Aspirin Dosing in Acute Myocardial Infarction
For a patient presenting with acute MI, administer 162 to 325 mg of non-enteric-coated aspirin immediately, which should be chewed or crushed for rapid buccal absorption, followed by a maintenance dose of 81 mg daily indefinitely. 1
Initial Loading Dose
The loading dose should be 162 to 325 mg of non-enteric-coated aspirin, administered as soon as possible after presentation. 1
- The aspirin must be chewed or crushed rather than swallowed whole to achieve rapid buccal absorption and immediate antiplatelet effect 1
- This applies to both STEMI and NSTEMI presentations 1
- The 162 mg dose has Level of Evidence A support, while 325 mg has Level of Evidence C support 1
- Aspirin should be initiated within 24 hours of hospital arrival, though ideally immediately upon diagnosis or even in the prehospital setting when ACS is suspected 1
- This loading dose achieves a 23% relative risk reduction in 5-week vascular mortality (2.4% absolute risk reduction) in STEMI patients 1, 2
Maintenance Dosing
After the initial loading dose, continue with 81 mg daily indefinitely. 1, 2
- The 81 mg daily maintenance dose is preferred over higher doses (such as 325 mg) to minimize bleeding risk while maintaining efficacy 1, 2
- Following stent placement, some protocols have used 325 mg daily for 1 month after bare-metal stents and 3-6 months after drug-eluting stents, but current evidence supports 81 mg daily even post-PCI due to excess bleeding with higher doses 1
- This maintenance dose should be continued indefinitely for secondary prevention 1, 2
Evidence Supporting This Dosing Strategy
The mortality benefit is substantial and immediate:
- Aspirin administered within the first 24 hours reduces vascular mortality by 23% at 5 weeks 1, 2
- Long-term aspirin therapy reduces serious vascular events by 1.5% per year in secondary prevention 1, 2
- Meta-analyses show similar efficacy across doses from 75 mg to 1500 mg daily for long-term prevention, but doses below 75 mg show reduced benefit 1, 3
Comparative Safety Data
The 162 mg loading dose may be safer than 325 mg without sacrificing efficacy:
- A large analysis of 48,422 STEMI patients treated with fibrinolytics found no difference in 24-hour, 7-day, or 30-day mortality between 162 mg and 325 mg initial doses 4
- However, the 325 mg dose was associated with significantly increased moderate/severe bleeding (OR 1.14,95% CI 1.05-1.24) 4
- For maintenance therapy, bleeding risk increases dose-dependently: 2.0% major bleeding with <100 mg daily, 2.3% with 100-200 mg, and 4.0% with >200 mg daily 1
Critical Implementation Points
- Do not use enteric-coated formulations for the initial loading dose, as they delay absorption 1
- Aspirin should be given immediately upon suspicion of MI, even before definitive diagnosis 1, 5
- The only contraindications are aspirin allergy, active bleeding, or documented aspirin intolerance 1
- Aspirin can be initiated in the prehospital setting by emergency medical services 1