Accidental hypothermia is an unintentional drop in core temperature to 35°C or below. Core temperature is best measured by esophageal probe. If core temperature cannot be measured, the degree should be estimated using clinical signs. Treatment is to protect from further heat loss, minimize afterdrop, and prevent cardiovascular collapse during rescue and resuscitation. The patient should be handled gently, kept horizontal, insulated, and actively rewarmed. Active rewarming is also beneficial in mild hypothermia but passive rewarming usually suffices. Cardiopulmonary resuscitation should be performed if there are no contraindications to resuscitation. CPR may be delayed or intermittent.
Key points
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Hypothermia can be life-threatening. Rescuers should attempt to minimize further heat loss and begin rewarming of hypothermic patients in the field while minimizing afterdrop and preventing circumrescue collapse.
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Some patients are cold and dead but other cold patients who are apparently dead can be resuscitated with full neurologic recovery.
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Unless there are definite contraindications, rescuers should do their best to resuscitate hypothermia patients, even if they appear to be beyond hope.
Introduction
Definition
Accidental hypothermia is an unintentional drop in core temperature, which is the temperature of the heart and central circulation, to 35°C or below. Accidental hypothermia can be caused by environmental exposure and by diseases or conditions that decrease thermoregulatory responses. Iatrogenic accidental hypothermia can occur during resuscitation in emergency settings. Accidental hypothermia is a disease of wars and other disasters, as well as a condition that can affect people who are outdoors for work or recreation or because they are homeless. Accidental hypothermia can occur during any season and in any climate, including subtropical or tropical. Accidental hypothermia can also be caused by trauma, sepsis, or other diseases that decrease metabolic heat production or affect thermoregulation.
Therapeutic hypothermia, a form of targeted temperature management, can be induced to protect the brain in resuscitated cardiac arrest patients who remain unconscious after return of spontaneous circulation (ROSC). Hypothermia for neuroprotection may also be induced for cardiac surgery. Induced hypothermia is not discussed in this article.
Physiology
In normal conditions, the human body maintains a core temperature of 37° plus or minus 0.5°C. In response to input from peripheral and, to a lesser degree, central thermoreceptors, the hypothalamus regulates autonomic reflexes that increase body cooling or warming. The main physiologic warming responses to defend against hypothermia are shivering and peripheral vasoconstriction. Peripheral vasoconstriction can be triggered centrally or by decreased local skin temperature.
Hypothermia is the result of net heat loss. Heat can be lost by conduction, convection, radiation, and evaporative mechanisms. Heat always flows from a warmer object or medium to a cooler object or medium. Conduction is the direct transfer of heat between objects that are touching each other. Convection is the transfer of heat from an object to a gas or liquid that is in motion. Radiation is the transfer of heat by electromagnetic energy between 2 objects that are exposed to each other; the body can be warmed by the sun or cooled by exposure to the night sky. Evaporation causes heat to be lost by the endothermic reaction of vaporizing water in sweat or wet clothing. Heat loss due to evaporation also accounts for insensible losses from skin and from breathing.
Humans are adapted to tropical climates. Human physiologic responses to cold have limited potential to protect against hypothermia. In a well-nourished person, if conditions are mild or insulation is adequate, exercise and shivering can raise the metabolic rate enough to prevent hypothermia. In colder conditions, humans must depend on behavioral responses to wear insulating clothing and to take shelter.
Pathophysiology
Cooling of the body results in decreased resting metabolism and decreased neurologic function. Shivering is induced by skin cooling, even when the core temperature is normal. Shivering increases metabolism directly by increased muscle activity and indirectly by increased ventilation and cardiac output. Shivering increases as core temperature decreases and is maximal at a core temperature of about 32°C. Shivering decreases below 32°C and ceases by about 30°C. Below 32°C, metabolism generally decreases with decreasing core temperature.
The main clinical effects of hypothermia are due to decreases in brain and cardiorespiratory functions. Brain cooling causes impaired function beginning at about 34°C and worsening with further cooling. Clinical signs are irritability, confusion, apathy, poor decision-making, lethargy, somnolence, coma, and finally death. Most of these changes, other than death, are reversible. Even patients in coma often recover neurologically intact. Decreased metabolic requirements of a cold brain can be neuroprotective, especially during anoxic conditions such as drowning. Cold-induced diuresis, plasma leak, and decreased fluid intake decrease circulating blood volume. Cooling of the heart causes decreased cardiac output and, usually, bradycardia. As the heart cools to 30°C and below, atrial dysrhythmias and premature ventricular contractions become common and ventricular fibrillation (VF) can occur. Especially below 28°C, VF can be easily induced by acidosis, hypocarbia, hypoxia, or rough movement. Hypoventilation and respiratory acidosis result from decreased ventilatory sensitivity to carbon dioxide (CO 2 ).
