I have had many issues with exercising, although I get a splitting headache once my heart rate gets up and nearly faint.

I have also done the eliptical, recumbent bike and swimming. My Dr's keep telling me that my issues are only due to muscle spasms in my neck that are affecting my optic nerve and causing these symptoms.

So now I am trying to do exercises that are low impact and easily controllable and don't make my neck or upper back/neck tighten.

Needless to say, I really miss working up a sweat.

Although....I did go to see a personal trainer who had me do an hours worth of monitored easier strengthening exercises and I really worked up a sweat. I had to sit down every once in a while so I didn't faint, but it was the best workout I have had in a year and a half.

Doing core work (vertical) on BOSU ball was really good, increases your heart rate and makes you feel a little more like you did before!

Good luck