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Using ASA to Prevent Heart Attacks
Step 1: The Critical Appraisal - Reading & Recommendations
One of the many effects of ASA on the body is that it reduces the ability of the blood to clot by reducing the stickiness of platelets in the blood. A myocardial infarction is typically caused by thrombus formation in one of the major coronary arteries, typically at a site of plaque build-up and arterial intimal wall damage. By reducing the ability of the blood to clot, the chance of a thrombus forming in the vessels supplying the heart is reduced. A stroke on the other hand can be either thrombotic or hemorrhagic. It important to know which type a patient has had before considering ASA therapy as hemorrhagic stroke could be made worse by a decreasing clotting tendency.
Given the potential benefits of taking ASA, several very large trials have been conducted on 20,000 physicians in the U.K., 52,000 male physicians in the U.S. and 121,700 nurses in the U.S. aimed at determining benefits from taking ASA1, 2, 3, 4, 5
These trials found little benefit in taking 325 mg of ASA daily because of gastrointestinal bleeding and excessive bleeding in people who had a haemorrhagic stroke. These two problems almost completely cancelled the benefit from taking ASA daily.
During the last few years three new trials have changed the way of thinking about use of ASA to prevent heart attacks. A trial done in Italy in more than three hundred family practices on all persons over the age of 50 who had at least one risk factor for coronary heart disease and no previous heart trouble took 100mg of ASA daily for three and one half years. The results caused the study to be stopped after three and one half years for ethical reasons. The ASA was found so beneficial, that it was considered unethical to withhold the low dose ASA from those in the control group. The heart attack rate was reduced by 44%, and the risk of death from cardiac causes by 23%. It was very important that blood pressure in everyone taking ASA be kept in the normal range of 140/90 or less. This step reduced the risk of hemorrhagic stroke.
The use of low dose ASA either 81 mg in North America or 100 mg in Europe was important in reducing the risk of GI bleeding which occurred when 325 mg or more of ASA is used daily.6 Two other studies on low dose ASA done in different settings found similar results.7,8 The fact that one trial was carried out in general practice, where all eligible patients over the age of 50 were enrolled, gives family doctors confidence that these benefits will be experienced by their own patients.9 A ten year study on nearly 100,000 women found an insignificant 9% reduction in cardiovascular events in women over 45 and a significant 17% reduction in strokes. However women over 65 in this study had significant reduction in cardiovascular events. This study suggests a starting age for women be older than the 50 prescribed for men.11
A more recent meta analysis of 6 trials confirmed the benefits in reducing cardiovascular risk but not on all cause mortality.10
The Critical Appraisal - Summary
The three studies confirm the benefit of low dose aspirin in preventing heart attacks. To minimize risks men over the age of 45 to 79 with at least one cardiovascular risk factor should take a low dose ASA daily (North America 81 mg, Europe 100 mg). Women age 55 to 79 could also consider taking ASA but women under the age of 65 may not receive the same preventive benefits. Individuals taking the ASA regularly should have their blood pressure in the normal range of less than 140/90.
The Critical Appraisal - Other Recommendations
The Canadian Task Force on the Periodic Health Examination gives the recommendation to take 325 mg of ASA daily to prevent heart attack or stroke a "C" recommendation. This recommendation is presently under review given the new evidence.
The U.S. Preventive Services Task Force (USPSTF) strongly recommends that clinicians discuss aspirin chemoprevention with adults who are at increased risk for coronary heart disease.
Rating: A recommendation.
Summary of Recommendations.
- The USPSTF recommends the use of aspirin for men age 45 to 79 years when the potential benefit due to a reduction in myocardial infarctions outweighs the potential harm due to an increase in gastrointestinal hemorrhage. Grade: A recommendation.
- The USPSTF recommends the use of aspirin for women age 55 to 79 years when the potential benefit of a reduction in ischemic strokes outweighs the potential harm of an increase in gastrointestinal hemorrhage. Grade: A recommendation.
- The USPSTF concludes that the current evidence is insufficient to assess the balance of benefits and harms of aspirin for cardiovascular disease prevention in men and women 80 years or older. Grade: I statement.
- The USPSTF recommends against the use of aspirin for stroke prevention in women younger than 55 years and for myocardial infarction prevention in men younger than 45 years. Grade: D recommendation.
Selected References
There is no Cochrane review on this topic.
1. Peto, R Grey R, Collins R, et al. Randomized trial of daily aspirin in British male doctors. BMJ 1988; 296: 313-316. Steering Committee of the Physician's Health Study Research group: Preliminary report: findings of the Aspirin component of the ongoing Physicians Health Study. New Engl J Med 1988; 318: 262-264.
2. Steering committee on the Physician's Health Study Research group. Final report on the aspirin component of the ongoing Physicians Health Study. New Engl J Med 1989; 321: 129-135.
3. Manson JE, Grobbee DE, Stampfer MJ, et al. Aspirin in the primary prevention of angina pectoris in a randomized trial of the United States physicians. Am J Med 1990; 89: 772-776.
4. Manson JE, Stampfer MJ, Colditz GA, et al. A prospective study of aspirin use and primary prevention of cardiovascular disease in women. JAMA 1991; 266(4): 521-527.
5. Collaborative group of the Primary Prevention Project. Low-dose aspirin and vitamin E in people at cardiovascular risk: a randomized trial in general practice. The Lancet 2001;397: 89-95.
6. Hennekens CH, Peto R. Hutcheson JB, et al. An overview of the British and American Aspirin studies. [letter] New Engl J Med 1988; 318: 923-924.
7. Mead TW. Determination of who may derive most benefit from aspirin in primary prevention; subgroup results from a randomized controlled trial. BMJ 2000;321:13-17.
8. Hansson L, Zanchetti A, Carruthers G, et al. Effects of intensive blood pressure lowering and low dose aspirin in patients with hypertension: principal results of the Hypertension Optimal Treatment (HOT) randomized trial. Lancet 1999;351:1755-1762.
9. Rosser W. Aspirin for primary prevention of cardiovascular events. Lancet 2001;357:84-86.
10. Bartolucci AA, Howard G Meta-analysis of data from the six primary prevention trials of cardiovascular events using aspirin. Am J Cardiol. 2006 Sep 15;98(6):746-50.
11. Ridker PM, Cook NR, Lee IM, Gordon D, Gaziano JM, Manson JE, Hennekens CH, Buring JE: A randomized trial of low-dose aspirin in the primary prevention of cardiovascular disease in women. N Engl J Med. 2005 Mar 31;352(13):1293-304.