What is the recommended initial dosing and monitoring approach for Coumadin (warfarin) in a hospitalized patient with atrial fibrillation?

Starting Warfarin (Coumadin) in Hospital for Atrial Fibrillation

For hospitalized patients with atrial fibrillation requiring anticoagulation, initiate warfarin at a low starting dose (2-5 mg daily) with concurrent heparin bridging therapy, targeting an INR of 2.0-3.0, and continue heparin until therapeutic INR is achieved for at least 24 hours. 1, 2

Immediate Anticoagulation Strategy

Heparin Bridging (Start Immediately)

• Administer unfractionated heparin as an IV bolus (typically 60-80 units/kg, maximum 5000 units) followed by continuous infusion (12-18 units/kg/hour) adjusted to maintain aPTT at 1.5-2 times control (typically 60-80 seconds). 2
• Low molecular weight heparin (enoxaparin 1 mg/kg subcutaneously twice daily) is a reasonable alternative to unfractionated heparin. 2, 3
• Continue heparin until warfarin achieves therapeutic INR (2.0-3.0) for at least 24 consecutive hours. 2

Warfarin Initiation Dosing

• Start with 2-5 mg daily in elderly, frail patients, or those with renal insufficiency. 1
• A 5 mg starting dose is generally preferred over 10 mg in AF patients, as they are typically older and more frail than venous thromboembolism patients. 1
• Avoid routine 10 mg loading doses in hospitalized AF patients given their age and comorbidities. 1

Critical Timing Considerations Based on AF Duration

AF Duration <48 Hours AND Low Stroke Risk

• Heparin may be started immediately before or after cardioversion if planned. 1
• Transition to warfarin and continue for at least 4 weeks post-cardioversion. 1

AF Duration ≥48 Hours OR Unknown Duration

• Mandatory anticoagulation with heparin bridging to warfarin for at least 3 weeks before any planned cardioversion and 4 weeks after. 1, 2
• This applies regardless of whether cardioversion ultimately occurs. 2

Hemodynamically Unstable Patients

• Perform immediate cardioversion without delay for prior anticoagulation, but start heparin bolus and infusion concurrently. 1, 2
• Continue anticoagulation for at least 4 weeks post-cardioversion once stabilized. 2, 3

Monitoring Protocol

INR Monitoring During Initiation

• Check baseline INR, then daily until therapeutic range achieved. 1
• Target INR: 2.0-3.0 for nonvalvular atrial fibrillation. 1
• Once stable, transition to less frequent monitoring (weekly, then monthly). 1

Dose Adjustments During Maintenance

Use the following algorithm for INR-based dose adjustments: 1

• INR <1.5: Increase weekly dose by 15%
• INR 1.6-1.9: Increase weekly dose by 10%
• INR 2.0-2.9: No change (therapeutic)
• INR 3.0-3.9: Decrease weekly dose by 10%
• INR 4.0-4.9: Hold 1 dose, restart with 10% weekly dose reduction
• INR ≥5.0: Hold until INR 2.0-3.0, restart with 15% weekly dose reduction

Alternative to Warfarin: Direct Oral Anticoagulants

Consider dabigatran, rivaroxaban, or apixaban as alternatives to warfarin in eligible patients without mechanical valves, severe renal failure, or advanced liver disease. 1

• These agents do not require heparin bridging or INR monitoring. 1
• Dabigatran demonstrated superior stroke reduction compared to warfarin in patients with good INR control. 4

Common Pitfalls to Avoid

Do Not Use Subtherapeutic "Low-Dose" Warfarin

• Fixed low-dose warfarin (INR target <2.0) leads to more thromboembolic events without reducing major bleeding compared to adjusted-dose therapy (INR 2.0-3.0). 5
• Always target INR 2.0-3.0, not lower ranges. 1

Do Not Stop Heparin Prematurely

• Continue heparin until INR is therapeutic (2.0-3.0) for at least 24 hours, as warfarin takes 3-5 days to achieve full anticoagulant effect. 2
• Early heparin discontinuation creates a prothrombotic window. 1

Do Not Forget Duration of Therapy

• Anticoagulation must continue for at least 4 weeks post-cardioversion regardless of whether sinus rhythm is maintained. 1, 2
• Long-term anticoagulation decisions should be based on stroke risk (CHA₂DS₂-VASc score), not rhythm status. 1

Special Circumstances

If Bleeding Risk Develops During Hospitalization

• Hemorrhagic complications (e.g., hemorrhagic pancreatitis, active GI bleeding) require temporary anticoagulation hold. 3
• Resume anticoagulation within 48-72 hours once bleeding controlled, as stroke risk increases substantially with prolonged interruption. 3
• For high-risk patients (CHA₂DS₂-VASc ≥4, prior stroke), minimize interruption duration. 3

TEE-Guided Cardioversion Approach

• If cardioversion needed urgently and patient has AF ≥48 hours, perform transesophageal echocardiography to exclude left atrial thrombus. 1
• If no thrombus identified, administer heparin immediately before cardioversion, then continue anticoagulation for 4 weeks. 1, 2