Heparin Dosing for Angina
For unstable angina, initiate heparin with a 60-70 units/kg IV bolus (maximum 5,000 units) followed by a continuous infusion of 12-15 units/kg/hour (maximum 1,000 units/hour), targeting an aPTT of 1.5-2 times control (approximately 50-70 seconds). 1, 2
Dosing Algorithm
The weight-based approach is superior to fixed dosing for achieving therapeutic anticoagulation rapidly:
- Initial bolus: 60-70 units/kg IV (cap at 5,000 units)
- Continuous infusion: 12-15 units/kg/hour (cap at 1,000 units/hour)
- Target aPTT: 1.5-2 times control or 50-70 seconds 3, 1
This lower-dose regimen for acute coronary syndromes differs from the higher doses used for venous thromboembolism (80 units/kg bolus, 18 units/kg/hour infusion), reflecting the increased bleeding risk when heparin is combined with aspirin and other antiplatelet agents in coronary disease 1, 4.
Monitoring Strategy
Check aPTT at baseline and approximately every 4 hours initially, then at appropriate intervals once therapeutic range is achieved 2. The FDA label explicitly states this monitoring frequency for continuous IV infusion.
Additionally, monitor:
- Platelet counts periodically (to detect heparin-induced thrombocytopenia)
- Hematocrit
- Occult blood in stool 2
Critical Nuances
The dose for unstable angina is deliberately lower than for venous thromboembolism because bleeding risk increases substantially when heparin is combined with aspirin, which is standard therapy in acute coronary syndromes 3, 1. The American College of Chest Physicians guidelines emphasize that doses of 60-70 units/kg bolus with 12-15 units/kg/hour infusion are specifically recommended for unstable angina and non-ST-elevation MI 1.
Weight-based dosing achieves therapeutic aPTT faster than fixed dosing (within 6-7 hours vs 22+ hours), though it carries higher risk of initial supratherapeutic levels 5. Research shows that 42-78% of patients on weight-based regimens may initially overshoot the therapeutic range 5, 6, but this has not translated to increased bleeding in most studies when appropriate dose caps are used.
Common Pitfalls to Avoid
Do not use the higher VTE dosing regimen (80 units/kg bolus, 18 units/kg/hour) for angina—this substantially increases bleeding risk in the setting of concurrent antiplatelet therapy 1
Respect the maximum dose caps: 5,000 units for bolus and 1,000 units/hour for infusion. These caps prevent excessive anticoagulation, particularly in elderly or lower-weight patients 1, 4
Adjust for special populations: Elderly patients and those with prior warfarin use within 7 days are at higher risk for supratherapeutic aPTT 6. Consider starting at the lower end of the dosing range (60 units/kg, 12 units/kg/hour).
Obese patients may require lower weight-adjusted doses to achieve target ACT, though this data comes primarily from PCI studies 7
Alternative Administration Routes
If continuous IV infusion is not feasible, the FDA label provides alternative regimens 2:
- Intermittent IV: 10,000 units initial dose, then 5,000-10,000 units every 4-6 hours
- Subcutaneous: 5,000 units IV bolus, then 10,000-20,000 units SC every 8 hours
However, continuous IV infusion is strongly preferred for unstable angina as it provides more consistent anticoagulation 3.