What does your doctor say? You can try searching hre or Googling for "chair workouts". You could just sit in a chair and do punching jabs, alternating arms, as fast as you can. I know that can get my HR up if I do it enough (I'm usually standing, but it will probably work sitting.)
I am 5'4".
Starting Weight (4.6.11) 164.6
First Goal 130 - Reached September 2011
Currently maintaining under 120 and working on changing my mental image of myself!
| current weight: 136.0