Introduction
Definition
Accidental hypothermia is an unintentional drop in core temperature, which is the temperature of the heart and central circulation, to 35°C or below. Accidental hypothermia can be caused by environmental exposure and by diseases or conditions that decrease thermoregulatory responses. Iatrogenic accidental hypothermia can occur during resuscitation in emergency settings. Accidental hypothermia is a disease of wars and other disasters, as well as a condition that can affect people who are outdoors for work or recreation or because they are homeless. Accidental hypothermia can occur during any season and in any climate, including subtropical or tropical. Accidental hypothermia can also be caused by trauma, sepsis, or other diseases that decrease metabolic heat production or affect thermoregulation.
Therapeutic hypothermia, a form of targeted temperature management, can be induced to protect the brain in resuscitated cardiac arrest patients who remain unconscious after return of spontaneous circulation (ROSC). Hypothermia for neuroprotection may also be induced for cardiac surgery. Induced hypothermia is not discussed in this article.
Physiology
In normal conditions, the human body maintains a core temperature of 37° plus or minus 0.5°C. In response to input from peripheral and, to a lesser degree, central thermoreceptors, the hypothalamus regulates autonomic reflexes that increase body cooling or warming. The main physiologic warming responses to defend against hypothermia are shivering and peripheral vasoconstriction. Peripheral vasoconstriction can be triggered centrally or by decreased local skin temperature.
Hypothermia is the result of net heat loss. Heat can be lost by conduction, convection, radiation, and evaporative mechanisms. Heat always flows from a warmer object or medium to a cooler object or medium. Conduction is the direct transfer of heat between objects that are touching each other. Convection is the transfer of heat from an object to a gas or liquid that is in motion. Radiation is the transfer of heat by electromagnetic energy between 2 objects that are exposed to each other; the body can be warmed by the sun or cooled by exposure to the night sky. Evaporation causes heat to be lost by the endothermic reaction of vaporizing water in sweat or wet clothing. Heat loss due to evaporation also accounts for insensible losses from skin and from breathing.
Humans are adapted to tropical climates. Human physiologic responses to cold have limited potential to protect against hypothermia. In a well-nourished person, if conditions are mild or insulation is adequate, exercise and shivering can raise the metabolic rate enough to prevent hypothermia. In colder conditions, humans must depend on behavioral responses to wear insulating clothing and to take shelter.
Pathophysiology
Cooling of the body results in decreased resting metabolism and decreased neurologic function. Shivering is induced by skin cooling, even when the core temperature is normal. Shivering increases metabolism directly by increased muscle activity and indirectly by increased ventilation and cardiac output. Shivering increases as core temperature decreases and is maximal at a core temperature of about 32°C. Shivering decreases below 32°C and ceases by about 30°C. Below 32°C, metabolism generally decreases with decreasing core temperature.
The main clinical effects of hypothermia are due to decreases in brain and cardiorespiratory functions. Brain cooling causes impaired function beginning at about 34°C and worsening with further cooling. Clinical signs are irritability, confusion, apathy, poor decision-making, lethargy, somnolence, coma, and finally death. Most of these changes, other than death, are reversible. Even patients in coma often recover neurologically intact. Decreased metabolic requirements of a cold brain can be neuroprotective, especially during anoxic conditions such as drowning. Cold-induced diuresis, plasma leak, and decreased fluid intake decrease circulating blood volume. Cooling of the heart causes decreased cardiac output and, usually, bradycardia. As the heart cools to 30°C and below, atrial dysrhythmias and premature ventricular contractions become common and ventricular fibrillation (VF) can occur. Especially below 28°C, VF can be easily induced by acidosis, hypocarbia, hypoxia, or rough movement. Hypoventilation and respiratory acidosis result from decreased ventilatory sensitivity to carbon dioxide (CO 2 ).
Out-of-hospital evaluation of hypothermia
Classification of Hypothermia Based on Core Temperature
Hypothermia is classified, based on core temperature, as mild, 35°C to 32°C; moderate, 32°C to 28°C; or severe, less than 28°C. Some investigators have also included another category, profound hypothermia, less than 24°C or less than 20°C, in which survival is significantly lower. Clinical effects of hypothermia vary greatly among individuals. Some patients are cold but not hypothermic. Because shivering is triggered by skin cooling, a patient can feel cold and be shivering but have a core temperature greater than 35°C. A patient who is cold and shivering but not hypothermic is considered to be cold-stressed.
