<P> CPR involves chest compressions for adults between 5 cm (2.0 in) and 6 cm (2.4 in) deep and at a rate of at least 100 to 120 per minute . The rescuer may also provide artificial ventilation by either exhaling air into the subject's mouth or nose (mouth - to - mouth resuscitation) or using a device that pushes air into the subject's lungs (mechanical ventilation). Current recommendations place emphasis on early and high - quality chest compressions over artificial ventilation; a simplified CPR method involving chest compressions only is recommended for untrained rescuers . In children, however, only doing compressions may result in worse outcomes . Chest compression to breathing ratios is set at 30 to 2 in adults . </P> <P> CPR alone is unlikely to restart the heart . Its main purpose is to restore partial flow of oxygenated blood to the brain and heart . The objective is to delay tissue death and to extend the brief window of opportunity for a successful resuscitation without permanent brain damage . Administration of an electric shock to the subject's heart, termed defibrillation, is usually needed in order to restore a viable or "perfusing" heart rhythm . Defibrillation is effective only for certain heart rhythms, namely ventricular fibrillation or pulseless ventricular tachycardia, rather than asystole or pulseless electrical activity . Early shock when appropriate is recommended . CPR may succeed in inducing a heart rhythm that may be shockable . In general, CPR is continued until the person has a return of spontaneous circulation (ROSC) or is declared dead . </P> <P> CPR is indicated for any person unresponsive with no breathing or breathing only in occasional agonal gasps, as it is most likely that they are in cardiac arrest . If a person still has a pulse but is not breathing (respiratory arrest) artificial ventilations may be more appropriate, but, due to the difficulty people have in accurately assessing the presence or absence of a pulse, CPR guidelines recommend that lay persons should not be instructed to check the pulse, while giving healthcare professionals the option to check a pulse . In those with cardiac arrest due to trauma, CPR is considered futile but still recommended . Correcting the underlying cause such as a tension pneumothorax or pericardial tamponade may help . </P> <Table> <Tr> <Th> Type of Arrest </Th> <Th> ROSC </Th> <Th> Survival </Th> <Th> Source </Th> </Tr> <Tr> <Td> Witnessed in - hospital cardiac arrest </Td> <Td> 52% </Td> <Td> 19% </Td> <Td> </Td> </Tr> <Tr> <Td> Unwitnessed in - hospital cardiac arrest </Td> <Td> 33% </Td> <Td> 8% </Td> <Td> </Td> </Tr> <Tr> <Td> Out - of - hospital cardiac arrest overall </Td> <Td> 59% </Td> <Td> 10% </Td> <Td> </Td> </Tr> <Tr> <Td> Unwitnessed out - of - hospital cardiac arrest </Td> <Td> 21% </Td> <Td> 4% </Td> <Td> </Td> </Tr> <Tr> <Td> Witnessed out - of - hospital cardiac arrest </Td> <Td> 41% </Td> <Td> 15% </Td> <Td> </Td> </Tr> <Tr> <Td> Witnessed and "shockable" with bystander CPR </Td> <Td> 53% </Td> <Td> 37% </Td> <Td> </Td> </Tr> <Tr> <Td> Bystander compression - only resuscitation </Td> <Td> - </Td> <Td> 13% </Td> <Td> </Td> </Tr> <Tr> <Td> Bystander conventional CPR </Td> <Td> - </Td> <Td> 8% </Td> <Td> </Td> </Tr> </Table>

When do you attempt to give cpr (cardio pulmonary resuscitation)
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