I had the same experience. I had climbed an 8% hill in summer. My rate was about 160. After the second time I stopped took a cold drink of water. My monitor went to about 240. I climbed the hill once more thinking that it was an error with the inexpensive Polar HRM. On the way home I rode by a hospital and got them to hook me up to a monitor. Next thing I knew I was on a gurney in the ER.
That was about two years ago and mine is getting worse. It starts on its own now. I am 53 years old. It is difficult to ride when it does it. I feel a little bit sick and don't have any strength.
