Whatever happened to cyanide capsules? General Erwin "Desert Fox" Rommel used one to commit suicide in 1944, likewise Heinrich Himmler and Hermann Goering after World War 2 ended. It is my understanding that they caused almost instant death. How about the suicide pill that airman Francis Gary Powers was carrying when his U-2 spy plane was shot down over Russia in 1960? (He survived and didn't use it - I don't have any problem with that, he was a great American hero and deserved to keep on living). Maybe someone can put the Tennessee prison officials in touch with experts who are familiar with this technology.
In Canada, the world leader in assisted suicide, they use pentobarbitol, which is what they used in Tennessee, too. Maybe they didn't get the dosage right.