I'm not encouraging, but it's not as bad as thinkable. Unless your Oxygen deprived for long time. Opiods have very low toxicity compared to other substances like alcohol, stimulants, paracetamol ECT ect.

The heroin won't damage you, might damage hearing tho for a while but other then that, nothing. No organ failure...

However... Opiods decrease breathing. So heroin indirectly causes brain damage through oxegyn deprivation. But tht heroin isn't directly neurotoxic like MDMA is.

You could get bad damage if you stop breathing for a long while, like few minutes and still live. Then yeah some noticable brain damage.

You can get coma, seizures and death from oxygen deprivation. But that's for long time, few minutes. However your doing a high dose since you want to die. So that could very well happen

The thing is, i or someone else can't tell you if you will get.

It is like playing Russian roulette ، there is a chance especially at really high doses

And btw, opiods are pain killers so you don't notice pain from not breathing

So you might kill way more brain cells and still survive...