In the 1890’s, scientists at Bayer developed a new medication they called aspirin. In 1899, Bayer began selling aspirin. For many decades, aspirin was used to treat pain, fever and inflammation. However, in the 1970’s, aspirin was found to have properties as a blood thinner. It is the blood thinning properties which makes aspirin beneficial for heart disease. Despite being in existence for more than 100 years, there are still questions about aspirin, such as the optimal dose and who should take it for the prevention of heart disease.
Aspirin’s ability to thin the blood centers around its effect on platelets. Platelets are cells which circulate in the bloodstream. If there is a tear in the wall of a blood vessel (a cut) or if a cholesterol-laden plaque in an artery breaks open, the platelets rush to the site of the injury and initiate the blood-clotting cascade. The resulting clot prevents bleeding from a cut or produces a clot within a heart artery, stopping the flow of blood and causing a heart attack. Aspirin inhibits the platelet’s ability to form a blood clot. This puts the person taking aspirin at risk for bleeding (since the platelets cannot trigger a blood clot) and the “blood is thinned”. Aspirin inhibits platelets for the life of the platelet, about 7 to 10 days. The most serious side effect of aspirin is bleeding. Aspirin may cause bleeding in the stomach (from an ulcer), the colon (from polyps) or the brain.
Aspirin is the mainstay of treatment for heart disease. In patients having an acute heart attack, aspirin is given immediately (in fact the patient is asked to chew it so it may be absorbed more quickly than if it was swallowed) to counteract the acute blood clot forming in the heart artery. After a heart attack, an “adult” aspirin (325 mg) is given daily to prevent another heart attack. This is termed secondary prevention. In this setting, aspirin has been shown to reduce the risk of heart attack, stroke or death by 33%. Similarly, aspirin is given to patents with chest pain due to heart artery disease, stroke patients, patients after heart bypass surgery and patients with a heart stent. In all of these scenarios, aspirin effectively reduces the risk of a second event balanced by a small increase in the risk for bleeding.
The optimal dose of aspirin is still not known. Doses below 75 mg are not effective, so the lowest dose in use now is “baby” aspirin (81 mg). Aspirin doses between 75 and 1500 mg are all felt to be equally effective. Low dose aspirin (81 mg) is associated with less risk for bleeding and less gastrointestinal intolerance. For patients with an acute heart attack, a fresh stent, a recent stroke or after bypass surgery, the initial dose of aspirin should be 325 mg daily for one month, followed by aspirin 81 mg daily indefinitely.
Can aspirin be used to prevent heart disease in people who have not yet had a heart attack (primary prevention)? The US Preventive Services Task Force recommends a low dose (81 mg) aspirin for adults aged 50 to 69 years old, who have a 10% or greater risk for cardiovascular disease (based on the American College of Cardiology cardiovascular risk calculator- http://tools.acc.org/ascvd-risk-estimator/), are not at increased risk for bleeding, and have a life expectancy of at least 10 years.
So, shouldn’t everyone be on an aspirin? Probably not. For those who are at low risk for heart disease, the risk of gastrointestinal bleeding and bleeding into the brain outweighs the benefit of even a low dose of aspirin. For those who have heart disease or a stroke or for those who are at high risk for heart disease, an aspirin a day can save a life and keep you away from your local heart hospital.