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Hypothermia: The Cold Facts
Joseph U Becker, MD | November 21, 2023
| Contributor Information
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Image from Wikimedia Commons | Adorabutton. [Creative Commons Attribution-Share Alike 4.0 International license (CC by-SA 4.0 DEED).]
Hypothermia: The Cold Facts
Joseph U Becker, MD | November 21, 2023 | Contributor Information
In the above image, Marines participate in a study on hypothermia and rewarming.
Hypothermia, defined as a core body temperature below 35°C,[1] usually occurs because of exposure to a cold environment. Hypothermia may arise in a variety of conditions, from those found on wilderness outings to inner-city and aquatic environments. Between 2018 and 2020, the highest US mortality rates attributed to excessive cold or hypothermia were in rural, as opposed to large central metropolitan, areas, with the rate in rural areas for females and males being 0.40 per 100,000 and 0.93 per 100,000, respectively, during that period.[2]
Primary hypothermia occurs in a person with intact thermoregulation and the ability to generate heat but who is exposed to severe cold that eventually overwhelms the body's heat-generating abilities. The speed at which hypothermia develops depends on many factors, including age, presence of insulation (eg, clothing, body fat), and local conditions (eg, precipitation, wind speed).[1] Small children and the elderly are at increased risk for hypothermia.
Secondary hypothermia results from impaired thermoregulation in patients with an acute medical condition (eg, sepsis, hypothyroidism, adrenal insufficiency). Alcohol and medications (eg, salicylate overdose) can also impair thermoregulation.
Image of thermoregulation during strenuous activities from Betts JG, Young KA, Wise JA, et al. OpenStax. [Creative Commons Attribution License 4.0 license (CC by 4.0).]
Hypothermia: The Cold Facts
Joseph U Becker, MD | November 21, 2023 | Contributor Information
Heat loss occurs via the following mechanisms:
Evaporation - Vaporization of body water
Radiation - Radiation of infrared heat away from the body
Convection - Transfer of heat to currents of air or water
Conduction - Direct transfer of heat to an adjacent surface or object
Under dry conditions, most heat loss (~50-60%) occurs via radiation.[3] Convective heat loss to cold air and conductive heat loss in water are the most common mechanisms of accidental hypothermia.
Under normal physiologic conditions, the hypothalamus regulates heat through conservation (vasoconstriction and stimulation of behavioral responses) and production (elevation of the metabolic rate and activation of skeletal muscle to induce shivering).
Image from Flickr | Taco Witte (background) [Creative Commons Attribution License 2.0 Generic (CC by 2.0)] / Dreamstime (figure).
Hypothermia: The Cold Facts
Joseph U Becker, MD | November 21, 2023 | Contributor Information
Physiology
Central nervous system metabolism decreases linearly with temperature. The consequence of this decrease is that at temperatures below 33°C, cerebral metabolism is markedly reduced. Therefore, patients with hypothermia may initially manifest confusion, which can progress to delirium and coma. The decrease in cerebral metabolism associated with moderate-to-severe hypothermia may be neuroprotective in some cases, and many case reports exist of patients with severe hypothermia or hypothermic cardiac arrest who make a complete or near-complete neurologic recovery.[4]
Of note, pupils will not react to light if core body temperature is below 28°C. Thus, pupillary reaction cannot be relied on for assessment of neurologic function in hypothermia.[5]
Joseph U Becker, MD | November 21, 2023 | Contributor Information
Initially, the body's response to immersion in a cold environment involves vasoconstriction in peripheral tissues, such as the extremities, shunting blood flow to the core. As a result, the kidneys experience a sudden increase in circulating volume. Simultaneously, hypothermia also impairs renal concentration of urine. These factors contribute to an early diuresis of dilute urine in hypothermic patients, which may in turn contribute to dehydration and electrolyte abnormalities.
Another important physiologic effect of hypothermia is that it causes a leftward shift in the oxyhemoglobin disassociation curve, reducing oxygen release to the tissues. Despite reduced oxygen demand by tissues with lowered metabolic rates, overall tissue oxygen content may be inadequate as a result.
Image of medical students participating in a hypothermia hands-on training from US Department of Defense | Petty Officer 1st Class Adam Eggers [public domain].
