Most cases of cardiac arrest occur outside of hospital and less than one in 10 of these will survive. However a new study has shown that performing CPR for at least 38 minutes can improve a patient's chances of survival.

Cardiac arrest refers to the sudden loss of function of the heart. It occurs when there is an abrupt disturbance in the heart's rhythm. This can cause the heart to stop beating.

A person whose heart has stopped beating will fall unconscious and stop breathing normally. If they do not get immediate medical assistance, sudden cardiac death will follow. An estimated 5,000-6,000 deaths in Ireland every year are due to cardiac arrest.

CPR (cardiopulmonary resuscitation) is a first aid technique which should be administered as soon as possible to a person who has suffered a cardiac arrest.

The first few minutes after a cardiac arrest are vital - if the pumping activity of the heart is not restored within a couple of minutes the brain suffers irreversible damage. CPR involves a combination of mouth-to-mouth rescue breathing and chest compressions to keep oxygenated blood flowing to the brain and other organs.

While research has shown that the early return of spontaneous circulation - that is the body being able to pump blood on its own - is important for people to survive a cardiac arrest without suffering brain damage, there has not been a lot of research done on the time between a cardiac arrest and the return of spontaneous circulation.

Japanese researchers decided to look into this further. They used a registry that tracked all cardiac arrests that occurred outside of hospital in Japan between 2005 and 2011. They analysed how much time had passed between the patient's initial collapse and the return of their spontaneous circulation.

They also looked at how well the patients' brains functioned one month after the incident.

The patients were considered to be doing well neurologically if they were able to return to normal activities, or if they were left with a moderate disability, but were still able to work part-time and live independently.

The researchers found that the time between the initial collapse and the return of spontaneous circulation was 13 minutes in those who were considered to have fared well. For those who suffered a severe brain disability, the time between the collapse and the return of spontaneous circulation was around 21 minutes.

The study found that the odds of surviving a cardiac arrest outside of hospital without suffering severe brain damage fell by 5% for every 60 seconds that passed before circulation was restored.

Based on their findings, the researchers calculated that if a person has collapsed as a result of a cardiac arrest, it is advisable to carry out CPR for 38 minutes or more.

"It may be appropriate to continue CPR if the return of spontaneous circulation occurs for any period of time," they said.

Bystanders are advised to carry out CPR until medical assistance arrives.

Details of these findings were presented at the American Heart Association's Scientific Sessions 2013.