A 19-day-old boy died after being given four times the solution needed to stop his heart during surgery, a coroner has ruled.
Zack Hider, from Portsmouth, was born with a congenital heart defect.
A "mistake" was made during the heart operation at Southampton General Hospital on 11 November last year, the inquest in Winchester was told.
Zack's parents said they did not blame any individual but hoped lessons were learned to prevent similar mistakes.
Recording a narrative verdict, coroner Graham Short said pressure from the excess dose of the heart-stopping solution had been the primary cause of death.
Moments after the mixture of blood and chemicals was injected, Zack's heart turned "stiff" and looked "bruised", surgeon Michael Kaarme told the inquest.
"I didn't know what was happening," he said, adding, "I didn't have any experience like this."
The man operating the machinery that controlled the solution, perfusionist Richard Hartshorne, did not appear in court.
In a statement, he said it had not been made clear a procedure requiring the heart to be stopped was being carried out, until part way through the operation.
An inquiry found Zack had been given a dose of the solution for a much larger child, like the previous patient in the theatre.
Mr Short said: "The volume was not adjusted prior to delivery for reasons that I've not been able to fully establish."
He said even when the operation was abandoned, Zack's heart could not be started and he died in hospital nine days later.
"By that time, the damage had been done," he said.
In a statement, Zack's parents, Gary and Sarah Hider, said they "did not blame any individual but only hope lessons are learned to make sure similar mistakes are not made in future."
Neil Pearce, associate medical director for patient safety at University Hospital Southampton NHS Foundation Trust, said: "We accept full responsibility for the incident and have apologised unreservedly for it.
"The error made was very rare and led to a severe reaction in Zack's heart muscle which took away his chance of surviving the complex surgery he was due to undergo."
He said an "immediate investigation" had led to the introduction of a number of "important new safety measures" which he added had subsequently been shared with all paediatric cardiac centres in the UK "to try to eliminate the risk of a similar incident occurring again".