Hypothermia: The Cold Facts
Joseph U Becker, MD | November 21, 2023 | Contributor Information
Mild hypothermia is commonly defined as a core temperature between 32°C and 35°C. In mild hypothermia, the hypothalamus attempts to reestablish thermal homeostasis by stimulating the metabolic rate and shivering, leading to tachycardia, tachypnea, and increased cardiac output. Vasoconstriction in the extremities raises the central circulating volume and thereby increases renal filtration and diuresis.[3]
In moderate hypothermia (core temperature: 28°C-32°C), shivering ceases and metabolic processes slow, resulting in reduced heat production. Patients in this stage require active rewarming because they are not themselves producing adequate heat to independently rewarm. Clinical conditions in moderate hypothermia can include mental status depression and decreasing pulse and respiratory rate. Cardiac arrhythmias (eg, atrial fibrillation [AF]) as well as loss of reflexes and muscle tone may be noted.[3]
Image of ice rescue from the US Coast Guard [public domain].
Hypothermia: The Cold Facts
Joseph U Becker, MD | November 21, 2023 | Contributor Information
Persons with severe hypothermia (core temperature: <28°C) are at risk for dysrhythmias (eg, ventricular fibrillation [VF], pulseless electrical activity [PEA], asystole). Coma or a severely depressed mental state is usually present.[3]
The Revised Swiss System (RSS) utilizes the AVPU (Alert, Verbal, Painful, Unconscious) consciousness scale, as well as the presence or absence of vital signs, to predict which patients are at elevated risk for cardiac arrest and thus require more aggressive rewarming measures such as extracorporeal membrane oxygenation (ECMO) or cardiopulmonary bypass (CPB).[6]
Image from Mine T, Sato I, Kishima H, Miyake H. J Med Case Rep. 2012;6:429. [Open access.] PMID: 23272675, PMCID: PMC3760449.
Hypothermia: The Cold Facts
Joseph U Becker, MD | November 21, 2023 | Contributor Information
Diagnosis and Laboratory Studies
The diagnosis of hypothermia must be based on accurate core temperature readings. Many thermometers in common use are unable to correctly read temperatures below 34°C; an appropriately calibrated low-temperature thermometer must be used. However, invasive temperature probes (rectal, bladder, distal esophageal) provide a closer measure of core body temperature.[3,5]
Hypothermia impairs the function of potassium channels and thus causes a slowing of impulse conduction in cardiac myocytes and a prolongation of electrocardiographic (ECG) intervals, including the PR, QRS, and QT. Osborne J waves (sudden positive deflections after the QRS waves, at the J points [leads V4-V6 of segment A, above, arrows]) are often noted, with the amplitude of the Osborne waves proportional to the degree of hypothermia.[3,5]
Cardiac pacemaker cells also experience impaired function with decreasing core temperature, initially leading to bradycardia. As hypothermia advances further, sinus bradycardia may lead to AF, possibly with a slow ventricular response; eventually to VF; and ultimately to asystole.
Electrocardiogram depicting hyperkalemia (8.2 mmol/L) from Wikimedia Commons | Drs Michael-Joseph F Agbayani and Eddieson Gonzales (Manila, Philippines). [Creative Commons Attribution 4.0 International license (CC by 4.0 DEED)].
Hypothermia: The Cold Facts
Joseph U Becker, MD | November 21, 2023 | Contributor Information
Hypothermia may cause significant laboratory abnormalities. Hypothermic patients commonly manifest metabolic acidosis (often from lactic acidosis).[5] Furthermore, hypothermia typically causes an increase in hematocrit. A decline in the serum potassium level has been noted in hypothermic individuals, but many patients may manifest hyperkalemia in the presence of comorbid conditions such as renal failure, rhabdomyolysis, or, as may occur in avalanche victims, crush injuries.[7,8]
Pancreatitis is common in hypothermic patients and may contribute to hypoinsulinemia and hyperglycemia.[9]
Arterial blood gas (ABG) values change with hypothermia as well; hydrogen ion concentration, arterial carbon dioxide tension (PaCO2), and arterial oxygen tension (PaO2) all decline.
For any hypothermic patient, it is important to be aware of comorbid or concomitant conditions such as sepsis, intoxication, or metabolic/endocrine derangements (eg, diabetes/diabetic ketoacidosis, adrenal insufficiency, or hypothyroidism) and to direct diagnostic testing accordingly.
Image of navy personnel undergoing wet-clothing hypothermia Basic Underwater Demolition/Seal (BUD/S) training from US Navy, Naval Health Research Center [public domain].
