Ohio's untried execution method, the first of its kind in the nation, will cause the condemned killer of a pregnant woman "agony and terror" as he struggles to breathe, attorneys trying to stop the execution argued in federal court.

The two-drug combination won't sedate death row inmate Dennis McGuire properly, and he will experience a suffocation-like syndrome known as air hunger, the attorneys said in filings Monday and Tuesday.

The drugs were chosen because of a shortage of other lethal injection drugs.

Lawyers had also asked governor John Kasich to spare McGuire on the grounds that a jury never got to hear the full details of his chaotic and abusive childhood and abuse. Kasich rejected that request without comment Tuesday. The governor typically does not give a reason when he turns down clemency requests by death row inmates.

McGuire, 53, is scheduled to die on 16 January for the 1989 rape and fatal stabbing of Joy Stewart in Preble County in western Ohio.

McGuire's lawyers asked federal judge Gregory Frost to delay the execution while they challenge the proposed lethal injection system.

"McGuire will experience the agony and terror of air hunger as he struggles to breathe for five minutes after defendants intravenously inject him with the execution drugs," the inmate's attorneys said in a Monday court filing.

They also said McGuire exhibits several symptoms of sleep apnea, which could exacerbate the problem.

The dose planned for McGuire isn't enough to properly sedate him, meaning he'll experience "the horrifying sensation" of being unable to breathe, Harvard anesthesiology professor David Waisel said in a Tuesday filing in support of the inmate.

A message was left with the Ohio attorney general's office, which was expected to oppose McGuire's filing. Frost scheduled a Friday hearing.

Supplies of Ohio's former execution drug, pentobarbital, dried up as its manufacturer put it off limits for executions. It's a challenge facing other death penalty states as well.

Missouri gave up attempts to use propofol over concerns the move could create a shortage of the popular anesthetic if the European Union, which opposes the death penalty, restricted its export.

In Georgia, the state's attempt to use a non-federally regulated dose of pentobarbital is the subject of a lawsuit.

Instead, Ohio's department of rehabilitation and correction plans to use a dose of midazolam, a sedative, combined with hydromorphone, a painkiller, to put McGuire to death.

That combination of drugs has never been used in a US execution. They are included in Kentucky's backup execution method, while Florida uses midazolam as part of its three-drug injection process.

In its ruling last month, the parole board criticized McGuire's attempts over the years to evade responsibility and said that a recent letter he sent Kasich describing the slaying as a lovers' quarrel gone wrong was disingenuous. It also questioned his claims of childhood abuse and instead focused on the brutality of Joy Stewart's stabbing death.

"McGuire's crime is very disturbing in character, as it involved the rape and slaying of a nearly eight-month-pregnant woman," the board said.

McGuire was mentally, physically and sexually abused as a child and has impaired brain function that makes him prone to act impulsively, his lawyers said in a filing with the board.

"Dennis was at risk from the moment he was born," the lawyers wrote. "The lack of proper nutrition, chaotic home environment, abuse, lack of positive supervision and lack of positive role models all affected Dennis' brain development."