Board Thread:Manga Discussion/@comment-189.84.202.137-20180524152410/@comment-35409268-20180628105731

He died of suffocation not organ damage. None of his organs were damaged to our knowledge, just limbs, neck, and other non-vital parts of our body which were repaired by Machi. Given that there is a lot of noncombat superhuman activity without Nen, Killua's advanced resistance to poison and Brain control, it would be possible that Hisoka's brain can take far more time than most human's without oxygen. Although, that is only speculation and I have absolutely no idea on how he would train that.