CPR training can help people develop the skills and confidence to provide effective chest compressions during a cardiac arrest. Shakirov Albert/ShutterstockWhen someone has a cardiac arrest, the actions of people nearby can make the difference between life and death. For years, the message has been clear: start cardiopulmonary resuscitation (CPR) as quickly as possible, even if you’ve never been trained.That advice remains essential. But growing evidence suggests that how CPR is performed could also influence someone’s chances of survival.Cardiac arrest happens when the heart suddenly stops pumping blood around the body, depriving the brain and other vital organs of oxygen. CPR involves pressing repeatedly on the chest to keep some blood circulating until emergency help arrives.Starting CPR promptly can double or even triple someone’s chances of survival. Yet survival remains low. In England in 2024, only 9.5% of people survived for 30 days following an out-of-hospital cardiac arrest in which ambulance crews attempted resuscitation.Survival depends on several factors, including how quickly CPR begins, whether the cardiac arrest is witnessed and whether a defibrillator is available. But researchers are increasingly interested in another question: how effectively are members of the public performing CPR before paramedics arrive?A 2026 Australian study examined 17,715 out-of-hospital cardiac arrests in New South Wales between 2021 and 2024. Researchers found that patients who received CPR judged by paramedics to be good quality had substantially higher odds of surviving for 30 days than those who received no CPR.CPR judged to be of a poor quality was not associated with a statistically significant improvement in survival compared with no CPR. This does not mean that poor-quality CPR is ineffective: the study could not establish whether it offered a benefit.However, these findings need careful interpretation. The study was observational, so it cannot establish that differences in perceived CPR quality directly caused the differences in survival. Paramedics’ assessments were also subjective, and CPR quality was not recorded for every case.Nevertheless, the findings reinforce the importance of giving members of the public the knowledge and support needed to perform effective chest compressions.What makes CPR effective?When the heart stops pumping, blood flow to the brain rapidly falls. Chest compressions help maintain circulation until paramedics can provide more advanced treatment. A public-access defibrillator can also assess the heart’s rhythm and deliver an electrical shock if one is needed.The UK’s Resuscitation Council guidelines recommend pressing firmly in the centre of an adult’s chest at a rate of 100 to 120 compressions a minute, to a depth of around 5 to 6cm.Compressions should be interrupted as little as possible, and the chest should be allowed to return to its normal position between each push. These recommendations are designed to help maintain blood flow while avoiding unnecessary interruptions.The difficulty of measuring CPR qualityIn England, national cardiac arrest reports record whether CPR was provided by a member of the public, but published figures do not routinely show how effectively chest compressions were performed. How to perform CPR. Paramedics arriving at the scene may be able to judge whether compressions are being delivered effectively. However, these observations are not always recorded consistently, making it difficult to compare cases or identify where improvements are needed.The Australian research offers one approach. Paramedics assessed bystander CPR as either good or poor quality, allowing researchers to examine its association with survival. But without a standardised, objective method of assessment, such judgments have limitations.More consistent recording could help ambulance services understand which aspects of CPR people find difficult and where additional training or guidance might help.For example, people may understand that they need to press on someone’s chest but be uncertain about how hard or how quickly to push. Better information about what happens during real emergencies could help shape training and improve the instructions given by emergency call handlers.And support is already developing. In the east of England, ambulance services have introduced a pilot using live video to help specialist paramedics guide people through CPR while waiting for an ambulance.This gives emergency staff an opportunity to see what is happening and offer more specific advice about chest compressions.Encouraging people to act remains essentialThe number of people receiving CPR from members of the public has increased in some countries. In England, bystanders provided CPR in 73% of out-of-hospital cardiac arrest cases in 2024.In Victoria, Australia, 80% of bystander-witnessed cases in which ambulance crews attempted resuscitation received bystander CPR in 2024-25, although the two figures cover different groups of patients.There is still considerable room for improvement. Across Europe, the average reported bystander CPR rate is 58%, with substantial differences between countries.Encouraging more people to intervene remains a public health priority. But in areas where bystander CPR is already common, improving its quality could offer another opportunity to increase survival.This should never become a reason for someone to hesitate. A person witnessing a cardiac arrest cannot be expected to perform CPR perfectly, particularly in a frightening and unfamiliar situation.Anyone who finds someone unresponsive should call 999 immediately. If the person is not breathing normally, the caller should begin CPR and follow the emergency call handler’s instructions. If a defibrillator is available, someone else can retrieve it while chest compressions continue.The priority must always be to get people to act quickly, rather than worry about getting every compression right. But understanding what happens when bystanders perform CPR in real emergencies could help ambulance services give better guidance and support.The aim is to make effective CPR easier for more people to deliver, and give those suffering a cardiac arrest a better chance of survival.Melinda McPherson receives funding from Anglia Ruskin University and Essex and Herts Air ambulance as part of the Vice Chancellors PhD scholarship. Hilary Bungay and Sarah McLachlan do not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and have disclosed no relevant affiliations beyond their academic appointment.