Anticoagulation for Mechanical Heart Valves

Mechanical heart valves need lifelong anticoagulation with a vitamin K antagonist, because the valve surface causes clot formation. Direct oral anticoagulants are contraindicated, because they have higher rates of valve thrombosis and stroke.

Targets by valve. Choose the INR target by valve position, valve type and thrombotic risk factors (atrial fibrillation, previous thromboembolism, left ventricular dysfunction, hypercoagulable state).

• Mechanical aortic valve (bileaflet or current-generation), no risk factors: warfarin with target INR 2.5 (range 2.0 to 3.0).
• Mechanical aortic valve with risk factors, mechanical mitral valve or tricuspid valve, and older-generation valves (caged-ball, tilting disc): target INR 3.0 (range 2.5 to 3.5).
• Specific aortic valve designs approved for lower targets (INR 1.5 to 2.0 after 3 months with aspirin 75 to 100 mg) only under specialist direction.
• After starting: warfarin with heparin cover (enoxaparin 1 mg/kg twice daily, or unfractionated heparin 80 units/kg then 18 units/kg/h adjusted to aPTT 1.5 to 2.5 times control) until INR is in range for 2 consecutive days.
• Surgical bioprosthetic aortic or mitral valve: warfarin with target INR 2.5 for 3 to 6 months, then aspirin 75 to 100 mg daily.
• Low-dose aspirin added to warfarin is not routine, and is considered only after thromboembolism despite a therapeutic INR and with low bleeding risk.

Initiation dose. Warfarin 5 mg daily for 2 days (2 to 3 mg in older patients, low weight, liver disease or interacting drugs), then adjust by INR.

Pregnancy. The highest-risk situation. Options directed by a specialist team: warfarin throughout with a dose of 5 mg daily or less (lowest fetal risk), or low molecular weight heparin twice daily with anti-Xa monitoring (peak 0.8 to 1.2 units/mL), with a change to unfractionated heparin from 36 weeks and warfarin avoided in weeks 6 to 12 if the dose is over 5 mg daily. Direct oral anticoagulants are contraindicated.

Kidney impairment. Warfarin remains the choice, with more frequent INR checks.

Interactions. Antibiotics, azoles, amiodarone, rifampicin, carbamazepine and alcohol alter INR. Check INR within 3 to 5 days of starting or stopping an interacting drug.

Special groups. In East Africa, rheumatic valve disease leads to valve replacement in young people, and access to INR monitoring is limited, so point-of-care INR and careful education are essential. Women of reproductive age need contraception counselling and a pregnancy plan before surgery.

Monitoring. INR twice weekly until stable, then at least every 4 weeks (every 12 weeks at most). Echocardiography at baseline and with new symptoms.

Referral and escalation. Seek urgent cardiology review for a new murmur, muffled valve click, heart failure, embolism or an INR outside 2.0 to 4.0 with bleeding. Suspected valve thrombosis is an emergency.

Stopping. Never stop warfarin without a plan. For procedures, bridge with heparin as directed by the specialist team.