Does Heavy Physical Exertion Trigger Myocardial Infarction? A Case-Crossover Analysis Nested in a Population-based Case-referent Study

Department of Public Health Sciences, Karolinska Institute, Sundbyberg, Sweden.
American Journal of Epidemiology (Impact Factor: 5.23). 04/2000; 151(5):459-67. DOI: 10.1093/oxfordjournals.aje.a010231
Source: PubMed


To study possible triggering of first events of acute myocardial infarction by heavy physical exertion, the authors conducted a case-crossover analysis (1993-1994) within a population-based case-referent study in Stockholm County, Sweden (the Stockholm Heart Epidemiology Program). Interviews were carried out with 699 myocardial infarction patients after onset of the disease. These cases represented 47 percent of all cases in the study base, and 70 percent of all nonfatal cases. The relative risk from vigorous exertion was 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 among sedentary subjects as in earlier studies, but the data also suggested a U-shaped association. In addition, the trigger effect was modified by socioeconomic status. Premonitory symptoms were common, and this implies risks of reverse causation bias and misclassification of case exposure information that require methodological consideration. Different techniques (the use of the usual-frequency type of control information, a pair-matched analysis, and a standard case-referent analysis) were applied to overcome the threat of misclassification of control exposure information. A case-crossover analysis in a random sample of healthy subjects resulted in a relative risk close to unity, as expected.

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    ABSTRACT: OBJECTIVES: The case-crossover study design was developed to examine triggers for the onset of myocardial infarction. This paper seeks to examine selected methodological issues when applying the case-crossover method to the study of traumatic injuries in the work environment. METHODS: Researchers known to be working on occupational case-crossover studies were invited to present at a workshop held at the National Occupational Injury Research Symposium in October 2000. Data from ongoing studies were used to illustrate various methodological issues involved in case-crossover studies of occupational injury. KEY FINDINGS AND ISSUES IDENTIFIED: To utilize the case-crossover design, investigators must clearly define the time during which a worker is at risk of injury, the period of time during which a particular transient exposure could cause an injury and carefully select control time periods that estimate the expected frequency of exposure. Other issues of concern are changing work tasks over time, correlated exposures over time and information bias. CONCLUSIONS AND FUTURE RESEARCH NEEDS: More case-crossover studies of occupational injury are needed to compare results from multiple studies. The validation of the timing of transient exposures relative to injury onset, whether done in a laboratory or field setting, should be conducted. Nested case-crossover designs in other epidemiological studies (case-control or cohort) can examine both transient and fixed risk factors for occupational injury, and should be attempted.
    Injury Prevention 10/2001; 7 Suppl 1:i38-42. · 1.89 Impact Factor
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