Well, if it's America's death rows you're talking about, the answer is, with difficulty.
There have been endless articles/discussions about whether or not the inmates suffer when subjected to the lethal injection; one man recently took two hours to die. Two whole hours. It takes a rare kind of genius to extend final suffering for that length of time. Burning at the stake might have been quicker.
And yet anyone who has ever had an anaesthetic knows that long before the patient has counted up to ten, he's out for the count. After that, a prisoner could be executed by any means the executing state chose. Moreover, anaesthetic drugs are widely available, used in all hospitals. Vets use them, too. I saw my horse being humanely put down within seconds, and a horse is a lot bigger than a human being.
Is the object to further punish someone who may already have been in solitary confinement half a lifetime (in Texas, anyway)? I'm beginning to think it might be. Whatever the reason, with this kind of barbaric treatment (putting aside the years and years of solitary misery already endured by some prisoners), a country that allows it cannot ever credibly condemn human rights abuse in other countries.
I'm thinking today of "my" prisoner, whose birthday it is next week. I cannot wish him a happy birthday; just the ability to endure. Until, of course, it's his turn.