Heparin Infusion Protocol
Initial Dosing
For venous thromboembolism (VTE), initiate heparin with an 80 units/kg IV bolus followed by 18 units/kg/hour continuous infusion. 1, 2, 3
Condition-Specific Dosing
VTE (DVT/PE): 80 units/kg bolus, then 18 units/kg/hour infusion 1, 2, 4, 3
Unstable angina/NSTEMI: 60-70 units/kg bolus (maximum 5,000 units), then 12-15 units/kg/hour infusion (maximum 1,000 units/hour) 1, 4
STEMI with fibrinolytics: 60 units/kg bolus (maximum 4,000 units), then 12 units/kg/hour infusion (maximum 1,000 units/hour) 1, 4, 3
Cardiac surgery/PCI: 100-150 units/kg bolus (target ACT >300 seconds) 2
Critical point: Weight-based dosing achieves therapeutic anticoagulation significantly faster than fixed-dose regimens (5,000-unit bolus/1,000 units/hour), with 97% of patients reaching therapeutic range within 24 hours versus only 77% with fixed dosing. 5 Subtherapeutic dosing in the first 24 hours increases recurrent VTE risk 10-22 fold. 2, 4
Monitoring Protocol
Check the first aPTT 6 hours after the initial bolus, then every 6 hours until therapeutic, then every 4 hours initially until stable. 1, 2, 3
Target aPTT Ranges
VTE: 1.5-2.5 times control (approximately 45-75 seconds, equivalent to anti-Xa 0.3-0.7 units/mL) 1, 2
ACS: 1.5-2.0 times control (approximately 50-70 seconds) 4
Important caveat: The therapeutic range varies significantly between aPTT reagents—with heparin levels of 0.3-0.7 anti-Xa units/mL, different reagents produce aPTT ratios ranging from 1.6-2.7 to 3.7-6.2 times control. 1 Your institution must calibrate its specific therapeutic range to the reagent used.
Additional Monitoring
Platelet count: Check daily to detect heparin-induced thrombocytopenia (HIT), which occurs in up to 5% of patients 2, 3
Hematocrit and stool occult blood: Monitor periodically throughout therapy 3
Dose Adjustment Nomogram
| aPTT Result | Action | Repeat aPTT |
|---|---|---|
| <35 seconds (<1.2× control) | 80 units/kg bolus, increase infusion by 4 units/kg/hour | 6 hours |
| 35-45 seconds (1.2-1.5× control) | 40 units/kg bolus, increase infusion by 2 units/kg/hour | 6 hours |
| 46-70 seconds (1.5-2.3× control) | No change | Next morning or per protocol |
| 71-90 seconds (2.3-3× control) | Decrease infusion by 2 units/kg/hour | 6 hours |
| >90 seconds (>3× control) | Hold infusion 1 hour, then decrease by 3 units/kg/hour | 6 hours |
Special Populations
Pediatric Dosing
Use preservative-free heparin in neonates and infants. 3
Initial dose: 75-100 units/kg IV bolus over 10 minutes 3
Maintenance infusion:
Target aPTT: 60-85 seconds (reflecting anti-Xa 0.35-0.70 units/mL) 3
Morbidly Obese Patients
For morbidly obese patients (BMI >40), use a modified dosing weight rather than actual body weight to avoid delays in achieving therapeutic anticoagulation. 6, 7
Recommended formula: Dosing weight = IBW + 0.3(ABW - IBW) or IBW + 0.4(ABW - IBW) 6
Alternative approach: Use the average of actual and ideal body weight 7
Rationale: Standard protocols with maximum dose caps (e.g., 5,000-unit bolus, 1,500 units/hour maximum) result in significant delays—one 388-kg patient required 55 hours to reach therapeutic aPTT. 6 Using actual body weight risks supratherapeutic levels, while ideal body weight produces subtherapeutic anticoagulation. 7
Renal Impairment
Monitor aPTT more frequently in patients with renal impairment, though dose adjustment is generally not required at therapeutic doses. 2 Heparin clearance involves both saturable (predominant at therapeutic doses) and nonsaturable mechanisms, with the saturable mechanism being independent of renal function. 2
Administration Routes
Continuous IV Infusion (Preferred)
Initial: 5,000-unit bolus, then 20,000-40,000 units/24 hours in 1,000 mL 0.9% sodium chloride 3
Advantage: Lower bleeding risk compared to intermittent injection 8
Intermittent IV Injection
Deep Subcutaneous Injection
Initial: 5,000-unit IV bolus, then 10,000-20,000 units subcutaneously every 8 hours OR 15,000-20,000 units every 12 hours 3
Technique: Inject above iliac crest or in abdominal fat layer using 25-26 gauge needle; rotate sites to prevent hematoma 3
Monitoring timing: Draw aPTT 4-6 hours after injection 1
Never use intramuscular route due to high risk of hematoma. 3
Transitioning to Oral Anticoagulation
Converting to Warfarin
Continue full-dose heparin for several days until INR reaches stable therapeutic range (2.0-3.0), then discontinue heparin without tapering. 3 Overlap warfarin with heparin for 4-5 days. 9
Converting to Direct Oral Anticoagulants (DOACs)
For continuous IV heparin, stop infusion immediately after administering first DOAC dose. 3 For intermittent IV heparin, start DOAC 0-2 hours before next scheduled heparin dose. 3
Common Pitfalls to Avoid
Using fixed-dose regimens instead of weight-based dosing: Results in subtherapeutic anticoagulation in 23% of patients within 24 hours and increases recurrent VTE risk 5-fold 5
Applying ACS dosing (60 units/kg, 12 units/kg/hour) to VTE patients: Leads to inadequate anticoagulation and higher recurrence rates 4
Delaying first aPTT beyond 6 hours: Prevents rapid dose optimization and prolongs time to therapeutic range 2
Failing to achieve therapeutic aPTT within 24 hours: Associated with 25% recurrent VTE rate and higher mortality in pulmonary embolism 4, 9
Exceeding maximum doses in ACS (>5,000-unit bolus or >1,000 units/hour): Increases bleeding risk without additional benefit 4
Not monitoring platelet counts daily: Delays detection of HIT, which can occur in up to 5% of patients 2
Inadequate mixing of heparin in infusion bags: Invert container at least 6 times to prevent pooling 3
Using preserved formulations in neonates: Risk of benzyl alcohol toxicity 3