LONDON—Prisoners executed by lethal injection in the United States may have experienced awareness and unnecessary suffering because they were not properly sedated, according to a research letter in this week's issue of the prestigious British medical journal, The Lancet.

The authors believe the use of lethal injection should cease in order to prevent unnecessary cruelty and call for a public review into anesthesia procedures during executions.

Lethal injection in the U.S. has eclipsed all other execution methods because of public perception that the process is relatively humane and does not violate the U.S. Constitution's prohibition against cruel and unusual punishment.

It generally consists of the sequential administration of sodium thiopental for anesthesia, pancuronium bromide to induce paralysis, and finally potassium chloride to stop the heart and cause death.

Pancuronium bromide paralyzes the skeletal muscles but does not affect the brain or nerves — a person injected with it remains conscious but cannot move or speak. With inadequate anesthesia, the subject would experience suffocation and excruciating pain without being able to move. Opponents of the drug's use say it is so cruel that some veterinarians won't use it to euthanize animals.

Leonidas Koniaris, of the University of Miami's Miller School of Medicine, and colleagues analyzed protocol information from Texas and Virginia, where around 45 per cent of executions are done. They found that executioners — typically medical technicians, since doctors are theoretically prohibited from participating by the Hippocratic oath — had no training in anesthesia, drugs were administered remotely with no monitoring of anesthesia and there were no data collection, documentation of anesthesia, or post-procedure peer review. The investigators also analyzed data from autopsy toxicology reports on 49 executions in Arizona, Georgia, North Carolina, and South Carolina. They found that concentrations of thiopental in the blood were lower than that required for surgery in 43 of the 49 executions, and 21 inmates had concentrations consistent with awareness.

Injection deaths fail to meet veterinary standards for putting down animals, the study says.

A Tennessee judge in a case brought by death-row inmates, quoted by The New York Times, wrote that in a worst-case scenario pancuronium bromide would produce "all the appearances of a serene expiration when actually the subject is feeling and perceiving the excruciatingly painful ordeal of death by lethal injection."

The drug, Judge Ellen Hobbs Lyle wrote, "gives a false impression of serenity to viewers, making punishment by death more palatable... to society."

Koniaris writes in The Lancet: "Our data suggest that anesthesia methods in lethal injection in the U.S. are flawed.

"Failures in protocol design, implementation, monitoring and review might have led to unnecessary suffering of at least some of those executed ..."

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