Therapeutic Hypothermia in the Field

It is essential to control shivering as soon as it is recognized during the cooling process. Shivering is the body’s only warming mechanism, and it needs to be controlled to blunt the body’s thermoregulatory defenses.¹⁴ A variety of pharmacologic strategies are available, and for EMS the most practical are either fentanyl or benzodiazepine boluses that can be followed by infusions as necessary. If these methods fail, propofol (10–50 mcg/kg/min) provides both vasodilation and an active lowering of the shivering threshold; it is used by some EMS and many specialty-care transport systems. Should these methods fail to suppress shivering, consider paralytics, depending on availability.⁴

Skin temperature is suspected to influence an increase in shivering by as much as 20%. Thus, applying transient skin warming methods (e.g., a blanket) can warm the skin and blunt shivering while allowing the core to remain cool.¹⁴ Interestingly, this raises question over the true benefit of direct cooling on the axilla, groin and neck, as these cooling methods may be blunted by the shivering they produce as the skin first cools.

Future Uses

Since at least 2006, physicians have been researching the benefits of cooling for patients experiencing acute stroke.³ In one study awake stroke patients were cooled to 33.4°C within 1.7 hours and maintained cold for 24 hours. None of these patients experienced complications; this seems to support the feasibility of using TH for stroke care.¹⁵ A 2007 review of animal studies found cooling the brain during acute ischemic events within 3 hours reduced infarct size and nearly completely abolished the ischemic region.¹⁶ This review concluded there are true beneficial neuroprotective properties to therapeutic hypothermia during stroke, but that the ideal cooling method, rate and duration were unknown.¹⁶ Finally, a multicenter study published in 2010 by the same author demonstrated that endovascular TH of acute stroke patients following tPA administration was safe.¹⁷

Hypothermia has been demonstrated to reduce intracranial pressure following severe traumatic brain injuries, particularly in children. At this point, however, its benefit for improving neurological outcomes has not been proven.² The hope is that with more research, appropriately applied hypothermia can reduce the secondary effects from evolving injuries and inflammation that occur following TBI. It is theorized to reduce mortality and improve neurological outcomes (having the greatest benefits when used for at least 48 hours) but not be as helpful when ICP is initially high or when barbiturates are used at high doses.⁴

Therapeutic hypothermia is also being researched for benefits to patients receiving PCI for STEMI.³ One animal study found that rapid cooling during a simulated left ventricular myocardial infarction preserved LV function, reduced injury size and preserved mitochondrial function.¹⁸

Since May 2005 the National Institute of Child Health and Human Development has supported the use of therapeutic hypothermia for neonates suffering from hypoxic-ischemic encephalopathy, while acknowledging more research is needed.²

Who Should be Cooled Down
Most studies of TH for post-cardiac arrest patients have excluded children, and some have excluded women of childbearing years.³ This does not mean these patients cannot be cooled, just that these patient groups haven’t been studied for TH following cardiac arrest. General inclusion criteria for patient cooling following prehospital cardiac arrest are as follows:⁴

• At least 18 years old;
• Time from arrest to ROSC less than 60 mins.;
• At least 5 minutes of CPR performed;
• SBP greater than 90 mmHg (MAP greater than 60) with only one vasopressor;
• GCS less than 9;
• Less than 6 hours since ROSC.

In the 2010 AHA guidelines, therapeutic hypothermia is described as a helpful therapeutic approach for patients who remain without meaningful response to verbal commands following ROSC.⁸ With these guidelines, TH between 32°–34°C for 12–24 hours was listed as a Class I, LOE B recommendation for patients presenting in VF, and a Class IIb, LOE B for patients presenting in PEA and asystole. This last recommendation was further supported in 2011 with a study demonstrating that TH does not reduce mortality for patients with asystole/PEA.⁹

Currently TH is recommended to be administered within 6 hours of ROSC, with a preference for the earliest possible initiation. There is clear data showing that the earlier TH is initiated, the better outcomes become.¹⁰