The use of mild therapeutic hypothermia for victims of SCA who have been resuscitated but who remain unresponsive has been recommended since 2002.20-22 Mild therapeutic hypothermia (TH) is defined as cooling the body to 32-34 degrees C for 12-24 hours after resuscitation from SCA. Mild TH has been shown to confer a survival benefit for patients who first presented in VF when provided up to six hours after the arrest.21 In addition, there is now a small amount of evidence that SCA victims with other presenting rhythms may also benefit.7
Existing knowledge of reanimation23 demonstrated higher survival and better neurologic outcome.21 This result along with studies demonstrating the feasibility of providing TH in the prehospital setting20,24 prompted TH use by a few EMS systems. Recent clinical studies show that prehospital cooling using ice-cold saline can lower core temperatures by 0.8 to 1.0 degree C on emergency department arrival, but these studies did not find any survival benefit from initiation of TH in the field.24-25 Initiating TH prior to ED arrival has served as an impetus for hospitals to develop TH protocols, and as a reminder to hospital personnel to consider continuing the therapy.
While it would seem logical that providing hypothermia therapy as early as possible following ROSC would improve neurological outcome, available data suggest that there is not a large clinical effect from instituting cooling in the prehospital setting following restoration of circulation.26 Additional clinical studies may improve current strategies for delivering hypothermia and better define the optimal rate of cooling induction, duration of cooling, determination of appropriate target temperatures, and perhaps, individual patient variables.24-25,27
In the prehospital setting, core temperatures can be reduced by 0.8 to 1.0 degree C with the rapid infusion of cold saline following ROSC.25Using either interossious infusion or a large bore IV, a large amount of fluid, 30-40 ml/kg, can be infused in a short time. Saline can be kept cold in the ambulance using low tech methods such as storage in a cooler packed with ice. An inexpensive “plug in” cooler can also be used to ensure that the saline is always cold.
Prehospital protocols sometimes recommend the use of a paralytic during infusion. Paralytic agents are considered during the provision of in-hospital TH if the patient is counteracting the cooling process by excessive shivering during the induction phase. Due to the usually short transport times to the hospital, and the added complexity of managing and monitoring a paralyzed patient, it is not necessary to use a paralytic for the induction of prehospital therapeutic hypothermia.
There may be other methods of cooling currently used in the prehospital setting. Cooling blankets and ice packs have been used, however, these are slow to produce a change in body temperature and, therefore, not very useful in the prehospital setting. More complex devices such as body cooling bags may be available, however, they are not widely used. Rapid infusion with cold saline provides a quick, safe, and simple method of decreasing core temperature in the prehospital setting.
It is essential that the EMS system providing TH has predetermined that the destination hospital has adequate resources to continue the hypothermia therapy begun in the field as well as the other supportive care needed during the post-resuscitation phase.7
While still considered controversial, there are EMS systems that provide intra-arrest cooling for all cardiac arrest resuscitations. This simplifies decision-making about who is eligible and increases the chance that an eligible subject will receive the treatment. To do this simply, some systems have opted to use cold saline as their primary resuscitation fluid.
Therapeutic Hypothermia Resources
The University of Pennsylvania has maintained a resource packed website with regard to many aspects of both in-hospital and out-of-hospital hypothermia therapy. Multiple examples of prehospital protocols and references are available at http://www.med.upenn.edu/resuscitation/hypothermia. The University of Pennsylvania resuscitation experts have been offering workshops at various locations around the country to help both EMS and hospital personnel to establish therapeutic hypothermia programs.
Order sets and algorithms for Minnesota Heart Institute partner hospital are available on the website maintained by the Abbott Northwest Hospital Minneapolis Heart Institute and can be found at http://www.mplsheart.com/Professionals/EmergencyProtocols/ TherapeuticHypoCardiacArrest.aspx.