For a decade, doctors have made hypothermia standard practice
DALLAS — Chilling the body after cardiac arrest has become a common practice to protect a blood-starved brain and heart from the rush of oxygen that comes with the resumption of normal cardiac rhythms. But studies presented November 17 during the American Heart Association Scientific Sessions call into question doctors’ recommendations on induced hypothermia and cast doubt on the necessity of a quick cooldown.
About 300,000 Americans suffer from cardiac arrest each year, and 80 percent of them die. Of those who survive, half are left with brain damage, which occurs when the heart starts beating again and a sudden rush of blood into oxygen-starved tissues triggers an avalanche of treacherous biochemical reactions.
In 2002, researchers reported that lowering a patient’s body temperature protected tissues from injury. The American Heart Association and others now recommend that comatose patients who arrive at the hospital in cardiac arrest be immediately chilled to a temperature between 32° and 34° Celsius (89.6° and 93.2° Fahrenheit).
A team that studied 36 intensive care units throughout Europe and Australia tested whether such a dramatic temperature drop is necessary. After reviewing the evidence from years of practice, said Niklas Nielsen of Lund University in Helsingborg, Sweden, “we thought optimal temperature had not been determined.” For the study, 950 patients in cardiac arrest were randomly assigned to be cooled to either 33° Celsius (91.4° Fahrenheit) or 36° Celsius (96.8° Fahrenheit), which is just slightly below normal body temperature.
Not only did the lower temperature fail to offer better protection to body tissues, it actually left patients more liable to suffer side effects such as pneumonia and internal bleeding. The study is the largest study so far to examine body cooling after cardiac arrest, and its results are “the opposite of what everyone believed,” Nielsen said. “I think we absolutely need to review the current guidelines.” The study was published November 17 in the New England Journal of Medicine.
Researchers not involved with the study were cautious about interpreting the results. “The question upon us today is how can this be?” said Benjamin Abella of the University of Pennsylvania in Philadelphia. The study had some elements that differed from the original 2002 research, he said. One example: A high number of patients in the new study had received CPR and therefore a restored heartbeat sooner. The findings could mean that patients at lower risk of injury to begin with may not benefit much from a cooldown.
A second study also presented November 17 examined whether a more immediate temperature drop improved patient health. It compared dropping body temperature while patients were on the way to the hospital with waiting until patients were in the emergency room. Researchers from the University of Washington in Seattle randomly assigned almost 1,400 patients to receive either an infusion of chilled saline in the ambulance or standard care. Nearly all patients had ventricular fibrillation, the kind of cardiac arrest that can be restored with a shock to the heart.
When compared with patients who were cooled immediately after arrival in the emergency room, the patients who were chilled sooner fared worse: Twenty-six percent experienced a second cardiac arrest before reaching the hospital, compared with 21 percent of the comparison group. Lead researcher Francis Kim suggested that the problem may have come from the pressure of extra fluid passing through the heart and not the cooling itself. That study appeared November 17 in JAMA.
“The take-home message is that if you start cooling after the heart is already beating, it’s OK to start later,” said Maaret Castrén of the Karolinska Institute in Stockholm, who was not part of the study. She recommended further investigation of dropping body temperature before blood circulation resumes.
Taken together, she said, the two studies highlight that a decade after therapeutic hypothermia was met with great enthusiasm, “there are a lot of open questions.”
N. Nielsen et al. Targeted Temperature Management at 33°C versus 36°C after Cardiac Arrest. New England Journal of Medicine. Published online November 17, 2013. doi: 10.1056/NEJMoa1310519.
N. Nielsen et al. Target Temperature Management 33°C versus 36°C after Out-of-hospital Cardiac Arrest, a Randomized, Parallel Group, Assessor Blinded Clinical Trial. American Heart Association Scientific Sessions, November 17, 2013, Dallas.
F. Kim et al. Effect of Prehospital Induction of Mild Hypothermia on Survival and Neurological Status Among Adults With Cardiac Arrest A Randomized Clinical Trial. JAMA. Published online November 17, 2013. doi: 10.1001/jama.2013.282173.
F. Kim et al. Randomized Clinical Trial of Pre-hospital Induction of Mild Hypothermia in Out-of-Hospital Cardiac Arrest Patients Using a Rapid Infusion of 4oC Normal Saline. American Heart Association Scientific Sessions, November 17, 2013, Dallas.
N. Seppa. Highlights from the American Heart Association Scientific Sessions, Los Angeles, November 3-7. Science News, Vol. 182, December 15, 2012, p. 9.
N. Seppa. Chill Out: Mild hypothermia aids heart attack recovery. Science News, Vol. 161, February 23, 2002, p. 115.