Accidental Hypothermia and Cardiac Arrest: Physiology, Protocol Deviations, and ECMO

Accidental hypothermia is a life threatening condition that can lead to a challenging resuscitation. The very young, old, and intoxicated patient are at high risk to developing hypothermia, even in temperate climates. The pathophysiologic changes from hypothermia make the standard ACLS approach insufficient to care for the hypothermic patient. This article will discuss the physiology of hypothermia and how you should alter your approach in the hypothermic patient, including early consideration of extracorporeal membrane oxygenation (ECMO).

How do we categorize hypothermia?

Accidental hypothermia is defined as a drop in core body temperature below 35^oC.¹ Accidental hypothermia can be primary due to environmental exposure, or secondary due to failed internal thermoregulation pathways due to a metabolic disorder, toxin, or other cause. Although easily missed, identification of hypothermia is important to improving outcomes, especially in patients with cardiac arrest.

Classically, hypothermia is categorized as mild, moderate, severe, and profound based on physical exam findings such as shivering. In practice, there is variability in clinical presentation across the stages of hypothermia. Because of this unpredictability, the stage of hypothermia is best determined by:^1,2

• Vital signs
• Core body temperature
• Level of consciousness

Stages of Hypothermia

What are common causes of hypothermia?

In the winter months, primary hypothermia becomes a greater concern in indigent populations and those involved in outdoor excursions.

Radiant and convective heat loss account for 90% of all heat loss.³ Prevention and treatment of mild to moderate hypothermia involves reducing each type of heat loss. Remove any direct contact to cool surfaces, such as metal backboards, remove wet clothing, increase the ambient room temperature, and provide barriers to radiant heat loss.

Secondary hypothermia is important to contemplate, as there is a higher risk of a missed diagnosis and delay in treatment. Secondary hypothermia can be multifactorial due to medications or toxins, metabolic or endocrine disorders, neurologic injuries, infection, and iatrogenic causes such as massive transfusion or prolonged exposure to cooled resuscitation rooms. Secondary causes of hypothermia should always be considered as these patients are less likely to respond to rewarming and supportive care and may require alternative treatments.

As hypothermia occurs, what physiologic changes do we expect to see?

General: As a patient’s core temperature decreases, the body attempts to preserve normothermia by multiple mechanisms. Initially, the metabolic rate increases, peripheral blood flow is shunted towards vital structures, and shivering initiates to increase heat production. If these mechanisms are inadequate, the core body temperature will continue to drop, leading to impairment of specific organ systems. As core body temperature drops below 35^oC, there is progressive CNS impairment, including poor judgment, amnesia, ataxia, and eventual brain death. These impairments may lead to paradoxical undressing, occurring in up to 30% of fatal hypothermic cases.¹

Cardiovascular: Along with peripheral vasoconstriction, there may be an initial increase in heart rate and blood pressure; however, as core temperature declines, progressive bradycardia and hypotension occurs. This bradycardia is considered physiologic to hypothermia and cardiac pacing is not needed unless hemodynamic instability occurs.² Atrial fibrillation and flutter are commonly seen in hypothermic patients, but typically revert once core temperature improves. ECG changes such as Osborne waves, or J point elevation, typically occur around 32^oC and may be accompanied by prolonged QT and PR segments.⁴ These physiologic cold responses are commonly seen in patients undergoing therapeutic hypothermia, and within mild to moderate hypothermia do not typically lead to dangerous arrhythmias.

As hypothermia becomes severe, myocardial irritability increases, with a risk for arrhythmia and cardiac arrest most significant below 28^oC.² Myocardial irritability places patients at risk for “rescue collapse”, or cardiac arrest occurring during transport and treatment thought to occur due to the substantial irritability of the myocardium.⁵ The hypothermic heart may be less responsive to medications or defibrillation.⁶ As core body temperature declines, hypothermic cardiac arrest occurs due to lost cardiac output either from a lethal arrhythmia or inability to sustain perfusion of the heart.

