How should I dose unfractionated heparin (and low‑molecular‑weight heparin if needed) for an acute ST‑segment‑elevation myocardial infarction patient undergoing primary percutaneous coronary intervention, including adjustments for weight, renal impairment, and contraindications?

Heparin Dosing for STEMI Undergoing Primary PCI

For STEMI patients undergoing primary PCI, administer unfractionated heparin as a 70-100 U/kg IV bolus (maximum 5,000 U) when used alone, or 60 U/kg IV bolus (maximum 4,000 U) when combined with glycoprotein IIb/IIIa inhibitors, targeting an ACT of 250-350 seconds (or 200-250 seconds with GP IIb/IIIa inhibitors), and discontinue immediately after the procedure. 1

Primary PCI Strategy (Preferred Approach)

Standard UFH Dosing

• Without GP IIb/IIIa inhibitors: Administer 70-100 U/kg IV bolus (maximum 5,000 U) 1, 2
• With GP IIb/IIIa inhibitors: Reduce to 60 U/kg IV bolus (maximum 4,000 U) 1, 2
• Target ACT: 250-350 seconds without GP IIb/IIIa inhibitors; 200-250 seconds with GP IIb/IIIa inhibitors 1, 2
• Duration: Discontinue heparin immediately at the end of the PCI procedure 2

Alternative: Enoxaparin for Primary PCI

Enoxaparin should be considered as an alternative to UFH for primary PCI, as it demonstrates superior outcomes with reduced death and major bleeding compared to UFH. 1

• Dosing: 30 mg IV bolus followed by 1.0 mg/kg subcutaneous every 12 hours 3
• Age restriction: Contraindicated in patients ≥75 years (Class III recommendation) 3
• Renal restriction: Contraindicated with creatinine >2.5 mg/dL (men) or >2.0 mg/dL (women) 3
• Evidence: Meta-analysis of 23 PCI trials (30,966 patients) showed enoxaparin reduced death and major bleeding compared to UFH, particularly in primary PCI 1

Alternative: Bivalirudin

• Dosing: 0.75 mg/kg IV bolus followed by 1.75 mg/kg/hour infusion 1
• Additional bolus: 0.3 mg/kg if needed 1
• Renal adjustment: Reduce infusion to 1 mg/kg/hour if creatinine clearance <30 mL/min 1
• Preferred over UFH with GP IIb/IIIa inhibitors in high bleeding risk patients 1
• Evidence: HORIZONS-AMI trial showed 40% reduction in major bleeding and 1% lower 30-day mortality, though with increased acute stent thrombosis 1

Fibrinolytic Strategy (When Primary PCI Not Available)

UFH Dosing After Fibrinolysis

• Initial bolus: 60 U/kg IV (maximum 4,000 U) 1, 4, 3
• Maintenance infusion: 12 U/kg/hour (maximum 1,000 U/hour) 1, 4, 3
• Target aPTT: 1.5-2.0 times control (approximately 50-70 seconds) 1, 4, 3
• Duration: Minimum 48 hours, preferably throughout hospitalization up to 8 days or until revascularization 4, 3, 2

Monitoring Requirements

• aPTT checks: At 3,6,12, and 24 hours after initiation 4, 3, 2
• After dose adjustment: Recheck aPTT 4-6 hours later 4, 3
• Platelet monitoring: Daily platelet counts to detect heparin-induced thrombocytopenia 4, 3, 2

Enoxaparin After Fibrinolysis (Preferred Alternative)

For patients <75 years with normal renal function receiving fibrinolytic therapy, enoxaparin is superior to UFH, reducing death and reinfarction by 17% with acceptable bleeding risk. 5

• Age <75 years: 30 mg IV bolus followed by 1.0 mg/kg subcutaneous every 12 hours 3, 2
• Age ≥75 years: Omit IV bolus; start with 0.75 mg/kg subcutaneous every 12 hours 3, 2
• Renal impairment (CrCl <30 mL/min): 1.0 mg/kg subcutaneous every 24 hours 3
• Duration: Throughout hospitalization, up to 8 days 3
• Evidence: EXTRACT-TIMI 25 trial (20,506 patients) showed 17% reduction in death/MI (P<0.001) and 33% reduction in reinfarction (P<0.001) compared to UFH 5

