Inert gas asphyxiation...
Fainting, then the systems shut down due to lack of oxygen.
Cheap too...
Death by Beiber
In an acoustic echo room play Justin Bieber music loud enough to kill.
I’ve heard that the big issue is the last minute reprieves from a governor or judge. There is a big effort to make sure that the condemned is 100% recoverable with no side effects until he is dead. Many of the painless methods used for animals like barbiturate overdose or inert gas will leave the condemned brain damaged or crippled if stopped half way through. So the current method is to give a sedative with no side effects and then a drug which produces a quick heart attack to minimize the amount of time when the executioner would have to tell the governor “he’s still alive but it’s too late to stop it.” If it takes five minutes for your beloved pet to go to sleep, stop breathing and the die peacefully no one will complain, but try to explain that to a governor who waited past the last minute.
Yeah, but they’re hyperventilating for O2.