Recommended Aspirin Dose for Suspected MI in the Field
Give 162-325 mg of non-enteric-coated aspirin, chewed immediately, for any adult with suspected myocardial infarction in the prehospital setting.
Specific Dosing Algorithm
The American Heart Association and American College of Cardiology recommend administering 162-325 mg of non-enteric-coated aspirin as soon as MI is suspected, with the patient instructed to chew the tablet(s) before swallowing 1, 2, 3.
Dose Selection Within the Range
- 162 mg has Level of Evidence A support, while 325 mg has Level of Evidence C support 2, 3
- The 162 mg dose may be as effective as and potentially safer than 325 mg, with one large study (n=48,422) showing no mortality difference but significantly less bleeding with 162 mg (adjusted OR 1.14 for moderate/severe bleeding with 325 mg, p=0.003) 4
- Either dose within the 162-325 mg range achieves rapid platelet inhibition within 30 minutes when chewed 5
Critical Administration Details
Formulation Requirements
- Must use non-enteric-coated aspirin - enteric-coated formulations delay absorption by hours and should never be used in acute MI 1, 2, 3
- The patient must chew the aspirin - chewing accelerates buccal absorption and onset of antiplatelet effect compared to swallowing whole 1, 3, 5
Timing Considerations
- Administer immediately upon suspicion of MI, even before definitive diagnosis 2
- Call EMS first, then give aspirin while awaiting arrival - do not delay EMS activation 1
- Aspirin should be given within 24 hours of symptom onset, with earlier administration associated with better outcomes 1, 2
Evidence Supporting This Approach
The ISIS-2 trial demonstrated that aspirin alone reduces 35-day mortality by 23% in acute MI, and when combined with thrombolytic therapy, mortality reduction reaches 42% 6. A dose of 160 mg or more produces rapid and near-total inhibition of thromboxane A2 production 6. Meta-analyses show similar efficacy across doses from 75-1500 mg for long-term prevention, but doses below 75 mg show reduced benefit 2, 7.
Absolute Contraindications in the Field
Do not give aspirin if the patient has: 1, 2
- Known aspirin allergy or documented aspirin intolerance
- Active bleeding on arrival or within the past 24 hours
- Current warfarin/Coumadin use
- Prior physician advice not to take aspirin
If any uncertainty exists about contraindications, wait for EMS without giving aspirin 1.
Common Pitfalls to Avoid
- Never use enteric-coated aspirin - this is the most common error and delays therapeutic effect by hours 1, 3
- Never allow the patient to swallow the tablet whole - chewing is essential for rapid buccal absorption 1, 3
- Never use sublingual administration - there is zero evidence supporting this route in MI 3
- Never delay EMS activation to administer aspirin - call EMS first, then give aspirin while waiting 1
Alternative Routes When Oral Not Possible
If the patient cannot take oral aspirin due to severe nausea/vomiting, rectal administration of 325 mg suppository is an acceptable alternative 6, 3. However, this is rarely applicable in the prehospital field setting.