The classes of hypothermia correlate with the ability of the body to thermoregulate. In mild hypothermia, shivering is effective in increasing metabolic rate and body temperature. Shivering increases in intensity as core temperature declines. A mildly hypothermic patient who is healthy, has sufficient caloric reserves, and is protected from further heat loss, can rewarm to a normal core temperature by shivering. Below about 32°C, the body requires exogenous heat to rewarm to a normal core temperature. Shivering can still be strong in moderate hypothermia at 31°C but becomes progressively weaker until it ceases at about 30°C.
Below 32°C, most patients will have altered mental status and a decreased level of consciousness. Many patients are unconscious at a core temperature of 30°C and most are unconscious by 28°C.
Field Classification of Hypothermia
Standard field classification
When core temperature measurement is not feasible, it is useful to classify a patient using clinical signs as being cold-stressed, or as having mild, moderate, or severe hypothermia, corresponding to the same classifications based on core temperature ( Fig. 1 ). Field classification will help to optimize treatment. Patients who are shivering but who are fully alert and functioning normally are likely to be cold-stressed rather than hypothermic. A patient who is alert and shivering but not functioning completely normally is most likely to be mildly hypothermic. A patient with a decreased level of consciousness is likely to be moderately hypothermic. A moderately hypothermic patient may or may not be shivering. Unconscious patients should be considered to be severely hypothermic. Clinical staging of hypothermia serves as a useful guide but is not completely correlated with measured core temperatures. As with any clinical condition, individuals are highly variable in their responses to cold. In addition, a patient whose level of consciousness has been altered by a condition other than hypothermia, such as intoxication or brain injury, may have a higher core temperature than predicted clinically.
The Swiss system
An alternate system of field classification, the Swiss system, correlates clinical signs with the standard core temperature ranges of mild, moderate, and severe hypothermia, as well as with 2 additional stages: profound hypothermia and death due to hypothermia. Hypothermia (HT) stages are HT I, HT II, HT III, HT IV, and HT V, as follows:
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HT I: clear consciousness with shivering: 35° to 32°C
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HT II: impaired consciousness without shivering: 32° to 28°C
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HT III: unconscious: 28° to 24°C
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HT IV: apparent death: 24° to 13.7°C
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HT V: death due to irreversible hypothermia: less than 13.7°C? (<9°C?).
This system is widely used in Europe. The Swiss system shares the limitations of standard field classification but is also limited by inconsistencies. A patient with moderate hypothermia (HT II) is likely to be shivering if the core temperature is above 30° to 31°C. Rescuers who use the Swiss system should focus on level of consciousness rather than the presence or absence of shivering. A second inconsistency in the Swiss system involves patients with core temperature less than 24°C who have vital signs. There are many reports of such patients who may succumb to sudden death by VF. This inconsistency does not affect treatment. Based on the results of a study correlating measured core temperatures of hypothermia victims with staging by the Swiss system, adjustments of the estimated core temperature parameters of the stages have been proposed.
Associated conditions can complicate field assessment and treatment of hypothermia
Traumatic and medical conditions that cause decreased level of consciousness or abnormal vital signs may confuse the classification of hypothermia. These conditions may also suppress or abolish shivering, necessitating much more aggressive rewarming at a given core temperature than would otherwise be necessary.
Measurement of Core Temperature
Esophageal temperature
Esophageal temperature is the most accurate minimally invasive method of measuring core temperature. The probe must be inserted into the lower third of the esophagus, an average of 24 cm below the larynx in adults. For hypothermic patients with decreased level of consciousness, esophageal temperature monitoring is very helpful to guide evaluation and treatment. Before placing an esophageal probe, which may cause vomiting and aspiration, it is usually best to protect the airway with endotracheal intubation or the placement of a supraglottic airway. Heated, humidified oxygen does not affect esophageal temperature if the probe is inserted in the lower third of the esophagus. If the probe does not have markings, it can be marked at 24 cm before insertion in an adult or the correct length of insertion can be estimated visually for an adult or child and marked.
Epitympanic temperature
Epitympanic probes are soft probes in which a thermistor is placed near the tympanic membrane. Thermistor probes are more accurate than the common infrared tympanic thermometers. When an epitympanic probe is used properly, the reading correlates well with carotid artery temperature. In patients with normal cardiac output, carotid temperature approximates core temperature. When cardiac output is low, epitympanic temperature is lower than core temperature. Epitympanic temperature is often falsely lower than core temperature in out-of-hospital settings in which it is difficult to insulate the ear canal from a cold environment. For an accurate reading, there should be no cerumen or snow in the canal and the probe must have an insulating cap to block air entry. Epitympanic thermometers designed for use in the operating room are not accurate when used in field settings. An epitympanic probe can be useful to evaluate a patient if an esophageal probe is not available and are generally preferred to an esophageal probe for a patient whose airway is not protected.
