What is the initial heparin (anticoagulant) dose for new onset atrial fibrillation (AFib)?

Heparin Dosing for New Onset Atrial Fibrillation

For new onset atrial fibrillation requiring immediate anticoagulation, administer an initial intravenous bolus of unfractionated heparin followed by continuous infusion adjusted to maintain an aPTT of 1.5 to 2 times the control value. 1

Clinical Context Determines Urgency

Hemodynamically unstable patients (presenting with angina, MI, shock, or pulmonary edema) require immediate cardioversion with concurrent heparin administration without delay for prior anticoagulation. 1 In this scenario, give the heparin bolus immediately and proceed with cardioversion. 1

Hemodynamically stable patients with AF >48 hours or unknown duration should receive heparin as a bridge to oral anticoagulation, with the goal of maintaining therapeutic anticoagulation for at least 3-4 weeks before and after any planned cardioversion. 1

Specific Dosing Protocol

Standard Unfractionated Heparin Regimen

The FDA-approved dosing for atrial fibrillation with embolization includes: 2

• Initial bolus: 5,000 units IV, followed by
• Continuous infusion: 20,000-40,000 units per 24 hours (approximately 1,000-1,700 units/hour) in 1,000 mL of 0.9% sodium chloride 2

Alternative weight-based approach (commonly used in practice): Initial bolus of 80 units/kg IV followed by continuous infusion of 18 units/kg/hour, adjusted based on aPTT monitoring. 2

Monitoring Requirements

• Baseline: Obtain aPTT, INR, platelet count, and hematocrit before initiating heparin 2
• During continuous infusion: Check aPTT approximately every 4 hours initially, then at appropriate intervals once therapeutic range is achieved 2
• Target aPTT: 1.5 to 2 times the control value (typically 60-80 seconds if control is 30-40 seconds) 1, 2
• Platelet monitoring: Periodically monitor platelet counts throughout therapy to detect heparin-induced thrombocytopenia 2

Low Molecular Weight Heparin as Alternative

LMWH can be used as an alternative to unfractionated heparin in the peri-cardioversion period, though the evidence base is more limited. 1 The guidelines note that "limited data support subcutaneous administration of low-molecular-weight heparin in this indication." 1

Research supports LMWH safety and efficacy: One randomized trial showed that once-daily tinzaparin was associated with zero ischemic strokes compared to 5 strokes in the unfractionated heparin group during the first 48 hours. 3 LMWH offers practical advantages including no need for IV access, no laboratory monitoring, and potential for outpatient management. 4

Duration of Heparin Therapy

Continue heparin until therapeutic oral anticoagulation is established (INR 2.0-3.0 for warfarin). 1, 2 For conversion to warfarin, continue full heparin therapy for several days until the INR has reached a stable therapeutic range, then discontinue heparin without tapering. 2

Total anticoagulation duration: At least 4 weeks post-cardioversion regardless of whether cardioversion occurs. 1

TEE-Guided Approach

As an alternative to routine pre-anticoagulation, transesophageal echocardiography can be performed to screen for left atrial thrombus. 1

• If no thrombus identified: Administer heparin bolus immediately before cardioversion, followed by continuous infusion, then transition to oral anticoagulation for at least 4 weeks. 1
• If thrombus identified: Treat with oral anticoagulation (INR 2.0-3.0) for at least 3-4 weeks before cardioversion. 1

Critical Pitfalls to Avoid

Do not delay heparin in hemodynamically unstable patients waiting for laboratory results or cardioversion preparation. 1 The risk of stroke from delayed anticoagulation outweighs bleeding concerns in acute AF with instability. 5

Do not use intramuscular heparin due to frequent hematoma formation at injection sites. 2 Only use IV or deep subcutaneous (above iliac crest or abdominal fat layer) routes. 2

Do not underdose in patients on DOACs if they require catheter ablation or other procedures—these patients may need higher initial boluses (120-130 units/kg) to achieve target ACT. 6

Avoid premature discontinuation of anticoagulation before completing the full 4-week post-cardioversion period, as atrial stunning can persist and thrombus can form even after successful rhythm conversion. 1