A group of first aid trainees thought their instructor was role-playing during a class on how to perform life-saving CPR as he had a heart attack during a lecture.
David Knowles, 77, began to lose consciousness as he told his students how to resuscitate people at his local church in Exeter.
Some of the group thought Mr Knowles was giving a demonstration as he lay on the floor feeling faint and unwell.
They managed to save the St John Ambulance volunteer's life after he told them what to do just before he passed out.
He told the BBC: "I had just started my lecture and we were talking about cardiopulmonary resuscitation.
"The more senior members of the group had asked for a demonstration...she thought I was role playing, but I told her it was real and that I was going to lose consciousness."
Mr Knowles suffered a cardiac arrest but survived because his trainees dialled 999 and performed CPR until paramedics rushed to the scene.
The pensioner fell into a coma and had another heart attack after he was taken to the Royal Devon and Exeter Hospital.
He would have died at the scene if nobody was there to help him.
Mr Knowles said: "I'm told I came to, briefly, and was talking to the paramedics about my condition, but the next thing I remember is waking up in hospital, two and a half weeks later."
The retired nurse left hospital after five weeks and has since made "excellent progress" at his home in Newtown, Exeter, according to the BBC.
He could have suffered brain damage, but is said to be mentally alert and able to walk unaided.
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