Kathryn Beattie was put on the machine after brain surgery at the Southern General Hospital in Glasgow on 21 June, 2004 but died later that day.
Dr Douglas Walker told the inquiry at Glasgow Sheriff Court he carried out a sequence of brain stem tests to confirm her as dead after she stopped breathing on her own.
He said that when Kathryn stopped breathing a sound would have been heard from her life-support machine, although the turn of events was not recorded in her medical notes.
It was also heard that her family had no recollection of such a sound being made or being told these tests were to be carried out.Dr Walker also said that he could not give an exact time of death because there was a discrepancy over exactly what time the tests were carried out and when the results were noted.
The consultant anesthetist was giving a second day of evidence at the inquiry into Kathryn’s death.
Kathryn had suffered from mild flu like symptoms and was started on antibiotics days before her death but by the weekend she felt better.
She was taken to the Victoria Infirmary in Glasgow after she told her mother she wanted to say something but that the words would not come out, and her family wanted her to be checked.
Blood tests were carried out and the family say they were then told she had leukamia.
After a brain scan which showed bleeding in her brain, she was transferred to the Southern General Hospital and died later that day.
Dr Walker said he explained to her family that if Kathryn stopped breathing they would not re-ventilate her
The inquiry before sheriff Linda Ruxton continues.