Chill Out: Mild hypothermia aids heart attack recovery
By Nathan Seppa
It sounds cold, but icing down patients who have just had a heart stoppage may boost their survival chances and prevent brain damage in those who do pull through, two new studies show.
A heart attack, near-drowning, or other misfortune that leaves a person pulseless for even a few minutes can damage the brain because halted blood flow starves brain tissues of needed oxygen. Ironically, cardiac resuscitation and the abrupt restoration of blood flow often cause a second round of damage to the brain.
Experiments in animals during the past 2 decades showed that cooling the body after a shutdown of the heart prevented much of this secondary damage. Lowering body temperature, thereby cooling the blood, slows processes in the brain that ignite harmful biochemical chain reactions.
Cooling was tried as a heart attack treatment in the 1950s. After mixed results, doctors shelved the concept. In fact, warming patients during some surgeries has proven beneficial (SN: 4/12/97, p. 220: https://www.sciencenews.org/sn_arc97/4_12_97/fob1.htm). However, cooling has been used for years with good results on another set of patients–those who have had blood flow to the brain stopped intentionally during heart surgery.
To assess whether this so-called mild hypothermia could help resuscitated heart attack patients, researchers in Europe randomly assigned about half of 273 such patients to receive cooling after their initial collapse. All the people had been pulseless for at least several minutes before being revived, says Mattias Fischer, an anesthesiologist at Bonn University in Germany. He and his colleagues kept the cooled patients at 33C (91.4F) for 24 hours.
Six months after their heart attack, 41 percent of the patients receiving the mild hypothermia treatment had died, compared with 55 percent of those kept at normal body temperature, the researchers report in the Feb. 21 New England Journal of Medicine (NEJM). Of the cooled patients, 55 percent were well enough to live independently and work at least part-time, but only 39 percent those patients who had not been cooled were living on their own and holding a job. The average age in both groups was 59.
In another study reported in the same issue of NEJM, researchers working in and around Melbourne, Australia, instructed emergency teams to apply ice packs to comatose heart attack patients who had been resuscitated–but only on odd-numbered days. These patients were then kept between 32C and 34C for 12 hours. Patients shiver as their body temperature drops but stop when they reach this range, says study coauthor Stephen A. Bernard, an intensive-care physician at the Dandenong Hospital in Dandenong, Australia. On even-numbered days, comatose heart attack patients were kept warm.
The researchers found that 21 of 43 cooled patients (49 percent) were ultimately discharged to their homes or a rehabilitation center, compared with only 9 of 34 patients (26 percent) kept warm.
People who are comatose after being revived following a heart attack face grim odds. Three-fourths or more die from the heart attack, Bernard says. Dandenong Hospital and other facilities in the Melbourne area have begun cooling all revived, comatose heart attack patients in the ambulance, he says.
Protective effects of hypothermia have also been indicated by case studies of people who have fallen through ice, experienced a heart stoppage, and been revived–a situation with built-in cooling. However, such severe hypothermia can render a person vulnerable to infections and other problems.
On the other hand, mild cooling “is without side effects, safe, and inexpensive,” says Peter J. Safar, an anesthesiologist at the University of Pittsburgh Medical Center. “Mild hypothermia for cardiac arrest should be part of [physicians’] guidelines,” he says.
The secondary damage to the brain caused by the restoration of blood flow appears linked to a chain reaction that releases glutamate from nerve cells in the brain. Too much glutamate, an amino acid, overexcites and kills these cells. Keeping the blood cool curbs glutamate release, Safar says.
Other research suggests that mild hypothermia also limits the number of heart-muscle cells that die after a heart attack, possibly offering an additional benefit, says Robert A. Kloner, a cardiologist at Good Samaritan Hospital in Los Angeles.
