Diagnostics: Accidental Hypothermia

Exposure to cold weather occurs even in temperate areas, predominantly affecting the elderly, those experiencing homelessness or substance use disorders, and outdoor adventurers. Management of cold weather exposure is dependent on the  core body temperature of the affected person . We will discuss the definitions of hypothermia and the temperature dependent implication on evaluation, diagnosis and management of cold weather-related injuries. 

Classification and staging  

Hypothermia is defined as a core body temperature below <35°C (<95°F). A temperature outside of the typical physiologic range but greater than 35°C (95°F) may be referred to as cold-stressed. 

Stage	Temperature	Clinical Presentation
Cold-stressed	>35°C (>95°F)	Normal mentation - Normal functioning - Shivering Response - Able to care for self
Mild hypothermia	35°C to 32°C (95°F to 92°F)	Progressive confusion/lethargy - Shivering response - Tachypnea, tachycardia
Moderate hypothermia	32°C to 28°C (90°F to 82°F)	Altered mentation leading to unconsciousness - Bradypnea - Bradycardia
Severe hypothermia	<28°C (<90°F)	Unresponsive - Possible cardiovascular compromise with hypotension and/or shock - Dysrhythmias, such as ventricular fibrillation

Table 1: Stages of Hypothermia and associated clinical presentation

While a preceding history of cold exposure may be evident in patients presenting with cold-related injury and illness, there may be clinical scenarios in which the history may be more subtle. This may include patients for whom exposure occurred not due to environmental, but other routes such as workplace related exposures, such as food service.

Evaluation: 

Once a cold-related injury is suspected an accurate measurement of temperature should be performed as there are temperature dependent implications for rewarming. As a general rule, central/core temperature (foley, esophageal, rectal) is preferred given accuracy as opposed to a peripheral (skin, oral) temperature. 

The gold standard for core temperature measurement includes an esophageal probe, given it has the highest degree of accuracy. This method is generally reserved for moderate or severe hypothermia given it requires a patient to be intubated. Other central measures, such as rectal and bladder may be obtained via thermometer or temperature sensing foley. External peripheral measurements via oral or forehead sensing thermometers are not routinely recommended for suspicion of hypothermia given they are not as accurate and can not be transduced continuously to evaluate for clinical improvement, however, in cases of mild hypothermia they may be the preferred method given patient tolerance. When accurate temperature measurement is unavailable or unable to be obtained, clinical staging may be used as progression from mild to severe hypothermia is marked by a change in mental status, i.e. shivering → loss of shivering → decreased alertness → unconsciousness

Due to poor peripheral perfusion, difficulty with pulse oximetry may also be encountered. Trialing several different and more central locations including the forehead, nasal ala and earlobe may be required.

In severe cases, care should be taken during the initial evaluation as the hypothermic heart is very sensitive to movement, and jostling may precipitate arrhythmias, including ventricular fibrillation, that can be unresponsive to defibrillation and medications until rewarming is achieved.

Finally, evaluation for concurrent conditions such as trauma, intoxication, or precipitating medical conditions is incredibly important, particularly in cases where the patient is unable to provide their own history. Some clinical clues include tachycardia in hypothermia, unless it is the initial stage of cooling, as the expected response below 35°C is bradycardia; for example: at 33°C, a normal heart rate is typically 45-55 beats per minute. 

On electrocardiogram evaluation, there may be Osborn waves, also known as J waves. These are distinct, dome-shaped deflections at the end of the QRS complex (the J point) on the ECG. They are not unique to hypothermia and may also be seen in various conditions such as Brugada syndrome, hypercalcemia, and after cardiac arrest.

Management: 

The management of cold-related injuries will vary based on the degree of hypothermia. Passive external rewarming may be utilized in all patients and only requirement management in mild hypothermia, whereas, patients with severe hypothermia may require a great deal of resources and invasive interventions. See the drop-downs below for further details methods of rewarming. 

