Anticoagulant Drugs Flashcards
What are some indications for prescribing anticoagulant drugs?
Atrial fibrillation and venous thrombosis
What part of the haemostatic mechanism do anticoagulant drugs target?
The fibrin clot (secondary haemostasis)
Why does atrial fibrillation predispose to thrombosis?
Causes stasis of blood in the left atrium as the heart doesn’t pump properly
What is the action of heparin?
Potentiates antithrombin = has immediate effect
How is heparin given?
Parenteral = IV or SC
2 forms = unfractionated, low molecular weight
What is the action of LMWH?
Keeps thrombin/factor Xa complex together = more predictable action so can use fixed dose
What is the action of unfractionated heparin?
Binds to thrombin/anti-thrombin complex to keep it together
How is unfractionated heparin monitored?
Using the activated partial thromboplastin time
How is LMWH monitored?
Anti-Xa assay can be used = only done in complicated patients (e.g renal failure)
What effect does heparin have on the prothrombin and activated partial thromboplastin times?
Prolongs both
What are the complications of heparin?
Bleeding, heparin induced thrombocytopenia (monitor FBC), osteoporosis if long term
How is heparin reversed?
Stop heparin = short half life if unfractionated
Protamine sulphate in severe bleeding = reverses anti-thrombin effect
How effective is protamine sulphate at reversing heparin?
Completely reverses unfractionated heparin
Partially reverses LMWH
What are some examples of coumarin antagonists?
Warfarin, phenindone, acenocoumarin
What is the action of coumarin anatagonists?
Inhibit vitamin K = causes production of inactive clotting factors
What elements of the haemostatic mechanism are dependent on vitamin K?
Factors II, VII, IX and X
Proteins C and S
Why is warfarin prescribed alongside warfarin for the first 3-5 days?
Protein C and S levels fall before clotting factors do = causes transient increases in clot risk
Why is warfarin monitored?
It has a narrow therapeutic index = aim of INR of 2-3
How should warfarin be taken?
Can be started slowly or rapidly = takes 6hrs to work
Dose should be taken at same time every day
How does warfarin affect the prothrombin and activated partial thromboplastin times?
Prothrombin time more sensitive to warfarin so is prolonged by more
What is the INR?
Mathematical correction for differences in the sensitivity of thromboplastin reagents
What does the INR allow for?
Comparison of results between labs and standardises reporting of prothrombin time
What are the factors that influence bleeding risk with warfarin use?
Intensity of anticoagulation
Concomitant clinical disorders and use of other medications = beware of drug interactions
Quality of management
What are some bleeding complications that occur with warfarin use?
Mild = skin bruising, epistaxis, haematuria Severe = GI or intra-cerebral, significant drop in Hb
