It's all very strange.
Scientifically its simple and well known (in the industry) how to kill someone by lethal injection without pain - oddly its near no different to general anesthesia, one drug to knock you out, the other to stop everything working (its why your breathing is done by a machine under a general).
So why they cannot just do that in lethal injection cases I don't know, they would die from not breathing and they wouldn't be aware as they are unconscious.
Have a feeling its something to do with high up bible bashers in the US of A, and that's not far fetched imo - those sorts want 'retribution' for crimes and legal murder gives you an easy to do method........you can make them suffer all you like and who is going to stop you? not like a dead person will take anyone to court - seems they messed up the suffering part - they probably want them to suffer quietly for hours on end then die, as long as they appear dead, job done, no one will ever 'find out' will they?