Even if a person whose heart suddenly stops beating is resuscitated, serious brain damage may occur. Now two research teams report that temporarily lowering body temperature may improve the odds of survival and recovery of people who remain in a coma after being resuscitated.

Given that approximately 375,000 people experience cardiac arrest in Europe alone each year, the researchers predict that lowering body temperature -- a treatment called hypothermia -- could prevent serious neurological complications in thousands of people annually.

The term hypothermia may bring to mind a picture of a hiker whose body temperature plunges after becoming lost in the snowy woods. This type of body temperature drop can be deadly, but milder forms of hypothermia can be beneficial. For instance, lowering body temperature during some types of heart surgery can protect the brain.

And several animal studies have shown that hypothermia may improve survival odds after cardiac arrest.

Now researchers in Europe and Australia report that hypothermia seems to be safe and effective in people who have gone into cardiac arrest, a situation in which the heart quivers with uncoordinated contractions and can't pump blood. A patient in cardiac arrest will not survive unless the heart is defibrillated, or shocked back into a normal rhythm.

In one of the studies, researchers randomly assigned 77 patients who were comatose after cardiac arrest to hypothermia or normal treatment.

Within 2 hours of being resuscitated, patients in the hypothermia group had ice packs applied to their body until their temperature dropped to 33 degrees Celsius (91.4 degrees Fahrenheit). This reduced temperature was maintained for 12 hours.

Patients treated with hypothermia were more likely to survive. Nearly half of the hypothermia group survived and went home or to a rehabilitation facility, compared with only a quarter of patients who did not receive hypothermia treatment.

When the researchers took into account factors that could have affected the results including age and the length of time a patient's heart was stopped, they found that patients treated with hypothermia were about five times more likely to have a good outcome.

In the other study, researchers also found that hypothermia led to better outcomes for patients who were comatose after cardiac arrest. This study, which was conducted at five hospitals in nine European countries, included 136 patients randomly assigned to hypothermia and 138 to normal care.

A cold-air-delivering mattress was used to lower body temperature to 32 to 34 degrees C (89.6 to 93.2 F). Patients were kept at this temperature for 24 hours -- twice as long as in the Australian study.

Patients in the hypothermia group were more likely to experience a good recovery and were less likely to die. Six months after treatment, 55% made a good recovery or had only mild neurological damage compared with 39% of patients who did not receive hypothermia.

During cardiac arrest, the brain is deprived of oxygen, and permanent damage begins to occur after about 5 minutes. Even when paramedics are able to restart the heart and get a person to a hospital, the patient remains unconscious due to brain damage about 75% of the time.

But the study shows that treatment with hypothermia can raise the percentage of patients who experience a good recovery to nearly 50%.

But as is the case with many medical treatments, early treatment is best. This presents an obstacle when patients experience cardiac arrest outside of the hospital. The researchers are currently studying the effects of administering an ice-cold intravenous fluid soon after cardiac arrest.

Hypothermia is thought to protect against brain damage by reducing the amount of oxygen the brain needs, but it seems more likely that hypothermia provides protection against numerous deleterious biochemical mechanisms.

Despite the benefits of the treatment, it is still possible that hypothermia may have some harmful effects. The treatment needs to be studied more extensively.

The New England Journal of Medicine February 21, 2002;346:546,549-563,612-613