• External Rewarming

  Passive external rewarming 

  • Remove wet articles of clothing 

  • Put patient in warm environment (28°C/82°F)

  • Insulate with blankets and other warm materials 

  Active external rewarming 

  • Use external heat: forced air warming systems (Bair Hugger), heating pads, heated blankets, etc. (make sure to check for burns with heating pads)

  • Rewarm torso first to avoid core temperature afterdrop (ex: cover torso but leave extremities uncovered)

• Internal Rewarming

  Active internal rewarming

     Basic:

  • Warmed isotonic crystalloid (normal saline) to 42°C/107°F

  • Warmed and humidified oxygen 

     Advanced

  • Irrigation of pleural/peritoneal cavities with warmed isotonic crystalloid (40-42°C)

  • Endovascular temperature control catheter 

  • Extracorporeal membrane oxygenation

Mild Hypothermia 35°C - 32°C (95°F - 89.6°F): 

Shivering may effectively produce heat if patients are insulated from elements

• Start with passive external warming, may use active external warming to increase patient comfort and decrease metabolic demands 

• Passive external warming requires shivering, and if patients aren't able to appropriately meet that metabolic demand (sepsis, glycogen depletion, hypovolemia, elderly), then we must supplement

• If elderly, malnourished, or history of cardiovascular disease, treat as moderate hypothermia with active external warming  

• Monitor core temperature for a goal of 0.5 - 2°C/hr with passive external warming  

• If not achieving sufficient rewarming, proceed to next tier of intervention 

• Treat any concurrent hypoglycemia

• If no aspiration risk, patients may benefit from warmed, high-calorie liquids or food - ensure the patient remains seated, and then observe

Moderate hypothermia 32°C - 28°C (89.6°F - 85°F):

In addition to mild hypothermia interventions, treat with active external warming and basic active internal warming   

• Goal temperature increase of 2°C/hr with active external rewarming 

• Keep patients horizontal to avoid temperature afterdrop, however it’s less likely with active external rewarming

• Temperature afterdrop occurs when cold, acidemic blood from peripheries returns to the central circulation > drop in temp and pH

• Give warmed IV fluids, preferably normal saline, warmed to 40-42°C (104-107.6°F)

• For 3 main purposes: 

1. Combatting cold diuresis

2. Inadequate fluid intake 

3. Reduced circulating blood volume due to extravascular plasma shift

• There can be a ⅓ reduction in effective circulating volume in moderate/severe hypothermia, so give those fluids! 

• Normal saline > lactated ringers here! This is due to the markedly impaired ability of the hypothermic liver to process lactate > lactate build up

• Intubation: medications for induction and paralytics may have delayed onset > use standard dose & monitor for delayed symptoms for a core temp > 30°C, however once the core temperature is <30°C, you may half the dose of intubation meds, but may need to redose 

• Cold-induced trismus may occur > fiberoptic intubation 

• Choose suctioning and rewarming over atropine and glycopyrrolate for bronchorrhea (decreased effectivity due to lower temps) 

• For ventilated patients, you want 100% air humidification and a circuit heater set to 40-45°C. You may also give humidified air via CPAP.

• Atrial arrhythmias tend to resolve spontaneously during rewarming  

Severe Hypothermia <28°C (82.4°F)

In addition to above interventions, we’re now pursuing advanced active internal rewarming. 

• If hemodynamically stable:

• Endovascular temperature control catheter

• Pleural lavage (if patient needs a chest tube, can do this at earlier stage) with isotonic saline at 40 to 42°C

• NO role for gastric, colonic, or bladder irrigation due to fluid and electrolyte shifts

• Can pursue ECLS (Extra-Corporeal Life Support) without hemodynamic instability, especially when other methods are inadequate, if they have frozen extremities, or for rhabdomyolysis with severe hyperkalemia  

• If hemodynamically unstable, immediately activate ECLS (e.g. ECMO, bypass) 

• If no ECLS available within 6 hrs (including transfer) > pursue other active invasive rewarming (AIR) measures 

• Intubation: if core temp <30°C (85°F), you may half the dose of intubation meds, but may need to redose

• Modified ACLS

Modification of ACLS in hypothermia - yes, it’s different!  

• 1 minute central pulse checks 

• Use continuous-wave doppler, bedside echocardiogram (TTE), or transesophageal echocardiogram (TEE) if you have it available 

• Check for breathing for up to 1 full minute

• Notably, there is a lack of consensus of the number of doses of medications and the number of shocks for ACLS in severe hypothermia due to lack of evidence. 

