Failure to administer oxygen was one of the errors highlighted at an inquest into the death of a 44-year-old woman at the Cumberland Infirmary.

Sharon Grierson, died on November 14, 2016, following a procedure at the Carlisle hospital to remove a polyp from her right vocal cord.

The inquest at Cockermouth Coroner's Court has heard how the mother-of-four did not have her oxygen concentration turned up following the operation, which was intended to be a day case.

It previously heard another major error occurred when medics wrongly inserted a breathing tube into her esophagus - and subsequently failed to recognise this immediately, depriving her of vital oxygen.

Mrs Grierson underwent general anaesthetic and the surgery with no problems. It wasn't until after medics removed the tube that was helping her breathe that problems arose.

The operation was run on 50 per cent oxygen, which is usual. But during the process of extubation her oxygen was not turned up to 100 per cent as it should have been. The error wasn't spotted until a second consultant anaesthetist, Dr Christopher Dickson, was called in to help.

Mrs Grierson's oxygen saturation levels dropped and she started to deteriorate. Medics attempted to reintubate her but on two attempts the breathing tube was inserted into her esophagus, it wasn't until a third attempt when it was correctly sited in her windpipe.

Meanwhile medics were giving Mrs Grierson CPR as she suffer a cardiac arrest.

Dr Glen Pinto, a specially-trained year three (ST3) trainee in anaesthetics, was working alongside Dr Jenny Fraser, the hospital's lead head and neck anaesthetist, that day.

Dr Fraser assumed her trainee had turned the oxygen up when required to do so. Dr Pinto said his general recollection was that the oxygen was on 100 per cent on extubation, but now admits his recollection was incorrect.

"I honestly believed this was what I had done on this occasion," he said. "The data indicates I did not increase the oxygen concentration immediately prior to the extubation."

Dr Fraser said in her evidence: "I made an assumption that, being an ST3 trainee, he would have understood the requirement to keep the sevoflurane gas on and turn the oxygen up for a deep extubation.

"I accept that I should have been more precise in communicating my instruction and should have either checked his actions or operated the anaesthetic machine myself."

Dr Julian Brown, consultant in anesthesia and intensive care medicine who works at Southend Hospital, prepared a report at the request of Cumbria Police CID after considering witness statements.

He concluded "lapses in her care caused her death", though he did not believe gross negligence occurred.

"In my opinion failure to use 100 per cent oxygen did not materially affect the outcome, which was the need for reintubation," he said.

He would have expected Mrs Grierson to require reintubation regardless of the failure to administer 100 per cent oxygen.

"In my opinion by far the most significant error was not the failure to manage the extubation, or the need for reintubation, or even the esophageal intubation but the failure to recognise a failed intubation."

Pathologists said that the lack of oxygen to her brain for more than 40 minutes resulted in irreversible brain damage. Mrs Grierson died three days later.