Alright, I think I'll let you in on what I did know, Watcher, I think it's been long enough(actually, I just wanted to see if you would react to the claim that I did 'something' while I was inside)*reaches in pocket and pulls out a rather large (as in too large to realistically fit in my pocket large) syringe with needle*
You, er...it ('heart'), have been infected with a heart virus! When this virus comes in contact with blood (or whatever else it uses to keep itself alive), it contaminates the substance with a somewhat concrete substance, causing the it to dry and harden. And of course, when the fuel for the heart can't flow, well, then the heart just goes to sh*t. Think of it like Inflammation of Atherosclerosis, but a little different.
Now since the virus takes a little while to really take effect, then I needed to bide my time, so I just kept 'it' busy with random attacks. But know 'it' should be feeling the effects of the virus, so yeah. And don't try saying that it doesn't work because the heart is an astral projection (because if it was, then it wouldn't be capable of interacting physically with, well, physical matter), or that it is immune to ALL viruses (aka those unknown to existance).