"I heard my mam shout my name. You know when someone shouts your name and it frightens you? I ran downstairs and there's my dad on the couch, blacked out, blue lips, blue ear lobes, his fingers – everything. I thought: 'Right, that's CPR time'."
Aaron Groves, from Liverpool, was 15 when his dad David suffered a cardiac arrest. His quick thinking saved David's life.
CPR is when you press up and down on a casualty's chest and give them a series of breaths to help pump blood around their body when their heart can't.
To mark the British Heart Foundation's Restart a Heart Day, both Aaron and David have spoken to Radio 1 Newsbeat about what happened.
Aaron: He wasn't breathing, so I rang 999, dragged him down to the floor and started doing chest compressions on him.
I'd had some informal training at the scouts and an NHS day in Liverpool with school a couple of years before.
But I didn't know I could do it. I'd never practised. It was like an emotionless experience.
I was speaking to the operator and he did talk me through it.
After four minutes of doing chest compressions, the ambulance actually arrived.
They got the defibrillator out. After about four blasts on that we got a heartbeat.
David: I woke up in hospital, I had complete memory loss for a week. I felt very proud of Aaron really.
Aaron: I actually wrote him a letter for when he woke up as the doctor said he might have some short-term memory loss.
It just explained why he was there. It helped my dad and his recovery throughout the week, to try and get his head round it.
It's really hard for him to understand. One minute you're on the couch, next minute you're in hospital with no clue why you're there.
David: For me it was when I woke up and saw that note. Even now, when it gets read out, it sends a shiver down my back.
In some respects I felt very emotionally traumatised. Waking up in hospital and learning I had complete memory loss for a week.
I have a very special relationship with Aaron that's unique. I had no idea he had those CPR skills.
In some respects that's what made me even more emotional. Had he not had those skills, I wouldn't still be here.
Aaron: I think it's been strange for my dad to get over it. During the events he had no recollection of why he was there for a good two weeks.
It's only when we got back to the house and he got his memory back that we learnt to deal with that emotion and trauma. It has brought us closer together afterwards.
David: We've always had this wonderful relationship even before what happened. A lot of the things that have happened to me afterwards have been quite unusual.
Having an internal defibrillator fitted and learning the psychological aspect of that. I've turned it round from being a negative to being a positive.
Aaron's been a great inspiration for that. I was also surprised by his knowledge. He explains things to me. It just stunned me that he knew what he did and I can't get over it really. It's an amazing thing to be with him.
Aaron: The way I looked at it was, if I do nothing he's no better but if I do something he might be better.
That's my advice. If you do nothing they're not going to get better, but if you do something hopefully they will.
So even if you don't know CPR, call an ambulance and they will tell you on the phone. You've just got to do it. It's a matter of life and death.
For more information about CPR and Restart a Heart Day click here