When is a high pulse rate danerous while exercising?

If you have symptoms: Maximal predicted hr is 220 - age in years. Many fatigue at mphr. However athletes often capable of exceeding mphr. No absolute limit, but hr>200 associated with limited filling time and pump failure. Generally people develop limiting symptoms in their body before heart gives out. Not true if heart vessels are diseased... In that case one develops crushing chest pain which usually halts exercise.