Warfarin

How Warfarin Works

Warfarin sodium (Coumadin) is a pharmaceutical derivative of the natural plant anticoagulant known as coumarin. Warfarin, a vitamin K antagonist, is the most commonly prescribed oral (PO) anticoagulant that works by inhibiting the synthesis of Vitamin K dependent clotting factors (VII, IX, X, prothrombin) that are formed in liver. While warfarin works by preventing clots from forming, it does not work on existing clots. Warfarin is a highly protein bound drug with a low affinity for albumin; therefore, effortlessly resulting in increased free warfarin when in the presence of other high protein drugs with stronger affinities. This causes an increased anticoagulant effect potentially leading to toxicity. Warfarin is used for long term conditions such as stroke, cardiac dysrhythmias (Atrial Fibrillation), chronic heart failure, valve replacement treatment, immobility, and deep vein thrombosis. Warfarin requires careful monitoring of the prothrombin time/International Normalized Ratio (PT/INR), which is a standardized measure of the degree to which a patient’s blood coagulability has been reduced by the drug. A normal INR (without warfarin) is 1.0, whereas a therapeutic INR (with warfarin) ranges from 2 to 3.5, depending on the indication for use of the drug. Elderly patients older than 65 years may have a lower INR threshold and should be monitored accordingly.

http://www.mybloodthinner.org/research.html

Warfarin Administration

Because warfarin is used chronically to prevent clots, administration is not considered for an immediate effect. Warfarin is available in oral (PO) form only and the typical maintenance dosage is 2-10 mg/day as determined by PT/INR values. The onset of warfarin is 12 to 24 hours and therapeutic effect is typically reached in 3 to 5 days. Warfarin can be given with heparin to further its effect. Heparin therapy will destroy clots and the added use of warfarin will prevent further blood clots from forming. Before administering warfarin a nurse should assess for bleeding. Also, a nurse should know that the oral form must be started several days before heparin is discontinued, their patient should; be protected against injury and wear a medic alert device, avoid over the counter drugs containing aspirin, avoid Intramuscular injections if possible, and inform other healthcare providers before minor/major surgery. It is important to teach a patient receiving warfarin to be cautious of their alcohol intake, limiting it to 1-2 drinks per day, as it will affect their liver function. A consistent diet of vitamin K (avoiding a high intake) is also important as it affects the binding available effects INR levels. Also a diet high in protein must be monitored, the more proteins taken in result in higher amounts of warfarin binding resulting in lower free drug levels. Flavanoids (found in grapefruit and cranberries, etc.), inhibit warfarin metabolism causing the INR value to increase which may lead to warfarin toxicity.

http://www.drugs.com/warfarin.html

Is Warfarin Really Safe?

Potential side effects of warfarin therapy include hemorrhage, hematuria (blood in the urine), epistaxis (nose bleed), ecchymosis (bruising), tarry stools, bleeding gums, skin necrosis (indicates an allergic reaction), leukopenia (decrease in white blood cell count), and agranulocytosis (no white blood cell production).
Luckily, there is an antidote for the uncommon event of warfarin overdosing. Vitamin K (AquaMephyton) causes more vitamin K dependant clotting factors to be produced. Oral dosage is preferred, intramuscular (IM)/suncutaneous (SQ) available, and intravenous (IV) are to be used cautiously due to the high risk of allergic reactions. Procedures for overdoses can be determined by using the most recently high INR results as follows- If the INR is 3-9:hold Coumadin, if the INR is 9-20 & minor bleeding is occurring: administer vitamin K PO, if the INR is 20+ & acute active bleeding is occurring: administer vitamin K via IV.



Contraindications Of Warfarin

Warfarin is a category X drug, which means that its use is contraindicated in pregnancy. The use of warfarin is also contraindicated in the presence of an existing hemorrhaging; in which case heparin should be administered. IM dosing of warfarin is to be avoided because of risk of bleeding and bruising.
Warfarin’s effect can be increased by: highly protein bound drugs such as ASA (aspirin), Sulfa containing drugs (loop diuretics), antibiotics, NSAIDS and acetaminophen (in liver, also effects a Vit. K clotting factor). These drugs cause an increased potential for bleeding.
Warfarin’s effect is decreased by: Estrogen, Bile acid sequestrates, and antacids. These drugs reduce the absorption of warfarin at the stomach.

http://www.nlm.nih.gov/medlineplus/druginfo/meds/a682277.html

Lilley, L. L., RN, PhD., Harrington, S., PharmD., & Snyder, J. S., MSN, RN, BC. (2007). Pharmacology and the Nursing Process (K. Geen & J. Horn, Eds., 5th ed.). St. Louis, MO: Mosby Elsevier