Air Care Series: Accidental Hypothermia
/Accidental hypothermia is defined as an involuntary decrease in core body temperature below 35°C (95°F). Primary hypothermia results from exposure to a cold environment that overcomes the body’s capacity for heat production via mechanisms such as exercise or shivering. Secondary hypothermia occurs due to an underlying medical condition, such as hypothyroidism or hypoglycemia, and can occur even in a warm environment.
Hypothermia is classified on the basis of core temperature as mild (35°C-32°C), moderate (32°C-28°C), severe (28°C-24°C), or profound (<24°C). It can also be staged clinically using the Swiss staging system (Table 1), particularly in settings where accurate measurement of core temperature is impractical or impossible.
Prehospital Management
The absolute priority in the prehospital management of a hypothermic patient is the safety of the rescuers. Once a scene is established as safe, or the patient has been moved to a safe location, the patient should be assessed for the presence or absence of a pulse. Because hypothermic patients may be profoundly bradycardic and peripherally vasoconstricted, a pulse check should be performed for up to 60 seconds and ultrasound, if available, should be used to assess for organized cardiac activity. If the patient has a pulse, management should prioritize prevention of further heat loss, safe rewarming that minimizes potential core after-drop and attention to techniques that decrease the likelihood of peri-rescue cardiovascular collapse.
Hypothermia decreases the threshold for ventricular fibrillation, and lethal dysrhythmias may be triggered by even minor patient movement or jostling; this is particularly true at core temperatures less than 28°C. Accordingly, hypothermic patients should be handled as gently as possible to minimize the risk of inducing dysrhythmia. Changes in patient position during rescue may also precipitate cardiac arrest as a result of decreased venous return, termed rescue collapse. Patients may experience a continued drop in core temperature following removal from a cold environment, called afterdrop, as perfusion to cooler peripheral tissues results in the return of cold blood to the heart. Afterdrop can occur due to self-rewarming with patient exertion and can be clinically significant in patients who are at the threshold for cardiovascular instability. Current Wilderness Medical Society (WMS) guidelines recommend limiting patient effort during rescue to minimize afterdrop and keeping patients in a horizontal position to minimize the chance of rescue collapse. (1)
Figure 1: Hypothermia Burrito. (1-3) Layered Sleeping bags (4) Padding (5) Tarp
Preventing further heat loss is also crucial to the prehospital management of hypothermia, and primarily involves insulating a patient from cold, moisture, and wind. Removal of wet clothing in a cold or windy environment can accelerate heat loss, and clothing should instead be left in place with the patient wrapped in a vapor barrier (e.g. tarp, reflective blanket, garbage bag) to prevent conductive and evaporative cooling. The patient should then be wrapped in dry, insulating materials; such as a sleeping bag, blankets, or extra clothing; followed by an outer layer that is ideally both wind- and waterproof (Figure 1). In the context of HEMS transport, a wind barrier is especially important to protect the patient from rotor wash while loading and unloading.
Patients who are alert and shivering (HT I) may be treated in the field if they are otherwise uninjured; the notable exception is patients with secondary hypothermia, who typically warrant transport for evaluation and management of their underlying illness. Rewarming should occur in a warm environment to ensure the patient is able to retain the heat generated, and wet clothing should be removed and replaced to prevent ongoing heat loss. The mainstay of therapy for HT I patients is self-rewarming via shivering and active movement. Shivering is a highly effective means of heat production but requires considerable energy to sustain. HT I patients who are not at risk for aspiration should be given warm, sugary, non-alcoholic drinks and/or high-carbohydrate foods to provide calories to support shivering. Physical exertion also raises core temperature, although current WMS guidelines recommend deferring exercise for 30 minutes after rescue to limit afterdrop (1). Self-rewarming can be augmented by the application of external heat sources, such as large electric or chemical heat packs or blankets, to a patient’s axillae, chest, and back. Heat packs should never be applied directly to the skin due to the risk of thermal injury, and small heat packs (e.g. hand/foot warmers) should be avoided as they are ineffective in raising core temperature and carry an increased risk of burns.
