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Therapeutic Hypothermia

Cardiac arrest (CA) is associated with very high mortality and causes neurological dysfunction in the survivors4. Out-of-hospital cardiac arrest (OHCA) affects approximately 300,000 Americans every year18. CA is a devastating condition and associated with a mortality of more than 90%15. Although the in-hospital rate of survival for individuals hospitalized following OHCA improved by nearly 12% from 2001 to 2009, the in-hospital mortality rate remains close to 60%21. Even after successful return of spontaneous circulation (ROSC) following an OHCA, approximately 80% of patients remain comatose5 and the majority of deaths following return of spontaneous circulation (ROSC) are attributable to neurological injury14. To suffer a CA can be a lifechanging experience with a significant impact on one’s day-to-day life13. Many of the individuals who do survive have a wide range of subsequent neurologic impairments20. It has been shown that even a short CA of only a few minutes in duration can change the affected individual permanently, including diminished cognitive function regarding emotions and memories23.

Therapeutic hypothermia (TH) is recommended in post-CA patients who remain comatose after successful resuscitation. TH for patients who remain comatose following resuscitation from a cardiac arrest improves both survival and neurologic outcomes21. A decrease in metabolism in the brain from TH may be one of the most important mechanisms of neuroprotection4. Since 2003, International Liaison Committee on Resuscitation (ILCOR) recommends TH treatment for everyone with CA who regains spontaneous circulation22. TH is now a component of post-cardiac arrest care as per current advanced cardiac life support protocol by American Heart Association/American College of Cardiology19. A Cochrane review released in 2012 also reported the benefits of TH2.

TH has been defined as a body temperature of below 34°C4. Differences of opinion exist as to what the goal temperature should be, when should TH be initiated, which CA victims should have the treatment, and so on. Because the pathophysiology for neurologic injury following CA is similar regardless of rhythm or location of arrest, the decision to initiate TH should be made on an individual patient basis considering the etiology of the arrest, time before initiation of CPR, duration of CPR prior to ROSC, and overall prognosis based on underlying comorbidities21.

The following table lists some guidelines and recommendations for the use of TH following CA provided by the AHA, ILCOR, and the European Resuscitation Council Guidelines for Resuscitation21:


In the initial randomized control trials demonstrating benefit with TH, target temperature was achieved within 4-8 h post-ROSC12,3. However, the optimal timing for TH initiation as well as the time to goal target temperature remains uncertain21. Mooney et al. demonstrated that among individuals undergoing TH post-CA (both shockable and non-shockable rhythms) there was a 20% increase in mortality for every hour that initiation of TH was delayed. Wolff et al. demonstrated that time to coldest temperature was an independent predictor of good neurologic outcome. A third study demonstrated that reaching target temperature within 6 h (early group) was associated with a greater likelihood of favorable neurologic outcome when compared with individuals reaching target temperature after 6 h (delayed group)10. In contrast, Haugk et al. found that shorter time to target temperature was associated with unfavorable neurologic outcomes (median duration 158 min-unfavorable versus 209 min-favorable). Based on the current data from observational studies as well as randomized control trials, the optimal timing to initiate TH and achieve target temperature remains unclear21.


There are multiple methods that have been documented for cooling a patient. Administration of cold intravenous fluids, ice packs around the patient, cold air blankets, and even spraying the patient with water and having a fan blowing on the patient. Another method is the use of intra-arrest transnasal evaporation8. More controlled methods like a catheter being placed into a large central vein and temperature-controlled fluids being circulated through the catheter are in use. The cooled saline is never injected directly into the bloodstream, but stays contained within the central catheter cooling the blood as it flows by the catheter1.

Although the intravascular catheter has demonstrated improved control of maintaining target temperature, given the lack of head-to-head comparison for clinical outcomes data, there does not appear to be one cooling method that is clearly superior21.


Based on the original studies regarding TH, the AHA recommended 32-34°C as the target temperature for individuals undergoing TH post-ROSC following CA21. In the setting of unknown optimal target temperatures for TH following CA, the Targeted Temperature Management (TTM) Trial Investigators recently published the largest randomized control trial of TH in patients who remained unconscious following ROSC after an OHCA17. They randomized 950 patients post-CA to targeted temperature management at a target temperature of 33°C versus 36°C. At completion of the trial, there was no difference in mortality (50%-33°C, versus 48%-36°C, p=0.51) or poor neurologic outcome (52% for each group, p=0.87). Both groups were actively cooled as quickly as possible using ice-cold fluids, ice packs, and intravascular or surface temperature-management devices to maintain target temperature17.

It is possible that the degree of hypothermia may need to be titrated or adjusted to fit the severity of neurologic injury, and that there are likely sub-groups of patients who may derive benefit from a greater degree of hypothermia21. The most recent ILCOR update on TH also favors continuing to target a temperature of 33°C, though acknowledging that 36°C can be used and will provide similar neurologic benefit21.


TH could be an uncomfortable treatment modality. The patient should, during the treatment with hypothermia, be sedated until he or she is comfortable and without pain or risk of shivering9. Adequate sedation and analgesia must be maintained throughout the hypothermia protocol until the patient has been rewarmed and has returned to normothermia and short-acting medications are preferred given the frequent reduced clearance of these medications in this patient population21.

Shivering is a physiologic reaction to cooling that frequently occurs when patients are between 35°C and 37°C and it should be treated because it slows the rate of cooling and increases metabolic activity21.


Rewarming should be initiated at 12–24 h after TH initiation21. Rewarming should occur slowly at a rate of 0.25–0.5°C per hour until the patient reaches normothermia7. Rapid rewarming increases the risk of significant complications including hypotension from substantial vasodilation as well as hypoglycemia and hyperkalemia21.


Although TH has been part of clinical practice for over a decade and TTM now has class I recommendations from the AHA for use in all patients post ROSC, significant controversy and questions remain regarding several aspects of its implementation: (1) which patients post-CA should receive TH; (2) when should TH be initiated and how long should patients be cooled; (3) what is the ideal temperature for cooling, and should this decision be universally applied, or made on an individualized basis; and (4) how and when should we be making neurologic prognoses21.


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