Expert witness Dr Dewi Evans said he believes air “trickled” into the infant’s circulation via a connecting port on his intravenous drip.
He told Manchester Crown Court it could have taken “several minutes” for it to take effect before the youngster, Child M, rapidly deteriorated and almost died as staff battled for nearly 30 minutes to revive him.
Letby, 33, is accused of trying to kill Child M on the afternoon of April 9 2016 while he was being treated in nursery room one on the Countess of Chester Hospital’s neonatal unit.
The defendant co-signed for an antibiotic given via a port on the drip at 3.45pm – 15 minutes before Child M stopped breathing followed by a dip in his heart rate and oxygen levels.
Letby was near the doorway of room one, helping a colleague prepare medication for Child M’s twin brother, when the alarm sounded at 4pm, the court heard on Thursday.
Consultant paediatrician Dr Evans said using a syringe to inject air via a port would be slower than a direct injection into the bloodstream.
Prosecutor Nick Johnson KC asked: “Would it follow, if someone chose to do it that way, they would not necessarily be standing over the baby at the time of the collapse?”
Dr Evans replied: “Yes, because you would not necessarily get an instant collapse. It could have occurred over several minutes.
Ben Myers KC, defending, said: “If there was air in his system sufficient to cause cardiac arrest, there is not going to be a recovery as rapid as this within 30 minutes.”
Dr Evans said: “I disagree with that. The resuscitation was absolutely incredible. This was a very, very robust period of resuscitation that was required. This is something that is fairly consistent with a baby having air into the circulation. I can’t think of any other cause.
“The volume required is pretty small. No nurse or doctor would allow a bubble of air into the circulation.”
Dr Evans said any bubbles would disappear if cardiac massage was carried out.
Mr Myers put it to Dr Evans that he no had empirical research to support his opinion that air could vanish within 30 minutes.
Dr Evans said he relied on his knowledge of “basic anatomy and physiology”.
Mr Myers went on: “You don’t know as a matter of fact how much air is required to cause a collapse?”
Dr Evans replied: “No. ‘Very little’ is all I can say.”
Letby, originally from Hereford, denies murdering seven babies and the attempted murders of 10 others between June 2015 and June 2016.
The trial continues on Friday.