Altered patients (HT II - III) require evaluation at a hospital, with destination choice dependent on their hemodynamic stability. For patients with SBP >90, no cardiac dysrhythmias, and core temperature >28°C, transport to the closest facility is typically appropriate. Patients who are hemodynamically unstable or have a core temperature <28°C should be transported to an ECMO center if possible. The optimal cabin temperature during transport is at least 28°C, though temperatures >24°C are acceptable and may be better tolerated by crew. Active external rewarming should be initiated with large heat packs or warming blankets applied as above, and the patient should be wrapped in insulating materials with their head (but not face) covered. Interventions such as IV/IO access and advanced airway management should be performed if indicated, and do not require modification in the setting of hypothermia. Hypothermic patients often require volume resuscitation during rewarming, as decreased ADH production can result in a cold-related diuresis and vasodilation naturally accompanies rewarming. IV fluids should be warmed to 40°C-42°C. (1)
In-Hospital Rewarming
In the hospital setting, HT II and hemodynamically stable HT III patients should undergo active external rewarming, with forced-air surface rewarming (e.g. via a Bair Hugger device) as the first-line option. Forced-air rewarming is effective in raising core temperature (2) with comparable patient outcomes relative to more invasive techniques (3) and lower potential for complications. Use of the Arctic Sun temperature management system has also been described, though supporting literature is limited to case reports (4, 5). There is little evidence to suggest minimally-invasive rewarming techniques (e.g. warmed IV fluids, warmed humidified oxygen, bladder lavage) are effective in raising core temperature (6-8), however these can be considered as adjunctive rewarming options given their low associated risks.
Active external rewarming should also be initiated for unstable HT III patients, however more aggressive management with invasive internal or extracorporeal rewarming is also warranted. Veno-arterial extracorporeal membrane oxygenation (VA-ECMO) is the treatment of choice for patients with hemodynamic instability in the setting of deep hypothermia, with marked benefits in both morbidity and mortality relative to conventional rewarming strategies. Morita et al. found an 84.4% survival rate among severely hypothermic patients treated with VA-ECMO vs. 56.6% among patients rewarmed with forced air, inhalation, and body-cavity lavage (p<0.05), with significantly better neurologic outcomes in the VA-ECMO group (9). Extracorporeal rewarming should be strongly considered for HT III patients with cardiac dysrhythmias, hypotension, respiratory distress or failure, refractory acidosis, core temperature <28°C, or failure to improve with less invasive methods (10).
In cases where ECMO is unavailable or is contraindicated (e.g. in a patient who is unable to be anticoagulated), alternatives such as thoracic-cavity lavage or endovascular rewarming can be considered, though evidence for these interventions is less robust. Warm-water pleural lavage via tube thoracostomies can be rapidly implemented in an emergency department setting, and good anecdotal outcomes have been reported (11). Endovascular rewarming (e.g. via the Alsius CoolGard system) has been used with modest success in hypothermic trauma patients (12), though overall outcomes are comparable to those achieved with less-invasive methods (3).
