Hemostasis Flashcards
How does the vascular endothelium reduce unprovoked thrombosis?
Healthy endothelial cells possess antiplatelet and anticoagulant activity that functions to inhibit clot formation. The negatively charged vascular endothelium repels platelets, and endothelial cells produce potent platelet inhibitors such as prostacyclin (prostaglandin I2) and nitric oxide that prevent adhe- sion of quiescent platelets. An adenosine diphosphatase (CD39) expressed on the surface of vascular endothelial cells also serves to block platelet activation through degradation of adenosine diphosphate (ADP), a potent platelet activator. The vascular endothelium also plays a pivotal anticoagulant role by expressing several inhibi- tors of plasma-mediated hemostasis. Endothelial cells can increase activation of protein C, an anticoagulant, via surface expression of thrombomodulin (TM), which acts as a cofactor for thrombin-mediated activation of protein C, making its activation 1000 times faster. Endothelial cells also produce tissue factor pathway inhibitor (TFPI), which inhibits the procoagulant activity of factor Xa and the TF–VIIa complex. Finally, the vascular endothelium synthesizes tissue plasminogen activator (tPA), which is responsible for activating fibrinolysis, a primary counter-regulatory mechanism limiting clot propagation.
Describe what occures with platelets after an endothelium injury
Damage to vascular endothelial cells exposes the underlying
extracellular matrix (ECM), which contains collagen, von
Willebrand factor (vWF), and other platelet-adhesive
glycoproteins. Platelet receptors for vWF (glycoprotein
Ib-IX-V complex) and collagen (integrin α2β1) facilitate
platelet adhesion to the site of vessel injury. Absence of either vWF (von Willebrand disease) or glycoprotein Ib-IX-V complex receptors (Bernard–Soulier syndrome) results in a clinically significant bleeding disorder.
In addition to promoting their adhesion to the vessel wall, the platelet interaction with collagen serves as a potent stimulus for the subsequent phase of platelet activation. During the activation phase, platelets secrete agonists such as thromboxane A2
(TxA2 ) and release granular
contents, resulting in recruitment and activation of
additional platelets and propagation of plasma-mediated
coagulation. Platelets contain two specific types of storage granules: α-granules and dense bodies. α-Granules contain numerous proteins essential to hemostasis and wound repair, including fibrinogen, coagulation factors V and VIII, vWF, platelet-derived growth factor, and
others. Dense bodies contain the adenine nucleotides
ADP and adenosine triphosphate (ATP), in addition to calcium, serotonin, histamine, and epinephrine. Redistribution of platelet membrane phospholipids during
activation exposes newly activated platelet surface receptors and binding sites for calcium and coagulation factor.
activation complexes, which is critical to propagation of
plasma-mediated hemostasis. During activation, platelets
also undergo structural changes to develop pseudopodlike membrane extensions and to release physiologically active microparticles, which serve to dramatically increase the platelet membrane surface area.
During the final phase, platelet aggregation, activators
released during the activation phase recruit additional
platelets to the site of injury. Newly active glycoprotein
IIb/IIIa receptors on the platelet surface gain higher affinity for fibrinogen, thereby promoting crosslinking and aggregation with adjacent platelets. The importance of these receptors is reflected by the bleeding disorder associated with their hereditary deficiency, Glanzmann thrombasthenia
Describe the extrinsic pahtway of coagulation
The extrinsic pathway of coagulation is now understood to represent the initiation phase of plasma-mediated
hemostasis and begins with exposure of blood plasma to
tissue factor (TF).
After vascular injury, small concentrations of factor VIIa circulating in plasma form phospholipid-bound activation complexes with TF, factor X, and calcium to promote conversion
of factor X to Xa. Additionally, the TF/factor VIIa complex activates factor IX of the intrinsic pathway, further demonstrating the key role of TF in initiating hemostasis
Describe the intrinsic pathway of coagulation
The intrinsic pathway begins when factor XII or the Hageman factor is exposed to collagen, kallikrein, and high molecular weight kininogen (HMWK) and is subsequently activated. Factor XIIa activates factor XI into XIa. With a calcium ion, factor XIa activates factor IX. Then, factor IXa, factor VIIIa, and calcium form a complex to activate factor X. Factor VIII is found in the blood and is often activated by thrombin (factor IIa)
Describe the common pathway of Coagulation
The final pathway, common to both extrinsic and intrinsic coagulation cascades, depicts thrombin generation and subsequent fibrin formation.
