My heart does not have room to effectively pump so I am unable to do much cardio? What exercise will help me lose weight?

I have pectus excavatum and my heart is turned in such a way that it is not able to effectively pump when I exercise. As a result I get dizzy and nauseous. I had a baby about 1 1/2 years ago and I am unable to get rid of an extra 20 pounds. How can I do that without overexerting? Don’t I need the cardio in order to lose weight?