Olympic silver medallist swimmer Michael Jamieson says doctors had to restart his heart on Wednesday morning.
The 25-year-old Scot revealed the news on social media site Twitter and said he had been cleared to resume training.
Jamieson described his medical care at Royal United Hospital Bath as "amazing".
"Not really sure the reasons behind it happening, but I went into an irregular beat after reaching 203 heart rate in a session (max is 193)," said Jamieson.
"Specialist said he's only seen this three times, all Olympic medallists - pretty cool!"
Jamieson came to prominence by finishing second in the 200m breaststroke at London 2012
and most recently
won silver in the same event at the Beijing World Cup.
A British Swimming spokesperson said: "Michael experienced a minor irregularity in his heart rhythm this week and was seen immediately by specialists.
"He has been cleared to train following medical treatment and is already back in the water."