Signal amplification results from activation of factor X by both intrinsic (FIXa, FVIIIa, Ca2+) and extrinsic (TF, FVIIa, Ca2+) tenase
complexes. The tenase complexes in turn facilitate
formation of the prothrombinase complex (FXa, FII [prothrombin], FVa [cofactor], and Ca2+), which mediates a surge in thrombin generation from prothrombin. Thrombin proteolytically cleaves fibrinogen molecules to generate fibrin monomers, which polymerize into fibrin strands to form a clot. Finally, factor XIII is activated by
thrombin and acts to covalently crosslink fibrin strands producing an insoluble fibrin clot resistant to fibrinolytic
degradation
Which are the major contrarregulatory pathways of coagulation?
Four major counterregulatory pathways have been identified
that appear particularly important for downregulating
hemostasis: fibrinolysis, tissue factor pathway inhibitor (TFPI) , the protein C system, and
serine protease inhibitors (SERPINs)
In vivo, plasmin generation
is most often accomplished by release of … from the vascular endothelium
tissue plasminogen activator (tPA) or urokinase
How the normal fibrinolysis is limited to the area of clot formation?
Activity of tPA and urokinase is accelerated in the presence of fibrin, thereby limiting fibrinolysis to areas of clot formation
How does plasmin acts?
Promotes enzymatic degradation of fibrin and fibrinogen and inhibits coagulation by degrading essential cofactors V and VIII and reducing platelet glycoprotein surface receptors essential to adhesion and aggregation.
What is the role of the serine protease inhibitors (SERPINs) in the coagulation process?
excessive fibrinolysis is prevented by the function
of two key SERPINs, namely plasmin-activator inhibitor-1 (PAI-1) and α2‐antiplasmin. PAI-1 serves as the primary inhibitor of tPA and urokinase, thereby decreasing plasmin generation, whereas α2‐antiplasmin directly
inactivates circulating plasmin
In addition, one of the most significant SERPINs regulating hemostasis is antithrombin (AT, formerly antithrombin III).
AT can inhibit all procoagulant proteases of the blood clotting cascade, but its primary targets appear to be thrombin and factors Xa (FXa) - less efficiently IXa (FIXa) XI, XII and the others.
Heparin binds AT, causing a conformational change that
accelerates AT-mediated inhibition of targeted enzymes
by over1000-fold
What is the tissue factor pathway inhibitor role in the coagulation?
TFPI binds and inhibits factor Xa through the formation of membrane-bound complexes. These factor Xa–TFPI complexes also act to inhibit TF/factor VIIa complexes, thereby downregulating the extrinsic coagulation pathway
How the protein C system acts?
The protein C system proves particularly important in regulating coagulation through inhibition of thrombin and the essential cofactors Va and VIIIa.
After binding to thrombomodulin on the surface of the endothelial cell, thrombin’s procoagulant function decreases and instead its ability to activate protein C is
augmented. Activated protein C (APC), complexed with the cofactor protein S, degrades both factors Va and VIIIa. Loss of these critical cofactors limits formation of tenase and prothrombinase complexes essential to formation of factor Xa and thrombin, respectively
Von Willebrand factor function
Under normal conditions vWF plays a critical role in platelet
adhesion to the extracelular matrix and prevents degradation of factor
VIII by serving as a carrier molecule.
Von Willebrand Disease presentation and labs
Classically, patients with vWD describe a history of easy
bruising, recurrent epistaxis, and menorrhagia, which are characteristic of defects in platelet-mediated hemostasis.
