The nurse Lucy Letby tried to kill a premature baby by giving him fluids contaminated with insulin, her murder trial has heard.
The boy's blood sugar levels fell dangerously low and his heart rate rose sharply after Letby intentionally added insulin to his intravenous feed during a night shift, Manchester Crown Court was told.
Letby, 32, is accused of attempting to kill the boy, known as Child F, less than 24 hours after she allegedly murdered his twin brother, Child E, by injecting air into his bloodstream.
The children's real names cannot be revealed for legal reasons.
The nurse, who is originally from Hereford, denies murdering seven babies and the attempted murders of 10 others between June 2015 and June 2016.
Peter Hindmarsh, professor of paediatric endocrinology at University College London, told the court on Friday that poisoning was the only reasonable explanation for Child F's sudden deterioration at the Countess of Chester Hospital's neonatal unit in the early hours of 5 August 2015.
After the infusion of nutrients, which began after midnight, the boy's heart rate surged to 200 beats per minute and his blood sugar fell to an "extremely low" reading.
Intravenous refers to a way of giving a substance through a needle or tube inserted into a vein.
Prof Hindmarsh said that a commonly used synthetic human insulin known as Actrapid - a colourless solution - was administered via the infusion.
He said the brain is reliant on a "constant supply" of glucose to function and the dangers of low blood sugar included seizures, coma and sometimes death.
The boy's glucose levels stayed low throughout the day shift of 5 August, even after the intravenous line and the connected bag containing the nutrients were replaced.
'Low blood sugar for 17 hours'
Child F's blood glucose only rose to safe levels after a decision to stop the nutrients from a second bag at 6.55pm and give him extra sugar independently, the court was told.
Prof Hindmarsh said it was apparent that the boy's low blood sugar was "persistent" for those 17 hours.
During that time, Child F received twice the amount of glucose normally given to correct hypoglycaemia in a baby, he said.
Child F made full recovery
The witness agreed with Letby's lawyer, Ben Myers KC, that the blood sample reading of high insulin came from the second stock bag so could not show what level was in the first bag attached in the early hours of 5 August.
But Prof Hindmarsh said similar blood glucose readings around the same period from one person were likely to mean they had a similar amount of insulin earlier in their system.
Child F went on to make a full recovery and was later discharged, the court heard.
The trial will continue on Monday.