Warfarin (Coumadin)

Last updated 26.09.12

 

Index

 

-  Action

-  Dose

-  Start

-  Maintenance

-  Overcoagulation

-  Duration

-  Interaction

 

Features

-  Anticoagulant

-  In-pts: Pharmacy to educate pt re use & effects

-  Coumadin or Maervin brands not bioequivelant

-  Coumadin used in hospital

-  If pt on Maervin keep on during admission

 

Action

-  Disrupts vitamin K synthesis --> less clotting factors (II, VII, IX, X) & protein C & S

-  Anticoagulation effect delayed 72-96hrs

-  Due to initial protein C & S effect wo clotting factors

-  Commonly covered by Enoxaparin

 

Indication

-  AF

-  Mechanical heart valves

-  DVT, PE

 

 

 

Warfarin Dosing

<Warfarin>

 

Features

-  Orally active

-  Increments of 1g (or 0.5 if needed)

-  In-pts

-  Ordered in special section of med chart

-  Dose at 1600 hrs each day for consistency

-  Before initiation ensure INR & plts are normal +/- LFTs

-  Blood levels monitored by INR (PT)

-  Dose given reflected in INR 72-96hrs later

-  Post op do not reload, start pre-op dose

 

Starting Dose

-  Generally requires anticoagulant cover until INR therapeutic ie: Enoxaparin

-  First dose 5mg

-  Day 1 INR

-  < 1.4: 5mg

-  Day 2 INR

-  < 1.8: 5mg

-  1.8-2.0: 1mg

-  > 2.0: Nil

-  Day 3 INR

-  < 2.0: 5mg

-  2.0-2.5: 4mg

-  2.6-2.9: 3mg

-  3.0-3.2: 2mg

-  3.3-3.5: 1mg

-  > 3.5: Nil

-  Day 4 INR

-  < 1.4: 10mg

-  1.4-1.5: 7mg

-  1.6-1.7: 6mg

-  1.8-1.9: 5mg

-  2.0-2.3: 4mg

-  2.4-3.0: 3mg

-  3.1-3.2: 2mg

-  3.3-3.5: 1mg

-  > 3.5: Nil

 

Maintenance Dose

-  INR < 2.0: increase by 5-20%

-  INR 2.0-3.0: Nil change

-  INR 3.0-3.5: decrease by 5-15%

-  INR 3.6-4.0: WH until INR therapeutic, decreased next dose by 10-15%

-  INR 4.0-6.0: WH until INR therapeutic, decreased next dose by 10-20%

 

 

 

Overcoagulation with Warfarin

<Warfarin>

 

- INR 4-6 + nil bleed

-  WH 2/7

-  Restart at reduced dose

-  Nil reversal

-  Check INR 1-2/7

- INR 6-10 + nil bleed

-  WH 1-2/7

-  Restart 10-20% reduced dose when INR therapeutic

-  Check INR next day

- INR >10 + nil bleed

-  WH 2/7

-  Restart 10-20% reduced dose when INR therapeutic

-  Reverse

-   Vitmain K 3-5mg IV/SC/PO

-   +/- 1000 IU Prothrombinex (as needed)

-  Check INR next day

- Significant bleed or high risk

-  Any INR (usually >6)

-  Stop Warfarin

-  Restart at reduced dose (clinical decision)

-  Reverse

-   Vitmain K 5mg IV/SC/PO

-   25-50 IU/kg Prothrombinex (as needed)

-   15 ml/kg FFP

-  Check INR every 1-2hrs until bleeding ceased, then next day

 

 

 

Duration of Warfarin Therapy

<Warfarin>

 

- DVT

-  Transient RF: 3/12

-  Non-transient: 6/12

- PE

-  Transient RF: 6/12

-  Non-transient: 12/12

- AF

-  Lifelong

-  Balance against risks

- Replaced Heart Valves

-  Mechanical: lifelong

-  Tissue

 

 

 

Drug Interactions with Warfarin

<Warfarin>

 

- Teratogen

- Metabolised by cytochrome P450

-  CYP 1A2-6 & CYP 3A4

- Many druges effect serum levels (670+)

- May increase of decrease therapeutic effect

- Drugs

-  Beta blockers

-  Metoprolol, Carvedilol

-  Aspirin, Clopidogrel

-  NSAIDs

-  Digoxin

-  Frusemide

-  Atorvastatin, Simvastatin

-  Metformin

-  Lesinopril

-  Esomeprazole, Omeprazole

-  Amlodapine

-  Prednisolone

-  Antibiotics

-   esp Macrolides & Éazoles

-  ..