I don't think lethal injection is necessarily the best choice (as has been discussed here a number of times, massive heroin OD or nitrogen asphyxiation seem like better and cheaper methods), but I notice that the article starts out with the following:
He had received simple instructions: Interpret the levels of an anesthetic in the blood to determine whether the inmates were conscious during their execution.
So the doc is tasked by lawyers opposing the death penalty with determining if the executed prisoners were conscious during their execution, but instead goes down the path of investigating pulmonary edema?
I don't see any additional reference to his initial analysis in the article, which leads me to believe he probably found that they were suitably sedated. If that's the case, even if the prisoner's lungs were laboring and their body was undergoing a process that would be very painful to a conscious person they are not experiencing that suffering through the sedation. Unless he also found that the prisoners weren't properly sedated, this shows that the drug selection was properly constructed to avoid suffering.
If we are supposed to discount the sedation, wouldn't we prosecute surgeons for torturing people? After all, cutting people open is associated with extreme agony ...