A free app that enables potentially life-saving first aid before ambulances arrive is being rolled out across Western Australia.

The First Responder app alerts people who are trained in first aid that somebody is seriously injured or having a heart attack nearby in the critical minutes before paramedics arrive.

According to St John's Ambulance, when a person is suffering from cardiac arrest, every minute that goes by during which they do not receive help cuts their chances of survival by 10 per cent.

Royal Perth Hospital cardiologist Nik Stoyanov said the first moments after someone had a heart attack were critical.

"It is absolutely vital to get help as soon as possible, in this scenario, every minute would be absolutely crucial," he said.

"Every minute that the brain is starved of oxygen or other vital organs are starved of oxygen could be absolutely crucial."

Those who sign up to be first responders must be trained and provide evidence they have completed a recognised first aid course within the past three years, or else are qualified doctors, nurses or other medical professionals.

Dr Mark Wilson is a neurologist and primary care physician in the UK who helped develop a similar app there in 2014.

"You're probably no more than a few hundred metres, certainly in urban areas from a doctor, nurse, paramedic, someone who is trained in holding an airway open and basic resuscitation skills," Dr Wilson said.

"How do we alert the people who are just around the corner from someone who has been hit by a car etc, it really came from a trauma perspective but it has a much bigger role in cardiac arrest — where starting early CPR is absolutely vital to survival.

"Usually we don't know about it until it is too late — this is a way of providing emergency care before the ambulance service arrives."

How does the First Responder app work?

Tony Ahern from St John Ambulance said the app could be the difference in a medical emergency. ( ABC News: Sarah Collard )

When someone dials triple zero for an ambulance, then nearby users of the app can be notified.

The nearest trained responder registered with St John Ambulance is notified and a team can be dispatched to the exact location through the app.

More than 18,000 people have downloaded the UK app GoodSam and Dr Wilson said new technology and triage care can work together to deliver life-saving help.

"The change potentially in medicine because of tech, mobile technology and and internet is really incredible," Dr Wilson said.

"And I think when we look back in 20 years' time, the big leap won't be because of a new drug that has been invented, it is going to be because of logistics and our ability to deliver care in a more timely manner."

The app can be used to request help in an emergency. ( ABC News: Robbie Koenig Luck )

The First Responder app had 1,300 downloads during a pilot process in WA, with many more expected once it is officially launched.

In addition to alerting first aiders that someone is having a medical emergency, the app reveals the location of defibrillators.

St John Ambulance chief Tony Ahern said the technology was about ensuring those trained in first aid could link up to those needing assistance as quickly as possible.

"So currently we could have a situation where there was a defibrillator in this building and people could use it and someone across the road could be calling an ambulance and not realise that. This app closes that gap," he said.

"It would allow someone here to have their phone activated with the app and they would know that there is an ambulance being called across the road — so they'd grab the defibrillator and go across."

Similar apps are in use in New Zealand and the UK.