Warfarin overdose

Warfarin, sold under the brand name Coumadin among others, is a medication that is used as an anticoagulant (blood thinner). It is commonly used to prevent blood clots such as deep vein thrombosis and pulmonary embolism, and to prevent stroke in people who have atrial fibrillation, valvular heart disease, or artificial heart valves. Less commonly, it is used following ST-segment elevation myocardial infarction and orthopedic surgery. It is generally taken by mouth, but may also be used intravenously.

The common side effect is bleeding. Less common side effects may include areas of tissue damage and purple toes syndrome. Use is not recommended during pregnancy. The effects of warfarin typically should be monitored by checking prothrombin time (INR) every one to four weeks. Many other medications and dietary factors can interact with warfarin, either increasing or decreasing its effectiveness. The effects of warfarin may be reversed with phytomenadione (vitamin K₁), fresh frozen plasma, or prothrombin complex concentrate.

Warfarin decreases blood clotting by blocking an enzyme called vitamin K epoxide reductase that reactivates vitamin K₁. Without sufficient active vitamin K₁, clotting factors II, VII, IX, and X have decreased clotting ability. The anticlotting protein C and protein S are also inhibited, but to a lesser degree. A few days are required for full effect to occur, and these effects can last for up to five days. Additionally, because the mechanism involves enzymes such as ''VKORC1'', patients on warfarin with polymorphisms of the enzymes may require adjustments in therapy to account if the genetic variant that they have is more readily inhibited by warfarin, thus requiring lower doses.

Warfarin first came into large-scale commercial use in 1948 as a rat poison. Warfarin was formally approved for human use by the U.S. Food and Drug Administration to treat blood clots in 1954. In 1955, warfarin's reputation as a safe and acceptable treatment was bolstered when President Dwight D. Eisenhower received warfarin following a massive and highly publicized heart attack. Eisenhower's illness kickstarted a transformation in medicine where coronary artery disease, arterial plaques, and ischemic strokes were treated and prevented by using anticoagulants such as warfarin. It is on the World Health Organization's List of Essential Medicines. Warfarin is available as a generic medication. In 2020, it was the 58th most commonly prescribed medication in the United States, with more than 11million prescriptions.

Medical uses

Warfarin is used to decrease the tendency for thrombosis, or as secondary prophylaxis (prevention of further episodes) in those individuals who have already formed a blood clot (thrombus). Warfarin treatment can help prevent formation of future blood clots and help reduce the risk of embolism (migration of a thrombus to a spot where it blocks blood supply to a vital organ).

Warfarin is best suited for anticoagulation (clot formation inhibition) in areas of slowly running blood (such as in veins and the pooled blood behind artificial and natural valves), and in blood pooled in dysfunctional cardiac atria. Thus, common clinical indications for warfarin use are atrial fibrillation, the presence of artificial heart valves, deep venous thrombosis, and pulmonary embolism (where the embolized clots first form in veins). Warfarin is also used in antiphospholipid syndrome. It has been used occasionally after heart attacks (myocardial infarctions), but is far less effective at preventing new thromboses in coronary arteries. Prevention of clotting in arteries is usually undertaken with antiplatelet drugs, which act by a different mechanism from warfarin (which normally has no effect on platelet function). It can be used to treat people following ischemic strokes due to atrial fibrillation, though direct oral anticoagulants (DOACs) may offer greater benefits.

Dosing

Dosing of warfarin is complicated because it is known to interact with many commonly used medications and certain foods. These interactions may enhance or reduce warfarin's anticoagulation effect. To optimize the therapeutic effect without risking dangerous side effects such as bleeding, close monitoring of the degree of anticoagulation is required by a blood test measuring an INR. During the initial stage of treatment, INR is checked daily; intervals between tests can be lengthened if the patient manages stable therapeutic INR levels on an unchanged warfarin dose. Newer point-of-care testing is available and has increased the ease of INR testing in the outpatient setting. Instead of a blood draw, the point-of-care test involves a simple finger prick.

Maintenance dose

Recommendations by many national bodies, including the American College of Chest Physicians, have been distilled to help manage dose adjustments.

