What is the recommended dosage of heparin (unfractionated heparin) in the management of myocardial infarction?

Heparin Dosage in Myocardial Infarction

Primary Dosing Strategy Based on Reperfusion Approach

For patients with ST-elevation myocardial infarction (STEMI) receiving fibrin-specific thrombolytics (alteplase, reteplase, tenecteplase), administer unfractionated heparin as a 60 U/kg IV bolus (maximum 4000 U) followed by 12 U/kg/hour continuous infusion (maximum 1000 U/hour), targeting an aPTT of 50-70 seconds (1.5-2.0 times control). 1, 2

For Primary PCI

• Administer an initial IV bolus of 100 U/kg (or 60 U/kg if GPIIb/IIIa inhibitors are used) 2
• Maintain activated clotting time (ACT) of 250-350 seconds during the procedure (200-250 seconds if GPIIb/IIIa antagonists are used) 2
• The infusion is typically terminated at the end of the procedure 2

For Fibrinolytic Therapy

• With fibrin-specific agents (alteplase, reteplase, tenecteplase): 60 U/kg IV bolus (maximum 4000 U) followed by 12 U/kg/hour infusion (maximum 1000 U/hour) for 24-48 hours 2, 1
• With non-selective agents (streptokinase, anistreplase, urokinase): Routine IV heparin within 6 hours is NOT recommended for patients not at high risk for systemic embolism 2
• For high-risk patients receiving non-selective agents: subcutaneous heparin 7500-12,500 U twice daily until ambulatory 2

For Patients NOT Receiving Reperfusion Therapy

• Administer 60 U/kg IV bolus (maximum 4000 U) followed by 12 U/kg/hour infusion (maximum 1000 U/hour), targeting aPTT 50-70 seconds 2, 1
• Alternative: subcutaneous heparin 7500 U twice daily (IV heparin is preferred) 2
• Continue anticoagulation for the duration of hospital stay up to 8 days 2

Monitoring Requirements

• Check aPTT at 3 hours after initiation, then at 6,12, and 24 hours 2
• Adjust dosing to maintain aPTT at 50-70 seconds (1.5-2.0 times control) 1, 3
• Monitor platelet counts daily to detect heparin-induced thrombocytopenia 3
• Check hematocrit and occult blood in stool periodically 3

Duration of Therapy

• Continuation of heparin infusion beyond 48 hours should be restricted to patients at high risk for systemic or venous thromboembolism 2
• High-risk features include: large anterior MI, atrial fibrillation, left ventricular thrombus on echocardiography, history of previous embolic event 2
• For patients transitioning to warfarin, continue full-dose heparin until INR reaches therapeutic range (2.0-3.0) for several days 3

Critical Dosing Distinctions

The key difference in heparin dosing depends on the thrombolytic agent used. Non-selective fibrinolytics (streptokinase, anistreplase, urokinase) produce systemic depletion of coagulation factors V and VIII and massive fibrin degradation products that themselves act as anticoagulants, making adjunctive heparin less necessary and potentially harmful 2. In contrast, fibrin-specific agents (alteplase, reteplase, tenecteplase) produce minimal systemic coagulation effects, making concurrent heparin essential for preventing reocclusion 2.

Common Pitfalls to Avoid

• Never use fixed-dose heparin without weight adjustment—the effect is primarily mediated by patient weight 1
• Do not confuse therapeutic heparin vials with catheter lock flush vials, which have caused fatal medication errors 3
• Avoid intramuscular administration due to frequent hematoma formation 3
• Do not use enteric-coated or delayed-release heparin formulations for acute MI 3
• For patients ≥75 years receiving enoxaparin as an alternative, omit the initial IV bolus and start with reduced subcutaneous dosing (0.75 mg/kg every 12 hours) 1