Wilderness medicine entails the treatment of exposure-related maladies in the middle of nowhere. Well, not always. While this blossoming field of emergency medicine focuses on preparedness and the ability to treat all types of medical problems in austere locations, bona fide wilderness medicine cases can pop up on your urban doorstep. Whether you live in the jungle of New York City or the middle of true Montana backcountry, you can expect to treat a range of patients with “wilderness issues,” including hypothermia. It is extremely important to know how to manage hypothermia, both in remote locations and within the warm walls of your major tertiary hospital.
A 4-year-old male is rushed to a rural emergency department on a cold April day. He fell out of a fishing boat and was in the lake for quite a while before eventual submersion. Vitals on presentation are a BP 128/83, HR 57, RR 8, SpO2 75% on non-rebreather at 15 L/min, and a temperature of 24.3oC. He is pale and cool to the touch, and begins to seize. An intraosseous (IO) line is placed and lorazepam is given. He is intubated, and a foley, NG tube, and rectal temperature probe are placed. Warm packs are placed at the axilla and groin, and he is covered with a warming blanket while invasive rewarming is initiated with warmed IV fluids and stomach and bladder irrigation.
There are two stages in the treatment of the hypothermic patient – pre-hospital management and management in the ED.
Imaging workup reveals only findings consistent with water aspiration. After discussion with an accepting tertiary hospital, it is decided to transfer the patient before full rewarming is complete. Just prior to loading the patient on the helicopter, his HR is 116, and he is still hypothermic at 26.8oC.
In developed countries, hypothermia is more common in cities than in the wilderness, likely a function of population and homelessness.1,2 Regardless of the environment, the basic treatment of hypothermia patients is pretty logical – warm them up. The body doesn't like being cold. Hypothermia causes physiologic changes in the respiratory, renal, and CNS systems; however, effects on the heart become the most worrisome.
As the conduction system cools down, a decrease in the spontaneous depolarization of the pacemaker cells causes bradycardia. Conduction velocity decreases, leading to lengthening of the cardiac cycle and eventually ventricular tachycardia, fibrillation, and asystole. A decreased transmembrane resting potential puts hypothermic patients at high risk for dysrhythmia, even from minor stressors like being jostled during transport. The goal is to warm patients up at 0.5-2.0oC/hour or, if in a serious dysrhythmia, as fast as humanly possible. Dysrhythmias become much less likely once a temperature greater than 30-32oC is reached.2
There are two stages in the treatment of the hypothermic patient – pre-hospital management and management in the ED. While active field rewarming is difficult simply due to a lack of heating sources, several steps should be considered to improve outcomes. Further heat loss should be minimized by getting the victim into a warmer environment (ambulance/helicopter) and, if possible, cutting off wet clothing and replacing it with dry clothing, or “wrapping” the patient in an insulation system. To “wrap” a victim, a large tarp or plastic sheet is laid on the ground with a sleeping mat in the middle of it. A dry sleeping bag or blankets are then placed on the sleeping mat with the patient on top (Image 1). Warm water bottles can be placed in the groin or axilla before “wrapping” up the patient, layer after layer.
Prior to full wrapping, it is important to assess the victim for signs of trauma or other medical issues. Fractures should be splinted and pressure dressings applied to wounds.1 Intravenous access can be obtained and a 500-ml bolus of warmed 5% dextrose in normal saline administered, since most patients are volume-depleted, secondary to cold diuresis. The patient should be kept supine to avoid orthostatic hypotension. As previously mentioned, these interventions should be gently completed, as vigorous movement can provoke ventricular fibrillation and asystole.1,2
Once in the ED, patients should be hooked up to cardiac monitoring, and an esophageal or rectal temperature probe placed, if feasible. Rewarming options suited for the hospital environment abound, and range from external to very invasive techniques (Table 1). Monotherapy with passive external rewarming (PER), such as blankets, is used in only the mildest hypothermia cases (>32oC), when the concern for dysrhythmia is low. Shivering thermogenesis disappears at around 30-32oC, making PER much less efficacious, necessitating the need for active rewarming.
It is extremely important to know how to manage hypothermia, both in remote locations and within the warm walls of your major tertiary hospital.
