Tennessee death row inmate Christa Pike was unconscious, intubated and on a ventilator on Thursday night after authorities failed to execute her by lethal injection, her lawyers have said in court filings.

Pike, 50, was injected with two lethal doses of pentobarbital on Wednesday but remained alive. She was then sent to hospital in an ambulance.

Pike remained critically ill and was being treated at a Nashville-area hospital.

Lawyers for Pike, who was sentenced to death in 1996 for the murder of Colleen Slemmer, are seeking to have her sentence commuted after the failed execution attempt.

Pike's lawyers filed an emergency motion on Friday to preserve "any and all evidence" related to her botched execution.

Hospital staff are working to save her life and clear the drug used, pentobarbital, from her system, the filing states.

During the execution attempt, prison officials tried to start intravenous (IV) lines by using at least seven needles on Pike, according to her lawyers.

At one point, Pike herself offered guidance of where they could try.

"One needle was bent in a 90-degree angle when it was removed," the filing states.

While receiving the two doses, "she could be heard crying, whimpering, and breathing loudly throughout the procedure", her lawyers said.

Tennessee Governor Bill Lee ordered an independent review into Pike's botched execution and he halted all executions in the state for the year.

The state's department of correction has defended the method it used, saying "the protocol does not allow for additional procedures beyond what was carried out".

Authorities are yet to say whether they will try again to execute Pike. They have not not offered any explanations about what went wrong.

Pike's attorneys previously warned that her conditions, including thrombocytosis (a blood-clotting condition) and small veins meant there was a risk she would experience "unnecessary" pain and suffering during a lethal injection.

In the latest filings, her lawyers allege that "at no point did any member of the execution team realise that the IV lines were not correctly placed or that the veins had blown and that the pentobarbital was, in whole or in part, entering Ms Pike's body".

They are requesting that all relevant physical, written and electronic evidence is preserved.

Dr Joel Zivot, an anaesthesiologist advising Pike's defence team, said two doses of pentobarbital was enough that "she would have surely died" had it entered her bloodstream in normal circulation.

The only possible explanation for her survival, Zivot said, "is that the pentobarbital blood level was never high enough to cause death" and that blisters on her arm after the botched execution indicated a ruptured vein.

So, instead of entering Pike's bloodstream, he said, the pentobarbital was more likely to have spread across the tissue under her skin.

Pike's medical condition of thrombocytosis may have also played a role, forming blood clots at the injection site that would potentially block pentobarbital from entering her veins.

Regarding the possibility the pentobarbital was degraded, Zivot explained that if this was the case, it could be due to factors including poor manufacturing or improper storage.

The attempted execution went ahead after multiple failed, last-minute legal challenges by Pike's lawyers.

The US Supreme Court thwarted two separate efforts to stop the lethal injection.

Tennessee's governor also denied a clemency request from Pike, the only woman on the state's death row.

Pike was 18 when she and her then-boyfriend, Tadaryl Shipp, beat, tortured and killed 19-year-old Slemmer in 1995.

She was sentenced to death the following year after a media frenzy around the killing in Knoxville, Tennessee.

Before the killing, Pike had accused Slemmer of insulting her and trying to steal her then-boyfriend.

Pike's boyfriend and co-defendant Shipp was 17 at the time so was spared the death penalty under state law. He was given a life sentence with the possibility of parole and remains in prison. (BBC)

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