Venous Thromboembolism (VTE) - Warfarin

Question 1

Indications of Warfarin (3).

Answer 1

1. VTE - prevent clot extension and recurrence.
2. AF - prevent embolic complications.
3. Heart Valve Replacement - prevent embolic complications (short-term for tissue valve replacement and long-term for mechanical valve replacement).

Question 2

Mechanism of Action of Warfarin.

Answer 2

1. Inhibits hepatic production of vitamin K-dependent coagulation factors and co-factors.
2. Inhibits Vitamin K Epoxide Reductase - which reactivates oxidised Vitamin K which usually is a cofactor for the carboxylation of clotting factors 1972 and Protein C.

Question 3

Contraindications of Warfarin.

Answer 3

1. Immediate Risk of Haemorrhage e.g. Surgery, Trauma.
2. Pregnancy - 1st Trimester (Foetal, Cardiac, Cranial Abnormalities).
3. Pregnancy - 3rd Trimester (Maternal Haemorrhage).

Question 4

Main Adverse Effect of Warfarin.

Answer 4

Bleeding : small increase - increases risk of bleeding following minor trauma or existing abnormalities or large increase - trigger spontaneous haemorrhage e.g. epistaxis or retroperitoneal haemorrhage.

Question 5

Surgery and Warfarin.

Answer 5

Stop 5 days before surgery and wait for INR to be less than 1.5.

Question 6

INR Targets and Warfarin.

Answer 6

1. Mechanical Heart Valves (Mitral Valves INR > Aortic Valves).
2. VTE (Target 2.5; 3.5 if Recurrent).
3. AF (Target 2.5).

Question 7

What is INR?

Answer 7

International Normalised Ratio - Ratio of Patient’s PT over Normal PT.

Question 8

What factors may potentiate warfarin?

Answer 8

1. Liver Disease.
2. P450 Enzyme Inhibitors e.g. Amiodarone, Ciprofloxacin.
3. Cranberry Juice.
4. Drugs to Displace Warfarin from Plasma Albumin e.g. NSAIDs.
5. Platelet Inhibitors e.g. NSAIDs.

Question 9

Other Adverse Effects of Warfarin.

Answer 9

1. Teratogenic - can be used in breastfeeding.
2. Skin Necrosis (Protein C - temporary procoagulant state - thrombosis in venules).
3. Purple Toes.

Question 10

What is required with Warfarin initially?

Answer 10

Cover with LMWH for 5 days when starting - warfarin is initially prothrombotic.