What anticoagulation regimen, target INR range, monitoring schedule, and bridging strategy are recommended for a patient with an ON‑X mechanical heart valve (aortic or mitral)?

ON-X Mechanical Valve Anticoagulation

Initial Anticoagulation (First 3 Months Post-Surgery)

All ON-X mechanical valve patients—whether aortic or mitral position—require standard-intensity warfarin with a target INR of 2.5 (range 2.0–3.0) for the first 3 months after surgery, combined with aspirin 81 mg daily. 1

• This initial period allows for valve endothelialization and establishes baseline anticoagulation before any dose reduction is considered. 1
• Aspirin 81 mg daily should be started immediately and continued indefinitely. 1

Long-Term Anticoagulation Strategy (After 3 Months)

ON-X Aortic Valve WITHOUT Thromboembolic Risk Factors

After 3 months, patients with an ON-X aortic valve and NO thromboembolic risk factors may be transitioned to low-dose warfarin with a target INR of 1.5–2.0 (plus aspirin 81 mg daily). 1

Thromboembolic risk factors that preclude low-dose therapy include: 1

• Atrial fibrillation
• Previous thromboembolism
• Left ventricular systolic dysfunction
• Hypercoagulable state
• Older-generation valve design

Important caveats about the low-dose ON-X aortic regimen:

• This recommendation is based on a single RCT (PROACT Aortic) that showed equivalent outcomes, but the control group had an unusually high bleeding rate of 3.2% per patient-year. 1
• More recent observational data from 510 patients followed for median 3.4 years showed a 57% reduction in the composite endpoint (thromboembolism, valve thrombosis, major bleeding) with INR target 1.8 (range 1.5–2.0) compared to standard-dose warfarin. 2
• The low-dose regimen achieved major bleeding reduction of 85% with similar thromboembolic event rates and zero valve thrombosis. 2

ON-X Aortic Valve WITH Thromboembolic Risk Factors

Maintain standard-intensity warfarin with target INR of 2.5 (range 2.0–3.0) indefinitely, plus aspirin 81 mg daily. 1

ON-X Mitral Valve (All Patients)

All patients with an ON-X mechanical mitral valve require standard-intensity warfarin with target INR of 3.0 (range 2.5–3.5) indefinitely, plus aspirin 81 mg daily. 1

• Mitral mechanical valves carry inherently higher thrombotic risk than aortic valves regardless of valve type. 1
• A recent RCT (PROACT Mitral) attempted to demonstrate noninferiority of low-dose warfarin (INR 2.0–2.5) versus standard-dose (INR 2.5–3.5) in ON-X mitral valves but failed to achieve noninferiority, with composite event rates of 11.9% vs 12.0% per patient-year. 3
• The confidence interval exceeded the 1.5% noninferiority margin, meaning low-dose warfarin cannot be recommended for mitral ON-X valves. 3

INR Monitoring Schedule

INR monitoring frequency depends on stability: 1

• Initial 3 months: Weekly INR checks until stable therapeutic range achieved, then every 2 weeks
• After stabilization: Monthly INR monitoring if consistently therapeutic
• Home INR monitoring is acceptable and shows equivalent safety outcomes in ON-X aortic valve patients on low-dose warfarin (56% reduction in composite endpoint). 2

Bridging Strategy for Procedures

Minor Procedures (Dental Extractions, Cataract Surgery)

Continue warfarin with therapeutic INR—do NOT bridge or interrupt anticoagulation. 1

• Bleeding is easily controlled in these procedures, and the thrombotic risk of interruption outweighs bleeding risk. 1

Major Procedures Requiring Warfarin Interruption

For ON-X mitral valves (high thrombotic risk): 4

• Stop warfarin 5–6 days before surgery
• When INR falls below 2.5, start therapeutic-dose LMWH (e.g., dalteparin 200 IU/kg daily or enoxaparin equivalent)
• Stop LMWH at least 24 hours before incision
• Resume therapeutic-dose LMWH or IV unfractionated heparin 24–48 hours post-surgery once hemostasis confirmed
• Restart warfarin on postoperative day 1 at usual maintenance dose
• Continue bridging until INR ≥2.5 on two consecutive measurements 4

For ON-X aortic valves without additional risk factors (low thrombotic risk): 1

• Bridging is NOT recommended for bileaflet or newer-generation mechanical valves in the aortic position without additional thromboembolic risk factors. 1
• Simply stop warfarin 5–6 days pre-procedure and resume postoperatively without heparin bridging. 1

Critical pitfall: The PERIOP-2 trial showed bridging increases bleeding risk without reducing thrombotic events in low-risk mechanical valve patients, but this was predominantly an aortic valve population (91% aortic vs 9% mitral). 4 Mitral valves still require bridging due to their intrinsically higher thrombotic risk. 4

Management of Thromboembolic Events Despite Therapeutic Anticoagulation

If stroke or systemic embolism occurs while INR is therapeutic:

ON-X Aortic Valve

Increase INR target from 2.5 (range 2.0–3.0) to 3.0 (range 2.5–3.5) OR add aspirin 75–100 mg daily if not already on it (assess bleeding risk). 1

ON-X Mitral Valve

Increase INR target from 3.0 (range 2.5–3.5) to 4.0 (range 3.5–4.0) OR add aspirin 75–100 mg daily if not already on it (assess bleeding risk). 1

• Before intensifying therapy, exclude infective endocarditis, document time in therapeutic range, screen for new-onset atrial fibrillation, and consider hypercoagulable workup. 1

Absolute Contraindications

Direct oral anticoagulants (DOACs) are absolutely contraindicated in all mechanical valve patients. 1

• The RE-ALIGN trial with dabigatran showed increased thrombotic and bleeding complications versus warfarin in mechanical valve patients. 1, 5
• Factor Xa inhibitors (rivaroxaban, apixaban, edoxaban) have not been adequately studied and are not recommended. 1, 5
• Low-dose rivaroxaban (10 mg daily) plus aspirin is specifically contraindicated after TAVI. 1

Management of Supratherapeutic INR

For INR >7 with minor bleeding (e.g., gum bleeding):

• Immediately hold warfarin
• Administer oral vitamin K 2.5–5 mg (NOT >5 mg to avoid prolonged warfarin resistance)
• Resume warfarin at lower dose once INR approaches therapeutic range 6

For life-threatening bleeding:

• Administer 4-factor prothrombin complex concentrate (onset 5–15 minutes) or fresh frozen plasma
• Add IV vitamin K 10 mg
• This patient population has extremely high thrombotic risk, so reversal must be balanced against valve thrombosis risk 1, 6

Critical pitfall: Never use high-dose vitamin K (>5 mg) in mechanical valve patients—it creates prolonged warfarin resistance lasting weeks and a prothrombotic state that is catastrophic for double-valve replacements. 6