors. Prognosis is related to extent of damage.
- Chronic liver disease due to alcohol, viral infection, autoimmune
disease etc is far more common than massive hepatic necrosis. Since the
liver produces most of the coagulation factors and clears the degraded
and inactivated products of coagulation it has been referred to as the
master organ of the hemostatic system. The coagulopathy of chronic liver
disease is complex, reflecting not only liver insufficiency but also hypersplenism
and in some instances platelet and bone marrow (especially with ETOH) suppression.
The following diagram outlines the multifaceted hemostatic defect associated
with chronic liver disease.
- Summary of the components of the hemostatic defect of chronic liver
disease
- Decreased platelet count, 2° splenic sequestration
- Decreased coagulation factors, 2° decreased production (exception
FVIIIc and fibrinogen, both acute phase reactants and conserved to end
stages of chronic liver disease)
- Increased FDP, 2° decreased clearance and low grade DIC (if
fibrinogen markedly decreased probably superimposed DIC secondary to infection
or other cause)
IX. PROPAGATION/TERMINATION: CIRCULATING
ANTICOAGULANTS
.
- Non-lmmune anticoagulants
- Heparin is a commonly used therapeutic anticoagulant. It is a sulfated
mucopolysaccharide which binds antithrombin III and by doing so increases
the avidity of the AT III for its substrates (eg thrombin and factor Xa)
1000 fold
- FDP's
- Immune anticoagulants (also referred to as inhibitors) (eg. IgG
>> IgM >>> IgA)
- Factor Specific
- Hemophilia - 25% of severe patients develop inhibitors and they
bleed!
- Non-Hemophilic (mainly associated with autoimmune & Lymphoproliferative
diseases. A vast majority involve inhibitors of FVIIIc and they bleed,
although inhibitors have been described to almost all known hemostatic
proteins and in instances where the inhibitor recognizes a physiological
anticoagulant like protein C the consequence can be thrombosis)
- Non-Factor Specific - lupus-like inhibitor (LLI) caused by antibody
directed against negatively charged phospholipid membrane/protein complexes
in the thromboplastin or partial thromboplastin reagents used for the PT
and PTT.
- Increased PTT more frequently than PT (usually do not bleed)
- Thrombosis in 10-25% (eg., deep venous thrombosis, pulmonary embolism,
recurrent abortion and arterial thrombosis.)
- Rarely be associated with low prothrombin levels or thrombocytopenia
in which case the patient may bleed.
- Diagnosis of Inhibitors vs. hereditary factor deficiencies (i.e.,
what to do with prolongation of the PTT and/or PT)
Step 1: Plasma mixing studies (abnormalities secondary to simple
factor deficiencies correct the PT and/or PTT to normal with equal part
mixture of normal plasma and patient plasma, whereas, abnormalities secondary
to inhibitors do not fully correct the PT and/or PTT). The plasma mixing
study is referred to as a "fifty-fifty mix" .
Step 2: Specific assays
Factor assays if mixing study completely corrects to normal
Phospholipid dependent assay, if mixing study does not correct, to
exclude a Lupus Like Anticoagulant.
X. APPENDIX: DIAGRAMMATIC REVIEW OF NORMAL
HEMOSTATIC MECHANISMS
Back to Top
Go Back to Hemostasis
and Thrombosis
Go Back to Course
Outline
Questions?
Comments? Send a message to the CATS guru: jkessler@salus.uvm.edu