Hypothermia: The Cold Facts
Joseph U Becker, MD | November 21, 2023 | Contributor Information
Clinical Management
The management of mild hypothermia in patients who are conscious and alert, with intact shivering, is straightforward and involves removing the patient from the cold environment, removing wet clothing, and providing insulation in the form of warmed blankets or heat-reflective barrier blankets. In neurologically intact patients who meet the criteria for mild hypothermia, a hot, sweet, nonalcoholic drink can be provided. While this will not raise the patient's core temperature, the carbohydrates will fuel shivering.
Although removal of wet clothes is desirable, it should not be performed in a cold or windy environment. If the patient is still in the environment that caused hypothermia, the individual should be wrapped in a sealed vapor barrier, and an external heat source (eg, forced–hot-air blankets, chemical heat packs, warm water bottles) should be applied. To minimize the risk of burns, do not apply heat sources directly to the skin.[5] Warmed intravenous (IV) fluids may be considered.
Images of warming blankets for localized rewarming from Costanzo S, Cusumano A, Giaconia C, Mazzacane S. Biomed Res Int. 2014;2014:136407. [Open access.] PMID: 25485278, PMCID: PMC4251640.
Hypothermia: The Cold Facts
Joseph U Becker, MD | November 21, 2023 | Contributor Information
Patients with mild-to-moderate hypothermia who cannot generate their own heat (ie, those who are not shivering and may have deranged vasoregulation) require active rewarming, which involves actively returning heat to the patient instead of simply preventing further heat loss. This may be accomplished via different approaches, including IV infusion of warmed (40°C-42°C) fluids and the use of forced–hot-air blankets.
Pleural lavage of warmed fluids is effective in elevating core body temperature in moderate hypothermia. Warmed normal saline or lactated Ringer solution is cycled through the thoracic cavity via thoracostomy tubes. Endovascular rewarming probes are another active rewarming technology and are increasingly being employed in patients with moderate or severe hypothermia.[10]
Adequate oxygenation is critical for preventing cardiac arrest; thus, warmed supplemental oxygen should be provided.[8]
Image from Wikimedia Commons | Blausen Medical Communications, Inc. [Creative Commons Attribution 3.0 Unported License (CC by 3.0 DEED).]
Hypothermia: The Cold Facts
Joseph U Becker, MD | November 21, 2023 | Contributor Information
Patients with severe hypothermia should be kept immobile, and unnecessary invasive procedures or maneuvers should be limited; movement at core temperatures lower than 30°C can incite dysrhythmias or cardiac arrest.[3,8] Hemodialysis, CPB (shown), or ECMO is appropriate for these patients and has been shown to be effective.[11] Patients with moderate-to-severe hypothermia and respiratory distress (as in near drowning) or severely depressed mental status (either from hypothermia or related injury or intoxication) may require endotracheal intubation.
In the process of rewarming, the following effects may be noted and should be taken into account:
Afterdrop phenomenon - As the periphery is warmed (via forced–hot-air blankets or other methods that unselectively rewarm the core and the periphery simultaneously), cold blood is returned to the core, contributing to a potential initial decrease in core body temperature; accordingly, in moderate-to-severe hypothermia, rewarming efforts should emphasize core rewarming over peripheral rewarming.[11]
Volume depletion - Peripheral vasodilation from peripheral rewarming as well as relative volume depletion from renal cold diuresis may lead to initial hypovolemia and possibly shock. During rewarming, hypothermic patients may require volume support in the form of boluses of crystalloid to support perfusion.
Rewarming acidosis - This effect, analogous to the afterdrop phenomenon, occurs as lactic acid is returned to the core from the periphery during peripheral rewarming.
Image of military medical personnel connecting a patient to an ECMO system from US Air Force | Staff Sgt Kevin Iinuma [public domain].
