Outline of Lecture
Thrombosis
Most common cause of death and disability in the United States
Arterial thrombosis
Myocardial infarction: 600,000 deaths per year
Stroke: 200,000 deaths per year
Venous thrombosis
Pulmonary embolism: 50,000 deaths per year
Thrombosis
Formation of a solid mass of platelets, RBCs, WBCs and fibrin within a living blood vessel or the heart.
Definitions
Thrombus: intravascular, living vessel or heart
Blood clot: extravascular, hematoma, post-mortem, in vitro
Embolus: fragment of thrombus that moves to a new site
Arterial Thrombosis
High speed blood flow
High shear and turbulence
Thrombus primarily composed of platelets with smaller amounts of fibrin and other cells
Thrombus associated with vascular abnormalities (atherosclerosis) most often
Arterial Thrombosis
Venous Thrombosis
Slow blood flow
Low shear
Thrombus primarily composed of fibrin with layers of platelets and RBCs
Most often occurs in cases of stasis (inadequate flow) or biochemical abnormalities
Venous Thrombosis
Thromboembolism
Arterial: often fragment of thrombus from heart wall or heart valve, travels downstream to smaller vessel – may lead to stroke or MI
Venous: fragment of venous thrombus that breaks off and travels upstream towards the heart, may lead to pulmonary embolism
Embolus
Pathophysiology of Thrombosis
Injury or Activation of Endothelium
Alteration in Blood Flow
Alteration in Hemostatic System
Biochemical Abnormalities
Endothelium
Injury or Activation of Endothelium
Atherosclerosis
Life style – smoking, obesity
Immune mediated
Heparin induced thrombocytopenia
Antiphospholipid Antibody Syndrome (Lupus Inhib)
Trauma
Artificial Surface (vascular graft)
Inflammation/Infection
Alteration in Blood Flow
Arterial Thrombosis
Turbulence, shearing forces, platelet binding
Atherosclerosis, vascular graft
Venous Thrombosis
Stasis, slow flow
bed rest, sitting for long trip
Alteration in Hemostatic System
Coagulation System
Fibrinolytic System
Platelets
Vessel Wall/Endothelium
Thrombophilia
Unusual or unexplained thrombosis – spontaneous, idiopathic, essential
Young age (20s, 30s, 40s)
No risk factors
Recurrent, unusual site (arm), multiple sites, extensive
Trigger – pregnancy, trauma, surgery
Coagulation Regulation
Accelerated Coagulation due to reduced inhibition of coagulation
Accelerated Coagulation due to elevated levels of coagulation factors
Coagulation Regulation – Strong Risk Factors
Antithrombin/Heparin Complex
Antithrombin/Endothelial Complex
Antithrombin Function
Regulates coagulation by inhibiting thrombin, FIXa, FXa
Forms 1:1 complex with active site of factor
Antithrombin is consumed in the process
Complexes are cleared by the liver
Activity increased 1000 fold by heparin
Hereditary Antithrombin Deficiency
Antithrombin Clinical
Increased risk of venous thromboembolism
First episode typically in 20s to 40s associated with pregnancy, trauma or surgery
Most common sites for thrombosis
Lower extremities
Pulmonary embolus
Mesenteric vein thrombosis
Superior sagittal sinus thrombosis
Cause of decreased Antithrombin
Heparin therapy
Nephrotic syndrome
Asparaginase therapy
DIC
Hereditary deficiency
Reduced production
Abnormal molecule
Heparin and Antithrombin
Antithrombin Deficiency – Case 1
19 y/o woman with worsening abdominal pain, peritonitis, hematemesis
Surgery revealed small bowel infarction due to mesenteric vein thrombosis
Past History – three episodes of deep venous thrombosis
Family History – father had recurrent DVT, died of mesenteric vein thrombosis at age 29
Antithrombin Deficiency – Case 1
Laboratory evaluation
Antithrombin activity = 48%
Antithrombin antigen = 50%
Protein C antigen = 115%
Follow up
Died of hemorrhagic cerebral infarct
Protein C is activated by
Thrombin/thrombomodulin complex
Activated Protein C inactivates FVa
Protein C System
Protein C and Protein S are vitamin K dependent proteins
Protein C is activated by thrombin/thrombomodulin on endothelial cells
Protein S is a co-factor
Activated protein C + protein S destroys factor Va and factor VIIIa – blocking coagulation
Protein C System – 3 abnormalities
Protein C deficiency
Protein S deficiency
Mutation of factor V cleavage site (activated protein C resistance)
Hereditary Protein C deficiency
Autosomal dominant inheritance with incomplete penetrance
most patients heterozygous – 1 gene abnormal
