Heparin Dosing for Arterial Occlusion
For acute arterial occlusion, initiate unfractionated heparin with a weight-based bolus of 80 units/kg followed by a continuous infusion of 18 units/kg/hour, targeting an aPTT of 1.5-2.5 times control (approximately 60-85 seconds). 1
Initial Dosing Strategy
The weight-based approach is superior to fixed dosing for achieving rapid therapeutic anticoagulation:
- Bolus dose: 80 units/kg IV 1
- Initial infusion: 18 units/kg/hour 1
- This regimen achieves therapeutic aPTT significantly faster than lower-dose protocols (60 units/kg bolus with 12 units/kg/hour), with 36% of patients reaching therapeutic range by 6 hours versus only 16.7% with lower dosing 2
The evidence strongly supports aggressive initial dosing—patients who achieve therapeutic aPTT within 24 hours have significantly lower mortality rates compared to those who do not 1
Monitoring Requirements
Check aPTT at 6 hours after initiation, then every 6 hours until two consecutive therapeutic values are obtained, then daily. 1
- Target aPTT: 1.5-2.5 times control (typically 60-85 seconds) 1, 3
- Obtain baseline platelet count, aPTT, and INR before starting therapy 3
- Monitor platelet counts daily to detect heparin-induced thrombocytopenia (HIT), which can paradoxically cause arterial thrombosis despite anticoagulation 4, 3
Dose Adjustments for Special Populations
Renal Impairment
Unfractionated heparin does not require dose adjustment for renal dysfunction because it undergoes primarily hepatic metabolism, making it the preferred anticoagulant when creatinine clearance is <30 mL/min 3, 5
Bleeding Risk Factors
Reduce initial dosing in patients with:
- Age >60 years 1
- Recent surgery, trauma, or invasive procedures 1
- Hepatic dysfunction 1
- Multiple comorbidities 1
For high bleeding risk patients, consider starting with 60-70 units/kg bolus (maximum 5,000 units) followed by 12-15 units/kg/hour (maximum 1,000 units/hour). 1, 6 This lower-dose regimen is specifically recommended for acute coronary syndromes where bleeding risk is elevated 1
Obesity
Do not cap doses based on arbitrary maximums in obese patients—underdosing is common and delays therapeutic anticoagulation 7. Each 1 unit/kg/hour reduction in infusion rate delays therapeutic anticoagulation by 0.75-1.5 hours 7. Calculate the full weight-based dose even if it exceeds 2-3 times typical doses 7
Critical Safety Considerations
Heparin-Induced Thrombocytopenia (HIT)
- Occurs in up to 5% of patients receiving unfractionated heparin 3
- Suspect HIT if platelet count drops to 30,000-40,000/mm³ between days 4-20 of therapy 4
- Paradoxically causes arterial thrombosis through platelet aggregation 4
- Immediately discontinue heparin if HIT is suspected and switch to alternative anticoagulants (argatroban, bivalirudin, or fondaparinux) 5, 8
Bleeding Risk
- Risk increases proportionally with heparin dose 1
- Concomitant use with fibrinolytics or glycoprotein IIb/IIIa inhibitors substantially increases bleeding risk 1
- Major bleeding occurs in approximately 2-3% of patients on therapeutic heparin 1
- Keep protamine sulfate immediately available for reversal (1 mg protamine neutralizes approximately 100 units of heparin) 8
Practical Algorithm
Confirm no absolute contraindications: active bleeding, severe thrombocytopenia (<50,000/mm³), recent intracranial hemorrhage 8
Calculate weight-based dose: 80 units/kg bolus + 18 units/kg/hour infusion 1
Adjust for bleeding risk: If high risk present, reduce to 60-70 units/kg bolus + 12-15 units/kg/hour 1, 6
Check aPTT at 6 hours: Adjust infusion rate by ±2-4 units/kg/hour based on result 1
Monitor platelets daily: Discontinue immediately if count drops >50% or below 100,000/mm³ 3, 4
The key pitfall is underdosing—particularly in obese patients or when physicians arbitrarily cap doses due to bleeding concerns 7. The evidence clearly demonstrates that inadequate initial dosing delays therapeutic anticoagulation and increases thrombotic complications without reducing bleeding risk 1, 2.