Thrombosis and Risk Factors Flashcards
What is Virchow’s triad?
Fundamental factors that contribute to thrombosis:
- Blood flow: stasis (predominantly venous)
- Damage in vascular endothelium: hypercoagulability (arterial)
- Changes in blood** **constituents: hypercoagulability (venous)
What is usually the primary pathological abnormality in arterial thrombosis?
Atherosclerosis
How can atherosclerosis of the vessel wall lead to arterial thrombosis?
- Rupture of atheromatous plaques
- Endothelial injury – expresses tissue factor
- Platelet aggregation and platelet thrombi lead to fine vessel occlusion
what are the risk factors for arterial thrombosis?
- Smoking
- Hypertension
- Hypercholesterolaemia
- Diabetes
- Family history
- Obesity
- Physical inactivity
- Age
- Male
What are the 2 major contributors to the pathophysiology in venous thrombosis?
- Venous stasis
- Hypercoagulable states
Composition of venous thrombi?
Predominantly composed of fibrin (with a lesser role for platelet accumulation and aggregation than arterial thrombi)
Composition of arterial thrombi?
Mostly composed of fibrin and platelets
What is the most common clinical manifestation of venous thrombosis?
DVT
Why is a DVT frequently unrecognised?
As 80% are clinically silent – prevention is key
50% of patients with a proximal DVT have an asymptomatic PE. How has this occurred?
Thrombus travelled from the femoral/pelvic vein to a pulmonary artery;
- DVT is found in 80% of patients with PE
- Sudden death is the presentation in 20% of PE
What is post-thrombotic syndrome?
Can happen to people who have had DVT of the leg. The condition can cause chronic pain, swelling, and other symptoms in your leg due to damage to veins during DVT formation (causing sluggish flow).
What % of people will have recurrent VTE?
30%
What defines a ‘hospital acquired VTE’?
VTE within 90 days of discharge
What proportion of all VTE does hospital acquired VTE account for?
2/3
What is the;
a) prophylaxis
b) treatment
for reducing the incidence of hospital acquired VTE?
- Prophylaxis: consistent risk assessment with tool (for both bleeding and thrombosis), appropriate prophylaxis (LWMH)
- I.e. a patient may not be suitable for a blood thinner
- Treatment: prompt diagnosis, unified care
Care pathway for hospital acquired VTE?
- Patient admitted to hospital
- Assess VTE risk and bleeding risk
- Balance risks of VTE and bleeding: offer VTE prophylaxis if appropriate, but n**ot** if the risks of bleeding outweighs the risk of VTE
- Reassess risks of VTE and bleeding within 24 hours of admission
- Reassess whenever clinical situation changes e.g. bleeding complication, unanticipated immobility
Risk factors for VTE?
- Active cancer/cancer treatment
- Age over 60 years
- Critical care admission
- Dehydration
- Thrombophilia
- Medical comorbidity: heart disease, metabolic, endocrine, respiratory pathologies, acute infectious diseases, inflammatory conditions
- Surgery
- Major trauma
- Personal history of VTE
- HRT or oestrogen-containing contraceptive therapy
- Varicose veins with phlebitis
- Obesity (BMI over 30kg/m2)
- Pregnancy and postnatal period
- Immobility
- First degree relative with VTE
What are the 3 most important anti-coagulant factors in the blood?
- Protein C
- Protein S
- Anti-thrombin III
What can a deficiency in anti-coagulant factors lead to an increased risk of?
Venous thrombosis/PE; will often get at a young age (N.B. this is a rarer cause of VTE than the risk factors)
Function of the fibrinolytic system?
The fibrinolytic system functions to;
a) remove the clot after the vasculature is repaired
b) to degrade clots that form in the bloodstream.
The final step in this pathway is the plasmin-mediated cleavage of fibrin, creating fibrin degradation products.
What can a deficiency in the fibrinolytic system lead to?
More prone to clots (but no clear clinical evidence of link to venous thrombosis)
General advice given to patients in hospital to reduce VTE?
- Do not allow dehydration unless clinically indicated
- Encourage mobilisation
- Aspirin and antiplatelets are not adequate prophylaxis for VTE
- Compression stockings
What are compression stockings?
Elastic stocking exact pressure on the leg at different compressions, aiding venous blood movement up the leg.
What drugs are given for pharmacological prophylaxis of thrombosis?
Blood thinners (given in lower doses than we would for treating a thrombotic event:
-
Low dose LMWH (low molecular weight heparin). More predicatable anti-coagulant effect than the old-fashioned heparin.
- Once a day subcutaneous injection
- Most common
-
Fondaparinux (synthetic pentasaccharide)
- Substitute for LMWH;
- Allergies
- Heparin is derived from mucosal tissues of slaughtered animals such as pig’s lungs –> patients may be unwilling/allergic/religious
- Given parenteral
- Substitute for LMWH;
N.B. the above medications cannot be given orally.
What is Fondaparinux a substitute for?
LMWH
Newer anticoagulants can be given orally. What are these?
- Direct inhibitors of Factor 10a: rivaroxaban (apixaban) - oral
- Directly thrombin inhibitors: dabigatran – oral
What is rivaroxaban? Mechanism?
- Oral anticoagulant
- Direct inhibitor of factor 10a
What is dabigatran? Mechanism?
- Oral anticoagulant
- Direct inhibitor of thrombin
Describe the difference in mechanism of old fashioned fractionated heparin and LMWH?
- Heparin;
- binds to natural anticoagulant factor called antithrombin
- heparin enhances the action of antithrombin by binding to it
- antithrombin inhibits thrombin and factor Xa (works against factor II as well - prothrombin)
- Modern heparin (LMWH);
- have much greater effect when they bind to antithrombin on factor Xa than they do on factor IIa
- i.e. function more as inhibitors of factor Xa anti-coagulants than anti-thrombin anti-coagulants)
- have much greater effect when they bind to antithrombin on factor Xa than they do on factor IIa
- Old fashioned unfractionated heparin;
- has equal effect in inhibiting both thrombin and factor Xa
- will not be effective in inhibiting the effects of thrombin that is bound to clot that already exists
What does LMWH bind to? Effects of this?
Have much greater effect when they bind to antithrombin on factor Xa than they do on factor IIa i.e. function more as inhibitors of factor Xa anti-coagulants than anti-thrombin anti-coagulants)
What does unfractionated heparin bind to? Effects of this? What is this less effective against?
has equal effect in inhibiting both thrombin and factor Xa
will not be effective in inhibiting the effects of thrombin that is bound to clot that already exists
Mechanism of Fondaparinux?
- smaller molecule than LMWH
- will only inhibit factor Xa when bound to antithrombin –> no effect on thrombin at all
Mechanism of Rivaroxaban and Apixaban?
- These are oral agents; more convenient and easier to extend treatment after discharge
- Work directly on factor Xa without needing to bind to antithrombin
Mechanism of Dabigatran?
- Taken orally
- Direct inhibitor of thrombin resulting in;
- decreased formation of fibrin
- reduced thrombin-stimulated platelet aggregation; prevents the formation of thrombi.