rare severe homozygous – purpura fulminans
Incidence heterozygous – 1:200
Incidence symptomatic – 1:1000
Activity levels 50% of normal
Increased risk of venous thrombosis
Causes of decreased Protein C
Warfarin therapy
Ongoing thrombosis
Vitamin K deficiency
Liver disease
Post-operative state
Hereditary deficiency
Reduced production
Abnormal molecule
Protein S
Co-factor of Protein C, produced in megakarocytes and endothelium
Vitamin K dependent – activity reduced more than antigenic level
60% bound to C4B-binding protein (inactive)
Protein S Assays
Total Protein S Antigen
Free Protein S Antigen – used at UW
Protein S Activity
Protein S deficiency
High C4B-binding protein – common
normal total, low free, low activity
Abn gene – low PS production – 1:2000
low total, low free, low activity
Abn gene – abnormal molecule – rare
normal total, ?free, low activity
Protein C/Protein S Clinical
Increased risk of venous thrombosis
First episode – 20s to 40s, associated with pregnancy, trauma, surgery
Warfarin associated skin necrosis
occurs 24 – 48 hrs after starting warfarin
Neonatal purpura fulminans
homozygous PC or PS deficiency – rare
Protein C and Protein S on Warfarin
Protein C and Protein S activity decrease along with Factors II, VII, IX and X
Protein C antigen falls to less than 50%
Protein S antigen falls to about 80%
Difficult to diagnose hereditary deficiency on warfarin
Protein C Deficiency – Case Study
APC Resistance – Mutant Factor V
Activated Protein C (APC) destroys factor Va by cleaving it at arginine 506
Some patients have a mutated factor V with a glutamine at position 506, this prevents APC from cleaving factor Va and destroying it
Defect is termed Factor V Leiden or APC resistance
Increased risk of venous thrombosis
Assay for Factor V Leiden
APC Resistance Assay
Determine aPTT in plasma before and after addition of Activated Protein C
Dilute sample in Factor V deficient plasma
Interpretation of ratio (PTTapc/PTT)
>2.1 normal
1.5-1.8 heterozygous abn Factor V
<1.3 homozygous abn Factor V
Probability of Factor V Abnormality
European/Hispanic Americans
3-5% of healthy subjects
20% of patients with venous thrombosis
30-50% of patients with venous thrombophilia
African Americans <1%
Asian Americans <1%
Summary of Protein C System
Protein C deficiency
decreased production
abnormal protein
Protein S deficiency
low free PS due to high C4B-binding protein
decreased production
abnormal protein
Factor V Leiden/APC Resistance
Coagulation Regulation – Mild Risk Factors
Prothrombin gene
Mutation of guanine to adenine at position 20210 in the 3’ untranslated region of prothrombin gene
Results in elevated levels of prothrombin in blood
Primarily leads to increased risk of venous thrombosis
Elevated Factor VIII
Other Coagulation Factors
Homocysteine Metabolism
Hyperhomocysteinemia
Hyperhomocystinuria – homozygous CBS deficiency – severe atherosclerosis
Hyperhomocysteinemia – modest elevation of homocysteine in plasma
Endothelial injury
Increased arterial thrombosis – OR 2
Increased venous thrombosis – ?? – not recommended
Venous Thrombosis Risk Model
Venous Thrombosis Risk Model
Multiple Hit Hypothesis
Development of venous thrombosis requires multiple risk factors or a strong stimulus
Hip surgery
Antithrombin deficiency + plane ride
Prothrombin mutation + oral contraceptives
Combined Genetic Defects
Cerebral Venous Thrombosis and Oral Contraceptives
Risk of First Venous Thrombosis
Healthy subjects, no defects 0.05%/yr age 60
Homocysteinemia 0.05%/yr (no change)
AT, PC, PS deficiency 1.0%/yr age 29
Increased FVIII, Pro, FVL 0.2%/yr age 40
Combined defects 2.5%/yr
Risk of bleeding on warfarin 2.0%/yr
Risk of Recurrent Venous Thrombosis
All patients 2.6%/yr
No Defects 1.3%/yr
Men vs women 2.7x
Oral contraceptive use 2.2x
AT, PC, PS deficiency 4.0x
FVL, Prothrombin, FVIII 2.0x
Hyperhomocysteinemia No change
Cancer, elderly
Bleeding on warfarin 2.0%/yr
Incidence of Coagulation Abnormalities
Protein C and Thrombosis
Thrombosis in Children
On average children have fewer spontaneous thromboses than adults
Children and young adults may require 3 or 4 risk factors before thrombosis occurs:
Central Venous Catheters
Disease (cancer, heart disease, infection etc)
Hereditary deficiencies
Hereditary Risk Factors in Children
Venous Thrombosis Pediatric Risk Model
