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Aspirin inhibits the activity of the enzyme cyclo-oxygenase inside platelets.
Cyclo-oxygenase is an enzyme whose activity is necessary for the formation of a
chemical, thromboxane A2, that causes platelets to aggregate. Aspirin, by
inhibiting the formation of thromboxane A2, prevents platelets from aggregating
and thereby prevents the formation of blood clots.
Aspirin alone has its greatest impact on improving survival among patients
with heart attacks. Numerous studies have shown that aspirin reduces mortality
(by 25%) when given to patients with heart attacks. Aspirin is easy to use, safe
at the low doses used for antiplatelet action, fast acting (with an onset of
action within 30 minutes), and cheap. Aspirin is given at a dose of 160 mg to
325 mg immediately to almost all patients as soon as a heart attack is recognized. It
also is continued on a daily basis indefinitely after the heart attack. The only
reason for not using aspirin is a history of intolerance or allergy to aspirin.
Aspirin is taken daily following a heart attack to reduce the risk of another heart
attack. (Preventing further heart attacks is called secondary prevention, while preventing
the first heart attack is called primary prevention). The ideal daily dose
of aspirin for secondary prevention has not been established. Some doctors recommend
160 mg; others recommend 81 mg. The reason for this difference has to
do with aspirin's occasional long-term side effect of bleeding (for example from
stomach ulcers). Even though the risk of major bleeding with long-term, moderate
dose aspirin (325 mg/day) is low (less than 1%), this risk can be
lowered slightly by using an even lower dose (160 or 81 mg/day).
Aspirin also benefits patients with forms of coronary heart disease other
than heart attacks. Aspirin has been shown to reduce heart attacks and
improve survival in the following patients:
Aspirin improves survival among patients with unstable
angina. Patients with unstable angina experience chest pains at rest or with
minimal exertion. These patients have critically narrowed coronary arteries
and are at imminent risk of having a heart attack.
Aspirin improves survival among patients with stable
exertional angina. (These are patients who experience chest pain only with
Aspirin prevents formation of blood clots at the site
of the PTCA (see below).
Aspirin prevents the formation of blood clots that can
occlude surgical bypass grafts. (Occlusion of bypass grafts can lead to heart
Aspirin in low doses (81 mg/day) has been shown to prevent first heart
attacks (primary prevention).