Onset of Action for Heparin in DVT/PE
Intravenous unfractionated heparin begins anticoagulation immediately upon administration, with peak anticoagulant effect achieved within minutes of the initial bolus. 1
Immediate Anticoagulation with IV Heparin
IV heparin provides an immediate anticoagulant effect when administered as a bolus, which is why it remains the preferred agent when rapid anticoagulation is required for acute venous thromboembolism 1
The pharmacokinetics show that after an IV bolus, heparin enters the bloodstream and binds to antithrombin immediately, producing anticoagulation within approximately 5-10 minutes 1
The biologic half-life varies with dose: approximately 30 minutes after a 25 units/kg bolus, 60 minutes after a 100 units/kg bolus, and 150 minutes after a 400 units/kg bolus 1
Standard Dosing for Immediate Effect
Initial dosing should be 80 units/kg IV bolus followed by 18 units/kg/hour continuous infusion to achieve rapid therapeutic anticoagulation 1
Alternative regimen: 5,000-10,000 unit IV bolus followed by 1,300 units/hour (or 18 units/kg/hour) continuous infusion 2
The critical principle is achieving therapeutic aPTT (1.5-2.5 times control or 45-75 seconds) within the first 24 hours, as failure to reach therapeutic levels within 24 hours is associated with a 25% recurrence rate 3
Monitoring Timeline
Check aPTT 4-6 hours after the initial bolus to confirm therapeutic anticoagulation has been achieved 2
Subsequent aPTT checks should occur 6-10 hours after any dose adjustment, then daily once therapeutic 2
The target is maintaining aPTT at 1.5-2.5 times control, which corresponds to plasma heparin levels of 0.3-0.7 IU/mL anti-factor Xa activity 1, 4
Subcutaneous Heparin: Delayed Onset
Subcutaneous administration has reduced bioavailability and delayed onset compared to IV administration 1
If immediate anticoagulation is required but subcutaneous dosing is planned, give an IV bolus of heparin in conjunction with the first subcutaneous dose 1
For therapeutic subcutaneous dosing: 5,000 unit IV bolus followed by 250 units/kg subcutaneously twice daily, or 333 units/kg initial subcutaneous dose followed by 250 units/kg twice daily 1
Critical Pitfall to Avoid
Never rely on subcutaneous heparin alone when immediate anticoagulation is needed for acute PE or extensive DVT, as the reduced bioavailability and delayed absorption can result in subtherapeutic anticoagulation during the critical first 24 hours 1
In hemodynamically unstable patients or those with high-risk PE, unfractionated heparin IV (not LMWH) must be used to ensure immediate anticoagulation 2, 5