An amount of air the “size of a teaspoon” would have been enough to kill a baby allegedly murdered by nurse Lucy Letby, her trial heard.

Two expert consultant paediatricians for the prosecution told Manchester Crown Court that, in their opinion, air had entered the circulation of the infant “on purpose” via an intravenous line.

Child A is said to be the first victim of Letby, 32, who allegedly caused his sudden collapse and death on the evening of June 8 2015, more than 24 hours after his premature birth at 31 weeks.

The Crown say the baby suffered an air embolism – where a blockage in the passage of blood occurs – at the hands of the defendant.

Several witnesses have told the court the twin boy had “unusual ski discolouration” of purple or pink blotches during the collapse which would “come and go”.

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Letby is on trial accused of the murders of seven babies and the attempted murders of 10 others at the Countess of Chester Hospital’s neo-natal unit.

Giving evidence on Tuesday, Dr Dewi Evans said: “Just before he collapsed (Child A) was in a stable condition. He was as well as he could be expected. All the markers of wellbeing were very satisfactory.

“By that time (Child A) had survived the most dangerous journey of his life really.”

However, he said air had “somehow” got into his circulation.

He said he said he formed the opinion at the time without knowing about the rash or any suggestion of an air embolism.

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Dr Evans said he ruled out other possibilities such as infection and smothering due to Child A’s stable vital signs prior to collapse.

He said that the positioning of various catheters and cannulas through intravenous lines during Child A’s treatment was also not the cause.

Nick Johnson KC, prosecuting, asked: “How does an air embolism kill somebody?”

Dr Evans said: “It interferes with the blood supply to the heart and lungs. It is the same mechanism as a clot. It blocks off blood supply and kills you.”

Mr Johnson said “As far as the means by which air could injected into a baby’s circulation, from what we know the way in which (Child A) was treated what are the possibilities?”

Dr Evans said: “Only one really. The air would have got through an intravenous line. That could only have happened in two ways – accidentally or on purpose.”

He added there was “no way” it could have been accidental because of the “failsafe systems, the alarms and the monitoring” of intravenous line equipment.

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Fellow expert Dr Sandie Bohin said it was “extremely unlikely” that air was administered accidentally.

She said: “My experience is that nursing staff who either put in a line or care for the line are absolutely meticulous in preventing any air getting into those lines.

“The lines and the connecting points are filled with saline so even the tiniest of air bubbles could not get in. “It is ingrained in nursing staff.”

She said the amount of air likely to be fatal in an infant such as Child A, who weighed just 1.6 kilos at birth, was a “teaspoon of air”.

Child A’s mother wept in the public gallery as Dr Bohin said she was left with only one “plausible explanation” for her son’s collapse and death which was an air embolism.

Dr Evans said the pattern of the collapse of Child A’s twin sister, Child B, on the following night shift was “very similar” and he believed that air was probably deliberately administered in her case.

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The court has heard that Child B made a “remarkable recovery” and was discharged the following month.

Dr Evans said: “The doctors did a really good job of saving her.

“Either the volume of air was less or the air had got through more slowly, or a combination of the two.”

Ben Myers KC, defending, asked Dr Evans: “Do you think you might have been influenced by the allegations rather than the facts of the case?”

“No,” said Dr Evans who explained he first became aware of the defendant’s name when it was published in the media following her arrest.

The consultant confirmed he was first contacted about the deaths by the National Crime Agency (NCA) but denied they provided him with the theory that deliberate harm had been inflicted to babies.

Dr Evans said: “All I was told was there was a hospital in Chester where lots of babies had died.

“This is a hospital where three or four babies died per year on average but somehow or other they had far more deaths which is a worry.

“Several of these were unexplained, even more of a worry.

“Several of these occurred in babies who were previously stable which adds to the worry and resuscitation was unsuccessful in some of the babies who collapsed.

“We had a constellation of worries here.”

Letby, originally from Hereford, denies all the alleged offences said to have been committed between June 2015 and June 2016.

The trial continues on Wednesday.