I gave my self a mini stress test to figure out my HR max so I could calculate my target heart rate range. I have hypertension and I am on beta blockers (my doctor gave me the okay to do what ever exercise I wanted to) since I can't do 220 - your age to find my THRR -d/t the BB I tried to find out when my HR max or when my HR plateaued which was 170- however- get this- when I approached my threshold and could push myself physically any longer my feet turned white and cold!? I do have raynaud's but during exercise? weird- can anyone please explain this.