This handout picture dated January 12, 2023 provided by the Tennessee Department of Correction shows convicted US murderer Christa Pike. [File, AFP]

The female inmate who survived a failed execution in a US prison last week is now conscious and speaking, but her prognosis remains unclear, her lawyers said Tuesday.

Christa Pike, 50, who was sentenced to death for a murder she committed when she was a teenager, received two supposedly lethal doses of the drug pentobarbital at a Tennessee prison a week ago.

She survived the injections, however, and was rushed to hospital, where she had been unconscious and on a ventilator in critical condition.

Pike's lawyers said Tuesday that "against all odds" she was now "conscious, speaking, and continues to receive critical medical care."

"Despite the severe damage she endured to her arms, she is still handcuffed and shackled to the bed," they said in a statement. "Her recovery is medically unprecedented. Her prognosis remains unclear but, at a minimum, we expect a long recovery."

Her lawyers repeated a call for Tennessee Governor Bill Lee to commute Pike's sentence to life imprisonment without the possibility of parole.

Lee suspended the carrying out of capital punishment in the southern state following the failed execution pending an independent investigation.

The prison chief has since resigned.

First to survive

Pike's legal team had previously objected to the execution's IV protocol, saying it would have caused needless suffering due to her small veins and a blood condition.

Medical experts had warned of needles becoming dislodged or piercing through the vein, causing lethal drugs to flow into Pike's arm instead of the bloodstream, lawyer Stephen Ferrell has said.

A court filing described pentobarbital as a caustic substance that burns skin, tissue and veins. Upon arrival at the hospital, both of Pike's arms were "swollen, burned, and blistered," it said.

The Death Penalty Information Center, which publishes data and analysis on capital punishment in the United States, said Pike was the first inmate ever to survive "after being injected with lethal doses of execution drugs."

Pike's case sparked debate over the death penalty for young offenders, as she was 18 when she and her boyfriend beat and killed 19-year-old Colleen Slemmer at a job training camp in 1995.

The killing gained national attention after Slemmer's body was found with a pentagram, often seen as a satanic symbol, carved in her chest.

Pike's lawyers sought clemency citing her age, childhood sexual abuse and mental health problems, including post-traumatic stress disorder.

The case had already attracted attention in the lead-up to the execution date as Pike would have been the first woman put to death by the state of Tennessee in more than 200 years.

The night of the planned execution, journalists gathered to witness Pike's last words and document her death.

Instead they exited the witness chambers stunned and reported that she was still breathing and even snoring after the lethal doses were administered.

In the United States, 18 women have been executed since 1976, according to the Death Penalty Information Center.