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Retired nurse David Knowles was teaching first aid to a group of churchgoers in Exeter, England, when his pulse started racing.

"I sat down to take my pulse. And at that point, one of the senior students came to me and asked if this was a scenario I was setting up, because I often work with scenarios," the St. John Ambulance trainer told As It Happens guest host Helen Mann.

"And I told her, 'No, there's no scenario. This is for real.'"

He asked the student, Karol Chew, to help him lie down on the floor and put his legs up on a chair.

"And the group suddenly realized that something actually was wrong and they were really shocked into relative immobility," he said.

But Knowles, who is 77, is experienced in such matters and remained calm.

He knew exactly what was happening to him, he said, and he started guiding Chew through the steps needed to save his life.

"Although I didn't want to really face the facts, I'm afraid I did very quickly come to the point where I told Karol, you know, 'Care for me, observe me, check my pulses now and again, and when I actually lose consciousness, don't do any CPR until you find I'm not breathing,'" he said.

Did you see our trainer David Knowles and Karol Chew, who saved his life on <a href="https://twitter.com/thismorning">@thismorning</a>? Learn how to do CPR here <a href="https://t.co/YzOzCwMGzs">https://t.co/YzOzCwMGzs</a> <a href="https://t.co/WnVFnNAoSH">pic.twitter.com/WnVFnNAoSH</a> —@stjohnambulance

Other members of the Belmont Chapel group helped however they could. One person aided Chew. Someone else called Knowles's wife, Nova. Another person went outside to direct the ambulances.

A few others, Knowles said, began to pray.

When Chew got 911 on the line, Knowles handled the call himself — or so he's told.

"I remember taking the phone from them, but I don't remember giving the message. But according to my group, I gave them a very detailed breakdown of how I was feeling at that moment," he said.

"I do remember warning the ambulance director that we needed help pretty soon because I felt like in the next few minutes, I was going to arrest."

Knowles is now being care for by his wife, Nova, also a retired nurse. (St. John Ambulance)

As he anticipated, he lost consciousness and stopped breathing soon after. Chew put the CPR skills he'd taught her to action, pumping his heart until the paramedics arrived.

Knowles awoke in a hospital 2½ weeks later, groggy and confused.

"It took me a little while to reconstruct the whole thing in my mind and, even then, I needed help now and again," he said. "It was most odd. It was far away and covered in dust in time. That's how it seemed to be to me."

As his memories came back to him, he learned he'd suffered a further cardiac arrest and a pulmonary embolism after being admitted to hospital on Feb. 16. Doctors used three stents to open his arteries and implanted an internal defibrillator in his chest.

He was discharged on March 22 and is now on the mend.

"I feel quite humbled by it all, really," Knowles said.

"Karol herself, as we're both Christian, she's obviously, spiritually speaking, a sister of mine, so I'm already quite close to her, and this has drawn us a little closer together as well. We did feel there was another great power at work, you know?"