So, at the time you are having a heart attack, a big part of the problem is the growth of the blood clot within the affected artery. Formation of this blood clot depends on the blood platelets, which are tiny blood cells whose job is to participate in blood clotting. It turns out that aspirin - in small doses - can rapidly and powerfully inhibit the activity of the platelets, and therefore can inhibit the growth of the blood clot.
Inhibiting the growth of the blood clot is critical if you're having an MI, since maintaining at least some blood flow through the coronary artery can prevent heart muscle cells from dying. This is why chewing and swallowing an aspirin is usually one of the first things you will be asked to do when you arrive in the emergency room with a suspected MI.
But time is of the essence -- minutes count. So if you think you might be having a heart attack, most experts now advise patients not to wait until they get medical help -- take an aspirin as soon as you are concerned enough to call the paramedics. In this way, you can begin therapy immediately.
How Much, What Type, and How To Take It
The current recommendation for people who may be having a heart attack is to chew and swallow one non-coated adult aspirin (325 mg) as soon as possible. Chewing or crushing the aspirin gets it into your bloodstream more quickly - within 4 to 5 minutes - and researchers have measured a significant effect on platelets within that short period of time. Swallowing a whole aspirin with water, as you normally would, takes 10 to 12 minutes to achieve the same effect. This time difference may seem small, but as we've said, minutes count when your heart is at risk.
Antman, EM, Hand, M, Armstrong, PW, et al. 2007 focused update of the ACC/AHA 2004 guidelines for the management of patients with ST-elevation myocardial infarction: a report of the American College of Cardiology/American Heart Association Task Force on Practice Guidelines. J Am Coll Cardiol 2008; 51:210.