9 - Thrombosis and Embolism Flashcards
What is the definition of a thrombosis?
Solid mass of blood within vessels or the heart (circulatory system)
What is Virchow’s triad?
- Thrombosis depends on three things: changes in vascular wall, blood flow and changes in blood
- Only need two of three for thrombosis
Why are you at risk of thrombosis in pregnancy?
- Hypercoagulable blood and stasis due to pressure on large veins of pelvis by uterus
Give some examples of endothelial damage and how they can cause a thrombus?
- Hypertension, scarred heart valves all are causing endothelial damage and if blood flow is slow not swift, thrombus will form
- This is due to vWF being exposed
Why does slow or turbulent flow lead to thrombosis?
- Gives platelets better chance to stick to endothelium and for clotting factors to accumulate
- Can damage endothelium leading to two of three of Virchow’s triad
- More likely to occur in veins and blood flow is slower
What are some conditions that lead to hypercoagulability of the blood?
- Smoking
- Pregnancy
- Post-op
- OC pill
- Inherited disorders
How would you describe this thrombus and where is it most commonly found?
- Platelets aggregate at the endothelium as they are small so carried along outside
- Lines of Zahn: white layer of platelets crosslinking by fibrin with red layer of red blood cells
- More obvious in arterial thrombi as blood flows over surface
What do thrombi look like post mortem?
- Not laminated
- Rubbery and shiny and not attached to intima
- Need to be able to distinguish if cause of death or not
What is thrombophlebitis?
Painful superficial thrombi in veins that have inflammation in the wall of the vein
What are the two classification of thrombi?
- Parietal: attached to wall of vessels and restrict lumen
- Occlusive: fill and completely obstruct lumen. mainly occurs over atheroscelrotic plaque
What is a thrombus on a heart valve called and what is the complications with it?
- Vegetation
- Easily embolises
- Can become infected, particularly in IV drug users due to microtrauma
- Mainly form on left heart valve because higher pressures and more microtrauma
What are the different outcomes of thrombi?
- Resolution: dissolved by lysis
- Propagation: thrombus grows
- Organisation: thrombus undergoes fibrous repair and forms scar on vessel wall
- Recanalisation: new channels run through occluding thrombus to restore blood flow but less blood than original can flow through
- Embolism: particularly in large veins of lower limb are dangerous when part of thrombus breaks off, thrombo-emoblism
Why do thrombi propagate as they move through veins?
- Veins get larger as you move upwards towards the heart
- Stagnation in the blood above the clot and when tributaries join they join abnormal flow so contribute to clot
What are the effects of thrombosis?
- Ischaemia and infarct due to occlusion
- Embolisation
- Congestion and oedema leading to pain and skin ulceration
- Repeated miscarriages due to thrombosis in uteroplacental often seen in inherited thrombophilias
What is an embolism?
Blockage of a blood vessel by solid, liquid or gas at a site distant from its origin due to blood current
e.g air, amniotic fluid, nitrogen, thromboemboli, body fat, medical equipment
Why can’t embolism occur in veins?
- Blood flows from smaller to larger vessels, therefore they will go to right heart and embolise in pulmonary arteries
If there is a thrombosis in the left and right circulation where will it end up?
- Left will go from aorta to anywhere in the body, e.g brain, renal
- Right will go from veins to lungs to embolise
What are saddle emboli?
Large emboli blocking the pulmonary trunk or bifurcation of other arteries, blocking both tubes, they result in sudden death
What happens when you have a large pulmonary emboli?
- Over 60% occlusion of pulmonary circulation leads to sudden death, right sided heart failure due to pulmonary hypertension or cardiovascular collapse
Why are lots of thrombi seen in the left heart and why is this a risk of embolism?
- Infarcts commonly affect left venricle so thrombi can form on necrotic endothelium
- Atrial fibrillation leads to less atrial contraction, dilation of left atrium and stagnation of blood so thrombus formation
- Vegetation of left valves
What is cardiac mural thrombi?
- Systemic thromboemboli in association with myocardial infarction or atrial fibrillation
- Adhere to vessels or heart causing obstruction but not blockage
What is a paradoxical emboli?
- Thromboemboli that form in systemic veins but embolise to systemic arteries
- Small emboli can fit through arterio/venous anastomoses in pulmonary circulation
- Large emboli can pass through defects in interventricular septum or patent foramen ovale during coughing, lifting or straining
RARE
How do you get an emboli from an atheroma?
- Atheroma is the necrotic material in an atherosclerotic plaque which can be released into blood when plaque opens during surgery or spontaneously
- Often affect intestine so abdominal pain
What are transient ischemic attacks?
- Episodes of neurological dysfunction that appearly suddenly and last a few minutes/hours then disappear
- Usually small atheromaemboli in carotid arteries
- Small so broken down quickly before severe damage
What is fat embolism and when does it occur?
- Usually after bone fracture or liposuction
- Fat escapes bone marrow by breaking into oil droplet and is sucked into venules ripped open by fracture
- Respiratory distress and neurological symptoms few days after fracture
- Can cause petechiae, renal failure, coma etc
How is air embolism caused?
- When inspiring there is negative pressure in veins of chest and head so veins can draw air in after trauma to neck or chest
- 100ml fatal
- Air transported to right heart where forms a frothy mass stopping circulation
- Common after labour
What is the bends?
- Form of gas embolism
- When at depth more gas dissolved in blood so if resurface to fast this gas comes out of solution as air leading to bubbles in blood
- Very painful as distorts tissues
- Nitrogen issue as lipid soluble so focal ischaemia in lipid rich tissues like lungs and joints
- When lungs affected it is the chokes
- Slow decompression in chamber
What is amniotic fluid embolism?
- Complication of labour where amniotic fluid enters maternal circulation
- Respiratory distress, hypotension, seizures, loss of consciousness, DIC (amniotic fluid contains prothrombotic substances)
- Emboli of fetal origin in lungs e.g lanugo
What are some risk factors of DVT?
How can you prevent thromboembolic disease?
- encourage mobility after surgery
- decompression stockings
- raised leg to increase venous return
- calf muscle stimulation
- anticoagulants
- umbrella in IVC to stop pulmonary emboli
What anticoagulants are used in preventing DVT and how do they work?
- Warfarin: interferes with vit K synthesis. dosage titrated to patients PT time
- Heparin: injected IV or subcutaneous. forms irreversible complexes with antithrombin II so it is activated
- Aspirin: irreversibly acetylates cyclooygenase so thromboxane A2 cannot be produced by platelets. For MI, stroke and long haul flights but can cause bleeding
What are the three tests used for clotting?
- Bleeding time: platelets
- PT: extrinsic
- APTT: intrinsic
Briefly outline the clotting cascade.
How does the clotting cascade activate fibrinolysis?
Fibrin activated tPA
What clotting factors are dependent on vitamin K for synthesis?
2, 7, 9, 10
What does clopidogrel do?
Antiplatelet
What factors are in each pathway of the clotting cascade?
- Extrinsic: 7 and 3
- Intrinsic: 8,9,11,12
What is the most common cause of microcytic anaemia?
Iron deficiency
What is trousseau’s syndrome?
What are d-dimers a measure of?
- Measure of crosslinked fibrin, linked to DVT
What is the summary of DIC?
Systemic activation of clotting and this consumes platelets so spontaneous bleeding as well as microthrombi
