DVT and pulmonary embolism Flashcards
What type of DVT (deep vein thrombosis) can be a big risk?
above knee DVT – associated with embolic events
What type of risk does DVT have?
Multifactorial risk
How does DVT form?
• Thrombus in deep vein around valves
• Multiple triggers – finely balanced system is destabilised
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What is a pulmonary embolus?
fragmentation of proximal clot which travels in venous system until it lodges in the pulmonary circulation
Where are the consequences of DVT?
Consequences locally in source limb and/or in heart or lungs after embolization
What is Virchow’s triad and what does it include?
contributing factors to thrombosis:
- Endothelial injury
- Stasis
• Blood components o Platelets o Coagulation factors o Coagulation inhibitors o Fibrinolytic factors
What happens as we get older?
We get more thrombotic as we get older
How does a haemostatic plug form?
- Release of tissue factor from vessel injury
- Coagulation cascade activated
- Vessel injury also platelet release reaction and vasoconstriction
- They all combine to form a stable haemostatic plug
What is present at the site of the clot?
- Tissue factor
- activating factors
- conversion of prothrombin to thrombin
- Activated platelets and platelet complexes
- Fibrinogen converted to fibrin
What forms the clot?
Cascade forms a clot
Cascade is self-perpetuating until something stops it
What are the phases of the cascade?
3 phases of cascade:
- initiation
- amplification
- propagation (this produces clot)
if these phases are interrupted the process is terminated
Which pathways can activate the cascade?
extrinsic and intrinsic pathways
What 2 things are part of the extrinsic pathway?
Tissue factor and vessel damage are part of the extrinsic pathway – more important than intrinsic pathway
What is crucial in the cascade regardless of the pathway?
conversion of factor 10 to 10a
What does factor 10a do and how does this lead to the production of a thrombus?
- It acts on prothrombin to thrombin, which activates conversion of fibrinogen to fibrin
- Activation of factor 13 and cross linked fibrin produces thrombus
What helps control the clotting cascade?
There’s inhibitors to balance:
• Proteins C and S – combine to make activated protein C
o Act on factor Va and VIIIa
• Anti-thrombin acts on X and thrombin
What can change the homeostatic point for the clotting cascade?
Defects in protein and C or S change the homeostatic point for clotting cascade
Why does the haemostatic plug form? and how does it form?
Haemostatic plug formation: Response to injury
- Vessel constriction
- Formation of unstable platelet plug
• platelet adhesion
• platelet aggregation - Fibrin stabilisation of the plug with fibrin
• blood coagulation - Dissolution of clot and vessel repair
• Intrinsic fibrinolysis
What happens in fibrinolysis?
- Tissue plasminogen activator converts plasminogen into plasmin
- It breaks down fibrin cross linking strands into D-dimer
- Raised D-dimer levels suggest tissue inflammation or active breakdown of fibrin clot
Where are clotting factors, fibrinolytic factors and inhibitors synthesised?
- liver
- endothelium
- megakaryocytes (platelets)
What are some measurements used in DVT/PE/clotting cascade?
- Prothrombin time – PT = PTR
- Partial thromboplastin time – APTT =APTTR
- Thrombin time - TT
What are the risk factors for venous thrombosis?
Stasis
• Prolonged immobility eg surgery, travel
• Stroke
• Cardiac failure
Coagulation abnormality • Surgery or major trauma • Pregnancy and puerperium • Oestrogen medication • Malignancy
Others • Age • Past history or family history of VTE • Obesity • COVID-19
What are the clinical features of DVT?
- Pain, tenderness of veins
- Limb swelling
- Superficial venous distension
- Increased skin temperature
- Skin discoloration
- All reflect obstruction to the venous drainage
- There are multiple differential diagnoses for these presenting features
How is DVT diagnosed?
- Risk assessment
- Evidence based pre-test probability score (well’s score) – combines clinical factors in an evidence based way (determines role for diagnostic imaging vs use of blood test for exclusion)
- D dimer for exclusion
- Diagnostic tests
- Compression ultrasonography
- Venography