A consultant doctor has told an inquest into the death of a premature baby that it was "impossible" to say why she forgot to move his feeding line into the correct position.
Cassian Curry weighed less than 2lbs when he was born at 28 weeks on the Jessop Wing in Sheffield on 3 April last year.
The inquest in Sheffield has heard how an umbilical venous catheter (UVC) was inserted in a "sub-optimal" position in his abdomen near his heart by two junior doctors.
He died two days later from a cardiac tamponade, when fluid builds up in the space around the heart, preventing it from pumping.
Neonatal consultant Dr Elizabeth Pilling described how she intended to have the line repositioned within 24 hours but waited because of the dangers of repeatedly handling a very premature baby. She then forgot to take action.
She said: "I've been round and round - why did I forget?
"It's impossible to remember why you forget things."
The consultant was starting work on her fifth consecutive 12-hour day shift on Easter Sunday when she was reminded by her registrar about the position of the line on an X-ray.
She said she did not think it was an urgent matter but had intended that it should be changed at some time during the day but "it went out of my head".
She said the only reason she could offer was how busy it was on the unit, saying: "I can't explain why I didn't do it in that situation, apart from the acuity of the unit."
Dr Pilling told the coroner she had never treated a cardiac tamponade in 13 years as a consultant and was shocked at Cassian's sudden deterioration because he had been doing well for his size and prematurity.
Asked about staffing levels, Dr Pilling said the unit was properly staffed, according to national guidelines and there were no absences.
But she told a coroner it was very busy that many of the staff on duty were junior and needed support.
Dr Pilling was asked by Ross Beaton, for Mr and Mrs Curry, whether she was aware some junior members of staff had submitted internal incident reports with concerns about staffing issues that bank holiday.
She replied: "I was aware, as they were coming in and telling me how hard it was, how busy it was. They were asking me if I can do anything to help."
Dr Porus Bustani, the clinical lead of the Jessop Wing neonatal unit, told the inquest that the staffing levels that weekend exceeded the national guidelines by some margin.
But Dr Bustani said the unit had reviewed the way its consultants worked so one person did not cover the whole of a bank holiday weekend. And he said he was working towards having two consultants on at weekends.
He added that he had been a consultant for 20 years and had only seen a tamponade caused by a line location once before.
"They are extremely rare but, when they happen, they are catastrophic and that's what happened with Cassian," he told the inquest.
The medical director of Sheffield Teaching Hospitals NHS Foundation Trust, Dr Jennifer Hill, has said the trust is "so very sorry for what happened" to Cassian, admitting there was "human error in terms of the management of Cassian's umbilical venous catheter."
The Care Quality Commission (CQC) has raised concerns about maternity services at the Jessop Wing and rated them as inadequate a month before Cassian was born, confirming this judgment earlier this month.
But Sheffield Teaching Hospitals NHS Foundation Trust has stressed that the neonatal unit did not form part of these maternity inspections and judgments.