Anticoagulation for ON-X Mechanical Aortic Valve
For patients with a new ON-X mechanical aortic valve without thromboembolic risk factors, warfarin should be targeted to INR 2.5 (range 2.0-3.0) for the first 3 months, then may be reduced to INR 1.5-2.0 (range 1.5-2.0) plus aspirin 81 mg daily for long-term management. 1
Initial 3-Month Period
- Start warfarin targeting INR 2.5 (range 2.0-3.0) immediately after valve replacement 1
- Add aspirin 81 mg daily in conjunction with warfarin 1
- This standard-intensity regimen is maintained for the first 3 months post-surgery to allow valve endothelialization 1
Long-Term Management (After 3 Months)
For Patients WITHOUT Thromboembolic Risk Factors:
Lower-intensity anticoagulation may be considered beginning 3 months after surgery 1:
- Target INR 1.5-2.0 (specifically INR 1.8, range 1.5-2.0) 1, 2
- Continue aspirin 81 mg daily 1
- This represents a Class IIb recommendation from ACC/AHA guidelines 1
Recent evidence strongly supports this lower INR strategy: A 2024 prospective registry of 510 ON-X aortic valve patients demonstrated that INR 1.8 (range 1.5-2.0) plus aspirin resulted in a 57% reduction in the composite endpoint of thromboembolism, valve thrombosis, and major bleeding compared to standard-dose warfarin (2.31% vs 5.39% per patient-year) 2. Major bleeding was reduced by 85% with similar thromboembolic event rates and zero valve thrombosis 2.
For Patients WITH Thromboembolic Risk Factors:
Maintain standard-intensity anticoagulation indefinitely 1:
Risk factors requiring higher INR include 1:
- Atrial fibrillation
- Previous thromboembolism or stroke
- Left ventricular dysfunction
- Hypercoagulable conditions
- Left atrial enlargement
Critical Caveats
The lower INR strategy (1.5-2.0) is specific to the ON-X valve only and should not be extrapolated to other mechanical valves 1. The ACC/AHA recommendation is based on the PROACT trial showing equivalent outcomes with significantly lower bleeding (1.6% vs 3.9% per patient-year, p=0.007) 3. However, the ESC guidelines do not endorse this lower INR target due to concerns about statistical power and the trial's unusually high bleeding rate in the control group 1.
Never use direct oral anticoagulants (DOACs) in patients with mechanical valves 1. A 2023 trial comparing apixaban to warfarin in ON-X valve patients was stopped early due to excess thromboembolic events with apixaban (4.2% vs 1.3% per patient-year) 4.
If choosing the lower INR strategy, ensure the patient has:
- No atrial fibrillation
- No prior thromboembolism
- No left ventricular dysfunction
- No hypercoagulable state
- Ability to maintain close INR monitoring 1
The lower INR approach requires meticulous INR monitoring as the PROACT trial used weekly home monitoring, which may not be feasible in all practice settings 1. If reliable monitoring cannot be ensured, maintain standard INR 2.5 (range 2.0-3.0) indefinitely 1.