Does heavy physical exertion trigger myocardial infarction? A case-crossover analysis nested in a population-based case-referent study

To study possible triggering of first events of acute myocardial infarction by heavy physical exertion, the authors conducted a case-crossover analysi s (1993-1994) within a population-based case-referent study in Stockholm Co unty, Sweden (the Stockholm Heart Epidemiology Program). Interviews were ca rried out with 699 myocardial infarction patients after onset of the diseas e. These cases represented 47 percent of ail cases in the study base, and 7 0 percent of all nonfatal cases. The relative risk from vigorous exertion w as 6.1 (95% confidence interval: 4.2, 9.0). The rate difference was 1.5 per million person-hours, and the attributable proportion was 5.7 percent. The risk was modified by physical fitness, with an increased risk being seen a mong sedentary subjects as in earlier studies, but the data also suggested a U-shaped association. In addition, the trigger effect was modified by soc ioeconomic status. Premonitory symptoms were common, and this implies risks of reverse causation bias and misclassification of case exposure informati on that require methodological consideration. Different techniques (the use of the usual-frequency type of control information, a pair-matched analysi s, and a standard case-referent analysis) were applied to overcome the thre at of misclassification of control exposure information. A case-crossover a nalysis in a random sample of healthy subjects resulted in a relative risk close to unity, as expected.