What is the initial dosage for heparin infusion in adults?

Heparin Infusion Dosage for Adults

For therapeutic anticoagulation in adults, initiate heparin with an 80 units/kg IV bolus followed by a continuous infusion of 18 units/kg/hour, with dose adjustments based on aPTT monitoring to maintain levels 1.5-2.5 times control (typically 45-75 seconds). 1, 2, 3

Initial Dosing Regimen

Weight-based dosing is superior to fixed-dose regimens and achieves therapeutic anticoagulation more rapidly with better clinical outcomes. 4

Standard Weight-Based Protocol:

• Bolus dose: 80 units/kg IV push 1, 2, 3
• Initial infusion rate: 18 units/kg/hour continuous IV 1, 2, 3
• This regimen applies to venous thromboembolism (DVT, PE), unstable angina, and acute coronary syndromes 1, 2

Alternative Fixed-Dose Option (when weight unavailable):

• Bolus: 5,000 units IV 2, 3
• Infusion: 32,000-40,000 units/24 hours (approximately 1,300-1,700 units/hour) 2, 3

Monitoring and Dose Adjustments

Check the first aPTT 4-6 hours after the initial bolus, then 4-6 hours after any dose change, and daily once therapeutic range is achieved. 1, 2, 3

Target aPTT Range:

• 1.5-2.5 times control value (typically 45-75 seconds) 1, 2, 3
• This corresponds to anti-Factor Xa activity of 0.3-0.7 units/mL 1

Dose Adjustment Protocol:

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

Duration and Transition to Oral Anticoagulation

• Continue heparin for minimum 5 days with overlap of warfarin for at least 4-5 days 1, 2, 3
• Discontinue heparin only when INR ≥2.0 for at least 24 hours 1, 2, 3

Critical Clinical Considerations

Achieving therapeutic aPTT within 24 hours is associated with significantly lower mortality in pulmonary embolism patients. 2, 5 Failure to reach therapeutic levels within 24 hours increases the risk of recurrent thromboembolism to 25%, compared to only 2% when therapeutic levels are achieved. 6, 4

Special Populations:

Pediatric dosing (use preservative-free formulations in neonates/infants): 3

• Bolus: 75-100 units/kg IV over 10 minutes 3
• Infusion:
  • Infants <1 year: 28 units/kg/hour 2, 3
  • Children 1-15 years: 20 units/kg/hour 2, 3
  • Adolescents ≥15 years: 18 units/kg/hour 2, 3

Cardiovascular surgery: 150-300 units/kg for procedures <60 minutes, or 400 units/kg for procedures >60 minutes 3

Common Pitfalls to Avoid

• Using fixed-dose regimens instead of weight-based dosing leads to subtherapeutic anticoagulation in up to 23% of patients and increases recurrent thromboembolism risk. 2, 4
• Delaying first aPTT check beyond 6 hours can miss early supratherapeutic or subtherapeutic levels. 1, 3
• Discontinuing heparin before INR is therapeutic for 24 hours creates a gap in anticoagulation coverage. 1, 2, 3
• Failure to use standardized dose adjustment protocols results in inconsistent therapeutic achievement. 1, 2
• Administering intramuscular injections should be avoided due to high risk of hematoma formation. 3

Risk Factors for Supratherapeutic Response:

Patients aged ≥67 years, those on warfarin within 7 days prior to heparin initiation, and those with high initial infusion rates are at increased risk for supratherapeutic aPTT responses. 7 Consider starting at 15 units/kg/hour in these populations, though this is not standard guideline recommendation. 7

Heparin Resistance:

For patients requiring >35,000 units/day to achieve therapeutic aPTT, consider monitoring anti-Factor Xa levels (target 0.35-0.7 units/mL) instead of aPTT for dose adjustments. 1