What is the loading dose for MI patient?
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Here is a clear summary of antiplatelet loading doses used in acute MI (ACS/STEMI/NSTEMI):
Antiplatelet Loading Doses in Acute MI
1. Aspirin (ASA)
- Loading dose: 162-325 mg orally (chewed for faster absorption)
- Maintenance: 75-100 mg/day (must be <100 mg/day if used with ticagrelor)
- Given to ALL patients with ACS at presentation, unless contraindicated.
2. Clopidogrel (Plavix) - P2Y12 inhibitor (prodrug)
- Loading dose: 300 mg orally (standard) or 600 mg (for faster/greater inhibition before PCI)
- 300 mg: ~80% platelet inhibition within 5 hours
- 600 mg: faster onset (useful if PCI within 3-4 hours)
- Maintenance: 75 mg/day
- Requires hepatic activation via CYP2C19; poor metabolizers may have reduced response.
- Used in both STEMI and NSTEMI.
(Katzung's Basic and Clinical Pharmacology, 16th Ed.; Rosen's Emergency Medicine)
3. Ticagrelor (Brilinta) - P2Y12 inhibitor (direct, reversible) - Preferred agent
- Loading dose: 180 mg orally
- Maintenance: 90 mg twice daily
- NOT a prodrug - works directly without hepatic conversion
- Peak concentration at ~2.5 hours
- Preferred over clopidogrel due to mortality benefit in ACS (PLATO trial)
- Note: Maintenance aspirin dose must be kept <100 mg/day when used with ticagrelor
- Relative contraindications: bradycardia, severe reactive airway disease, prior hemorrhagic stroke
(Harrison's Principles of Internal Medicine 22E; Washington Manual of Medical Therapeutics)
4. Prasugrel (Effient) - P2Y12 inhibitor (prodrug, faster than clopidogrel)
- Loading dose: 60 mg orally
- Maintenance: 10 mg/day
- Only used in the invasive approach (PCI) and only after coronary anatomy is known
- Faster, greater, and more uniform platelet inhibition vs. clopidogrel
- Absolute contraindications: prior TIA or stroke, high bleeding risk
- Use with caution: age >75 years, body weight <60 kg
- No benefit shown for pre-treatment before PCI in NSTE-ACS
(Katzung's Basic and Clinical Pharmacology; Washington Manual; Rosen's Emergency Medicine)
Quick Reference Table
| Drug | Loading Dose | Maintenance | Route | Note |
|---|---|---|---|---|
| Aspirin | 162-325 mg | 75-100 mg/day | PO (chew) | All ACS patients |
| Clopidogrel | 300-600 mg | 75 mg/day | PO | Prodrug, CYP2C19 |
| Ticagrelor | 180 mg | 90 mg BID | PO | Preferred, direct |
| Prasugrel | 60 mg | 10 mg/day | PO | PCI only, post-anatomy |
Key Clinical Points:
- DAPT (Dual Antiplatelet Therapy) = Aspirin + one P2Y12 inhibitor, is standard in all ACS patients.
- Ticagrelor is the preferred P2Y12 inhibitor in most ACS scenarios due to its mortality advantage.
- Prasugrel is reserved for patients going to the cath lab where PCI is planned, after coronary anatomy is confirmed.
- In STEMI patients unable to take oral agents (e.g., post-cardiac arrest), cangrelor (IV P2Y12 inhibitor) can be considered - it has an immediate onset (<2 minutes) and short half-life (4-6 minutes).
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