Unfractionated Heparin Infusion Dosing
For therapeutic anticoagulation, administer UFH as a 60 U/kg IV bolus (maximum 4000 U) followed by an initial infusion of 12 U/kg/hour (maximum 1000 U/hour), adjusted to maintain aPTT at 1.5-2.0 times control (approximately 50-70 seconds). 1, 2, 3
Standard Weight-Based Dosing Protocol
The established dosing regimen from ACC/AHA guidelines and FDA labeling is:
- Initial bolus: 60 U/kg IV (maximum 4000 U)
- Initial infusion: 12 U/kg/hour (maximum 1000 U/hour)
- Target aPTT: 1.5-2.0 times control (50-70 seconds) 1, 4, 5, 4, 2, 3
This applies whether UFH is used:
- With fibrinolytic therapy for STEMI 2, 6
- For acute coronary syndromes without fibrinolytics 4, 5, 4
- For venous thromboembolism 7, 8
- For atrial fibrillation with thromboembolism 3
Monitoring Requirements
Check aPTT at baseline, then approximately every 4 hours initially until stable, then at appropriate intervals thereafter. 3
- Dosage is adequate when aPTT is 1.5-2.0 times control
- Alternative target: anti-factor Xa level of 0.3-0.7 IU/mL 8, 3
- Monitor platelet counts every 2-3 days from day 4-14 to screen for heparin-induced thrombocytopenia 7, 3
Critical Dosing Caps
Never exceed these maximum doses to avoid bleeding complications:
- Bolus: 4000 U maximum (even if weight-based calculation is higher) 1, 2
- Infusion: 1000 U/hour maximum initially 1, 2
The 2008 ACC/AHA performance measures explicitly define excessive dosing as:
- Initial bolus >70 U/kg OR total bolus >4000 U
- Initial infusion >15 U/kg/hour OR total infusion >1000 U/hour 1
Context-Specific Adjustments
For Primary PCI (without fibrinolytics):
- 70-100 U/kg IV bolus when no GP IIb/IIIa inhibitor is planned
- 50-60 U/kg IV bolus with GP IIb/IIIa inhibitors 9
For VTE Treatment:
Alternative dosing from VTE guidelines: 80 U/kg bolus followed by 18 U/kg/hour infusion 7
- However, this higher rate may lead to supratherapeutic levels more quickly 10
- Target aPTT ratio of 1.5-2.5 approximating anti-Xa 0.3-0.7 IU/mL 7
For Atrial Fibrillation:
Research suggests initial infusion rates of 9.7-11.0 U/kg/hour without bolus may reduce bleeding risk while achieving therapeutic anticoagulation 11
- Rates >11.0 U/kg/hour associated with increased bleeding (OR 1.95) 11
Special Populations
Obese Patients:
Lower weight-adjusted doses are required to achieve target ACT/aPTT 12, 13
- Consider using adjusted body weight rather than actual body weight
- Adjusted weight = Ideal body weight + 0.4 × (Actual weight - Ideal body weight) 14
- Despite higher absolute doses, obese patients need lower U/kg dosing 12
Pediatric Patients:
- Initial dose: 75-100 U/kg IV bolus over 10 minutes
- Maintenance:
- Infants: 25-30 U/kg/hour (infants <2 months need highest doses, average 28 U/kg/hour)
- Children >1 year: 18-20 U/kg/hour
- Target: aPTT 60-85 seconds (anti-Xa 0.35-0.70) 3
Common Pitfalls to Avoid
Don't use fixed dosing (e.g., 5000 U bolus, 1000 U/hour) - weight-based dosing provides more predictable anticoagulation 1, 4
Don't exceed maximum caps even for large patients - this increases bleeding risk without improving efficacy 1
Don't use intramuscular route - high risk of hematoma 3
Don't forget to cap doses at 48 hours - prolonged UFH beyond 48 hours increases HIT risk unless ongoing indication exists 1, 2
In obese patients, don't blindly use actual body weight - this leads to excessive dosing 12, 14, 13
Duration of Therapy
- Minimum 48 hours for patients receiving fibrinolytic therapy 2
- Preferably for duration of hospitalization, up to 8 days (use LMWH beyond 48 hours to reduce HIT risk) 1, 2
- For VTE: Continue until therapeutic on warfarin (INR 2-3 for at least 24 hours) 3
Alternative Monitoring
Anti-Xa assay monitoring may be superior to aPTT in critically ill patients 15
- Target anti-Xa: 0.3-0.7 IU/mL
- Less affected by coagulation factor abnormalities
- More consistent correlation with heparin levels