• Hold all meds and trial a single defibrillation until warmed to 30°C (ACLS and Wilderness Medical Society guidelines) 

• Hypothermia has decreased effects and metabolism of vasoactive & anti-arrhythmic meds 

• Large total doses of epinephrine from repeat dosing in ACLS are associated with increased mortality and local tissue injury 

• When patient rewarmed > 30°C (86°F), may give 1mg IV epinephrine but double the interval in between doses (every 6-10 minutes)

• At core temperatures ≥35°C (95°F) standard ACLS protocols should be followed with respect to defibrillation and medication dosing

• Bradycardia management

• Cardiac pacing generally not required unless there is hypotension or bradycardia persist despite rewarming to 32 to 35°C

• Transcutaneous pacing less hazardous than transvenous pacing

• Do not terminate resuscitation for low EtCO2 (<10 mmHg) as it may reflect a low metabolic rate rather than poor perfusion 

• Traditional signs of futility do not apply (fixed and dilated pupils, rigor mortis)

Contraindications to chest compressions in hypothermia

• Cardiac contractions are seen on TTE or TEE 

• Pulse detected with blood pressure monitoring or ultrasound 

• Verified DNR status

• Obvious lethal injuries 

• Frozen, uncompressible chest wall

• Avalanche burial > 35min and airway obstructed with snow or life (most likely cause of death: asphyxia) 

• Presence of any signs of life 

Challenges of management associated with safe rewarming of accidental hypothermia: 

• Cardiac arrhythmias

• Typically through hypothermia-induced prolonged repolarization > avoid QTc prolonging drugs 

• <28°C (severe hypothermia), cardiac membrane is prone to ventricular fibrillation, key is to minimize jostling 

• Uncontrolled bleeding

• Enzymes require certain temperature to function, therefore malfunction of coagulation system occurs with hypothermia

• Think of the triad of death here! Hypothermia > Coagulopathy > Acidosis

• Rewarming shock 

• Focus on early fluid resuscitation with warmed IV normal saline with moderate hypothermia or <32°C

• Can even occur after restoration of normal core temperature 

Failure of rewarming

If unable to rewarm > 0.7°C/hour:

• Address reversible causes (i.e., hypoglycemia)

• Empiric antibiotics, even without obvious source 

• Broad workup including: infection, adrenocortical insufficiency, hypothyroidism, toxins, CNS lesions 

Outcomes of resuscitation in hypothermia 

• Terminate resuscitative efforts once core body temp reaches 32°C (92°F) if there has been no return of spontaneous circulation (ROSC)

• Poor prognosticators 

• Hyperkalemia > 12mEq/L (no reported cases of survival)

• Fibrinogen < 50mg/dL

• Ammonia > 420mcg/dL

• Elevated lactate

• Elevated sodium

• Elevated creatinine (kidney injury)

• Asphyxia 

• Unwitnessed cardiac arrest

• Asystole 

• Age > 75 

Neurologic outcomes associated with hypothermia are largely dependent upon: 

• Time before rescue

• Quality of prehospital interventions

• Rewarming methods used 

Case report 1999, young female skier who fell down waterfall gulley, being continuously sprinkled with ice cold water, temp was 13.7°C (57°F) when rescued, lifeless > 9h of evacuation, transport, stabilization; at 5 months had excellent mental function and was returning to work 

Lab testing in cold exposure

• Typically refrain from lab evaluation until patient warmer than 32°C (89°F)

• Useful in evaluation of concurrent conditions, such as intoxication or precipitating medical conditions (however should treat clinical picture first)

• In severe hypothermia, may be inaccurate 

• May help with prognostication for severe hypothermia 

• Poor prognosticators 

• Hyperkalemia > 12mEq/L (no reported cases of survival)

• Fibrinogen < 50mg/dL

• Ammonia > 420mcg/dL

• Elevated lactate

• Elevated sodium

• Elevated creatinine (kidney injury

Key hypothermia management takeaways: 

• Obtain adequate temperature: esophageal preferred, rectal & bladder acceptable, however may lag up to 1 hour in severe cases  

• Insulation and vapor barrier from ground and elements are key to hypothermia treatment 

• Gentle handling required for moderate and severe cases to prevent dysrhythmia

• Normal saline preferred over lactated Ringer’s for fluid resuscitation due to impaired hepatic lactate metabolism in hypothermia

• Keep patients horizontal to prevent afterdrop and cardiovascular collapse in moderate to severe cases 

• Be vigilant for hypothermia secondary to another cause through identification and treatment 

• Volume replacement is essential during rewarming as circulating blood volume is decreased by 1/3rd 

• ACLS is modified in hypothermia

• Initiate ECLS early for severe hypothermia

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Post by : Caroline geels, MD

Dr. Geels is a PGY-1 in Emergency Medicine at the University of Cincinnati

Editing by : Ryan LaFollette, MD

Dr. LaFollette is an Assistant Program Director at the University of CIncinnati and Co-Editor of TamingtheSRU