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

Heparin Bolus Dosing in Myocardial Infarction

For patients with acute myocardial infarction, administer unfractionated heparin as a weight-based bolus of 60 U/kg (maximum 4,000 U) followed by an infusion of 12 U/kg/hour (maximum 1,000 U/hour), adjusted to maintain aPTT at 1.5-2.0 times control (50-70 seconds). 1, 2, 3

Dosing by Clinical Scenario

STEMI with Fibrinolytic Therapy

• Bolus: 60 U/kg IV (maximum 4,000 U) 1, 2, 3
• Infusion: 12 U/kg/hour (maximum 1,000 U/hour) 1, 2, 3
• Target aPTT: 1.5-2.0 times control (approximately 50-70 seconds) 1, 2, 3
• Duration: Continue for 48 hours, then discontinue unless high risk for systemic or venous thromboembolism exists 1, 2, 3

Primary PCI (No Prior Anticoagulation)

• Bolus: 70-100 U/kg IV (maximum 5,000 U) when used alone 2, 3
• Bolus with GP IIb/IIIa inhibitors: 60 U/kg IV (maximum 4,000 U) 2, 3
• Target ACT: 250-350 seconds when used alone; 200-250 seconds with GP IIb/IIIa inhibitors 3

Non-ST Elevation MI/Unstable Angina

• Bolus: 60-70 U/kg IV (maximum 5,000 U) 1, 4
• Infusion: 12-15 U/kg/hour 1, 4
• Target aPTT: 1.5-2.0 times control (50-70 seconds) 1, 4

Critical Dosing Principles

Weight-Based Dosing is Mandatory

• Body weight is the strongest predictor of heparin effect on aPTT 1, 5, 6
• Fixed-dose regimens (5,000 U bolus/1,000 U/hour infusion) result in marked overanticoagulation in most patients and should be avoided 6
• Weight-adjusted dosing achieves therapeutic aPTT more rapidly and reduces the need for dose adjustments 6

Maximum Dose Caps Must Be Respected

• Never exceed 4,000 U bolus in patients receiving fibrinolytic therapy 1, 2, 3, 5
• Never exceed 1,000 U/hour initial infusion in patients >70 kg 1, 2, 3, 5
• These caps prevent excessive anticoagulation and reduce bleeding risk, particularly when heparin is combined with fibrinolytics or GP IIb/IIIa inhibitors 2, 5

Monitoring Requirements

aPTT Monitoring Schedule

• Check aPTT at 3,6,12, and 24 hours after initiation 2, 3, 5
• Recheck 4-6 hours after any dose adjustment 2, 3
• Adjust infusion rate to maintain aPTT at 1.5-2.0 times control 1, 2, 3

Platelet Monitoring

• Check daily platelet counts to detect heparin-induced thrombocytopenia 1, 2, 3, 5

Common Pitfalls to Avoid

Do Not Use Fixed-Dose Regimens

• Traditional 5,000 U bolus/1,000 U/hour infusion results in 95% of patients being above target aPTT at 6 hours 6
• This overanticoagulation increases bleeding risk and adverse outcomes 6

Do Not Exceed Maximum Dose Caps

• Exceeding 4,000 U bolus or 1,000 U/hour in patients >70 kg leads to excessive anticoagulation and increased bleeding, especially with concurrent fibrinolytics 2, 5

Do Not Discontinue Prematurely

• Continue heparin for minimum 48 hours unless contraindicated 1, 2, 3
• High-risk patients (large or anterior MI, atrial fibrillation, known LV thrombus) may require longer duration 1

Do Not Switch Between Anticoagulants

• Patients already receiving enoxaparin, bivalirudin, or fondaparinux should not receive standard UFH dosing 2, 5
• For patients already on UFH who require PCI, give additional boluses as needed rather than restarting the full regimen 5

Special Populations

Heparin-Induced Thrombocytopenia

• Use bivalirudin as alternative: 0.25 mg/kg bolus followed by 0.5 mg/kg/hour for 12 hours, then 0.25 mg/kg/hour for 36 hours 1, 2, 3

Patients <75 Years Without Renal Dysfunction

• Enoxaparin is an acceptable alternative: 30 mg IV bolus followed by 1.0 mg/kg subcutaneous every 12 hours 1, 3
• Do not use LMWH in patients ≥75 years receiving fibrinolytic therapy (Class III) 1, 3
• Do not use LMWH in patients with significant renal dysfunction (creatinine >2.5 mg/dL in men or >2.0 mg/dL in women) 1, 3