Cardiac Arrest Management
Accidental Hypothermia resulting in cardiac arrest is a unique clinical entity with management and prognosis that differ from normothermic arrest. Field determination of death in a hypothermic patient can be extremely challenging. Exam findings such as fixed and dilated pupils, dependent lividity, and rigor mortis are not reliable indicators of death in the setting of profound hypothermia, and patients have survived with good neurologic outcomes despite prolonged low- or no-flow times. Given this, resuscitation should be attempted in any pulseless hypothermic patient, regardless of core temperature, unless the patient has injuries that are obviously incompatible with life or is frozen with an incompressible chest wall. CPR should be started immediately, ideally with a mechanical CPR device as prolonged resuscitation may be required -- case reports describe neurologically intact survival following >3.5 hours of continuous CPR (13, 14). If scene factors or rescue considerations make immediate application of CPR unsafe or impossible, rescue may still be attempted, as patients have survived neurologically intact despite significant delays in CPR initiation (15) and receiving intermittent compressions during rescue (Boue). Airway management and IV/IO access should be performed as in normothermic patients, however recommendations regarding defibrillation and vasoactive medication administration vary. Current American Heart Association guidelines state it “may be reasonable” to follow the standard BLS and ACLS algorithms (16). In contrast, the European Resuscitation Council guidelines recommend no more than three attempts at defibrillation and withholding vasoactive medications entirely while a patient’s core temperature is below 30°C. Once the patient has been rewarmed to 30°C, standard defibrillation attempts can be resumed, however the dosing interval for medications should be doubled until the patient reaches a core temperature of 35°C. (17) The 2019 Wilderness Medical Society guidelines mirror the ERC guidelines regarding medication administration, but recommend only one attempt at defibrillation while core temperature is <30°C (1).
The optimal strategy for rewarming in hypothermic cardiac arrest is VA-ECMO, and HT IV patients should be transported to an ECMO center whenever possible. Morita et al. reported an 83.3% survival rate for cardiac arrest patients warmed with VA-ECMO, compared to 14.3% in patients who underwent conventional rewarming (p<0.05), with better neurologic outcomes in the ECMO cohort (9). Cardiopulmonary bypass (CPB) has also been used successfully in cases of hypothermic arrest, however ECMO confers a significant survival benefit relative to CPB (18) and is preferred when available. If extracorporeal rewarming is unavailable or contraindicated, thoracic-cavity lavage should be strongly considered (19).
Despite the relatively improved prognosis in hypothermic versus normothermic cardiac arrest, aggressive resuscitation is still likely to be futile in some hypothermic patients. Pasquier et al. identified six independent predictors of survival to hospital discharge in hypothermic arrest: age, gender, serum potassium, core temperature at admission, mechanism of cooling, and duration of resuscitation (20). These variables were incorporated into the HOPE score (www.hypothermiascore.org), which gives the probability of survival in a patient undergoing extracorporeal rewarming. Though not a substitute for clinical judgement, the HOPE score can be used to guide termination of resuscitation decisions; a score <10% has a negative predictive value of 97% for survival to hospital discharge (21).
Conclusion
Accidental hypothermia may occur as a result of environmental exposure or an underlying medical condition, and ranges in severity from mild to profound. The safety of responders is paramount during rescue of a hypothermic patient, and management priorities should include gentle handling with avoidance of rescue collapse, afterdrop, and ongoing heat loss. Patients with HT I can typically be treated in the field with self-rewarming, calories to support shivering, and external rewarming with hot packs or heated blankets. Stable HT II - III patients warrant hospital evaluation with active external rewarming as the mainstay of therapy. Extracorporeal rewarming should be strongly considered for HT II - III patients with hemodynamic instability, core temperature <28°C, or who fail to improve with less invasive measures. Resuscitation should be attempted for all patients in hypothermic cardiac arrest, with the exception of those who are frozen solid or have injuries obviously incompatible with life, as good neurologic survival is possible even with prolonged down times. VA-ECMO is the rewarming strategy of choice for patients in hypothermic arrest, and the HOPE score can be used to guide resuscitation decisions in such patients.
AUTHORED BY Katherine Connelly (@kmconnel78)
Dr. Connelly is a third-year Emergency Medicine resident at the University of Cincinnati interested in EMS.
Post by ADAM GOTTULA, MD (@LAERTEZZ)
Dr. Gottula is a fourth-year Emergency Medicine resident at the University of Cincinnati and future Anesthesia Critical Care Fellow at The University of Michigan with an interest in critical care and HEMS.
FACULTY EDITORS Conal Roche, MD
Dr. Roche is a faculty member in The Department of Emergency Medicine who is fellowship trained in wilderness medicine.
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