The prison commissioner in the southern US state of Tennessee has resigned over the botched execution of convicted murderer Christa Pike, Governor Bill Lee said on Saturday.

Lee said in a statement that commissioner Frank Strada would leave his post this month.

Pike’s lawyers say she is in critical condition, unconscious and on a ventilator in a hospital, after her execution by lethal injection went wrong on Wednesday.

Pike, 50, survived two injections of the normally lethal substance pentobarbital as punishment for the murder of Colleen Slemmer in 1995.

It was the second failed execution in Tennessee this year. The state halted an execution in May after medical staff failed to tap a vein in the prisoner’s arm.

Lee said he thanked Strada for “his willingness to put the interests of Tennesseans first during this difficult moment”.

In the same statement, Strada said, “I continue to believe the Department (of Correction) carried out its responsibilities in accordance with the established protocol.”

Lee, who previously denied Pike clemency, has halted further executions through the rest of the year while a review is conducted.

Had the drugs killed Pike, she would have been the first woman executed by the state in more than two centuries.

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