I’m a 999 call supervisor at the London Ambulance Service and I’ve instructed more than a hundred people on how to give cardiopulmonary resuscitation (CPR) over the phone. But I’d never imagined in a million years that I’d end up using those skills on my own family.

I was asleep after a night shift when my mum started screaming for me. My dad, Eddie, had collapsed and wasn’t responsive. He was making these weird noises – like the air was being pushed out of his body. I’d never actually seen a cardiac arrest, but I knew instinctively that he was about to stop breathing.

Whenever I take a call, I visualise walking into the situation and try to imagine what’s happening. That morning was surreal: this time the story that’s normally in my head was my story. When I rang for an ambulance, I recognised the call handler’s voice from work and that’s when what was happening really hit me. At the same time my mum was shouting, my sister was screaming. And my dad wasn’t breathing. He went cold and his face got dark. I knew he was gone.

Call handlers are trained in giving CPR, so I knew it was difficult, but it was still harder than I’d thought. You can practise as much as you want, but when you’re doing it for real, you’ve got your emotions to deal with as well. The responsibility felt like a big weight on my shoulders, given my job. I knew how much was riding on it – and also the outcomes. Fewer than one in 10 people who have a cardiac arrest [out of hospital] survive.

I was doing chest compressions for 13 minutes before the first responder, Beth, got to me. It felt like for ever. At one point my arms gave way and I begged my mum to take over. But I could see her compressions weren’t as effective. I told her to get off and I got back to it.

When Beth arrived there was a moment when we looked at each other I’ll always remember. I saw in her face that this was real. When she started cutting his shirt open to use the defibrillator, I just ripped it off.

It didn’t quite work the first time, but the second shock was effective. Then my dad started breathing and within a few seconds he was talking. And then it was as if I needed to take a breath as well. I had to run to the window and just stick my head out. I felt as if I had a big ball of fire in my belly.

Beth said my chest compressions had been fantastic. But I felt it was her shocking him that started him breathing again, so that’s what I’m thankful for. He was out of hospital after a week and is in great health.

When I went back to work I was anxious about talking someone through CPR again, and the first time I took a cardiac arrest call, it did affect me differently. I had a flashback of my dad being in that way and it made me nervous because I understood how the caller was feeling. Before I was just guessing – now I actually know.

Since then I’ve been fine on those calls. And now I feel I can sometimes tell when something isn’t right. I took a call where someone had fallen and was unconscious. The lady wasn’t saying the right things to trigger me to give CPR instructions, but I got that feeling I’d had with my dad. I told her to go back and tell me exactly what was happening. When she did, I was able to establish that the patient was in cardiac arrest.

Sometimes callers are afraid to do what we tell them, or they think it’s not going to work. It does frustrate me when I know first-hand how effective our instructions are. I can’t tell callers about my experience, but I hope raising awareness will help give people courage if they ever need us. We always reassure people doing CPR, but it’s more meaningful when I do it now. “You’re doing well, keep going” – I know the value of that.

• Estelle Williams is a 999 call supervisor at the London Ambulance Service.