Coincidentally, more from JAMA on this topic.....

Exercise-related Heart Attacks Are Different

An examination was made of over 1,000 patients who suffered a
myocardial infarction following exercise. It showed that the
patients were more likely to be male, hyperlipidemic, and smokers
than those who suffered an infarct not related to exercise. They
were much more likely to present with ventricular fibrillation and a
large thrombus in the infarct artery. They were over ten times more
likely
to suffer a myocardial infarction during exercise than at other
times.

Reference: Giri, S., Thompson, P., Kiernan, F. et al.1999. Clinical
and angiographic characteristics of exertion-related acute
myocardial infarction. J. Am. Med. Assoc. 282(18):1731-1736.
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[log in to unmask] (Liza May)