Rectal, urinary bladder, and oral temperatures
Placement of a rectal or bladder probe thermometer in the field requires exposing a possibly hypothermic patient causing further heat loss. Rectal or bladder temperature should not be performed until a patient is in a warm environment. The only use for oral temperature is to rule out hypothermia, because oral temperature is lower than core temperature by a variable amount. Thermometers that contain liquid (mercury or alcohol) usually cannot measure temperatures less than 35.6°C. If a nonelectronic thermometer is used it must be a low-reading thermometer.
Although rectal or urinary bladder temperature is often called a core temperature, they can lag behind core temperature changes by up to 1 hour. During rewarming, monitoring of rectal or bladder temperature can give the false impression that the patient is still cooling.
Temporal artery temperature
So-called temporal artery thermometers are too inaccurate to be used in the assessment of a possibly hypothermic patient.
Heat flux thermometer
The noninvasive heat flux or double sensor thermometer combines a skin temperature sensor with a heat flux sensor. Readings accurately reflect core temperature in operative and intensive care unit settings. The heat flux thermometer is a promising noninvasive method of core temperature measurement that may be useful for out-of-hospital evaluation and treatment of hypothermia if accuracy is demonstrated in field situations.
Safety of Rescuers and Initial Priorities
The first priority during rescue is the safety of the rescuers. If a potentially hypothermic patient has an obvious fatal injury, it may not be necessary for rescuers to enter the scene. Even after it is safe for rescuers to enter the scene, it may be advisable to move the patient to a safer place before further evaluating the patient and initiating treatment.
Once the safety of the rescuers is assured, rescuers should determine whether the patient is in cardiac arrest. If the patient has vital signs but is not completely alert, rescuers should avoid causing cardiovascular collapse by gentle handling and by keeping the patient horizontal as much as possible. Attempt to minimize further decrease in core temperature and begin rewarming. If the patient is in cardiac arrest, rescuers should start resuscitation, if indicated.
Treatment of a Cold-Stressed Patient Who is Not Hypothermic
A patient who is completely alert and shivering, well-nourished, and not hypothermic is not at risk for significant afterdrop or circumrescue collapse. The patient may remove wet clothing and put on dry clothing without shelter. The patient does not need to be horizontal, may sit up to eat and drink, and may ambulate.
Core Temperature Afterdrop
After a patient is removed from a cold environment, core cooling continues due to conductive heat loss from the warm core to cool peripheral tissue and convective heat loss to blood when increased blood flow to peripheral tissue causes increased cooling of blood that returns to the central circulation. Conductive cooling is only minimally affected by treatment. Convective cooling can be increased by movement or rewarming of the extremities that results in an increased rate of blood flow to the periphery. Increased peripheral blood flow allows an increased volume of cooled blood to return to the central circulation. This decreases core temperature and increases the workload on the heart. Afterdrops as high as 5° to 6°C have been reported in hypothermic patients during prehospital care. For a patient with moderate or severe hypothermia, afterdrop can contribute to cardiovascular instability.
Circumrescue Collapse
Syncope or sudden death that occurs in victims of cold-water immersion during or after rescue and removal from water is called circumrescue collapse. Circumrescue collapse may be due to sudden hypotension or sudden onset of VF. Removal from the water decreases hydrostatic pressure and allows blood to pool in dependent areas. This decreases blood return, causing hypotension or cardiovascular collapse. Blood returning from the periphery is cooled and contributes to afterdrop. If the victim has to perform work there is an increased risk of circumrescue collapse. Fatal circumrescue collapse was observed in shipwrecked sailors subsequent to climbing ladders to board rescue boats. A combination of mechanical stimulation of the heart, acidosis from blood returning from the extremities, and afterdrop may cause VF. Theoretically, imminent rescue may contribute to mental relaxation of a hypothermia victim, causing a decrease in catecholamine levels that results in hypotension. In the water this could precipitate loss of consciousness and drowning. Fatal circumrescue collapse has been documented in terrestrial rescue, as well as in immersion hypothermia.
Handling a Hypothermic Patient During Rescue
A hypothermic patient should be kept horizontal, especially after rescue from water, to minimize the effects of decreased hydrostatic pressure. Rescuers should avoid having the patient make any physical effort, but should encourage the victim to stay alert and focus on survival to minimize the chance of circumrescue collapse.
