Haemostasis and thrombosis Flashcards
What is a typical presentation of a patient with DVT
Immobile for 3 weeks after major surgery
Right calf swollen & collateral superficial veins present
Palpation - localised tenderness & pitting oedema
Immobility (i.e after surgery) is a key risk factor for the development of a thrombus and therefore DVT.
What investigations should be carried out if you suspect that the patient has a DVT
Measure vital signs- although DVT will not alter these
Two-level Wells score- survery- 9 points assessed (e.g entire leg is swollen, pitting oedema confined to symptomatic leg, collateral superficial veins (non-varicsose). Scored out of 10 - closer to 10 -greater probability of DVT.
Take blood for D-dimer testing- measures fibrin degradation products
Arrange a proximal leg vein scan- ultrasound to look for thrombosis.
Summarise the diagnosis and treatment for DVT
Positive D-dimer test - diagnosis of deep-vein thrombosis (DVT) interim treatment with parenteral anticoagulant
Ultrasound scan confirms DVT maintenance treatment with oral anticoagulant
In which type of veins does DVT occur
And the most common disorder is deep vein thrombosis (DVT). DVT is the formation of a blood clot within one of the deep veins commonly occurring in the femoral or popliteal veins of the leg.
Summarise the cell based theory of coagulation
- Initiation – small scale production of thrombin.
a. Targeted by ANTI-COAGULANTS. - Amplification – large scale production of thrombin (on platelet surfaces).
a. Targeted by ANTI-PLATELETS. - Propagation – generation of fibrin strands by thrombin.
a. Targeted by THROMBOLYTICS.
Describe the initiation phase of the coagulation cascade
§ STEP 1 – Initiation – small scale thrombin production:
o Tissue factor:
§ TF-bearing cells activate F10 and F5 forming the prothrombinase complex.
o Prothrombinase complex:
§ This actives F2 (pro-thrombin), forming thrombin.
o Antithrombin (AT-III):
§ AT-III inactivates F10a and thrombin.
Essentially, what is thrombin
Thrombin is F2a
Describe the role of thrombin
Thrombin (factor IIa) cleaves fibrinogen, producing fragments that polymerise to form fibrin. It also activates factor XIII, a fibrinoligase, which strengthens fibrin-to-fibrin links, thereby stabilising the coagulum. In addition to coagulation, thrombin also causes platelet aggregation, stimulates cell proliferation and modulates smooth muscle contraction.
Describe the orally active direct thrombin inhibitors (or non-vitamin K dependent anti-coagulants)
Inhibit factor IIa
Dabigatran (oral) - factor IIa inhibitor
Inhibit factor Xa
Rivaroxaban (oral) - factor Xa inhibitor
Summarise the role of heparins as anti-coagulant drugs
Increase activity of AT-III
Heparin (IV, SC) - activates AT-III (fIIa & fXa)
Low-molecular weight heparins (LMWHs, e.g.Dalteparin) - activate AT-III (fXa)
Describe the mechanism of action of heparin
Heparin inhibits coagulation, both in vivo and in vitro, by activating antithrombin III. Antithrombin III inhibits thrombin and other serine proteases by binding to the active site. Heparin modifies this interaction by binding, via a unique pentasaccharide sequence, to antithrombin III, changing its conformation and increasing its affinity for serine proteases
Explain the difference in pharmacology between heparin and LMWHs
To inhibit thrombin, it is necessary for heparin to bind to the enzyme as well as to antithrombin III; to inhibit factor Xa, it is necessary only for heparin to bind to antithrombin III (Fig. 25.6). Antithrombin III deficiency is very rare but can cause thrombophilia and resistance to heparin therapy.
The LMWHs increase the action of antithrombin III on factor Xa but not its action on thrombin, because the molecules are too small to bind to both enzyme and inhibitor, essential for inhibition of thrombin but not for that of factor Xa
Explain the route of administration for heparins
Heparin is not absorbed from the gut because of its charge and high molecular-weight, and it is therefore given intravenously or subcutaneously (intramuscular injections would cause haematomas).
Summarise the use of warfarins as anti-coagulants
Reduce levels of other factors
Warfarin (oral) - vitamin K antagonist
Vitamin K - required for generation of factors II, VII, IX & X
Describe the mechanisms of action of warfarin
Vitamin K antagonists act only in vivo and have no effect on clotting if added to blood in vitro. They interfere with the post-translational γ-carboxylation of glutamic acid residues in clotting factors II, VII, IX and X. They do this by inhibiting vitamin K epoxide reductase component 1 (VKORC1), thus inhibiting the reduction of vitamin K epoxide to its active hydroquinone form (see Fig. 25.5). Inhibition is competitive (reflecting the structural similarity between warfarin and vitamin K; see Fig. 25.3).
