Anticoagulants - How Do They Work?
Anticoagulants, also known as blood thinners, work by interfering with the body's natural blood clotting process. When a blood vessel is damaged, clotting factors in the blood are activated to form a clot to stop bleeding. Anticoagulants disrupt this clotting process in a targeted way to prevent dangerous clots from forming while still allowing normal bleeding to occur.
There are two main mechanisms by which Anticoagulants work. The first is by inhibiting vitamin K-dependent clotting factors in the liver. Vitamin K is required to activate clotting factors II, VII, IX and X. Anticoagulants that work this way include warfarin and newer oral anticoagulants. The second main mechanism is by directly inhibiting thrombin or factor Xa. Thrombin and factor Xa are enzymes central to the clotting cascade. Drugs like heparin, enoxaparin, and the direct oral anticoagulants (DOACs) apixaban, rivaroxaban and dabigatran use this method.
Different Anticoagulant Medications
Heparin
Heparin is usually given by injection and works immediately by directly inhibiting thrombin and factor Xa. It has a short half-life in the body of 1-2 hours, so it needs to be administered often, usually several times a day via subcutaneous injection. Heparin is commonly used in hospital settings for short-term treatment.
Low Molecular Weight Heparin (LMWH)
LMWHs like enoxaparin (Lovenox) have less binding to other proteins than unfractionated heparin so they have longer half-lives, allowing once or twice daily dosing. LMWHs are useful for outpatient treatment and for certain high risk situations where very predictable drug levels are needed.
Warfarin
Warfarin is a vitamin K antagonist that works indirectly to inhibit clotting factors II, VII, IX and X. It takes 3-5 days to reach its full effect in the body due to its mechanism of action. Careful dose adjustments and ongoing blood monitoring with an INR test is required when using warfarin to make sure clotting is properly inhibited without causing too much bleeding risk.
Direct Oral Anticoagulants (DOACs)
DOACs like apixaban (Eliquis), rivaroxaban (Xarelto), edoxaban (Savaysa) and dabigatran (Pradaxa) work directly on thrombin or factor Xa without the need for monitoring. They have fewer food and drug interactions than warfarin. DOACs have largely replaced warfarin for most patients due to their greater predictability and convenience.
Monitoring Anticoagulant Therapy
For anticoagulants that work by inhibiting vitamin K-dependent clotting factors, monitoring is important to ensure clotting is properly inhibited without causing excessive bleeding risk.
Patients on warfarin need regular INR testing, which measures how long it takes blood to clot compared to normal. The goal INR range depends on the reason for anticoagulation but is typically between 2-3. Frequent INR testing is needed in the initial stabilization phase while the proper dose is determined, then monitoring can become less frequent once a stable dose is established.
Direct acting anticoagulants like LMWHs and DOACs do not require routine monitoring in most cases since their effects are more predictable. However, medical tests may still be needed periodically to check kidney and liver function since these can impact drug levels in some patients. Medication levels may also be measured in emergency situations where bleeding risk needs to be accurately determined.
Managing Anticoagulation Safety
While anticoagulants are highly effective for preventing dangerous clots, they also increase the risk of bleeding. Therefore proper management is important. Key aspects of anticoagulant safety include:
- Careful patient education on potential bleeding risks and what to do/who to contact if bleeding occurs.
- Close medication compliance and not missing or doubling up on doses which can increase bleeding risk.
- Avoiding risk factors for bleeding like excess alcohol, medications that interact, contact sports etc.
- Seeking prompt medical attention if any bleeding or head injury occurs while on an anticoagulant.
- Having a coordinated care plan between prescriber, patient and pharmacy/lab for ongoing safe use.
- Monitoring for signs of excess anticoagulant effect through regular testing if required by the specific medication.
With diligent management anticoagulants can safely reduce thrombotic risks, but also, importantly, reduce bleeding risks. An individualized plan tailors anticoagulation to each patient's needs and lifestyle.
Transitioning Between Different Anticoagulants
In some cases it becomes necessary to transition a patient from one anticoagulant medication to another. Common reasons for switching include inadequate clot protection, side effects, cost issues or convenience factors. Proper planning is important when transitioning between different anticoagulant drugs.
When switching from warfarin to a DOAC, warfarin should be overlapped with the DOAC for 4-5 days to ensure adequate anticoagulation during the transition. For DOAC to DOAC switches, having a buffer of 1-2 days between stopping one and starting another is reasonable.
For injectable to oral switches like heparin to warfarin or DOAC, the heparin would usually be stopped once a therapeutic oral regimen is reached as indicated by INR results. Transitioning from an oral drug to an injectable may involve gradually increasing injectable doses while decreasing the oral drug over a few days.
Close monitoring by a healthcare provider familiar with anticoagulation medication options helps safely transition patients between therapies for optimal thrombosis protection with minimal bleeding risk. Proper planning removes uncertainties and improves medication adherence.
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