I think they were pretty stupid to use the drugs they did. I don't think the drugs were appropriate in that they were not strong enough. Midazolam (versed) was used, this was just the sedative to keep him asleep. I guess they thought that the hydromorphone (dilaudid) would kill him right off the bat. I don't know how much pain he was feeling, though it was obviously distressing to watch him struggle for air. Reports say that he died of a heart attack, but I wonder if he really had asphyxial attack, in which the heart stops because of lack of oxygen. That is, the medication basically made him suffocate to death. Suffocation is a tough way to die and could violate the "cruel and unusual punishment" part of the constitution.
I just read that they had to give him 15 doses of medication. The execution protocol stated that the injection should be 50 mg midazolam and 50 mg hydromorphone. They ended up giving him 750 mg of each. This has never been tried before, and it could fall under medical experimentation on a human.
Turns out that in a high proportion of regular lethal injections, where three drugs were used, that the sedative given at the start was given in lower quantities than required in surgeries (88% of the cases). About 21% of the people executed had a high chance of being aware and in distress during the execution. This was taken from a study done by U of Miami, and followup work a few years later reinforced that this indeed was happening.
There apparently have been quite a few **** ups with lethal injection, and they all stem from people who don't have the proper education or training to administer the drugs or even the proper training to start an IV. One guy who was to be executed had people stick him for 2 hours trying to find a vein anywhere in his arms or legs that could be used, and they eventually gave up. I don't think they had the training to use a vein in his neck or his groin. The guy who came up with the lethal injection protocal stated that it never occured to him that they would have complete idiots administering the drugs.