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.
"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.
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.
McGuire, 53, is scheduled to die Jan. 16 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.
A message was left with the Ohio attorney general's office, which was expected to oppose McGuire's filing.
Gov. John Kasich has yet to rule on McGuire's separate request for mercy.
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.
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 U.S. execution. They are included in Kentucky's backup execution method, while Florida uses midazolam as part of its three-drug injection process.
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