How many times do films or TV dramas feature someone kneeling over their collapsed friend, desperately pumping up and down on their chest, breathing into their mouth and begging them to wake up? After a few tense moments, there’s some coughing and spluttering from the person on the ground, they open their eyes and their life has been saved. We can all rest easy.

But is it like that in the real world? Cardiopulmonary resuscitation, or CPR, is designed to help someone who’s had an unexpected cardiac arrest – in other words their heart has stopped beating strongly enough to pump blood around the body, depriving the brain of blood flow. The idea of CPR is not to start the heart beating again, but to get oxygen into a person’s lungs to prevent brain damage. To restart the heart would usually require an electric shock.

This is why it’s essential not to give up on CPR before medical help arrives. It buys you time, but it needs to be done hard and fast, which soon tires you out, so you need to get yourself into a position where you can continue for a long time, or ideally take turns with someone else. If someone without a pulse appears to recover and regain a pulse through CPR then the chances are that their heart was still beating faintly all along.

So don’t be surprised if someone doesn’t immediately come back to life. It doesn’t mean you’re doing it wrong; it’s rare for CPR to rouse someone from unconsciousness straight away.

You wouldn’t get that impression from fiction, though. In studies of medical dramas, resuscitation was shown to be successful 75% of the time on US television, whereas there was a more realistic 25% success rate in British dramas. Even that 25% rate is higher than many places actually achieve. The success of CPR depends a lot on where you live and how fast you can get medical help. A review of studies across many countries found a survival rate of just 5.35% in New York compared with 37% in the German city of Heidelberg.

Hold your breath?

Over the years there’s been a lot of debate over the best way of doing CPR. For a long time most guidelines recommended 15 compressions on the chest, followed by two breaths into the person’s mouth, followed by 15 compressions again, repeating this sequence until medical help arrives. In 2005, many organisations including the American Heart Association, changed their recommendations to 30 compressions followed by two breaths. But now there’s research suggesting the breaths might not even be needed at all, a finding which might well encourage more people to come to the aid of a stranger in the street. At the University of Arizona, Professor Gordon A Ewy and his team demonstrated in laboratory studies using animals that compressions alone were just as effective as compressions with breaths.

But what would happen with people in real-life emergencies? It’s not easy studying this – since you don’t know who’s going to collapse and where, it’s impossible to randomise passers-by to do CPR with or without breathing into the person’s mouth. However, in 2004 Arizona began a public information campaign explaining that people didn’t have to give breaths in order to attempt CPR with a video showing you how to do it (see below). The result was a dramatic increase in the number of bystanders prepared to have a go, leading to an increase in survival rates from 18-34%, Not only did just as many people survive even with no breaths, but giving compressions without interruption in this study protected against damage to the brain.