Aspirin Dosing in Acute Myocardial Infarction
Give 162 to 325 mg of non-enteric-coated aspirin immediately, chewed or crushed, with the 162 mg dose having the strongest evidence (Level A) and representing the optimal balance between efficacy and bleeding risk. 1, 2
Initial Loading Dose
Administer 162 to 325 mg of aspirin as soon as a heart attack is suspected, ideally in the prehospital setting or immediately upon presentation. 1, 2
The aspirin must be chewed or crushed (non-enteric-coated formulation) to achieve rapid buccal absorption and immediate platelet inhibition. 1, 2
The 162 mg dose carries Level of Evidence A support from the ACC/AHA, while the 325 mg dose has only Level of Evidence C support. 1, 2
Research directly comparing these doses demonstrates that 162 mg is as effective as 325 mg for mortality reduction but causes significantly less bleeding (9.3% vs 12.2% moderate/severe bleeding, adjusted OR 1.14 for 325 mg). 3
Evidence Supporting the 162 mg Dose
In the GUSTO I and III trials (n=48,422 STEMI patients), 162 mg aspirin showed equivalent 24-hour mortality (2.8% vs 2.9%), 7-day mortality (4.9% vs 5.2%), and 30-day mortality (6.5% vs 7.1%) compared to 325 mg, with no statistical difference after adjustment. 3
The 162 mg dose produces rapid and near-total inhibition of thromboxane A2 production, achieving the full clinical antithrombotic effect needed in acute MI. 1
When given within 24 hours of STEMI onset, aspirin reduces vascular mortality by 23% at 5 weeks. 2, 4
Maintenance Dosing After the Acute Event
Switch to 81 mg daily indefinitely after the initial loading dose. 1, 2, 4
The 81 mg maintenance dose is explicitly preferred over higher maintenance doses (such as 325 mg daily) to minimize bleeding risk while maintaining full antiplatelet efficacy. 1, 2, 4
This maintenance dose reduces serious vascular events by 1.5% per year in long-term secondary prevention. 2, 4
Critical Implementation Points
Never use enteric-coated aspirin for the loading dose as it delays absorption when immediate platelet inhibition is essential. 1, 2
Aspirin can and should be administered by first aid providers or EMS personnel in the prehospital setting while awaiting transport. 1
The only contraindications are documented aspirin allergy, active bleeding, or prior physician instruction not to take aspirin. 1
If the patient has already taken aspirin before presentation, do not give an additional loading dose—continue with the maintenance regimen. 1
Common Pitfalls to Avoid
Do not delay aspirin administration waiting for diagnostic confirmation—give it immediately when acute MI is suspected based on symptoms. 1, 2
Do not use higher maintenance doses (such as 325 mg daily) long-term, as meta-analyses show equivalent efficacy across doses from 75 to 1500 mg daily for secondary prevention, but higher doses increase bleeding without additional benefit. 5, 6
Do not give aspirin to patients with systolic blood pressure below 90 mmHg until hemodynamic stability is achieved, though this relates more to concurrent nitrate administration than aspirin itself. 1