In more severe cases (i.e., type 3 vWD), concomitant
reductions in factor VIII may lead to serious spontaneous
hemorrhage, including hemarthroses.
Routine coagulation studies are generally not helpful in the diagnosis of vWD, as the platelet count and prothrombin time (PT) will be normal in most patients
and the activated partial thromboplastin time (aPTT) may
demonstrate mild-to-moderate prolongation depending
on the level of factor VIII reduction. Initial screening tests
for vWD involve measurement of vWF levels (vWF antigen) and vWF platelet binding activity in the presence of the ristocetin cofactor, which leads to platelet agglutination.
Presentation and labs of hemophilia A and B
Hemophilia A (factor VIII deficiency) and hemophilia B
(factor IX deficiency) are both X-linked inherited bleeding disorders most frequently presenting in childhood as
spontaneous hemorrhage involving joints and/or deep
muscles.The severity of the disease is dependent
on an individual’s baseline factor activity level. Severe disease, defined by less than 1% of coagulation factor activity, occurs in approximately two-thirds of patients with hemophilia A and one half of patients with hemophilia B.
Classically, laboratory testing in patients with hemophilia reveals prolongation of the aPTT, whereas the PT, bleeding time, and platelet count remain within normal limits. However, a normal aPTT may also be seen in
mild forms of hemophilia, and it is important to exclude
vWD as a cause of factor VIII deficiency
What is the hemophilia C?
Factor XI deficiency, known as hemophilia C or Rosenthal syndrome (prevalence: 1 in 1,000,000), is characterized by isolated prolongation in aPTT and variable bleeding severity. Factor XI activity levels, however, do not correlate well with bleeding risk. Most individuals
do not experience spontaneous bleeding, hemarthrosis, or
muscle hematomas, though bleeding episodes can occur
under situations of hemostatic challenge such as trauma,
surgery, or childbirth
Presentation and coagulogram of Factor XIII deficiency
Factor XIII is involved in stabilizing the fibrin clot. Factor
XIII deficiency (prevalence: 1 in 2,000,000) presents with delayed bleeding after hemostasis, impaired wound healing, and, occasionally, pregnancy loss.
Laboratory evaluation in these patients will demonstrate normal aPTT and PT, but the diagnosis can be confirmed by measurement of factor XIII activity levels
Explain the vitamine F deficiency ant it’s aassociation with bleeding
Vitamin K is an essential fat-soluble vitamin that is required for the carboxylation of factors II, VII, IX, and X and proteins C and S. Without carboxylation, these factors cannot bind to the phospholipid membrane of
platelets and participate in hemostasis.
How does liver diseas affects coagulation?
The liver is the primary site for production of procoagulant factors, including fibrinogen; prothrombin (factor
II); factors V, VII, IX, X, XI, and XII; the anticoagulants
protein C and S; and AT. Severe liver disease impairs synthesis of coagulation factors, produces quantitative and
qualitative platelet dysfunction, and impedes clearance
of activated clotting and fibrinolytic proteins. Laboratory
findings commonly associated with liver disease include
a prolonged PT and possible prolongation of the aPTT,
suggesting that these individuals are at increased risk of
bleeding. However, these abnormal values only reflect
decreases in procoagulant factors and do not account
for parallel decreases in anticoagulant factors (protein
C, protein S, and AT). As a result, patients with chronic
liver disease are thought to have a rebalanced hemostasis
and actually generate amounts of thrombin equivalent to
healthy individuals.6
Similarly, thrombocytopenia from platelet sequestration in the spleen is often observed in patients with liver disease and portal hypertension. However, levels of the plasma metalloprotease ADAMTS13, responsible for cleaving vWF multimers, are also decreased in chronic liver disease and result in high circulating levels of large
vWF multimers, which promote platelet aggregation.
Consequently, this increase in vWF may partially correct
for thrombocytopenia and platelet dysfunction but can
also result in a prothrombotic state and increase clotting
risk.
