TRIGGERING OF ACUTE MYOCARDIAL-INFARCTION BY HEAVY PHYSICAL EXERTION - PROTECTION AGAINST TRIGGERING BY REGULAR EXERTION

Background. Despite anecdotal evidence suggesting that heavy physical exertion can trigger the onset of acute myocardial infarction, there h ave been no controlled studies of the risk of myocardial infarction du ring and after heavy exertion, the length of time between heavy exerti on and the onset of symptoms (induction time), and whether the risk ca n be modified by regular physical exertion. To address these questions , we collected data from patients with confirmed myocardial infarction on their activities one hour before the onset of myocardial infarctio n and during control periods. Methods. Interviews with 1228 patients c onducted an average of four days after myocardial infarction provided data on their usual annual frequency of physical activity and the time , type, and intensity of physical exertion in the 26 hours before the onset of myocardial infarction. We compared the observed frequency of heavy exertion (6 or more metabolic equivalents) with the expected val ues using two types of self-matched analyses based on a new case-cross over study design. The low frequency of heavy exertion during the cont rol periods was validated by data from a population-based control grou p of 218 subjects. Results. Of the patients, 4.4 percent reported heav y exertion within one hour before the onset of myocardial infarction. The estimated relative risk of myocardial infarction in the hour after heavy physical exertion, as compared with less strenuous physical exe rtion or none, was 5.9 (95 percent confidence interval, 4.6 to 7.7). A mong people who usually exercised less than one, one to two, three to four, or five or more times per week, the respective relative risks we re 107 (95 percent confidence interval, 67 to 171), 19.4 (9.9 to 38.1) , 8.6 (3.6 to 20.5), and 2.4 (1.5 to 3.7). Thus, increasing levels of habitual physical activity were associated with progressively lower re lative risks. The induction time from heavy exertion to the onset of m yocardial infarction was less than one hour, and symptoms usually bega n during the activity. Conclusions. Heavy physical exertion can trigge r the onset of acute myocardial infarction, particularly in people who are habitually sedentary. Improved understanding of the mechanisms by which heavy physical exertion triggers the onset of myocardial infarc tion and the manner in which regular exertion protects against it woul d facilitate the design of new preventive approaches.