What is vitamin K essential for
It is essential for the formation of clotting factors II, VII, IX and X, which are glycoproteins with γ-carboxyglutamic acid (Gla) residues
How long do the effects of warfarin take to develop
The effect of warfarin takes several days to develop because of the time taken for degradation of preformed carboxylated clotting factors. Onset of action thus depends on the elimination half-lives of the relevant factors. Factor VII, with a half-life of 6 h, is affected first, then IX, X and II, with half-lives of 24, 40 and 60 h, respectively.
Describe the importance of the different anti-coagulant drugs having different routes of administration.
Oral -improved patient adherence, less management required.
I..V/S.C- faster onset- useful in emergencies.
Outline the treatment pathway for patients with DVT.
interim treatment with parenteral anticoagulant. DALTEPARIN- fast-acting while the patient is in hospital
maintenance treatment with oral anticoagulant RIVAROXABAN / WARFARIN -dabigatran rarely used- associated with G.I bleeding.
What are the indications for these anti-coagulants
Venous thromboembolism (DVT + PE)
Prevent thrombosis during surgery
Atrial fibrillation – prophylaxis of stroke
What is a common consequence of DVT
Pulmonary embolsim
the thrombus embolises and is carried up through the venous system to the right side of the heart- where it then gets lodged in the lungs.
Describe the typical presentation of DVT
Chest pain Dyspnoea & Tachypnoea Low oxygen saturation <96% High RR >16 Low BP <120/80
How can we diagnose P.E
Two-level Wells score = 6
Multiple-detector computed tomographic pulmonary angiography (CTPA)
Outline the treatment for P.E
CTPA confirms pulmonary embolism (PE) DALTEPARIN / HEPARIN - as a general rule heparin is more potent- give I.V>S.C for rapid action
Ultrasound scan confirms DVT maintenance treatment - oral anticoagulant RIVAROXABAN / WARFARIN
Define thrombosis
Thrombosis is the pathological formation of a ‘haemostatic’ plug within the vasculature in the absence of bleeding (‘haemostasis in the wrong place’).
What determines how likely you are to develop a thrombosis
Virchow’s triad
Describe Virchow’s triad
Rate of blood flow
Blood flow slow/stagnating no replenishment of anticoagulant factors & balance adjusted in favour of coagulation (e.g when patient is immobile)
Consistency of blood
Imbalance between pro-coagulation & anticoagulation factors (e.g FV Leiden)
Blood vessel wall integrity
Damaged endothelia blood exposed to pro-coagulation factors (e.g in HTN- endothelia exposed to high shear stress).
Summarise Virchow’s triad
Over a century ago, Rudolph Virchow defined three predisposing factors – ‘Virchow triad’: injury to the vessel wall – for example, when an atheromatous plaque ruptures or becomes eroded; altered blood flow – for example, in the left atrial appendage of the heart during atrial fibrillation, or in the veins of the legs while sitting awkwardly on a long journey; and abnormal coagulability of the blood – as occurs, for example, in the later stages of pregnancy or during treatment with certain oral contraceptives (see Ch. 36).
Describe a typical presentation for acute coronary syndrome
History of hypertension & hyperlipidaemia
Shortness of breath, sweating, dizziness & chest pain
Describe some key investigations for acute coronary syndrome
Blood pressure = 122/83, Pulse rate = 68 bpm, Respiratory rate = 21 breaths per minute, SpO2 = 92%
No changes on ECG & elevated troponin.
Elevated troponin key- indicated damage to cardiac myocytes.
Describe arterial thromboses
Arterial thromboses do not generally occur due to the deposition of a clot on the blood vessel wall within the lumen. In arteries a clot tends to form within an atherosclerotic plaque and this gives rise to a thrombus composed mainly of platelets and leukocytes often referred to as a ‘white thrombus’.
Clot forms within the vessel wall -appears white due to foam cells- which are macrophages that take up cholesterol from the vessel wall- giving the white appearance.
Form due to damaged endothelia.
Describe venous thrombi
Venous thromboses (red thrombus) are mainly composed of fibrin and erythrocytes with a small platelet component. Around 1 in 1000 people are affected by venous thrombosis in the UK. And the most common disorder is deep vein thrombosis (DVT). DVT is the formation of a blood clot within one of the deep veins commonly occurring in the femoral or popliteal veins of the leg. This condition arises due to a stagnating blood flow (point 1 in Virchow’s triad) and is associated with air travel but can often remain undetected when there are no symptoms.
