Ohio's prison department said it would increase the amount of two death penalty drugs it uses in executions after reviewing a case in which a condemned inmate was seen convulsing and gasping for breath for some time after being injected.

Dennis McGuire, 53, who admitting raping and killing a pregnant woman, was executed in January with a sedative-painkiller combination never before used in the United States, where lethal injection is the preferred method of execution.

The execution witnessed by reporters and McGuire's adult children took about 25 minutes to complete, amid reports that he gasped for an unusually long 15 minutes while clenching his fists and that his stomach had churned up and down visibly.

The Ohio Department of Rehabilitation and Correction (DRC) said on Monday its use of a two-drug combination of the sedative midazolam and the pain killer hydromorphone to execute McGuire had been within constitutional bounds and that his movements had been consistent with the drug effects and other factors.

The DRC said it would increase the midazolam from 10 mg to 50 mg and the hydromorphone from 40 to 50 mg, saying there was "no reason not to increase the dosage levels to reaffirm that the drugs will, without doubt, cause profound general anesthetic and ventilatory depressant effects".

"The massive doses of drugs given to McGuire rendered him unconscious before any of the irregular bodily movements were observed. There is no evidence that McGuire experienced any pain, distress, or anxiety," the DRC added in a statement.

McGuire's children have sued Ohio, alleging the state violated his constitutional right for protection against cruel and unusual punishment.

Ohio and other states that have the death penalty have increasingly been forced to seek alternate drugs and sources of drugs for executions as pharmaceutical companies have raised objections to their products being used in capital punishment.

Executions have gone forward in Missouri, Texas, and Oklahoma despite appeals raising concerns over the drugs.

Ohio turned to the two drugs after a shortage of its primary execution drug, the fast-acting barbiturate pentobarbital.

Ohio Governor John Kasich in February delayed the next scheduled execution until November to give the state prison department time to complete a review of McGuire's execution.

Gregory Lott, 52, had been scheduled to die on March 19 by a lethal injection of the same combination used on McGuire.

(Reporting by Eric M. Johnson in Seattle; Editing by Gareth Jones) nL2N0NL094