Eight out of 355 cardiac arrest patients who do not immediately wake up after hospitalisation, have benefited from being cooled down to a temperature of 33°C for as long as 48 hours. However, this does not provide researchers from Aarhus University and elsewhere with evidence to conclude that 48–hour cooling is preferable to the typical 24 hours when it comes to preventing brain damage. The level of uncertainty is too high and the difference too small to reach this conclusion.

A five per cent difference in favour of the 48 hours. This is the result of a study from Aarhus University that has spent several years examining the possible benefits of cooling down patients suffering cardiac arrest to 33°C for 48 hours, instead of the 24–hours that is standard practice in most places.

"We cannot rule out that there is a smaller clinical benefit of cooling down patients for two days, and the indications are that patients under the age of sixty in particular may benefit from the extra 24 hours. But a statistically significant result will require a randomised and blinded study with 3,000 contributing patients, and we do not have the opportunity to undertake such a study," says Professor Hans Kirkegaard from the Research Center for Emergency Medicine at the Department of Clinical Medicine, Aarhus University. He has spent eight years on the randomised and blinded study, which includes 355 out of hospital cardiac arrest patients aged 18 to 80, who have been admitted to ten different intensive care units in six different countries. "It is an extensive study involving a lot of preparation in relation to preparing and aligning practice in the participating countries, which in addition to Denmark include Norway, Finland, Estonia, Germany and Belgium. More than three years passed from the first patient being included in 2013 until we registered the final one in the summer of 2016. We have also stipulated a range of requirements for the suitability of patients in the study," adds Hans Kirkegaard about the study, which was published in The Journal of the American Medical Association.

The researchers followed the survivors and after six months measured how they were managing in relation to what is known as a CPC Score, which is an incremental scale from 1 to 5. CPC stands for Cerebral Performance Category Score and the scale describes the spectrum from: ‘A return to normal cerebral function and normal living'; to 'Some disability but sufficient function for independent activities of daily living’; to ‘Severe disability, limited cognition, inability to carry out independent existence’; to 'Coma', often simply referred to as a vegetative state; and finally, to ‘Brain death’. In the research project, CPC scores of 1 and 2 are combined in a positive outcome – with the above–mentioned five per cent advantage in favour of the 48 hours – while scores of 3,4 and 5 are included in the study as a negative result.

It has been common knowledge for many years that cooling down the body after cardiac arrest can reduce the degree of brain damage. The practice of cooling down patients is also used in maternity wards, where new–born babies who have not received sufficient oxygen during birth are often cooled for as long as 72 hours. In practice, patients are cooled down either by placing them in special thermo suits that have cold water circulating through them, or by placing a catheter in a vein before circulating cold salt water through this to keep the body temperature down.

In the old days, the method was simply to cover the person with ice particularly around the groin and neck areas, which is where the large arteries lie just beneath the skin. According to Hans Kirkegaard, there are of course side effects associated with cooling down people. Some catch pneumonia and others arrhythmia, which is an irregular heart rhythm for as long as the cooling lasts.