We thank Dr Čulić for his insightful comments on our paper. We agree that our observations suggest that even though the relative risk for the association between moderate exertion and acute myocardial infarction (MI) onset is smaller than that for higher levels of exertion, the absolute number of MI cases that occur in association with moderate exertion is likely larger because of the higher frequency of such activity.1,2 Nonetheless, the absolute risk of MI onset associated with each bout of vigorous activity is very low, and based on our data the risk appears to be even smaller for moderate exertion.
Comparing our study with earlier work,1 it is important to note that unlike the paper by Mittleman et al., we specifically evaluated the effect of moderate level activities and the exposure–response relationship between increasing intensities of physical exertion and the risk of acute MI onset.
We also agree that the finding of a stronger association between strenuous exertion and MI onset when exertion was performed outdoors is intriguing. Dr Čulić's hypotheses regarding the reasons for this observation are well founded. However, it is important to note that although meteorologic and air pollution effects have been reported to play a role in the onset of MI,3,4 the magnitude of their effects make the possibility that our observed association was primarily driven by confounding due to the co-occurrence of outdoor activity and transient fluctuations in these environmental conditions extremely unlikely. Despite this, it is conceivable that, for example, traffic-related exposure5 may modify the effects of outdoor strenuous exertion, as has been also suggested by experimental studies.6 The possibility that the effects of exertion might be modified by the external environment (and vice versa) is an important consideration that certainly merits further study.
. Triggering of acute myocardial infarction by heavy physical exertion. Protection against triggering by regular exertion. Determinants of Myocardial Infarction Onset Study Investigators. N Engl J Med 1993;329:1677-1683.