How did the traditional three-drug lethal-injection procedure work?

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Padded gurney with black restraints and IV stands, used for lethal injection, sits in a bare room with white cinder block walls at the New Mexico state penitentiary. — capital punishment Lethal-injection room.

The traditional three-drug lethal-injection procedure, used widely in the United States until the late 2000s, involved the sequential administration of three specific chemicals to a prisoner strapped to a gurney:1

Sodium thiopental: A barbiturate anesthetic intended to induce deep unconsciousness within approximately 20 seconds.1
Pancuronium bromide: A total muscle relaxant that paralyzes all voluntary muscles, which leads to suffocation when administered in sufficient dosages.1
Potassium chloride: A chemical that induces irreversible cardiac arrest.1

When performed as intended, the process typically took about five minutes, with death occurring less than two minutes after the final injection. However, this protocol has faced significant criticism and legal challenges regarding the Eighth Amendment prohibition of cruel and unusual punishment. Concerns have been raised that if the anesthetic fails to render the prisoner fully unconscious, the subsequent drugs can cause excruciating pain while the prisoner remains unable to signal distress due to paralysis.1