What is the appropriate heparin (unfractionated heparin) dosing regimen for a typical adult patient with deep vein thrombosis or pulmonary embolism and no significant impaired renal function?

Unfractionated Heparin Dosing for DVT/PE in Adults

For treatment of deep vein thrombosis or pulmonary embolism in adults with normal renal function, administer an initial IV bolus of 80 units/kg (or 5,000 units) followed by continuous IV infusion of 18 units/kg/hour (or 32,000-40,000 units per 24 hours), adjusted to maintain aPTT at 1.5-2.5 times control. 1, 2

Initial Dosing Regimen

Weight-based dosing is superior to fixed dosing and should be the preferred approach:

• IV bolus: 80 units/kg (maximum 5,000 units) 1, 2
• Continuous infusion: 18 units/kg/hour 1, 2

Alternative fixed-dose regimen (if weight-based unavailable):

• IV bolus: 5,000 units 1, 2
• Continuous infusion: 32,000-40,000 units per 24 hours 1, 2

The weight-based nomogram approach reduces recurrent thromboembolism rates compared to fixed dosing, as patients achieve therapeutic aPTT levels more rapidly. 1

Monitoring Requirements

Check aPTT 4 hours after initiation and 4 hours after any dose adjustment, then at appropriate intervals once therapeutic. 2

Target aPTT: 1.5-2.5 times control value 1, 2

• This corresponds to heparin levels of 0.2-0.4 U/mL by protamine titration 1
• Or anti-factor Xa levels of 0.30-0.7 U/mL 1

Additional monitoring throughout therapy (regardless of route):

• Platelet counts periodically to detect heparin-induced thrombocytopenia 2
• Hematocrit 2
• Occult blood in stool 2

Duration and Overlap with Oral Anticoagulation

Continue heparin for at least 5 days AND until INR is therapeutic (2.0-3.0) for at least 24 hours on two consecutive measurements. 1, 2

• Start warfarin within 24 hours of initiating heparin 3
• Overlap heparin and warfarin for minimum 4-5 days 1, 3
• Do not discontinue heparin until INR is stable in therapeutic range 2

Alternative Routes (When IV Access Limited)

Subcutaneous administration is an acceptable alternative:

• Initial: 5,000 units IV bolus, followed by 17,500 units (or 250 units/kg) subcutaneously every 12 hours 1, 2
• Adjust dose to maintain aPTT in therapeutic range 1
• Check aPTT 4-6 hours after injection 2

Critical Pitfalls to Avoid

Failure to achieve therapeutic aPTT within 24 hours is associated with 25% recurrence rate of venous thromboembolism. 1 This is the most common and dangerous error.

Do not use prophylactic doses for established thrombosis. Prophylactic dosing (5,000 units every 8-12 hours subcutaneously) is only for prevention, not treatment. 1, 2

Confirm vial strength before administration - fatal medication errors have occurred from confusion between treatment-dose vials and catheter lock flush vials. 2

Avoid intramuscular administration due to high risk of hematoma formation. 2

When to Use Unfractionated Heparin Over LMWH

While LMWH is generally preferred for DVT treatment 1, unfractionated heparin is specifically indicated when:

• Severe renal dysfunction (creatinine clearance <30 mL/min) 1
• High bleeding risk requiring rapid reversibility 1
• Hemodynamic instability 4
• Morbid obesity 4
• High-risk pulmonary embolism with cardiogenic shock 1

For pulmonary embolism, either unfractionated heparin or LMWH is appropriate for initial treatment. 1

Dose Adjustment Algorithm

If aPTT is below therapeutic range (<1.5 times control):

• Increase infusion rate and recheck aPTT in 4 hours 2

If aPTT is above therapeutic range (>2.5 times control):

• Decrease infusion rate and recheck aPTT in 4 hours 2
• Consider holding infusion temporarily if significantly elevated 2

The therapeutic window is narrow, requiring vigilant monitoring to balance efficacy against bleeding risk. 1