Hi, I recently took part in a bicycle time trial which consisted of 10 miles. I have ridden these before and swim 5 times a week run and bike, basically a triathlete. I am 47 years old man and have been training seriously since my teenage years. On this last time trial I was riding at my total max effort and beyond, only this time my vision started to blur felt light headed an for a split second kind of passed out then immediately regained then passed out 3 or 4 more times then regained again. I am assuming oxygen was starved from my brain in order for this to happen I was very well hydrated. I felt perfectly well okay after I finished the race, and suffered no ill effects. Just curious if I should be concerned. f