Willie Brown Jr, 61, was executed early on Friday by lethal injection at North Carolina's Central Prison for the 1983 killing of a convenience store clerk after a robbery.
In what is believed to be the first case of its kind, a machine to monitor brain waves was used to determine whether Brown was unconscious before he was injected with paralytic and heart-stopping drugs.
State officials brought in the machine after a judge ordered the prison to ensure Brown felt no pain during the execution - in line with the US constitution which outlaws cruel and unusual punishments.
However, the decision has proved controversial because the code of ethics of the American Medical Association (AMA) prohibits doctors from participating in executions.
According to the AMA the monitoring of a brain wave machine relies on a physician's skill and expertise, and is therefore forbidden.
"The use of a physician's clinical skill and judgment ... undermines a basic ethical foundation of medicine - first, do no harm"
Dr Priscilla Ray,
American Medical Association
Dr Priscilla Ray, head of the association's Council on Ethical and Judicial Affairs, said:"The use of a physician's clinical skill and judgment for purposes other than promoting an individual's health and welfare undermines a basic ethical foundation of medicine - first, do no harm.
"Therefore, requiring physicians to be involved in executions violates their oath to protect lives and erodes public confidence in the medical profession."
The case comes amid increased scrutiny of lethal injections across the US.
Willie Brown Jr was executed
for a 1983 murder
In other states such as Florida and California, executions have been delayed while courts ponder whether lethal injections cause excessive pain.
Lawyers for condemned inmates have argued that executioners without medical training often do a poor job of administering the complex mixture of chemicals, leading to the prisoner suffering unneccesarily.
In February, the execution of an inmate in California was halted after San Quentin prison failed to find trained anesthesiologists willing to participate.
A judge had ordered the presence of anaesthesiologists to ensure the inmate's death was painless.
Prior to Friday's execution, Brown's lawyers had argued that the methods used by North Carolina and 36 other states did not fully ensure inmates were unconscious before lethal drugs were injected.
If inmates were not fully sedated, they could experience an agonizing death, defence lawyers said.
That could result in cruel and unusual punishment prohibited by the Eighth Amendment to the US Constitution, they argued, adding that only medical professionals trained to administer anaesthesia could ensure Brown was unconscious.
In response US District Judge Malcolm Howard Howard ruled earlier in the week that for the execution to proceed a brain wave monitor supervised by medical personnel could be used instead.
North Carolina state law has long required a physician to be present at executions. Their identities are confidential by law.