MDBG says 心脏 can be counted with either 顆 or 个...
Thank you for checking! In fairness, I suppose you usually only get one at a time.

I'm wondering if what you're referencing is something about "heart" as, like, "core" / "kernel" / "inmost center"? Because that would make sense!
Yes! In the books everything has a heart of sorts called a "kernel" which can be used to affect the whole, sounds a lot like 颗 words would apply ;)