Inhibition of haemostasis Flashcards
What are anticoagulants
- act w/in coag cascade
- inactivate factors that have been activated
Describe the mechanism of Tissue factor pathway inhibitor (TFPI)
[Physiological inhibitors of 2dary haemostasis]
- produced by endo.c & MK
- TFPI alpha: K1 & K2 domains bind & inhibit FVIIa; K3 domain bind to protein S => enhances FXa inhibition (by TFPIα)
- TFPI beta: K1 & K2 bind & inhibit FXa
Describe the mechanism of Antithrombin-glycosaminoglycan pathway
[Physiological inhibitors of 2dary haemostasis]
- (SERPIN: Serine Protease Inhibitor)
- antithrobmin (AT) & Heparin cofactor II (minor AT) > both inhibit thrombin = inhibit plt aggregation
- & inhibit FIIa, FXa, FIXa, FVIIa
Describe Protein C in the Protein C/S system
[Physiological inhibitors of 2dary haemostasis]
- Endothelial Protein C receptor (EPCR) binds to Protein C -> presents to thrombin
- activated via thrombin => Activated Protein C (APC) = prolong clotting time
- inactivated FVa & FVIIIa w/ protein S
Describe Protein S in the Protein C/S system
[Physiological inhibitors of 2dary haemostasis]
- free form only has anticoag. activity (C4b-bound is inactive)
- inactivate FVa & FVIIIa w/ protein C
- anticoagulant activity on FVa-FXa complex
Describe the mechanism of Protein Z/Z dependent protease inhibitor
[Physiological inhibitors of 2dary haemostasis]
- Protein Z dependent protease inhibitor (ZPI) + preotein Z (+Ca2) => inhibits FXa
- Protein ZPI inhibits FIXa, FXIa
describe what a pathological inhibitor of coagulation is (pathological anticoagulant)
- auto Aby or anti-phospholipid syndrome that inhibit normal coag. cascade => haemorrhagic diathesis
How do you dx Aquired Haemophilia A (AHA) (4 test/Ix)
[pathological inhibitors]
- Screening test: prolonged aPTT
- Mixing test ( to rule out deficiency)
- Test for anti-phospholipid Aby (to rule out antiphospholipid syndrome)
- Bethesda assay to confirm anti-FVIII aby
describe principle of the bethesda assay for testing acquired pathological inhibitors
- FVIII inhibitors quantified by mixing plasma w/ known FVIII amount
- incubated @ 37C for 2h
- amount of inhibitor = control (plasma & buffer) - results
* 1 Bethesda unit = inhibitor that will inactivate half of the factor in the mixture (pts & normal plasma) in 2 hrs
What is Antiphospholipid syndrome
[pathological inhibitors]
- autoimmune disorder
- prothrombic factors interact => arterial & venous thrombosis
- can be associated w/ other autoimmune disease = make antiphospholioid Aby
How do you dx Lupus anticoagulant (LA) (4 test/Ix)
[pathological inhibitors]
- Prolong phospholipid-dependent coagulation test
- Dilute Russell Viper Venom Test (dRVVT)
- Phospholipid-neutralisation test (PNT)
- Partial thromboplastin time (PTT)
- Confirm w/ LA-sensitive reagent to demonstrate phospholipid dependence
Describe how warfarin works
[pharmacological inhibitors]
- inhibit synthesis of functional Vit-K-dependent coag factors (II, VII, IX, X)
(- bc Vit K = carbosylation of pCoag protein = fuctional Ca2+ binding sites = activated)
Describe how heparin works
[pharmacological inhibitors]
- unfractionated heparin (UFH) & Low MW Heparin (LMWH)
- binds to antithrombin = anticoag activity
How can you monitor heparin treatment (UFH & LMWH)
[pharmacological inhibitors]
UFH: aPTT (ratio range 1.5-2.5)
LMWH: snti-Xa more infarmative than aPTT but has limitations
Which pharmacological anticoagulant is used with Venous thromboembolism (important??)
- first episode treated w/ warfarin along w/ parenteral anticoag (UFH or LMWH)
- treat for >= 5d until INR is >=2.0 for 24+hrs
Describe how fondaparinux works
- anti-Xa activity higher than LMWH
- & longer halflife of LMWH
Describe how FXa inhibitors works (direct oral anticoag: DOAC)
- Rivaroxiban, Apixaban, …aban
- inhibit thrombin generation
Describe how Thrombin inhibitors works (direct oral anticoag: DOAC)
- Dabigatran, …atran
- inhibit thrombin action (thrombiun not bind to thrombin active site)
The difference b/w these inhibitors:
a) Physiological
b) pathological
c) Pharmocaological
a) prevent intravascular thrombosis/coagulation
b) ineffective coagulation => haemorrhage
c) therapeutically manipulate haemostasis e.g. for medical procedures or VTE
What happens if there’s a deficiency in Physiological inhibitors of 2dary haemostasis?
predispose Venous Thromboembolism
Give some examples of Acquired antithrombin (AT) deficiency
dec production (liver cirrhosis); inc loss (nephrotic syndr.); enhanced consumption (DIC & prolonged UFHeparin)
Once aquired AT deficiency have been ruled out, what test can you use to Dx AT deficiency?
- Functional AT assay
- Antigenic AT assay
= allows differentiation b/w type I & Type II deficiency
What’s the purpose of mixing studies (50:50 aPTT) i.e. the indication if corrected or not
Corrected: deficiency is present (& inhibitor is present e.g. heparin)
Not corrected: Lupus anti-coag. is present
What’s the purpose of the Platelet Neutralising procedure (PNP) & the indication of results i.e. short/long time
- uses lysed plt suspension (excess phospholipids = use up lupus Aby) = allow remaining plts to aggregate = shorter time
- Shorter time (<3s than control) = LAC
- Control: use saline = lupus Aby bind to plts = shorter time
What’s the purpose of the diluted Russell Viper Venom Time (dRVVT) & the indication of results i.e. ratio of test/normal
- direct FXa from RVV bypass intrinsic factors (FVIII, IX or XI deficiency not affect result)
- LAC prolong time by clotting to phospholipid
- N ratio: 0.9-1.05
- If result is >1.05 = suggest presence of LAC but could also be due to abnormality in FII, V, X or fibrinogen or another inhibitor.
