A student died during an underwater challenge at his brother’s 21st birthday party, an inquest heard.
Dominic Hamlyn, 24, became unresponsive while swimming at the family home in Crundale, Kent, on 28 July.
His father, neurosurgeon Peter Hamlyn, said he had been performing the same “party trick” since he was a child.
Coroner Scott Matthewson said the Cambridge graduate had a pre-existing heart condition and died from “sudden adult death syndrome”.
The inquest heard Dominic had challenged a family friend to swim lengths of the pool underwater and had swum about two-and-a-half while holding his breath before he was seen to be not moving.
Mr Hamlyn told the inquest in Maidstone he gave his son emergency treatment while waiting for the paramedics to arrive.
Dominic was taken to William Harvey Hospital in Ashford but he died about 15 hours later.
His father, who treated boxer Michael Watson after his near-fatal brain injury during his Chris Eubank fight in 1991, said his son had been unresponsive for “seconds rather than minutes” before being pulled out of the pool.
A medical student at the party initially performed CPR before Mr Hamlyn took over until paramedics arrived at about 03:45 GMT.
Peter Hamlyn said his eldest son could usually swim four or five lengths underwater and would take a series of deep breaths before doing so.
“It was an entirely routine thing which I had seen him do since he was a child,” Mr Hamlyn told the court.
He said his son had been an extremely fit athlete, played rugby, cricket and rowed for Downing College, Cambridge. The inquest also heard that alcohol was not relevant
‘Lack of awareness’
Pathologist Dr Olaf Biedrzycki said he could have lost consciousness underwater caused by a lack of oxygen to the brain, but “on the balance of probability” it was “an underlying heart condition”.
“If he wasn’t trying to do more than he had done before, that is slightly more likely than shallow water blackout,” he said.
He gave the cause of death as acute cardiac arrhythmia – sudden adult death syndrome.
In a statement, the 24-year-old’s family said there was a “lack of medical awareness” about the syndrome.
“As a result, victim’s families will continue to go unscreened and readily treatable warning signs missed.”
“In life Dominic gave so much to others. It is up to us to ensure his legacy is not just a family immersed in our grief but that his loss brings awareness and change,” they said.