What is acute coronary syndrome
Any condition brought on by sudden, reduced blood flow to the heart
i.e myocardial infarction
Describe a NSTEMI
Non-ST elevation myocardial infarction
This is caused by partial occlusion of a coronary artery and it can lead to stable angina
Treatment with Anti-platelets (e.g. clopidogrel and aspirin)
Describe a STEMI
ST elevated myocardial infarction
‘White’ thrombus fully occluded coronary artery
Treatment: antiplatelets & thrombolytics
What can cause acute coronary syndrome
White thrombi: Caused by: Damage to endothelium Atheroma formation Platelet aggregation
Summarise the amplification stage of coagulation
§ STEP 2 – Amplification – large-scale production of thrombin (on platelets):
o Thrombin:
§ Thrombin activates platelets in a +ve feedback effect.
o Activated platelet:
§ Changes shape. (discoid to stellate)
§ Platelets become “sticky” and attaches other platelets. and migrate to the thrombus to cause further occlusion.
Explain, in detail, how thrombin causes platelet activation
Thrombin - binds to protease-activated receptor (PAR) on platelet surface.
PAR activation rise in intracellular Ca2+
Ca2+ rise exocytosis of adenosine diphosphate (ADP) from dense granules
ADP receptors
ADP activates P2Y12 receptors platelet activation/ aggregation (paracrine and autocrine effect)
Cyclo-oxygenase
PAR activation liberates arachidonic acid (AA)
Cyclo-oxygenase (COX) generates thromboxane A2 (TXA2) from AA
Glycoprotein IIb/IIIa receptor (GPIIb/IIIa)
TXA2 activation expression of GPIIb/IIIa integrin receptor on platelet surface
GPIIb/IIIa - involved in platelet aggregation
What type of receptor is PAR
GPCR.
Summarise the anti-platelet drugs
Aspirin – irreversible COX1 inhibitor – it reduces the production of thromboxane by platelets (oral)
Clopidogrel – irreversible ADP (P2Y12) receptor antagonist (oral)
Abciximab (IV/SC) – monoclonal antibodies directed at GlpIIb/IIIa (limited clinical use - tend to only be used by specialists).
What is important to remember about the dosing of aspirin
NB: High doses no more effective BUT more side-effects
such as G.I bleeding
What are the indications for the use of anti-platelet drugs
Indications for drug use – ARTERIAL THROMBI:
o Acute coronary syndromes – MI.
o Atrial fibrillation – prophylaxis of strokes.
Summarise aspirin
Low-dose aspirin (see Ch. 27) in chronic use profoundly (>95%) inhibits platelet TXA2 synthesis, by irreversible acetylation of a serine residue in the active site of cyclo-oxygenase I (COX-1). Oral administration is relatively selective for platelets partly because of presystemic drug elimination (Ch. 10). Unlike nucleated cells, platelets cannot synthesise proteins, so after administration of aspirin, TXA2 synthesis does not recover fully until the affected cohort of platelets is replaced in 7–10 days.
Describe clopidogrel
Clopidogrel and prasugrel inhibit ADP-induced platelet aggregation by irreversible inhibition of P2Y12 receptors (Ch. 17) to which they link via a disulfide bond
Summarise COX-1 inhibitors such as aspirin
Irreversibly acetylates a serine residue on the COX enzyme. More effective anticoagulant at lower concentrations due to a reduced effect on endothelial COX, which is involved in the production of PGI2.
What is important to remember about clopidogrel
An oral antiplatelet agent that irreversibly blocks the P2Y12 ADP receptor subtype. Acts as a pro-drug that must be metabolized by the liver to generate the active metabolite.
It is a prodrug and is converted into its active sulfhydryl metabolite by CYP enzymes in the liver including CYP2C19. Patients with variant alleles of CYP2C19 (rapid or poor metabolisers) are at increased risk of therapeutic failure from lack of efficacy or from bleeding. There is a potential for interaction with other drugs, such as omeprazole (Ch. 31), that are metabolised by CYP2C19 and current labelling recommends against use with proton pump inhibitors for this reason.
State some other ADP antagonists
Prasugrel
Ticagrelor
Cangrelor
State some other G2b/3a receptor antagonists
Tirofiban An intravenously administered non-peptide glycoprotein IIb/IIIa receptor antagonist.
Eptifibatide An intravenously administered peptide glycoprotein IIb/IIIa receptor antagonist.
When does the amplification phase of coagulation take place
The coagulation process only proceeds to the amplification phase when the TF- bearing cells come into contact (i) platelets which are only present within blood vessels and (ii) the factor VIII/ von Willebrand factor (vWF) complex, which is only released when the vascular endothelium is damaged. The amplification phase sets the stage for subsequent large-scale thrombin production and involves thrombin-mediated activation of factors V, VIII, IX on the surface of platelets.
What is a typical presentation of a stroke
Severe headache, dizziness & loss of coordination
Numbness in the face, arms and legs
What are the key investigations for stroke
Serum glucose & electrolytes - within normal ranges
CT scan – eliminate possibility of haemorrhagic stroke- this is an important exclusion- as it is a stroke caused by inappropiate bleeding and not occlusion - we therefore need to exclude this as if we treat for ischaemic drug (with fibrinolytics) but the patient actually has a hemorrhagic stroke- then this will make them worse.