Weight-Based Adjustments

UFH Bolus Calculations

• Standard patient (70 kg): 70-100 U/kg = 4,900-7,000 U (capped at 5,000 U) 1
• With GP IIb/IIIa (70 kg): 60 U/kg = 4,200 U (capped at 4,000 U) 1
• Maintenance infusion: Always cap at 1,000 U/hour for patients >70 kg 4, 3

Enoxaparin Weight-Based Dosing

• 50 kg patient: 50 mg subcutaneous every 12 hours 3
• 80 kg patient: 80 mg subcutaneous every 12 hours 3
• 100 kg patient: 100 mg subcutaneous every 12 hours 3

Renal Impairment Adjustments

Enoxaparin Contraindications and Modifications

• Absolute contraindication: Creatinine >2.5 mg/dL (men) or >2.0 mg/dL (women) – use UFH instead 3
• CrCl <30 mL/min: Reduce to 1.0 mg/kg subcutaneous every 24 hours (retain 30 mg IV bolus if <75 years) 3
• CrCl 30-60 mL/min: No dose adjustment required 3

UFH in Renal Impairment

• No dose adjustment required for bolus 4, 3
• Monitor aPTT more frequently (every 3-4 hours initially) 4
• Adjust infusion based on aPTT response 4

Bivalirudin in Renal Impairment

• CrCl <30 mL/min: Reduce infusion to 1 mg/kg/hour 1
• Bolus dose unchanged 1

Special Populations and Contraindications

Heparin-Induced Thrombocytopenia (HIT)

Bivalirudin is the recommended anticoagulant for STEMI patients with HIT. 1, 4, 3

• Dosing: 0.25 mg/kg IV bolus followed by 0.5 mg/kg/hour for 12 hours, then 0.25 mg/kg/hour for 36 hours 4, 3
• Monitoring: Reduce infusion if aPTT exceeds 75 seconds within first 12 hours 3

Elderly Patients (≥75 Years)

• Enoxaparin: Class III contraindication after fibrinolysis due to increased intracranial hemorrhage risk 3
• If enoxaparin unavoidable: Omit 30 mg IV bolus, use 0.75 mg/kg subcutaneous every 12 hours 3, 2
• UFH: No age-based dose adjustment required 4, 3

High Bleeding Risk Patients

• Prefer bivalirudin over UFH with GP IIb/IIIa inhibitors (Class IIa recommendation) 1
• Consider radial access over femoral 1
• Avoid combining enoxaparin with GP IIb/IIIa inhibitors (increased bleeding in ASSENT-3 trial) 3

Critical Pitfalls to Avoid

Dosing Errors

• Never exceed maximum bolus doses: 5,000 U for UFH alone, 4,000 U with GP IIb/IIIa inhibitors 1, 4
• Never exceed maximum infusion: 1,000 U/hour for patients >70 kg 4, 3
• Do not switch between UFH and LMWH – associated with increased bleeding 1, 3, 2

Monitoring Failures

• Inadequate aPTT monitoring leads to subtherapeutic or supratherapeutic anticoagulation 4, 3
• Missing platelet count monitoring delays HIT detection 4, 3
• Failing to adjust for renal function with enoxaparin causes bleeding complications 3

Timing Errors

• Discontinuing heparin too early in high-risk patients (large anterior MI, atrial fibrillation, LV thrombus) increases thrombotic complications 3
• Continuing heparin infusion after primary PCI unnecessarily increases bleeding risk 2
• Delaying first aPTT check beyond 3 hours after fibrinolysis misses early anticoagulation failures 4, 3

Drug Interaction Errors

• Failing to reduce UFH dose when adding GP IIb/IIIa inhibitors (should reduce from 70-100 U/kg to 60 U/kg) 1
• Using fondaparinux as sole anticoagulant for primary PCI (Class III recommendation due to catheter thrombosis) 1

Transition to PCI After Fibrinolysis

Patients on Enoxaparin

• Continue enoxaparin during PCI – do not switch to UFH 3
• No additional anticoagulation needed if last enoxaparin dose <8 hours prior 3

Patients on UFH

• Administer additional UFH bolus to achieve target ACT 250-350 seconds (or 200-250 seconds with GP IIb/IIIa inhibitors) 3, 2
• Check ACT before PCI to guide additional dosing 1