What is the recommended dose of heparin (unfractionated heparin) for a patient with myocardial infarction (MI)?

Recommended Heparin Dosing for Myocardial Infarction

For patients with myocardial infarction (MI), unfractionated heparin should be administered as a weight-based bolus of 60 U/kg (maximum 4000 U) followed by an initial infusion of 12 U/kg per hour (maximum 1000 U/hour), with dose adjustments to maintain activated partial thromboplastin time (aPTT) at 1.5 to 2.0 times control (approximately 50-70 seconds). 1

Dosing Based on Treatment Strategy

With Fibrinolytic Therapy

• Fibrin-specific agents (alteplase, reteplase, tenecteplase):

  • Initial bolus: 60 U/kg IV (maximum 4000 U)
  • Initial infusion: 12 U/kg/hour (maximum 1000 U/hour)
  • Target aPTT: 1.5-2.0 times control (50-70 seconds) 1

• Non-selective fibrinolytic agents (streptokinase, anistreplase, urokinase):

  • UFH is indicated for patients at high risk for systemic emboli (large or anterior MI, atrial fibrillation, previous embolus, or known LV thrombus) 1
  • May be reasonable to administer UFH to all patients receiving streptokinase (Class IIb recommendation) 1

With Primary PCI

• Initial bolus: 60-70 U/kg IV (maximum 5000 U) 2
• For PCI procedures: 100 U/kg if no GPIIb/IIIa inhibitors are used; 60 U/kg if GPIIb/IIIa inhibitors are used 1
• Target activated clotting time (ACT): 250-350 seconds (200-250 seconds if GPIIb/IIIa antagonists are used) 1

Without Reperfusion Therapy

• Initial bolus: 60-70 U/kg IV
• Initial infusion: 12-15 U/kg/hour
• Target aPTT: 1.5-2.0 times control (50-70 seconds) 1

Monitoring and Dose Adjustment

aPTT Monitoring

• First aPTT check: 4-6 hours after starting infusion 3
• Subsequent monitoring: Every 6 hours until therapeutic, then daily 3

Dose Adjustment Nomogram

aPTT (seconds)	Action
<35 (<1.2× control)	Bolus 80 U/kg; increase infusion by 4 U/kg/h
35-45 (1.2-1.5× control)	Bolus 40 U/kg; increase infusion by 2 U/kg/h
46-70 (1.5-2.3× control)	No change (therapeutic range)
71-90 (2.3-3.0× control)	Decrease infusion by 2 U/kg/h
>90 (>3.0× control)	Stop infusion for 1 hour, then decrease by 3 U/kg/h

Important Considerations

Safety Monitoring

• Monitor platelet counts daily 1
• Watch for signs of bleeding 3
• Higher doses and supratherapeutic clotting times increase bleeding risk 3

Special Populations

• Elderly patients (>75 years): Consider lower doses and careful monitoring
• Renal failure: UFH is preferred over LMWH; standard weight-based dosing with careful monitoring 3

Alternative Anticoagulants

• Bivalirudin: Consider in patients with heparin-induced thrombocytopenia

  • Dosing: 0.75 mg/kg IV bolus followed by infusion of 1.75 mg/kg/h during PCI 1
  • HERO-2 regimen for use with streptokinase: 0.25 mg/kg bolus followed by 0.5 mg/kg/h for 12 hours, then 0.25 mg/kg/h for 36 hours 1

• Low Molecular Weight Heparin (LMWH):

  • May be considered for patients <75 years without significant renal dysfunction receiving fibrinolytic therapy
  • Not recommended for patients >75 years receiving fibrinolytic therapy 1

Common Pitfalls to Avoid

• Failure to use weight-based dosing, which can lead to under or overdosing
• Not adjusting for maximum dose limits (4000 U bolus, 1000 U/hour infusion)
• Inadequate monitoring of aPTT leading to subtherapeutic or supratherapeutic anticoagulation
• Not monitoring platelet counts daily to detect heparin-induced thrombocytopenia
• Using LMWH in elderly patients (>75 years) or those with significant renal dysfunction who are receiving fibrinolytic therapy

The evidence strongly supports weight-based dosing of unfractionated heparin for optimal outcomes in MI patients, with careful monitoring and dose adjustments to maintain the therapeutic anticoagulation window.