With a cardiac arrest, the heart stops beating and as a result, there is no blood flow or oxygen delivered to the brain, resulting in nerve cell death. Brain injury can continue to evolve even after the blood flow is restored, a phenomenon called reperfusion injury.
Guidelines recommend cooling resuscitated cardiac arrest patients who remain comatose soon after hospital arrival for up to 24 hours in an intervention called therapeutic hypothermia. Body temperature is then slowly raised to normal levels.
“The hope is to give the brain a better chance to recover by starting cooling as soon as possible after resuscitation,” said Francis Kim, M.D., the study’s lead author and an associate professor of medicine in the cardiology division of the University of Washington in Seattle.
The Induction of Mild Hypothermia Following Out-of-hospital Cardiac Arrest Trial included 1,359 people successfully resuscitated after out-of-hospital cardiac arrest in Seattle and King County, Wash., in 2007 to 2012.
Researchers randomly assigned patients to standard care with or without cooling after resuscitation and on the way to the hospital.
Nearly all of the patients with ventricular fibrillation — a form of cardiac arrest due to fast, abnormal heart rhythm that can be treated with a shock from a defibrillator — received cooling once they were admitted to the hospital. This was regardless of whether they received cooling or not on their way to the hospital as part of the research study. The results of the study showed that:
- Among patients with ventricular fibrillation, 62.5 percent survived if they had early, prehospital cooling en route to the hospital, compared to 64.3 percent who did not.
- For patients whose cardiac arrest wasn’t due to ventricular fibrillation, 19.2 percent survived until hospital discharge if they had cooling en route to the hospital, compared to 16.3 percent who did not.
- Almost 43 percent of the early cooling patients with ventricular fibrillation had full brain function recovery compared to 46.4 percent of those who received only standard care.
- The full recovery rate for the non-ventricular fibrillation patients was 9.1 percent for those cooled early and 8.9 percent for those who received standard care.
- Twenty-six percent of the patients cooled early went into re-arrest, meaning their pulse stopped again en route, compared to 21 percent who received standard care.
“Under the conditions of our study, early cooling in the field didn’t improve neurologic outcomes or survival following resuscitation from out-of-hospital cardiac arrest,” Kim said.
Researchers will assess additional data gathered during this study to determine whether early cooling is associated with improved neurologic function six months after resuscitation.
Previous studies of cooling in the field have returned varying results and the field will continue to be a focus of research potentially using other methodologies.