Heparin Drip Rate Calculation
For a patient receiving 18 units/kg/hr with a 25,000 units in 250ml formulation, the heparin drip rate is 0.18 ml/kg/hr.
Calculation Method
To calculate the heparin drip rate in ml/hr, we need to follow these steps:
Determine the concentration of the heparin solution:
- Heparin formulation: 25,000 units in 250 ml
- Concentration = 25,000 units ÷ 250 ml = 100 units/ml 1
Calculate the flow rate using the prescribed dose:
- Prescribed dose: 18 units/kg/hr
- For each kg of body weight, the patient needs 18 units/hr
- To convert units/hr to ml/hr: (18 units/kg/hr) ÷ (100 units/ml) = 0.18 ml/kg/hr 2
Final calculation for specific patient:
- Multiply 0.18 ml/kg/hr by the patient's weight in kg
- Example: For a 70 kg patient: 0.18 ml/kg/hr × 70 kg = 12.6 ml/hr 2
Clinical Context
The dose of 18 units/kg/hr is consistent with standard weight-based heparin protocols recommended by the American College of Chest Physicians for venous thromboembolism treatment 2
This dosing approach is supported by evidence showing that weight-based heparin dosing achieves therapeutic anticoagulation more rapidly than fixed-dose regimens 3
Important Considerations
Initial bolus dose is typically 80 units/kg before starting the continuous infusion at 18 units/kg/hr 2
Monitoring of aPTT should begin 4-6 hours after the initial bolus and after any dose change 1
Target aPTT is typically 1.5-2.5 times the control value (approximately 45-75 seconds) 2, 1
Dose adjustments should be made according to a standardized protocol based on aPTT results 1
Special Populations
For morbidly obese patients, consider using a modified dosing weight rather than actual body weight to avoid supratherapeutic anticoagulation 4, 5
Some studies suggest that a lower initial infusion rate of 15 units/kg/hr may be appropriate for elderly patients or those with recent warfarin therapy to avoid supratherapeutic aPTT response 6
For pediatric patients, dosing requirements may be higher:
- Infants (<1 year): 28 units/kg/hr
- Children (1-15 years): 20 units/kg/hr
- Adolescents (≥15 years): 18 units/kg/hr 2
Common Pitfalls to Avoid
Using fixed-dose regimens rather than weight-based dosing can lead to subtherapeutic anticoagulation and increased risk of recurrent thromboembolism 1, 3
Failure to achieve therapeutic aPTT within 24 hours is associated with higher mortality in pulmonary embolism 1
Standard protocols with maximum dose caps may cause significant delays in achieving therapeutic anticoagulation in obese patients 5, 7