By Michelle Roberts
Health reporter, BBC News
Kenneth Biros was convicted of murdering Tami Engstrom
Convicted murderer Kenneth Biros has become the first person in the US to be executed by a new form of lethal injection.
Ohio overhauled its procedure after a failed execution attempt had to be halted in September.
Executioners tried for hours to find a usable vein for injection, hitting bone and muscle in as many as 18 tries.
The new regimen uses a single drug rather than three in combination, with a back-up plan should that fail.
If the new injection doesn't work, executioners can inject drugs directly into muscles instead of veins.
The move could put an end to lawsuits claiming that the state's three-drug system was capable of causing severe pain.
Injection experts and defence lawyers agree the single dose of thiopental sodium does not cause pain.
But death penalty experts say the new method takes longer than the old one.
The origin of the lethal injection can be traced back to 1977 in Oklahoma.
It was the brainchild of a medical examiner who wanted to find a more humane alternative to the electric chair.
Used in all 36 death penalty states in the US
Three-in-one injection contains an anaesthetic, muscle paralyser and a heart stopper
The new jab contains a large dose of anaesthetic alone
With the help of an expert in anaesthesia, the medic came up with a combination of three drugs, intended to produce a relatively pain-free death from respiratory and cardiac arrest.
The current US protocol, used in 34 of the 36 death penalty states, still relies on three drugs to give the same effect - a short-acting anaesthetic called thiopental, a muscle paralyser called pancuronium bromide, and an agent called potassium chloride to stop the heart beating.
But even when delivered by seasoned executioners, experience shows the regimen does not always work as efficiently as intended.
Some prisoners have taken many minutes to die, and others have become very distressed.
Researchers who have studied execution data and carried out lab tests believe the doses in the current regimen might be wrong.
The injection is designed to bring about a humane death
According to Leonidas Koniaris and colleagues from the University of Miami Miller School of Medicine, the anaesthetic element of the jab may not be fatal and the heart stopper can be too weak, meaning the patient can die from suffocation caused by the muscle paralyser.
They said in a medical journal: "The Eighth Amendment of the US Constitution prohibits cruel and unusual punishment.
"This paper suggests that current protocols used for lethal injection in the US probably violate this amendment."
The new lethal injection contains a massive dose of the anaesthetic agent thiopental sodium.
The same drug has been used for years to put patients to sleep for surgery, but in much smaller doses.
Dr Richard Birks, president of The Association of Anaesthetists of Great Britain and Ireland, said: "If you gave anyone enough thiopental it would kill them.
A hearse carries away the body of Kenneth Biros
"It will cause varying degrees of respiratory and cardiac depression, but the person would not be aware of this.
"They would go to sleep within seconds in the same way as someone who is having a general anaesthetic, but they wouldn't wake up again. It would be peaceful."
But he said it might take longer for them to be pronounced dead, since there would be no agent like potassium used to stop the heart beating.
However, Dr Birks pointed out that he or the association did not condone lethal injection.
In Ohio, inmates generally have taken about seven minutes to die after injection.
That is part of a total process lasting about 30 minutes, from the time witnesses start watching the insertion of the intravenous needles until the warden announces the time of death.
Anaesthetist Mark Dershwitz, who was consulted by Ohio, estimated death with the new lethal injection would come after 15 minutes.