Warfarin

CLASS

• anticoagulant

MECHANISM OF ACTION

• prevents the synthesis of vitamin K dependent clotting factors
• II, VII, IX & X in liver
• prevents the reduction of vitamin K 2, 3-epoxide -> vitamin K

PHARMACEUTICS

• tablets: 0.5, 1, 3 & 5mg
• racemic mixture of warfarin sodium

DOSE

• 0.2mg/kg to load with in paediatrics
• load with 5, 5, 5mg
• maximum effect at 24-72hrs

INR targets:

• DVT and PE: 2.0-3.0
• Atrial fibrillation: 2.0-3.0
• Bioprosthetic heart valves: 2.0-3.0
• Mechanical heart valves: 2.5-3.5

INDICATIONS

• venous or arterial thromboembolism prophylaxis or treatment

ADVERSE EFFECTS

• procoagulant effect with initiation of treatment (due to protein C and S depletion)
• bleeding
• teratogenic in pregnancy
• many drug interactions increased or decrease INR

PHARMACOKINETICS

• Absorption – 100% bioavailable
• Distribution – 99% protein bound, Vd = 0.15L/kg
• Metabolism – hepatic
• Elimination – urine and faeces, t ½ = 40 hrs

PATIENT EDUCATION

Patients on warfarin therapy should have the following explained:

• Reason for treatment
• Mechanism of action of warfarin
• Time of day to take warfarin (same time of day)
• The INR, target range and need for regular testing
• Signs and symptoms of bleeding
• Effect of illness, injury or any changes in physical status
• Potential effect of invasive procedure, surgery or dental work
• The effects of common over-the-counter (OTC) medication interactions
• The need for consistent intake of vitamin K-rich foods
• Effects of alcohol intake
• Appropriate action if diarrhoea or vomiting occurs

References and Links

Chris Nickson

Chris is an Intensivist and ECMO specialist at the Alfred ICU in Melbourne. He is also a Clinical Adjunct Associate Professor at Monash University. He is a co-founder of the Australia and New Zealand Clinician Educator Network (ANZCEN) and is the Lead for the ANZCEN Clinician Educator Incubator programme. He is on the Board of Directors for the Intensive Care Foundation and is a First Part Examiner for the College of Intensive Care Medicine. He is an internationally recognised Clinician Educator with a passion for helping clinicians learn and for improving the clinical performance of individuals and collectives.

After finishing his medical degree at the University of Auckland, he continued post-graduate training in New Zealand as well as Australia’s Northern Territory, Perth and Melbourne. He has completed fellowship training in both intensive care medicine and emergency medicine, as well as post-graduate training in biochemistry, clinical toxicology, clinical epidemiology, and health professional education.

He is actively involved in in using translational simulation to improve patient care and the design of processes and systems at Alfred Health. He coordinates the Alfred ICU’s education and simulation programmes and runs the unit’s education website, INTENSIVE.  He created the ‘Critically Ill Airway’ course and teaches on numerous courses around the world. He is one of the founders of the FOAM movement (Free Open-Access Medical education) and is co-creator of litfl.com, the RAGE podcast, the Resuscitology course, and the SMACC conference.

His one great achievement is being the father of three amazing children.

On Twitter, he is @precordialthump.

| INTENSIVE | RAGE | Resuscitology | SMACC