Three-month-old Robert Stephens died last November at the Birmingham Children's Hospital, after an anaesthetist administered an adult dose of the drug because of an administrative error.
Dr Richard Whittington, the Birmingham coroner, recorded a verdict of misadventure after hearing how the mix-up, during the operation to help reduce Robert's blood pressure, led to him receiving four times the child's dose of the drug.
Dr David Rushton, the pathologist, said the overdose contributed to the death of Robert, of St Austell, Cornwall. Doctors at the hospital and the anaesthetist involved in the operation had denied the overdose contributed to the death.
Robert's mother, Mandy Stephens, has said that she will consider suing the hospital over the incident.
The coroner said the hospital, the leading centre in Britain for the operation, had been "frank and honest" in its admission of the error.
He said Robert's heart condition almost inevitably led to death, but he added: "We cannot dismiss the evidence of Dr Rushton. One cannot accept that death was entirely due to natural causes."
Mark Thomas. the child's father, said in a statement read to the inquest that serious heart problems, including a hole in the heart, were diagnosed shortly after his son's birth in August last year.
Robert had major heart surgery two days after his birth and surgeons at the Birmingham Children's Hospital operated on him again in November.
Doctors gave the child an antidote to the overdose, which had caused cyanide poisoning.
But Mr Thomas said: "His face was blue and swollen. I was shocked when I saw it. It was quite unexpected to me when he passed away. I thought he was going to make it. It was not unexpected to the anaesthetist."
Dr Rushton said the overdose, of sodium nitroprusside, contributed to the brain damage that killed the baby but this was also partly caused by Robert's condition.
He said the overdose "may not have been the major factor, but I think that it is unlikely to do no harm at all to a child".
However, Dr Alistair Vale, director of the West Midlands Poison Unit, said in a statement that it was Robert's heart condition which had proved fatal, not cyanide poisoning from the overdose.
Alison Warren, a junior sister in the intensive care unit, noticed Robert's continued erratic blood pressure three hours after the operation. She checked his dose of the drug and discovered the error.
"Robert was critically ill. The drug was not working and I thought it may be toxic. He was blue and quite mottled," she said. An antidote was given but he died two days later.
The drug was administered by a trainee anaesthetist during the operation, under the direction of Charles Ralston, a consultant anaesthetist.
Mr Ralston said that when Robert was taken to intensive care after surgery he "assumed" the correct dosage was given but did not check. He said the poison level had considerably reduced by the next day and was not responsible for the child's death.
He added: "We now subsequently do check in the anaesthetics room prior to surgery and we have a large wall chart in the room showing recommended dosages."
Adrian Hickman, a legal representative for the family, said after the hearing that Robert's parents would now consider taking legal action against the hospital. He said the verdict was "satisfactory".Reuse content