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That decline is mirrored in the US. The death penalty is retained by 31 American states, but popular support has fallen to around 50% – its lowest level since 1972 - with young people in particular less likely to support it.
Hanging was the most common form of capital punishment in the US until the 1890s. Then, the electric chair became the most widespread method. In 1982, the first execution by lethal injection was carried out by the state of Texas, after which it gradually replaced the electric chair across the nation.
Today, other methods are very rarely used. Only Utah occasionally executes individuals by firing squad – the last time was in 2010.
Perhaps surprisingly, though, there is still no consensus on the exact combination of drugs and dosages to use for the lethal injection.
The drug midazolam – a sedative used by several states to cause unconsciousness – has proved so controversial that, in 2017, Alabama inmate Thomas D Arthur asked to be executed by firing squad. Arthur lodged an appeal with the Supreme Court to postpone his execution on the basis that midazolam, one of the drugs in Alabama’s three-drug lethal injection combination, could contribute to ‘prolonged torture’. The Supreme Court denied the appeal, and Arthur was executed (by lethal injection) in May 2017.
In Arkansas, the lethal injection comprises three chemicals: midazolam, to sedate; vecuronium bromide, to paralyse the muscles; and potassium chloride, to stop the heart.
Each of these is delivered at a dose that could theoretically kill the inmate; however, each drug has drawbacks. A cocktail of all three is used to mitigate the other drugs’ disadvantages.
At the time of execution, the inmate is strapped to a gurney, and IV tubes are inserted into both arms.
It was only later that the department director revealed that the single vein into which the drugs were being administered had ‘blown’.
The first drug given to the inmate is midazolam, a sedative, which is administered to render the inmate unconscious and prevent them from feeling any suffering.
Midazolam is a benzodiazepine. At low doses it has an anti-anxiety effect. At around 10mg, it knocks the inmate unconscious. On death row in Arkansas, the dose is 500mg.
The drug travels up the arm via the bloodstream to the brain. Within seconds, the inmate starts to black out.
It has been alleged, however, that some inmates who received midazolam have appeared to regain consciousness mid-execution.
Following McGuire's execution, Ohio raised its dosage to 50mg of each drug for executions.
An Associated Press reporter who witnessed the execution of Kenneth Williams in Arkansas in April 2017 reported that Williams lurched and convulsed 20 times after the injection of midazolam. According to the reporter, about three minutes into the execution, Williams’ body jerked forward "in a series of what seemed like involuntary movements," lurching violently against the leather restraint across his chest.
Dr Joel Zivot, an associate professor of anesthesiology and surgery at Atlanta's Emory University, said: “It was either a seizure that was predictable based upon Mr Williams’ co-existing medical conditions, or partial paralysis in an execution where the protocol itself was not followed. Or, more to the point, even if the protocol was followed, the protocol was fundamentally flawed.”
Williams’ attorneys and the American Civil Liberties Union of Arkansas called for an independent investigation. Arkansas's governor, Asa Hutchinson, dismissed the calls.
“I think it’s totally unjustified,” he said. “You don’t call for an independent investigation unless there’s some reason for it. Last night, one of the goals was there not be any indications of pain by the inmate, and that’s what I believe is the case.”
Separately, in a 2015 legal challenge brought by several death row inmates in Oklahoma, numerous experts testified that midazolam has no pain-relieving properties, and does not produce the deep, coma-like state of unconsciousness necessary to relieve suffering.
Placing the inmate in a state of deep unconsciousness is critical because the effects of the other drugs used in the lethal injection are believed to be extremely unpleasant.
Once the inmate is confirmed unconscious, 100mg vecuronium bromide is administered into their other arm.
Vecuronium bromide is used clinically in anaesthesia to paralyse the muscles – causing them to relax – so that surgery can be performed.
Unsedated, the inmate would feel the paralysis spread limb by limb. As the lung muscles are paralysed, the inmate would struggle to breathe. If conscious at this stage, “you’d feel you were suffocating,” Dr Stephen Morley, a forensic toxicologist at Leicester Royal Infirmary, told BBC Three.
At this stage, the inmate should be fully unconscious and no longer breathing. However, their heart may still be beating as it isn't affected by vecuronium bromide in the same way.
To stop the heart, potassium chloride is administered directly after the vecuronium bromide. Without proper sedation, this stage would be extremely painful. The feeling has been likened to 'liquid fire' entering veins and snaking towards the heart.
If the inmate is not fully paralysed, their muscles will also spasm uncontrollably, causing them to buck on the gurney, according to Dr Morley. This is because potassium sends signals to every muscle in the body to contract.
When the potassium reaches the inmate’s heart, it disrupts the delicate balance of sodium and potassium ions that keep the heart beating. The inmate’s heart would begin beating irregularly - and then stop.
If the procedure goes according to plan, the inmate should be dead less than 10 minutes after the first drug enters their system.
If there is a problem injecting the drugs, such as a blown vein or drugs being accidentally injected into muscle - or if the inmate regains consciousness - the procedure is stopped, the curtain for observers is closed, and staff work to restore the infusion site.
The prevention of unnecessary suffering is a major part of lethal injection protocol, according to the Arkansas Lethal Injection Procedure document, which states (in capital letters): “EVERY EFFORT WILL BE EXTENDED TO THE CONDEMNED INMATE TO ENSURE THAT NO UNNECESSARY PAIN OR SUFFERING IS INFLICTED BY THE IV PROCEDURE.”