Session 5: Thrombosis and Embolism Flashcards
Definition of thrombus.
A solid mass formed from the constituents of the blood within the heart or vessels during life.
Definition of thrombosis.
Process of formation of a thrombus.
What three things are thrombosis dependent on?
Virchow’s triad:
Changes in vascular wall (endothelial damage)
Changes in blood flow (turbulent or slow flow)
Changes in the blood itself (hypercoagulability)
Referring to Virchow’s triad, when is arterial and cardiac thrombi likely to form?
When there is a site of endothelial damage or turbulent flow.
Referring to Virchow’s triad, when is venous thrombi likely to form?
Often seen where there is stasis.
How many components of Virchow’s triad are needed to produce thrombus?
Seems like only two out of three like stasis and hypercoagulability.
Give common causes of endothelial damage in vessels.
Myocardial infarction
Secondary to the haemodynamic stress of hypertension
Scarred heart valves
Trauma
Surgery
Inflammation
Surface of atherosclerotic plaque when they break open.
Explain how endothelial damage can lead to thrombus formation.
When there is a breach in the endothelium as in clotting platelets will adhere to exposed von Willebrand factor/factor VIII complex.
This is usually no problem when the blood flow is swift like in an artery because the platelet thrombi won’t be able to grow. This is because the swift blood flow will continuously wash away platelets, chemical mediators and clotting factors.
However if there is also stasis in the vessel this can allow the thrombus to grow bigger.
Referring to the stasis and endothelial damage, where are you most likely to see thrombus formation?
In veins because the blood flow is slower in veins.
What else in veins increases risk of thrombus formation?
The valves causing pockets of stagnant flow and also turbulent flow.
Give examples of conditions where blood flow might be particularly slow/turbulent in the veins.
Pregnancy where the woman is not moving PAtiens on bed rest or immobilised Lack of muscular contraction in the calves result in blood stasis. Cardiac failure Ulcerated atherosclerotic plaques Within aneurysm Around abnormal heart valves After MI
What else can blood stasis/turbulent flow cause?
It can cause endothelial damage itself.
Causes of hypercoagulability
Pregnancy After surgery Fractures Burns Smoking Cancer Oral contraceptive pill Factor V Leiden Antithrombin III deficiency Protein C deficiency Protein S deficiency
How can pregnancy, post-surgery, trauma and burns result in hypercoagulability?
Increased circulating levels of fibrinogen and factor VIII leading to the increased coagulability
Where are you likely to see thrombus?
Platelets flow in the blood by the sides at the surface of the endothelium. This means that platelets are more likely to aggregate behind a valve. This is particularly common in endothelial injury or haemostasis. In surgery palettes aggregate more easily as well and platelet collection will grow more quickly.
Explain formation of a thrombus.
E.g. in haemostasis fibrinogen binds the platelets together and fibrin will grow out of the platelet layer. The fibrin will then trap red blood cells. This means a white layer (platelets) will be covered by a red layer (fibrin and red blood cells).
The surface of the red layer is thrombogenic and this means that platelets will adhere to the exposed fibrin. In this way a second white layer of platelets is formed. This happens over and over again, rinse and repeat.
Upon examination you can see the laminations by the naked eye. Thrombus is therefore a laminated structure.
What are the laminations called?
Lines of Zahn.
How do arterial thrombi and venous thrombi differ in appearance?
Arterial thrombi have more obvious lines of Zahn than venous thrombi.
Arterial: Pale Lines of Zahn Granular Low cell content
Venous: Soft Gelatinous Deep red Higher cell content
Post-mortem blood will clot and form thrombi.
How can you tell the difference between post-mortem thrombi and pre-mortem thrombi?
Post-mortem thrombi have no laminations at all.
They are also more rubbery and shiny than pre-mortem thrombi and they are not attached to the intima.
Pain is not always present in thrombus formation. However it can be, why would it be?
It is usually painful in the superficial veins.
What is thrombophlebitis?
Painful superficial thrombi with associated inflammation in the wall of the vein.
What is a parietal thombus?
When the thrombus is attached to the wall of the vessel and restricts the lumen.
What is an occlusive thrombus?
When the thrombus fill and obstruct the lumen.
What do arterial thrombi tend to be? Parietal or occlusive?
Arterial thrombi tend to remain parietal. This is because of the flow of the blood in the arteries.
When can occlusive thrombi form in the arteries?
They usually form over an atherosclerotic plaque that has cracked open.
What is a vegetation? What’s the danger of vegetations`
A thrombus on a cardiac valve. They easily embolise.
Where do vegetations most commonly form? Why?
On the valves of the left heart.
Because they are exposed to greater pressures and therefore also to microtrauma which exposes thrombogenic subendothelial tissue.
What are the outcome of thrombi?
Resolution Propagation Organisation Recanalisation Embolisation
Explain resolution.
The thrombus dissolves
Explain propagation.
Progressive spread of the thrombosis and the thrombi will grow bigger and bigger.
This happens distally in arteries and proximally in veins.
Explain organisation.
Thrombus undergoes fibrous repair and forms a fibrous scar on the wall of the vessel.
Explain recanalisation.
In an occluding thrombus new channels lined with endothelium can run through the occlusion itself and restore blood flow. However the blood flow will have significantly smaller capacity than original vessel.
Explain embolisation. (Briefly)
Part of the thrombus breaks of and embolisms. This is called a thromboembolism when the embolus comes from a thrombus.
Thrombi that form where are more likely to embolise?
In the lower limbs such as the femoral, iliac and popliteal veins.
Most common clinical effects of thrombosis.
Occlusion of an artery at the site of the thrombus leading to ischaemia and infarction like in a myocardial infarction.
Pulmonary embolism, cerebrovascular accident from embolism
Congestion and oedema can lead to pain and skin ulcerations.
Repeated miscarriage.
