**6. Complications and problems with prehospital therapeutic hypothermia**

The usual side effects pertaining to therapeutic hypothermia in general like arrhythmias, electrolyte abnormalities, bleeding, infection and other complications could also happen here, however these are discussed in a previous chapter. In this section, I will discuss the complications and problems pertinent to the prehospital phase of hypothermia induction. Overcooling is a potential problem in the field. It is very important to avoid overcooling below the target range because adverse events likely increase when patients are cooled to < 32°C.[73,74] In a retrospective review, investigators showed that unintentional overcooling below target temperature is common, and concluded that improved mechanisms for temperature control are required to prevent potentially deleterious complications of more profound hypothermia.[75] I also add that effective and accurate methods for prehospital temperature monitoring is important, such as tympanic or esophageal temperature monitors. Another important complication is shivering, especially in the EMS with some limitations on use of antishivering medications, such as neuromascular blockers and some sedatives. One important potential problem is the interference of inducing TH in the field with the actual CPR and ACLS ongoing on the patient. Some providers believe that basic resuscitation care should be prioritized over induction of TH, especially with no proven outcome benefit of prehospital TH. A survey of EMS physicians on the implementation rate of prehospital cooling in the United States reported that the most common barriers to prehospital hypothermia are the lack of ideal equipment and space in EMS vehicles to store the equipment that is used to initiate cooling, lack of credentialing for the use of paralytic agents, and difficulty in prioritizing for training and patient care.[76] Another problem noted from some of the clinical studies addressed above is that after induction of hypothermia in the field, some patients were transported to hospitals where TH is not systematically continued in the post resuscitation care occurring in-hospital. If a patient is cooled only to be rewarmed soon after transport to a facilty, then this may actually be worse than not cooling the patient to begin with, as this might reverse and maybe even cause a rebound in all of the mechanisms of secondary injury (ischemia-reperfusion) discussed above. Hence, it is very important that these patients be transported to a facilty staffed and equipped with the ability to continue inhospital therapeutic hypothermia for at least 12-24 hours in addition to the other bundles of resuscitative care.[7]

#### **7. Conclusion**

40 Therapeutic Hypothermia in Brain Injury

arrest period may improve rate of ROSC.

experimentation.[71,72]

patient. No other major adverse events occurred. In 2011, Garrett et al performed a retrospective analysis of individuals experiencing OHCA whereby six months into the study a prehospital intraarrest TH (IATH) protocol was instituted.68 In this protocol, patients received 2000 ml of ICF directly after obtaining intravenous access. 551 patients were analysed. Rates of prehospital ROSC were 36.5% versus 26.9% (OR 1.83; 95% CI 1.19–2.81) in patients who received IATH versus normothermic resuscitation respectively. While the frequency of survival to hospital admission and discharge were increased among those receiving IATH, the differences did not reach statistical significance.The secondary analysis found a linear association between the amount of cold saline infused and the likelihood of prehospital ROSC. They concluded that the infusion of 2000 ml of ICF during the intra-

These studies are either underpowered or due to study design do not allow conclusions regarding effects on outcome to be drawn, but the safety and feasibility of early cooling was demonstrated. Another major limitation in most of these studies is that TH is not systematically continued in the post resuscitation care occurring in-hospital. Therefore, it is not possible to evaluate the benefits of pre-hospital cooling alone, as the effect of TH has

Most of the trials described above (Table 1) used LVICF for induction of TH in the prehospital setting. All the studies that used LVICF showed that this method for cooling is safe and feasible. However, LVICF may portend some potential problems. In one study on cold fluids, it was shown that chilled fluids begin to warm during transit through intravenous tubing, but the rate was not rapid enough to be deemed potentially clinically significant.69 In addition, in some instances, time to transport from the field to the emergency department may be too short for LVICF to have a significant cooling effect. In a study by Spaite et al on prehospital cardiac arrest, the time to transport from the field to the hospital was about 7 minutes.70 In the study by Bernard et al above, 52% of the patients did not receive the goal of 2 L chilled saline because the transport time to the hospital was < 20 minutes.58 EMS systems with short transport times may not benefit from prehospital TH methods, esp chilled fluids, as much as systems that need longer time to get to their respective facilities. Another cooling method, used by Castren et al was transnasal cooling, with a machine that employs evaporation of an inert liquid sprayed in the posterior nasopharynx.[60] They did show that this method of transnasal cooling was safe and effective during arrest, with a rapid onset of TH in the prehospital setting. However, it is expensive and not widely available at this point. Another method used was cooling pads by Uray et al.[63] They used prechilled cooling pads that were stored in an insulated box with a cooling battery. They were able to achieve target temperature within about 50 minutes with only mild dermal erythema, which resolved soon after removal of the pads. Storm et al used cooling caps that proved feasible and with no significant adverse events.[65] Other promising new technologies include chilled perfluorocarbons and saline/ice ''slurries'', that are still at level of animal

been shown to necessitate a cooling period of at least 12 to 24 hours.

**5. Methods for induction of prehospital therapeutic hypothermia** 

Animal and laboratory data have suggested that there is significantly decreased neurological injury if cooling is initiated as soon as possible after resuscitation. Human clinical studies are either underpowered or due to study design do not allow conclusions regarding effects on outcome to be drawn, but the safety and feasibility of early cooling was strongly demonstrated. Prehospital cooling comes with its own logistic challenges, such as limitation of EMS vehicle space, lack of ideal equipment for induction of hypothermia and for temperature monitoring, lack of credentialing for use of paralytic agents by EMS teams that are not staffed by physicians, transport to facilities that are not equipped to continue inhospital therapeutic hypothermia and postresuscitation care, the potential for overcooling and shivering, and interference with basic resuscitation efforts in the field. Intraarest and postarrest bundles of care that include therapeutic hypothermia, as well as training of EMS teams, EMS physicians, emergency room staff, cardiologists and cardiac catheterization lab staff, and intensive care unit physicians and staff on these protocols and bundles are crucial for the success of these bundles and the implementation of this important therapy, whether cooling is initiated in the field or in the hospital setting. Clearly, large prospective randomized controlled trials of prehospital therapeutic hypothermia preferably as part of a cardiac arrest bundle of care are needed.

Prehospital Therapeutic Hypothermia for Cardiac Arrest 43

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