I have asked my cardiologist this same question and was told that pushing your heart above it's maximum predicted heart rate for long periods can damage the heart. Also, there is not benefit to your health by exercising at this rate, the most good is done between 75 85% of your max predicted heart rate.
I asked for the same reason, keeping my heart rate at that level got to the point that it did not seem like much of a workout. I do an hour a day on the treadmill and have to keep it at a lower speed than I am capable of to maintain the proper heart rate. It can be a little frustrating. She did say short bursts would not be harmful, you just have to watch for symptoms and make sure you are getting proper heart rate and blood pressure response to exercise.
Hope this helps,
You probably won't like my reply. I'm not a doctor or an expert at all, but I recently read a book called "Recapture your Health," by Dr. Walt Stoll. In it he says, "Excessive exercise can stress the body's reserves and lead to depletion and health problems." "Overtraining can lead to hormonal abnormalities." "Professional athletes are also known to have a shorter life span than other people and many must retire early because they "burn out." I don't know how credible Walt Stoll is, but I have heard the thing about professional athletes before. I think you should check with your own doctor about what a healthy amount of exercise is for you. Good luck!