The one time I have witnessed the euthanasia of an older pet, the process was extremely rapid and I do not believe the animal could have felt any pain. I don’t understand why three chemicals are needed for human executions.
The euthanasia solution we use is a combination of barbiturate and potassium. The animal is put to sleep and when the overdose of potassium hits the heart stops. The combination of excessive barbiturate and potassium does the job. There used to be a blue solution called T-61 that had that combination along with succinylcholine...then someone claimed that succinylcholine was inhumane and it then was eliminated. They use succinylcholine in putting the prisoners to sleep....go figure.