Active external rewarming (AER) involves techniques in which heat is delivered to the skin. These modalities are best used in young, healthy patients who have a temperature of <32oC, but who do not have a serious arrhythmia.1 The most common methods include hot water bottles or warmed saline bags placed in the axilla and groin, warm blankets, hot water bath immersion, and forced-air rewarming blankets (e.g., Bair Hugger). Rewarming blankets have been shown to provide significant heat transfer and are a common piece of equipment in many EDs.1 Warm water immersion is an alternative option but comes with many pitfalls and poses difficulties for cardiac monitoring, not to mention it makes CPR nearly impossible.
In sicker, colder patients, or those with serious dysrhythmia, active core rewarming (ACR) should be pursued. ACR methods vary greatly in degree of invasiveness and, although many are used concomitantly, care should progress from least to most invasive, based on the patient's condition. The two most commonly used and least invasive maneuvers are heated humidified air inhalation and heated IV fluids. Inhaled air can be given via mask or endotracheal tube, but should be humidified and heated to 40-45oC. Whether blood or crystalloid, all IVF used in hypothermia resuscitation should be heated to 40-42oC and infused through short or insulated IV tubing. Both of these procedures provide significant heat transfer and are important methods of less invasive rewarming.1
Progressing beyond air and IV fluids, ACR becomes much more invasive, including gastrointestinal irrigation, peritoneal lavage, thoracic lavage via thoracostomy tubes, and mediastinal irrigation. Extracorporeal blood rewarming can be accomplished with venovenous rewarming, continuous arteriovenous rewarming, cardiopulmonary bypass, or hemodialysis, which can be especially beneficial in the setting of concurrent renal failure or severe electrolyte abnormalities.1,2 Full description of these procedures is beyond the scope of this article but are described in detail in the cited sources.1-3
Dysrhythmias are common in severe hypothermia, and usually progressively deteriorate from bradycardia to atrial fibrillation, and then to ventricular dysrhythmias. Hypothermic bradycardia is resistant to atropine, and transvenous pacing can precipitate ventricular fibrillation.2 Bradycardia and atrial fibrillation are generally innocuous and usually resolve with rewarming alone, but can serve as a marker of severity of hypothermia. As in any other scenario, CPR should be started for pulseless VT, fibrillation, or asystole. Most ACLS medications have temperature-dependent effectiveness and should be considered to be ineffective until core temperature reaches 30oC, although IV magnesium sulfate may be of some benefit.1,2 Defibrillation can be attempted once, but the mainstay of treatment is CPR while rewarming continues. Once 30oC is reached, standard ACLS protocols can be resumed.2 Stories of amazing hypothermia resuscitations are out there – the lowest recorded temperature in accidental hypothermia with survival was a frigid 13.7oC.1 Until an accurate marker of death can be established, warm up your patient and remember the old adage: No one is dead until they are warm and dead.
Our 4-year-old patient had a brief episode of ventricular fibrillation requiring CPR while en route to the tertiary hospital, likely the result of his temperature of 26.8oC at time of transfer, coupled with a bumpy ride. Should the transfer have been delayed until the patient was warmed to a temperature above 30oC? It's a tough call. The treating physicians felt the risk of transfer was countered by the need to get him to a tertiary hospital. Thankfully, his arrhythmia resolved and he was discharged home a week later without significant neurologic impairment.
Table 1. Re-Warming Modalities
- Passive External Rewarming (PER)
- Dry clothing and blankets
- Active External Rewarming (AER)
- Radiant heat lamps
- Warm water bottles/packs
- Warm blankets
- Hot water immersion
- Forced-air rewarming
- Active Core Rewarming (ACR)
- Heated intravenous fluids
- Heated humidified oxygen
- Gastric, bladder, colonic lavage with heated fluids
- Mediastinal lavage
- Peritoneal lavage
- Thoracic lavage
- Venovenous rewarming
- Arteriovenous rewarming
- Cardiopulmonary bypass
- Danzl, DF: “Accidental Hypothermia,” in Wilderness Medicine, 5th Ed, Auerbach, PS, p 125, Mosby Elsevier, Philadelphia, 2007.
- Danzl, DF: “Accidental Hypothermia,” in Rosen's Emergency Medicine – Concepts and Clinical Practice, 7th Ed, Marx JA, Hockberger RS, Walls RM, et al (eds), p 1868, Mosby Elsevier, Philadelphia, 2010.
- Prendergast HM, Erickson TB: “Procedures Pertaining to Hypothermia and Hyperthermia,” in Roberts & Hedges' Clinical Procedures in Emergency Medicine, 6th Ed, Roberts JR, Custalow CB, Thomsen TW, et al (eds), p 1363, Elsevier Saunders, Philadelphia, 2014.