What is the starting dose of Unfractionated Heparin (UFH) for anticoagulation in patients?

Unfractionated Heparin Dosing for Therapeutic Anticoagulation

For therapeutic anticoagulation in acute coronary syndromes, administer UFH as an 80 units/kg IV bolus (maximum 4000 units) followed by 18 units/kg/hour continuous infusion (maximum 1000 units/hour), adjusted to maintain aPTT at 1.5-2.0 times control. 1, 2

Initial Dosing Regimen

The weight-based dosing strategy has been validated across multiple clinical contexts:

• Standard therapeutic dose: 80 units/kg IV bolus followed by 18 units/kg/hour infusion 1, 2, 3
• Maximum bolus: Cap at 4000 units for patients >50 kg 4, 1
• Maximum infusion rate: Cap at 1000 units/hour for patients >55 kg 4, 1

Context-Specific Dosing

For STEMI patients receiving fibrinolytic therapy (alteplase, reteplase, or tenecteplase):

• Use a lower dose: 60 units/kg bolus (maximum 4000 units) followed by 12 units/kg/hour infusion (maximum 1000 units/hour) 4
• This reduced dosing minimizes bleeding risk when combined with fibrinolytics 4

For patients without IV access:

• Alternative subcutaneous regimen: 333 units/kg loading dose, then 250 units/kg every 12 hours 1

For cardiovascular surgery:

• Minimum 150 units/kg for total body perfusion 2
• 300 units/kg for procedures <60 minutes or 400 units/kg for procedures >60 minutes 2

Monitoring Protocol

First aPTT measurement at 6 hours after bolus, then every 4-6 hours until stable in therapeutic range 1, 2:

• Target aPTT: 1.5-2.5 times control (approximately 50-70 seconds) 4, 1
• Once stable, check aPTT daily 1
• Monitor platelet counts daily throughout therapy 4, 2

Dose Adjustment Algorithm

Based on aPTT results 1:

• aPTT <35 seconds: Give 80 units/kg bolus, increase infusion by 4 units/kg/hour
• aPTT 35-45 seconds: Give 40 units/kg bolus, increase infusion by 2 units/kg/hour
• aPTT 46-70 seconds: No change (therapeutic range)
• aPTT 71-90 seconds: Decrease infusion by 2 units/kg/hour
• aPTT >90 seconds: Hold infusion for 1 hour, then decrease by 3 units/kg/hour

Evidence Supporting Higher Dosing

The 80:18 regimen achieves therapeutic anticoagulation significantly faster than the traditional 60:12 regimen 3, 5:

• At 6 hours, 36% of patients on 80:18 reached therapeutic range versus only 16.7% on 60:12 (p=0.03) 3
• 97% of patients on weight-based dosing exceeded therapeutic threshold within 24 hours versus 77% on standard care (p<0.002) 5
• Failure to achieve adequate anticoagulation (aPTT >1.5 times control) is associated with 25% risk of recurrent thromboembolism 6

Special Populations

Severe renal insufficiency (CrCl <30 mL/min):

• UFH is the preferred anticoagulant as it undergoes hepatic metabolism 4, 1

Pediatric patients (use preservative-free formulation) 2:

• Initial: 75-100 units/kg IV bolus over 10 minutes 1, 2
• Infants: 25-30 units/kg/hour (infants <2 months require highest doses, average 28 units/kg/hour) 1, 2
• Children >1 year: 18-20 units/kg/hour 1, 2
• Target aPTT: 60-85 seconds 1

Critical Pitfalls to Avoid

Excessive dosing errors that increase bleeding risk 4:

• Never exceed 70 units/kg bolus OR 4000 units total bolus 4
• Never exceed 15 units/kg/hour infusion OR 1000 units/hour total 4
• Avoid giving another anticoagulant (enoxaparin, bivalirudin, fondaparinux) prior to UFH 4

Monitoring failures 1:

• Different aPTT reagents have variable heparin responsiveness—use laboratory-specific therapeutic ranges 1
• Heparin clearance is saturable and dose-dependent, making anticoagulant response nonlinear 1
• For subcutaneous dosing, draw aPTT 4-6 hours post-injection 2

Contraindications:

• Absolute contraindication in active heparin-induced thrombocytopenia (HIT) 4, 1
• Use extreme caution in patients with history of HIT; consider direct thrombin inhibitors or fondaparinux instead 4