Cool treatment: advances in therapeutic hypothermia
Therapeutic hypothermia is widely used as a routine part of treatment for cardiac arrest, and there is evidence to suggest that it could also be beneficial in patients who have had a myocardial infarction (MI).
These are both highly active areas of research, and in this article we summarize the findings of some very recently published studies and reviews. In addition, we talk to Professor David Erlinge, a leading researcher investigating the potential of therapeutic hypothermia in cardiology, who provides an overview of current knowledge.
Hypothermia after cardiac arrest
The author of a review published in July in the Cleveland Clinic Journal of Medicine made the point that mild therapeutic hypothermia is a recommended intervention for out-of-hospital cardiac arrest due to ventricular fibrillation. However, "first-responders, emergency-room staff, and intensive-care teams have been slow to adopt and integrate it in to a comprehensive post-resuscitation strategy".
The review covered the proposed mechanisms by which hypothermia has a therapeutic effect, including a reduction in neuronal metabolism in the early stage of ischemic injury, a reduction in glucose and oxygen consumption by the brain (minimizing the supply-demand mismatch), and a decrease in the rate of release of excitatory amino acids that would normally trigger cytotoxic cascades after injury.
Overall, the author concluded that, based on published clinical data, "survivors of cardiac arrest due to ventricular tachycardia or ventricular fibrillation have improved neurologic outcomes if they are cooled to a core body temperature of 32-34 degrees Celsius for 24 hours as soon as possible after reaching the hospital".1
Meanwhile, a new study of 140 patients with out-of-hospital cardiac arrest confirmed the potential survival benefits of therapeutic hypothermia. The study, published in Circulation in July, found that there was a 20% increase in the risk of death (95% confidence interval 4% to 39%) for every hour of delay to the initiation of cooling.2
A separate small study of cardiac arrest cases at a single center found that cerebral performance category scores were significantly better at discharge and after 6 months in patients who had undergone cooling, compared with those who had not.3
Hypothermia after MI
While therapeutic hypothermia has been shown to improve outcomes, including the preservation of neurologic function, after cardiac arrest, the evidence for a beneficial role after MI is currently less strong. Can hypothermia help preserve myocardial function?
A good summary is provided by a systematic review published in June, in which the authors discussed findings from 2 feasibility trials and 3 randomized controlled trials published before July 2010. The authors noted that the mean infarct size ranged from 2% to 14.1% of the left ventricle in patients treated with hypothermia, and from 8% to 13.8% in control patients.
The incidence of major adverse cardiac events (MACEs) was from 0% to 6.2% in patients treated with hypothermia, and from 3.9% to 10% in controls. All-cause mortality, meanwhile, occurred at a rate of 0% to 3.4% in the hypothermia groups and 2.2% to 10% in the control groups.
The authors added that subgroup analyses indicated that therapeutic hypothermia may reduce infarct size in patients with anterior wall infarction. They concluded that, overall, “more evidence is needed to determine whether therapeutic hypothermia is associated with improved infarct size, MACEs, or all-cause mortality”.4
The specialist view
Some of this evidence on the effects of hypothermia in MI may be provided relatively soon. Commenting on the current understanding of the value of therapeutic hypothermia with regard to the treatment of cardiac arrest and MI, Professor David Erlinge, head of the Department of Cardiology at Lund University Hospital, Sweden, told GetInsideHealth:
“Therapeutic hypothermia is now recognised in all major guidelines as standard treatment for cardiac arrest, and it is has been successfully implemented in most of Europe for several years. In many countries specialists have reported increased survival of patients, typically going from 30% to 50% some years after introducing hypothermia. At a recent meeting in Japan a few weeks ago I heard similar stories coming from US sites.
"With hypothermia for MI, we are still at the research stage. We recently did a pooled analysis of the ICE-IT and RAPID MI-ICE studies, and when you combine the data you see significant improvements in infarct size. It looks promising, but we need a large trial: the CHILL-MI trial has just started, it will involve 10 sites in Europe, and we are the first site to enrol patients. We started a month ago and have included 6 patients so far. In total the trial should include 120 patients, and we think we will be able to report the first data in the first half of next year [2012]."
To learn more about some of the research on therapeutic hypothermia in acute MI being conducted by Prof. Erlinge, see the separate video interview conducted earlier this year at the American College of Cardiology’s annual meeting: http://www.getinsidehealth.com/en/Library/Articles/en/2011/May/ACC-2011-Hypothermia-being-actively-pursued-to-preserve-myocardium-in-acute-MI/.
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