Summarise the treatment for ischaemic strokes
Ischaemic stroke
Thrombolytic therapy ALTEPLASE (tPA)
Hemorrhagic stroke- surgery
What category of thrombi do strokes fall into
Don’t really fit into ‘red’ or ‘white’ classification as they form within the atria (often associated with A.F) and then lodge in the lumen of the cerebral arteries.
Describe the propagation phase of coagulation
Generation of fibrin strands
Activated platelets
Large-scale thrombin production
Thrombin
Factor IIa binds to fibrinogen and converts to fibrin strands
What is important to remember about anti-platelets and anti-coagulants
Anticoagulants & anti-platelets - DO NOT remove pre-formed clots
BUT they can still be used as maintenance therapy- it is just better to use thrombolytics in emergency situations.
- Antiplatelets used for prophylaxis, not specifically treatment of atherosclerosis
- Thrombolytics can be used to treat ruptured plaques but mainly indicated for ischaemic stroke
Summarise thrombolytics
Convert plasminogen plasmin
Plasmin - protease degrades fibrin
Alteplase (IV) - recombinant tissue type plasminogen activator (rt-PA)
Describe streptokinase
Plasimogen activator
Streptokinase is a plasminogen activating protein extracted from cultures of streptococci. Infused intravenously, it reduces mortality in acute myocardial infarction, and this beneficial effect is additive with aspirin (see Fig. 25.8). Its action is blocked by antibodies, which appear 4 days or more after the initial dose: its use should not be repeated after this time has elapsed.
Describe some other recombinant tPA analogues
Reteplase A non-glycosylated form of human tPA administered by i.v. injection and used to treat acute MI
Tenecteplase A genetically engineered variant of alteplase with three mutations designed to increase plasma halflife, potency and reduce PA inhibitor inactivation.
Summarise the treatment of thrombosis
The drugs that directly or indirectly target the clotting factors are known as anticoagulants and these will affect all three stages of coagulation
The antiplatelet drugs affect processes involved in platelet aggregation and these drugs will only act on stages 2 & 3 of the cell-based theory of coagulation
The thrombolytic (or fibrinolytic) drugs are able to dissolve the fibrin strands holding the clot together and these drugs will only affect the final stage of coagulation.
What is Bivalirudin
A specific and reversible inhibitor of thrombin with a short duration of action as is recommended as the first line agent for patients with HIT undergoing PCI.
How often must heparins be administered
Has a short half- life (~ 1hour) and must be given at regular intervals or by continuous infusion..
Describe some other indirect thormbin inhibitors
Fondaparinux A pentasaccharide derived from the portion of heparin that binds to antithrombin. It is a selective factor Xa inhibitor that requires daily subcutaneous injections.
Apixaban fXa inhibitor approved in 2014
What is coumarin
Natural version of Vitamin K.
What can blood be described as
However, blood is not a homogenous liquid and can be more accurately described as a heterogenous colloidal suspension, containing numerous cell types, proteins, lipoproteins and immunoglobulins
Summarise the composition of blood
The liquid component of blood is known as plasma and comprises around 55% of the total blood content. The remaining 45% is made of blood cells namely erythrocytes (red blood cells), leukocytes (white blood cells) and thrombocytes (platelets).
What is found in the blood plasma
Serum Clotting factors Water (90%) Electrolytes (e.g. Na+, Cl, HCO3) Proteins (e.g. albumin) Lipoproteins (e.g.cholesterol) Pro-coagulants Prothrombin Fibrinogen Clotting factors V, VII-XIII Anticoagulants Tissue factor pathway inhibitor Protein C Protein S Antithrombin III Plasminogen
Differentiate between haemostasis and thrombosis
When a vessel becomes damaged it is important that the body repairs this as soon as possible to minimise any blood loss that may occur. Haemostasis is an essential physiological process where an impermeable platelet and fibrin plug or clot is formed at the site of the vessel injury.
However, this process can also occur pathophysiologically i.e. within the lumen or the walls of a blood vessel that it not ruptured. The pathophysiological clotting of blood is known as thrombosis and to prevent this from occurring there are numerous physiological anticoagulants already present within the blood (see table 1).
What normally has to be damaged for a thrombus to form
The tunica intima
Describe the management of a NSTEMI
o Reduce lipid formation and platelet aggregation/activation – antiplatelets:
E.G. Clopidogrel, aspirin, Abciximab.
Describe the management of a STEMI
o Reduce lipid formation, platelet aggregation/activation and dissolve thrombus – antiplatelets and thrombolytics:
APs – Clopidogrel, aspirin, Abciximab.
TLs – Alteplase.
