What are the recommended loading doses for aspirin, P2Y12 inhibitors, unfractionated heparin, and glycoprotein IIb/IIIa inhibitors in acute myocardial infarction?

Loading Doses for Acute Myocardial Infarction

For acute MI, administer aspirin 162-325 mg (non-enteric, chewable preferred), a P2Y12 inhibitor loading dose (clopidogrel 600 mg, prasugrel 60 mg, or ticagrelor 180 mg), and unfractionated heparin 50-100 U/kg IV bolus depending on whether GP IIb/IIIa inhibitors are planned.

Aspirin Loading

• Loading dose: 162-325 mg non-enteric, chewable aspirin should be given immediately upon presentation 1, 2, 3
• For STEMI specifically, 162-325 mg before primary PCI 1, 2
• For NSTE-ACS, 162-325 mg promptly after presentation 3
• Non-enteric formulation is critical because enteric-coated aspirin has delayed and reduced absorption 3
• Maintenance: 81 mg daily is preferred (81-325 mg acceptable) 1, 2

P2Y12 Inhibitor Loading

Three options exist, with choice depending on clinical scenario:

Clopidogrel

• Loading dose: 600 mg as early as possible or at time of PCI 1, 2
• 300 mg loading if given with fibrinolytic therapy 1
• Less potent than prasugrel/ticagrelor but safer in patients with prior stroke/TIA 1, 2

Prasugrel

• Loading dose: 60 mg as early as possible or at time of PCI 1, 2
• Contraindicated in patients with prior stroke/TIA 1, 2
• Not recommended in patients ≥75 years or <60 kg body weight 4
• Should not be given <24 hours after fibrin-specific fibrinolytic 2, 5

Ticagrelor

• Loading dose: 180 mg as early as possible or at time of PCI 1, 2
• Preferred over clopidogrel for NSTE-ACS patients 3
• Critical caveat: Aspirin maintenance dose must be ≤100 mg daily when using ticagrelor 1, 6

Unfractionated Heparin Loading

Dosing depends on whether GP IIb/IIIa inhibitors are planned:

With GP IIb/IIIa Inhibitor Planned

• 50-70 U/kg IV bolus to achieve ACT 200-250 seconds 1, 2

Without GP IIb/IIIa Inhibitor

• 70-100 U/kg IV bolus to achieve ACT 250-350 seconds 1, 2
• Target ACT: 250-300 seconds (HemoTec) or 300-350 seconds (Hemochron) 2, 5

Alternative anticoagulant: Bivalirudin 0.75 mg/kg IV bolus followed by 1.75 mg/kg/h infusion is an acceptable alternative, particularly in high bleeding risk patients 1, 2

Glycoprotein IIb/IIIa Inhibitor Loading

Use is now more selective (Class IIa/IIb) given potent P2Y12 inhibitors:

Abciximab

• 0.25 mg/kg IV bolus, then 0.125 mcg/kg/min infusion (max 10 mcg/min) 1, 2
• Reasonable in STEMI patients undergoing primary PCI 1, 2

Eptifibatide

• 180 mcg/kg IV bolus (double bolus), then 2 mcg/kg/min infusion 1, 2
• Second 180 mcg/kg bolus given 10 minutes after first 1, 2
• Reduce infusion by 50% if CrCl <50 mL/min; avoid in hemodialysis 1, 2

Tirofiban

• 25 mcg/kg IV bolus (high-bolus dose), then 0.15 mcg/kg/min infusion 1, 2
• Reduce infusion by 50% if CrCl <30 mL/min 1, 2

Key consideration: GP IIb/IIIa inhibitors are reasonable in NSTE-ACS patients with high-risk features (elevated troponin) not adequately pretreated with clopidogrel 3. However, routine pre-catheterization laboratory administration in STEMI is not beneficial (Class III) 1, 2.

Critical Pitfalls to Avoid

1. Never use enteric-coated aspirin initially - delayed absorption negates rapid antiplatelet effect 3
2. Do not give prasugrel to patients with prior stroke/TIA - absolute contraindication 1, 2
3. Fondaparinux cannot be used as sole anticoagulant for PCI - requires additional anti-IIa agent due to catheter thrombosis risk 1, 2, 5
4. When using ticagrelor, aspirin maintenance must be ≤100 mg daily - higher doses reduce ticagrelor efficacy 1, 6
5. Adjust heparin dose based on GP IIb/IIIa use - failure to reduce dose increases bleeding risk 1, 2