In background, space marines usually take bionic replacements to recover from battle damage and not voluntarily (apart from perhaps techmarines and Iron Hands). When those replacements are needed is very obvious in the case of lost limbs or ruined senses because of appropriate critical hit results.

But when is getting a bionic heart appropriate? My guess is that a new heart would be needed after burning a fate point to survive an other-wise fatal critical hit to the body location. Or at least a percentage chance. What do you all think?

Well, not a bad topic.

Well, i would consider the posibility like then you can become Dreadnought in the higher ranks, a bionic heart might be possible in the case of 0 fate points left and "revived" through the replacement under "extreme conditions" to keep a player happy.

Often that option might be good choice for a player of the kill-team if he really loves his character, and to help the GM from introducing a new character.

If you're a Techmarine or Iron Hand, whenever the fancy strikes you.

Space Marines have a backup heart, so a situation where your main heart was destroyed and you need a bionic replacement doesn't necessarily have to be one where you had to burn a fatepoint.

Outside of a critical damage result that says your heart is destroyed?

When your GM says your heart is destroyed.

Duly noted all. I'm playing a Lamenters character and within two rounds of his first fight, he is down a leg already. His Chapter's luck is kicking in already! So this mightn't be a theoretical issue soon.