Aspirin

Aspirin

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, which 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 anti-platelet 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 1one percent), 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 an acute heart attack. 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 attacks.
• Aspirin improves survival among patients with stable exertional angina. (These are patients who experience chest pain only with exertion.)
• 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 attacks.)
• Aspirin in low doses (81 mg/day) has been shown to prevent first heart attacks (primary prevention)

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