Unfractionated Heparin Dosing for Pulmonary Embolism
For the treatment of pulmonary embolism, unfractionated heparin should be administered as an intravenous bolus of 80 U/kg followed by a continuous infusion at 18 U/kg/hour, with dose adjustments based on aPTT monitoring to maintain a target range of 1.5-2.5 times the control value (approximately 45-75 seconds). 1
Initial Dosing
- Weight-based dosing is superior to fixed dosing, as it achieves therapeutic anticoagulation more rapidly and effectively 1
- Initial bolus: 80 U/kg intravenously 1
- Maintenance infusion: 18 U/kg/hour as continuous IV infusion 1
- This weight-based approach is preferred over the older fixed-dose regimen (5,000-10,000 U bolus followed by 1,000 U/hour) 1
Monitoring and Dose Adjustment
- First aPTT check: 4-6 hours after starting the infusion 1
- Target aPTT: 1.5-2.5 times control value (approximately 45-75 seconds) 1
- This corresponds to heparin levels of 0.3-0.7 IU/mL by anti-factor Xa assay 1
Dose Adjustment Algorithm
When aPTT results are available, adjust the heparin dose according to this protocol 1:
| aPTT Result | Action |
|---|---|
| <35 seconds (<1.2× control) | 80 U/kg bolus; increase infusion by 4 U/kg/hour |
| 35-45 seconds (1.2-1.5× control) | 40 U/kg bolus; increase infusion by 2 U/kg/hour |
| 46-70 seconds (1.5-2.3× control) | No change |
| 71-90 seconds (2.3-3.0× control) | Reduce infusion by 2 U/kg/hour |
| >90 seconds (>3.0× control) | Stop infusion for 1 hour, then reduce by 3 U/kg/hour |
Duration of Therapy
- Continue heparin for at least 5-7 days 1, 2
- Overlap with oral anticoagulant (warfarin) for at least 4-5 days 1, 2
- Discontinue heparin after INR has been ≥2.0 for at least 24 hours 1
Alternative Subcutaneous Regimen
If intravenous administration is not feasible, subcutaneous unfractionated heparin can be administered as:
Important Considerations
- Monitor platelet counts every 2-3 days from day 4 to day 14 to screen for heparin-induced thrombocytopenia (HIT), which occurs in up to 5% of patients receiving UFH 1, 2
- Low-molecular-weight heparin (LMWH) is preferred over UFH for most stable patients with PE due to more predictable pharmacokinetics and lower risk of HIT 1, 3, 4
- However, UFH may be preferred in patients with severe renal impairment, those who may require procedures, or hemodynamically unstable patients 1
Potential Pitfalls
- Inadequate initial dosing is associated with higher rates of recurrent venous thromboembolism 1
- Failure to achieve therapeutic aPTT within 24 hours is associated with increased mortality 1
- Fixed dosing regimens (non-weight-based) often result in subtherapeutic anticoagulation 1
- Heparin resistance may occur in some patients with PE due to increased heparin binding proteins, requiring higher doses or monitoring with anti-Xa levels 5
Remember that rapid anticoagulation is critical in PE management, and weight-based dosing protocols have been shown to achieve therapeutic levels more consistently than fixed-dose regimens 1.