Leon Barnett has revealed an implant in his chest recorded his heart-rate at "just under 300 beats per minute" following Northampton's clash with Bury.
That game on October 2 revealed a heart condition and the 32-year-old defender was advised to retire.
Barnett admits he is lucky it was picked up in time and told the Cobblers' YouTube channel: "Straight away Northampton handled it quite well.
"They said 'we'll get you in touch with a cardiologist and just go down that route.'
"At that time I didn't think it was anything too serious. I had a device inserted into my chest and that monitors my heart-rate.
"It happened for a second time at the Bury game and after that game once again I started to feel it.
"At that time I've got the implant in my chest that tracks my heart-rate. I went to the cardiologist and they downloaded the data and they found that my heart-rate was going just under 300bpm, which is unbelievable.
"They said if I was a normal person in a regular job I would have collapsed, fainted, but obviously because you play football you are quite fit, you managed to stay on your feet, so I'm very fortunate they obviously found it and handled it quite well.
"It's a bit of a shock but it could have been a lot worse."
Barnett made over 350 appearances during his career and played in all four divisions of the English game for Luton, West Brom, Norwich, Wigan and Bury before joining the Cobblers.
Barnett played 24 times for Northampton before the game in October after signing on a free transfer in May 2017.