Not sure if this is the right forum for this, as it doesn't really require modification of bike, just habits. I have a heart condition - an enlarged heart. It's inherited, no amount of Cheerios is going to fix it. In hindsight there were clues my whole life but I didn't start actually getting chest pain until I was in my early thirties. This is common. It's why teen athletes with the same thing keel over dead one day... they had no idea there was a problem.
I'm on blood pressure medications, which limit heart rate. There have also been some blood sugar side effects which limit my diet, and make me feel like crap after eating any sort of bulk carbs. I sometimes get dizzy standing up. Exercising has therefore been frustrating. I never know setting out if I'm going to feel good enough to keep going, and I never get going too quickly. I have to stop once in a while and no one really wants to go slow enough for me to keep up with him. Getting heart rate up to 90% or burning a lot of calories is out of the question, as is any sort of weightlifting (builds heart muscle in the wrong place).
But I need to keep at it... because it's still exercise and it's still needed. It took me about two years from my diagnosis to really get back to a place where I was happy with my health. I dropped everything at first. But I missed cycling a lot and remembered there were lower gears. I've lost about 25 lb due to diet and lack of weightlifting, that has helped too. So if you are reading this and think that your heart problem means you get to sit on the couch and read the Internet all day - get back out there you big wuss.
Patients with Hypertrophic Cardiomyopathy Place Unnecessary Restrictions on Exercise