Aspirin Dosing in Acute Myocardial Infarction

Initial Loading Dose

The American College of Cardiology recommends administering 162 to 325 mg of non-enteric-coated aspirin immediately, which should be chewed or crushed for rapid buccal absorption, in patients presenting with acute MI, with the 162 mg dose having Level of Evidence A support and the 325 mg dose having Level of Evidence C support 1, 2, 3, 4
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, achieving a 23% relative risk reduction in 5-week vascular mortality in STEMI patients 3, 4, 5
The loading dose applies to both STEMI and NSTEMI presentations, and should be administered as soon as possible after presentation 1, 2, 3, 4

The American College of Cardiology recommends continuing with 81 mg daily indefinitely after the initial loading dose, as this maintenance dose is preferred over higher doses to minimize bleeding risk while maintaining efficacy 3, 4, 5, 6
The 81 mg daily maintenance dose should be continued indefinitely for secondary prevention, reducing serious vascular events by 1.5% per year 3, 5, 6

Aspirin administered within the first 24 hours reduces vascular mortality by 23% at 5 weeks, and long-term aspirin therapy reduces serious vascular events by 1.5% per year in secondary prevention 3, 5
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 4

The American Heart Association recommends not using enteric-coated formulations for the initial loading dose, as they delay absorption, and aspirin should be given immediately upon suspicion of MI, even before definitive diagnosis 1, 2, 4
Aspirin can be initiated in the prehospital setting by emergency medical services, and the only contraindications are aspirin allergy, active bleeding, or documented aspirin intolerance 1, 2, 4