What is the recommended heparin infusion dose for Acute Myocardial Infarction (AMI)?

Heparin Infusion Dosing for Acute Myocardial Infarction

For AMI patients receiving fibrinolytic therapy with alteplase, administer a 60 U/kg IV bolus (maximum 4,000 U) followed by 12 U/kg/hour infusion (maximum 1,000 U/hour for patients >70 kg), targeting aPTT 1.5-2.0 times control (50-70 seconds) for 48 hours. 1

Dosing Based on Reperfusion Strategy

With Fibrinolytic Therapy (Alteplase/tPA)

The ACC/AHA guidelines provide Class IIa recommendations for weight-based heparin dosing with fibrin-specific agents:

• Initial bolus: 60 U/kg IV (cap at 4,000 U maximum) 1, 2
• Maintenance infusion: 12 U/kg/hour (cap at 1,000 U/hour for patients weighing >70 kg) 1, 2
• Target aPTT: 1.5-2.0 times control value (approximately 50-70 seconds) 1, 2
• Duration: Continue for 48 hours minimum 1, 2

Critical monitoring protocol: Check aPTT at 3,6,12, and 24 hours after initiation, then recheck 4-6 hours after any dose adjustment 2. Monitor daily platelet counts to detect heparin-induced thrombocytopenia 2.

With Non-Selective Thrombolytics (Streptokinase, Anistreplase, Urokinase)

For high-risk patients (large or anterior MI, atrial fibrillation, previous embolus, or known LV thrombus):

• Delay heparin initiation: Withhold for 6 hours after thrombolytic administration 1, 3
• Begin when aPTT <2 times control (<70 seconds) 1
• Initial infusion rate: Approximately 1,000 U/hour 1
• Target aPTT: 1.5-2.0 times control 1
• Duration: 48 hours, then consider switching to subcutaneous heparin, warfarin, or aspirin alone 1

The rationale for delayed heparin with streptokinase is that these non-selective agents cause systemic fibrinolysis and prolonged aPTT, requiring time for coagulation parameters to normalize before safe heparin administration 3.

Primary PCI Strategy

For patients undergoing primary percutaneous coronary intervention:

• Without GP IIb/IIIa inhibitors: 70-100 U/kg IV bolus (maximum 5,000 U) 2, 4
• With GP IIb/IIIa inhibitors: 60 U/kg IV bolus (maximum 4,000 U) 2, 4
• Target ACT: 250-350 seconds when UFH used alone; 200-250 seconds with GP IIb/IIIa inhibitors 2

Non-ST Elevation MI (NSTEMI)

For NSTEMI patients not receiving thrombolytics:

• Initial bolus: 60-70 U/kg IV (maximum 5,000 U) 4
• Maintenance infusion: 12-15 U/kg/hour 4
• Target aPTT: 50-70 seconds 4
• Alternative: Intravenous UFH or subcutaneous LMWH (enoxaparin 1 mg/kg twice daily) 1

Patients NOT Receiving Thrombolytics

For AMI patients who do not receive reperfusion therapy:

• Standard prophylaxis: Subcutaneous UFH 7,500 U twice daily 1
• High-risk patients (large or anterior MI, atrial fibrillation, previous embolus, known LV thrombus): Prefer intravenous heparin using the dosing regimen above 1

Duration of Therapy

Standard duration is 48 hours for most patients 1, 2. However, continuation beyond 48 hours is recommended for high-risk patients with:

• Large or anterior MI 1
• Atrial fibrillation 1
• Previous embolic events 1
• Known left ventricular thrombus 1
• High risk for systemic or venous thromboembolism 1, 2

Alternative: Low-Molecular-Weight Heparin

Enoxaparin is an acceptable alternative to UFH for STEMI patients <75 years without significant renal dysfunction:

• Dosing: 30 mg IV bolus followed by 1.0 mg/kg subcutaneous every 12 hours 2, 3
• Class III contraindications (do NOT use): Patients ≥75 years receiving fibrinolytic therapy, or patients with significant renal dysfunction 2

Critical Pitfalls to Avoid

Common errors that increase complications:

• Failing to cap bolus doses at maximum limits (4,000-5,000 U depending on indication) increases bleeding risk 2
• Inadequate aPTT monitoring leads to subtherapeutic or supratherapeutic anticoagulation 2
• Switching between UFH and LMWH during acute phase increases bleeding complications 2
• Premature discontinuation in high-risk patients increases thrombotic events 2
• Using intramuscular route causes frequent hematomas and should be avoided 5
• Confusing vial strengths - always confirm correct concentration before administration 5

Special Populations

For pediatric patients with AMI (rare):

• Initial dose: 75-100 units/kg IV bolus over 10 minutes 5
• Maintenance: Infants require 25-30 units/kg/hour; children >1 year require 18-20 units/kg/hour 5
• Use preservative-free formulations in neonates and infants 5

For patients with heparin-induced thrombocytopenia:

• Alternative anticoagulant: Bivalirudin 0.25 mg/kg bolus followed by 0.5 mg/kg/hour for 12 hours, then 0.25 mg/kg/hour for 36 hours 2