The maintenance dose of warfarin can fluctuate significantly depending on the amount of vitamin K₁ in the diet. Keeping vitamin K₁ intake at a stable level can prevent these fluctuations. Leafy green vegetables tend to contain higher amounts of vitamin K₁. Green parts of members of the family Apiaceae, such as parsley, cilantro, and dill are extremely rich sources of vitamin K; cruciferous vegetables such as cabbage and broccoli, as well as the darker varieties of lettuces and other leafy greens, are also relatively high in vitamin K₁. Green vegetables such as peas and green beans do not have such high amounts of vitamin K₁ as leafy greens. Certain vegetable oils have high amounts of vitamin K₁. Foods low in vitamin K₁ include roots, bulbs, tubers, and most fruits and fruit juices. Cereals, grains, and other milled products are also low in vitamin K₁.

Self-testing

Anticoagulation with warfarin can also be monitored by patients at home. International guidelines on home testing were published in 2005. The guidelines stated:

A 2006 systematic review and meta-analysis of 14 randomized trials showed home testing led to a reduced incidence of complications (thrombosis and major bleeding), and improved the time in the therapeutic range.

Alternative anticoagulants

In some countries, other coumarins are used instead of warfarin, such as acenocoumarol and phenprocoumon. These have a shorter (acenocoumarol) or longer (phenprocoumon) half-life, and are not completely interchangeable with warfarin. Several types of anticoagulant drugs offering the efficacy of warfarin without a need for monitoring, such as dabigatran, apixaban, edoxaban, and rivaroxaban, have been approved in a number of countries for classical warfarin uses. Complementing these drugs are reversal agents available for dabigatran ( idarucizumab), and for apixaban, and rivaroxaban ( andexanet alfa). Andexanet alfa is suggested for edoxaban, but use of it is considered off label due to limited evidence. A reversal agent for dabigatran, apixaban, edoxaban, and rivaroxaban is in development (ciraparantag).

Contraindications

All anticoagulants are generally contraindicated in situations in which the reduction in clotting that they cause might lead to serious and potentially life-threatening bleeds. This includes people with active bleeding conditions (such as gastrointestinal ulcers), or disease states with increased risk of bleeding (e.g., low platelets, severe liver disease, uncontrolled hypertension). For patients undergoing surgery, treatment with anticoagulants is generally suspended. Similarly, spinal and lumbar puncture (e.g., spinal injections, epidurals, etc.) carry increased risk, so treatment is suspended prior to these procedures.Brayfield A (ed), Martindale: The Complete Drug Reference nlineLondon: Pharmaceutical Press ccessed on 24 April 2017/ref>

Warfarin should not be given to people with heparin-induced thrombocytopenia until platelet count has improved or normalised. Warfarin is usually best avoided in people with protein C or protein S deficiency, as these thrombophilic conditions increase the risk of skin necrosis, which is a rare but serious side effect associated with warfarin.

Pregnancy

Warfarin is contraindicated in pregnancy, as it passes through the placental barrier and may cause bleeding in the fetus; warfarin use during pregnancy is commonly associated with spontaneous abortion, stillbirth, neonatal death, and preterm birth. Retrieved on 15 December 2008 through Google Book Search. Coumarins (such as warfarin) are also teratogens, that is, they cause birth defects; the incidence of birth defects in infants exposed to warfarin ''in utero'' appears to be around 5%, although higher figures (up to 30%) have been reported in some studies. Depending on when exposure occurs during pregnancy, two distinct combinations of congenital abnormalities can arise.

First trimester of pregnancy

Usually, warfarin is avoided in the first trimester, and a low-molecular-weight heparin such as enoxaparin is substituted. With heparin, risks of maternal haemorrhage and other complications are still increased, but heparins do not cross the placental barrier, so do not cause birth defects. Various solutions exist for the time around delivery.

When warfarin (or another 4-hydroxycoumarin derivative) is given during the first trimester—particularly between the sixth and ninth weeks of pregnancy—a constellation of birth defects known variously as fetal warfarin syndrome (FWS), warfarin embryopathy, or coumarin embryopathy can occur. FWS is characterized mainly by skeletal abnormalities, which include nasal hypoplasia

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