Oxygen consumption: In addition to CNS and cardiovascular failure, severe hypothermia with a temperature less than 28^oC is associated with decreased oxygen consumption. As hypothermia continues, the demand for oxygen decreases faster than the availability. At 28^oC, oxygen consumption is decreased by 50%, leading to a potentially protective effect on the brain and vital organs if hypothermia develops before asphyxia.^2,4,6

Coagulopathy: Additionally, hypothermic patients are at risk for coagulopathy, impaired immune function, and a high potential for peripheral frostbite.⁶

Should I change my evaluation for cardiac arrest patients with hypothermia?

In patients presenting with hypothermic cardiac arrest, it is important to consider which came first, severe hypothermia or cardiac arrest. Evaluation for the “H’s and T’s”, electrolyte abnormalities, and myocardial ischemia is just as important to prevent anchoring on primary hypothermia as the cause of an arrest. Upon initial presentation, it may be unclear if a patient suffered a hypothermic cardiac arrest or there were underlying causes.

Your initial physical examination may be unreliable. Stage IV hypothermia may demonstrate signs of life, but the patient may have an irregular or small-volume pulse with unrecordable blood pressure. At extreme temperatures, the brain may be able to tolerate circulatory arrest for up to 10-times longer than normothermic temperatures. In rare cases, patients have survived with neurologic recovery after suffering core temperatures of <14^oC or CPR for as long as 6 hours.^7,8

What modifications should I make to my resuscitation?

Prehospital setting

In the field, prehospital care should center around high-quality BLS care, including maintaining good compression depth and rate, as well as decreasing hands-off time.⁹ Additionally, passive rewarming should be initiated by removing wet clothes, and active rewarming with blankets and warmed IV fluids or humidified oxygen if available, although these techniques alone are not effective. In the prehospital setting, further aggressive treatment of severe hypothermia remains controversial and many providers may not have the equipment to provide additional rewarming methods. Patients identified as having a hypothermic cardiac arrest should be considered for transport to an ECMO or cardiopulmonary bypass center.¹ Cessation of resuscitation should only occur in the field if there is obvious signs of a lethal injury, prolonged asphyxia, or if the chest is incompressible.²

Defibrillation

Theoretical physiologic changes in severe hypothermia have led to modifications in BLS and ACLS care.⁴ The hypothermic myocardium is theorized to be less likely to respond to medications or defibrillation, and although the temperature at which defibrillation should be attempted has not been established, defibrillation should be attempted at least once.⁹ Deferring further attempts is of uncertain value, and current European guidelines recommend 3 defibrillation attempts with core temperatures <30^oC.² Current AHA recommendations state it may be reasonable to continue further defibrillation attempts.⁹ Some animal models show improved response to defibrillation in hypothermic animals as low as 30^oC, however lower core temperature was not tested.¹⁰

Cardioactive medications

The effects of cardioactive medications are also unclear in hypothermic patients. Drug metabolism is slowed at lower temperatures, and there is a theoretic potential for toxic concentrations of medications if given at typical doses. Some porcine studies show that epinephrine may still be effective at increasing coronary perfusion pressure but does not appear to improve survival, and may lead to increased oxygen consumption.¹¹ Other medications, including amiodarone, are known to be less efficacious at lower temperatures. For this reason, European guidelines recommend to withhold ACLS medications until the patient reaches 30^oC.²

Airway management

When obtaining an advanced airway, providers should consider non-depolarizing paralytics to prevent artificial elevations in serum potassium levels.¹² Elevated potassium levels may impact ECMO candidacy in many centers, and an elevated potassium >12 mEq/L is an indicator to cease resuscitative efforts, even in hypothermic patients.

Rewarming techniques

Active rewarming should be continued in-hospital. In patients in cardiac arrest, extracorporeal rewarming is the preferred method as it provides circulation and oxygenation while reducing the risk of cardiopulmonary failure during the rewarming procedure.^13,14 ECMO can provide rewarming rates as high as 10^oC/hour, while other active rewarming methods can increase core temperature by 1-2^oC/hour.¹⁴

In locations without access to ECMO, forced-air rewarming and techniques such as thoracic or abdominal lavage, hemodialysis, and thoracotomy may be considered. Forced-air rewarming is the most commonly available method and has the least potential for complication, providing up to 1^oC/hour.¹⁵ Thoracic lavage has a high rate of complications including infection and requires a large volume of warmed saline or tap water as well as staff to provide continuous cavity lavage. Case reports identify both thoracotomy with internal cardiac massage and less invasive tube thoracostomy as both potential methods for lavage.¹⁶ In situations without access to specialist care, forced-air rewarming and warmed fluids are appropriate management while transfer is arranged, with the decision to perform lavage dependent on available resources.