Fibrinolysis of a formed clot is also aberrant in patients with liver disease. Excessive fibrinolysis is prevented by thrombin-activatable fibrinolysis inhibitor (TAFI), which blocks activation of plasmin from plasminogen. TAFI is synthesized by the liver, and because levels are decreased in patients with chronic liver disease,
it was believed that such individuals are at increased
bleeding risk because of hyperfibrinolysis. However,
levels of PAI-1, an inhibitor of tPA and urokinase, are
also increased in liver disease and may serve to normalize fibrinolytic activity. Thus, in patients with chronic
liver disease, hemostatic mechanisms are rebalanced,
but decreases in procoagulant and anticoagulant factors
create a tenuous equilibrium that is easily disrupted. As
a result, these patients are at risk for both bleeding and
inappropriate clotting.
How does renal disease affects the coagulation?
Platelet dysfunction commonly occurs in association
with chronic renal failure and uremia and has primarily been attributed to decreased platelet aggregation and
adhesion to injured vessel walls. Impaired adhesion is
likely the result of defects of glycoprotein IIb/IIIa, which
facilitates platelet binding of fibrinogen and vWF.
Additionally, accumulation of guanidinosuccinic acid
and the resulting increase in endothelial nitric oxide
synthesis further decrease platelet responsiveness. Red
blood cell (RBC) concentration has also been suggested
to contribute to impaired platelet activity, as correction
of anemia shortens bleeding times. This is thought to be
the result of the increased RBC mass displacing platelets
from the center of the vessel and bringing them into
close proximity of the endothelium, thereby promoting
adhesion
Presentation and labs of disseminated intravascular coagulation
Disseminated intravascular coagulation (DIC) is a pathologic hemostatic response to TF/factor VIIa complex that
leads to excessive activation of the extrinsic pathway, which overwhelms natural
nticoagulant mechanisms
and generates intravascular thrombin
Most often, DIC presents clinically as a diffuse bleeding disorder associated with consumption of coagulation factors and platelets during widespread microvascular thrombotic activity resulting in multiorgan dysfunction.
Laboratory findings typical of DIC include reductions in platelet count; prolongation of the PT, aPTT, and thrombin time (TT); and elevated concentrations of soluble fibrin and fibrin degradation products (D-dimers). However, DIC is both
a clinical and laboratory diagnosis, so laboratory data
alone do not provide sufficient sensitivity or specificity to confirm a diagnosis
Conditions associated with DIC
1) Infections:
- Bacterial (gram-negative bacilli, grampositive cocci)
- Viral (CMV, EBV, HIV, VZV, hepatitis)
- Fungal (histoplasma)
- Parasites (malaria)
2) Malignancy
- Hematologic (AML)
- Solid tumors (prostate cancer, pancreatic cancer)
- Malignant tumors (mucin secreting adenocarcinoma)
3) Obstetric causes
- Amniotic fluid embolism
- Preeclampsia/eclampsia
- Placental abruption
- Acute fatty liver of pregnancy
- Intrauterine fetal demise
4) Massive inflammation
- Severe trauma
- Burns
- Traumatic brain injury
- Crush injury
- Severe pancreatitis
5) Toxic/ immunologic
- Snake envenomation
- Massive transfusion
- ABO blood type incompatibility
- Graft versus host disease
6) Other
- Liver disease/fulminant hepatic failure
- Vascular disease (aortic aneurysms, giant hemangiomas)
- Ventricular assist devices
Explain the cardiopulmonary bypass-associated coagulopathy
Initial priming of the bypass circuit results in hemodilution and thrombocytopenia. Adhesion of platelets
to the synthetic surfaces of the bypass circuit further
decreases platelet counts and contributes to platelet
dysfunction. During CPB, expression of platelet surface
receptors important for adhesion and aggregation (GPIb,
GPIIb/IIIa) are downregulated and the number of vWFcontaining α-granules are decreased, thereby impairing
platelet function. Furthermore, induced hypothermia
during CPB results in reduced platelet aggregation and
plasma-mediated coagulation by decreasing clotting
factor production and enzymatic activity. Increased
plasmin generation may also occur during CPB, a process that accelerates clot lysis