Hypothermia: The Cold Facts
Joseph U Becker, MD | November 21, 2023 | Contributor Information
Patients with cardiovascular instability should undergo rapid core rewarming. A fall in core body temperature to below 32°C places the patient at risk for cardiac arrest. The risk significantly increases at temperatures below 28°C.[3,8] Resuscitation efforts should focus on high-quality cardiopulmonary resuscitation (CPR) and the provision of rapid rewarming.[12]
Patients in cardiac arrest because of hypothermia should be considered candidates for extracorporeal life support (ECLS). ECLS is effective because it provides continuous oxygenation and hemodynamic support while also enabling rewarming.[8] Patients who present to care with a core body temperature lower than 28°C, dysrhythmia, or hypotension should be transported to an ECLS-capable center.[11]
Admission x-ray and computed tomography (CT) scan showing suspected aspiration of a large amount of water in an adult found in 12°C seawater with a core temperature of 22°C (rectal) from Sawamoto K, Tanno K, Takeyama Y, Asai Y. Int J Emerg Med. 2012;5(1):9. [Open access.] PMID: 22300441; PMCID: PMC3287102.
Hypothermia: The Cold Facts
Joseph U Becker, MD | November 21, 2023 | Contributor Information
Hypothermic, asystolic patients should be rewarmed before being pronounced dead. This is true, of course, only when it may be assumed that asystole occurred as a result of hypothermia and not another associated or unrelated condition, such as trauma or asphyxia. Consider resuscitation until rewarming achieves a core temperature of 32°C-35°C.[3] Patients who have been effectively rewarmed and who have received appropriate and quality resuscitative interventions at this point may be assumed to be irretrievable.
Factors associated with poor outcomes in hypothermia include evidence of intravascular coagulation in tandem with deficient fibrinogen levels, as well as extreme lactic acidosis (>20 mmol/L). Cardiac arrest owing to another process (eg, asphyxia or trauma, as in an avalanche burial or drowning) is also associated with poor outcomes.[4]
Joseph U Becker, MD | November 21, 2023 | Contributor Information
Hypothermia in Children
Accidental hypothermia can also occur in children and is managed similarly to that in adults. Children are at increased risk for hypothermia due to having a greater body surface area-to-mass ratio than adults, as well as having a higher metabolic rate. Infants are at even more heightened risk because of their limited fat stores and their inability to shiver or move out of a cold environment.[13]
Rapid rewarming is indicated in mild hypothermia, and the advanced cardiovascular life support (ACLS) protocol is indicated in cardiac arrest. Pediatric patients in cardiac arrest secondary to hypothermia warrant expert consultation prior to cessation of resuscitative efforts, and they likely should be transferred to an ECLS center, if one is available.[13]
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Contributor Information
Author
Joseph U Becker, MD
Associate Professor, Department of Emergency Medicine
Stanford University School of Medicine
Palo Alto, CA
Disclosure: Joseph U Becker, MD, has disclosed no relevant financial relationships.
Editor
Olivia Wong, DO
Section Editor
Medscape Drugs & Diseases
New York, NY
Disclosure: Olivia Wong, DO, has disclosed no relevant financial relationships.
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References
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QuickStats: Death Rates* Attributed to Excessive Cold or Hypothermia,† by Urbanization Level§ and Sex - National Vital Statistics System, 2018-2020. MMWR Morb Mortal Wkly Rep. 2022 Feb 18;71(7):282. PMID: 35176009 [Full text].
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Debaty G, Moustapha I, Bouzat P, et al. Outcome after severe accidental hypothermia in the French Alps: a 10-year review. Resuscitation. 2015 Aug;93:118-23. PMID: 26095302
Paal P, Gordon L, Strapazzon G, et al. Accidental hypothermia-an update: The content of this review is endorsed by the International Commission for Mountain Emergency Medicine (ICAR MEDCOM). Scand J Trauma Resusc Emerg Med. 2016 Sep 15;24(1):111. PMID: 27633781
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Inoue H, Uemura S, Harada K, et al. Risk factors for acute pancreatitis in patients with accidental hypothermia. Am J Emerg Med. 2019 Feb;37(2):189-193. PMID: 29764735
Laniewicz M, Lyn-Kew K, Silbergleit R. Rapid endovascular warming for profound hypothermia. Ann Emerg Med. 2008 Feb;51(2):160-163. PMID: 17681640
Durrer B, Brugger H, Syme D, for the International Commission for Mountain Emergency Medicine. The medical on-site treatment of hypothermia: ICAR-MEDCOM recommendation. High Alt Med Biol. 2003 Spring;4(1):99-103. PMID: 12713717
Mair P, Gasteiger L, Mair B, Stroehle M, Walpoth B. Successful defibrillation of four hypothermic patients with witnessed cardiac arrest. High Alt Med Biol. 2019 Mar;20(1):71-77. PMID: 30801202
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