Monitoring

Even after initiation of rewarming and ROSC, patients require continued monitoring. Spontaneous hypothermia followed by hyperthermia after resuscitation is well known.^2,6 Such episodes of fever are associated with worse neurologic outcomes, so it is important to closely monitor temperature trends and treat with antipyretics or invasive temperature control devices.⁶

When should I be calling ECMO?

ECMO is the gold standard treatment for patients suffering from severe hypothermia with or without cardiac arrest.¹⁴

Indications and contraindications may vary by institution, similar to current variability in location dependent extra-corporeal cardiopulmonary resuscitation (eCPR) guidelines. Providers should familiarize themselves with their local institutional policies about initiating ECMO and eCPR for hypothermic patients. If such procedures are not already in place, providers should work with their hospital cardiac surgery teams to create site-specific guidelines.

Patients who meet indications for initiation of eCPR should have conventional CPR continued while obtaining sterile femoral arterial and venous access to be used for ECMO cannulation. The ECMO team will initiate the patient V-A ECMO to maintain perfusion of the brain, heart, and other major organs. Once initiated on ECMO, compressions can cease. The ECMO circuit can then be used to actively rewarm the patient while providing oxygenation and perfusion.

Patients placed on ECMO appear to have improved rates of survival. Unlike traditional ACLS care, ECMO allows for several days of hemodynamic support as the patient is warmed and stabilized. Systematic reviews suggest survival to discharge rates of 20.3% to 67.6% in patients presenting in hypothermic cardiac arrest.^13,14 High survival rates may be due to publication bias of successful case reports or patient selection as institutional policies vary. Patient centered outcomes, such as neurologic deficit at discharge, are not yet known. Complications from ECMO are rare, reported to occur in 1.2% of cases.¹³

Can I ever stop a resuscitation before rewarming?

“No one is dead until warm and dead” is classically taught. The idea is that every hypothermic cardiac arrest should undergo rewarming and continued resuscitation until reaching a goal temperature. Current literature has identified some indications to stop resuscitation, even for hypothermic patients, as follows:

• Obvious lethal injury
• Frozen body
• Hyperkalemia of 10-12 mmol/L^5,12–14,17
• Avalanche patients: Airway packed with snow and burial duration exceeded 35 minutes^9,12

However, the AHA does not currently endorse utilizing any of these indications prior to warming with the exception of avalanche patients.⁹

Summary

Both primary and secondary hypothermia can be difficult to identify. Although patients presenting in hypothermic cardiac arrest require evaluation for traditional causes of cardiac arrest, there are some changes to standard ACLS care. ECMO is the gold standard for active rewarming, but survival to discharge is difficult to predict, and patient-centered outcomes are not yet known. Most patients should undergo active rewarming prior to considering cessation of resuscitative efforts.

Alterations to ACLS:

• Up to 3 defibrillations should be attempted at core temperatures <28^oC, and it may be reasonable to continue defibrillation attempts, or withhold until a core temperature of 30^oC is reached.^2,9,12
• Epinephrine and other vasoactive medications may have decreased efficacy and increased concentrations during hypothermia.¹¹
• Consider withholding ACLS medications until a core temperature of 30^oC is reached, and then doubling the interval of dosing until normothermia is achieved.² The AHA does not currently have recommendations on medication administration.⁹
• Active rewarming should be initiated with a goal temperature of at least 30^oC.

Indications to consider cessation in hypothermic patients:

• Obvious lethal injury
• Frozen body
• Hyperkalemia of 10-12 mmol/L^5,12–14,17
• In avalanche patients: airway packed with snow and burial duration exceeded 35 minutes^9,12

Author information

Evan Kuhl, MD

Chief Resident
GWU Emergency Medicine Residency

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