The problem is with the left ventrical due to the abundance of androgen receptors there.
The amount of cardio you do is completely insignificant compared to the amount of test you run. (Or other highly androgenic products.)
Increased prostate size is another problem, although less deadly.
All of this information is easily available on the internet, just google it.
That is why I disagree with every other person on this board about running high test all the time. I don't disagree that more test will make you bigger, I'm sure that more is better in terms of size. But in terms of not dying, more test is not better. Whenever a test-free cycle or low-test cycle is mentioned, everyone says, "Wait, your body NEEDS testosterone, it just ain't SAFE not to run any." Ha... not safe. I guess that's why they need 700 mg of test per week, to be safe. The people who have dropped dead because of roid-induced cardic hypertrophy had been using roids quite a while - 10 years or more if I recall correctly - but a lot of guys here have been using roids that long haven't they?
Sometimes folks will say you gotta take more test than Deca, otherwise you'll be limp. I say reduce the test and take some Dostinex for the Deca Dick.
If you're a competitive bodybuilder, you have no choice, you have to take grams per week, including lots of androgens. But if you're just trying to bulk up for football or look good at the beach, you have to decide if the risk is worth it. I don't know why more people don't know this and reduce their androgens and compensate by putting the anabolic side through the roof.
The following study says that the enlarged heart will return to normal size after you give up the juice. But unfortunately that's after "several years". I don't know about you, but I don't want to go "several years" completely roid-free.
Are the cardiac effects of anabolic steroid abuse in strength athletes reversible?
In the end, people who wanna run high test all the time are gonna do just that and will probably be much bigger than me and get better lookin women, so there ya go.