What i did at first with the heart rate monitor is got on the treadmill or bike and got my heartrate higher untill i would start to get changes in head pressure , more headache, nausea, dizzeyness .
I would record the heartrate number and then each time i would keep my heartrate just under so as not to aggrevate the symptoms .
Also i would note if i got sickly the next day. I think my heartrate at which i could excersise was 115 beats per min. Its going to be different for everyone though. Then i would slowley push my threshold every week to see what happened .
Sometimes it would be ok to increase my heartrate other times i would go to fast and make myself sick
Its a learning process with lots of failures but now im winning more often .
You have to be patient and just work at it and never give up.