There was am article in Bicycling mag in October 2003 entitled Riding With (My) Heart where the author indicated that some, but not all, cardiologists advise there is a 7-fold increase in your risk of dying from the exercise when you exceed 70-80 percent of your maximum heart rate. The individual had some particular heart problems, but the advice applies to the public at large rather than just this individual based on his specific medical circumstances. Of course the paradox, as pointed out in a sidebar at the end of the article, is that heavy exercise such as cycling helps you increase your odds of avoiding death while at the same time you are at a heightened risk of death.
FWIW.