Movement and rewarming of the extremities should be avoided to prevent increased blood flow to cool peripheral tissue with resultant increase in return of cooled blood to the central circulation. Increased return of blood can stress a heart that is already impaired due to cold or can cause increased cooling that might precipitate VF, especially at core temperatures less than 28°C.
Protection from Further Heat Loss
In addition to minimizing afterdrop, rescuers should attempt to maintain core temperature by adding insulation and by providing a vapor barrier. Options for insulation include extra clothing, blankets, quilts, sleeping bags, and insulated pads. Insulated pads can prevent large conductive heat losses to the ground. The vapor barrier protects against heat loss from evaporation and convection. Materials that can be used to make a vapor barrier are bubble wrap, sheets of plastic, reflective blankets, and garbage bags, with a hole cut out for the face. Bubble wrap is a good vapor barrier but has minimal value as insulation. The vapor barrier is usually placed as the outermost layer but can also be effective if placed between wet clothing and outer dry layers. Extra insulation can make up for the lack of a windproof layer or a vapor barrier. Insulation can be placed inside 2 layers of vapor barrier.
Wet clothes should be cut off, rather than pulled off, to minimize movement of the extremities. The patient should be insulated from the ground. The head and neck do not lose heat faster than the rest of the body but may account for a large percentage of heat loss if they are the only areas not protected by insulation. Insulation should be pulled as tightly around the face as possible without interfering with breathing to decrease heat loss from the head and neck. If the patient is being evacuated by helicopter, a windproof layer, ideally a vapor barrier, should be used for protection from wind and from rotor wash.
Field Rewarming
After a hypothermic patient has been protected from further cooling, the patient should be rewarmed as much as possible. The best rewarming methods minimize afterdrop even though other methods might be faster overall. Rewarming is not required for mild hypothermia that is not complicated by other conditions but will make patients more comfortable. Patients with decreased level of consciousness require active rewarming.
Support shivering for a cold-stressed or mildly hypothermic patient
Shivering is the most effective method of rewarming a cold-stressed or mildly hypothermic patient. Because shivering is uncomfortable and can stress the cardiovascular system, active rewarming methods are preferred. If active rewarming is not possible, the patient should be insulated and given high carbohydrate drinks and food. Drinks can be warmed but should not be hot enough to cause burns. The heat content of warm high-carbohydrate drinks is negligible compared with the number of calories in the carbohydrates. Vigorous shivering increases heat production up to 5 to 6 times resting metabolic rate. Shivering can increase core temperature 1° to 3°C per hour in a well-insulated patient with adequate caloric reserves.
Delay standing or walking
A hypothermic patient who is found sitting or lying down should not be allowed stand right away. Standing and walking increase blood flow to the legs, worsening afterdrop, and may increase the risk of hypotension. The patient should be insulated, given calories, and observed for at least 30 minutes before exercising. A patient who tolerates standing without problems can be allowed to walk, slowly at first, then gradually increasing in speed as tolerated.
Active external rewarming
Active external rewarming is beneficial in an alert, shivering patient and is mandatory in a patient who has a decreased level of consciousness, even if shivering. Effective rewarming may be provided using large electric heat pads or blankets, large chemical heat pads, warm water bottles, or a Norwegian charcoal-burning HeatPac (Normeca, Loerenskeg, Norway). Providing heat to a shivering patient decreases shivering and does not change the rate of core rewarming but is more comfortable, requires fewer calories, and is less stressful for the heart than relying on shivering alone. Shivering or use of rewarming devices should be used in combination with insulation and vapor barriers. The HeatPac should only be used outdoors or with good ventilation to prevent carbon monoxide (CO) poisoning. The Hypothermia Protection and Management Kit (HPMK), developed by the United States Armed Forces, uses a heat blanket with 4 large chemical heat pads and a heat-reflective shell. The HPMK provides effective insulation and rewarming. The HPMK is commercially available. Small chemical heat packs may be helpful to protect hands and feet from frostbite but have insufficient heat content to treat hypothermia.
Body-to-body rewarming of a shivering person in a sleeping bag decreases shivering and does not increase the rate of rewarming. Body-to-body rewarming may make a cold patient more comfortable but requires an extra rescuer. Body-to-body rewarming should not be used if it will delay evacuation.
A warm shower or bath should not be used for rewarming. Vasodilation of the skin causes increased afterdrop and decreased blood pressure. Rewarming in a shower or bath has the potential to cause cardiovascular collapse.
Protection of cold skin
Cold skin is very susceptible to injury from pressure and heat. Anecdotal reports of skin damage associated with the use of hot water bottles filled with lukewarm water represent pressure injuries rather than burns but can still be devastating. The HPMK may cause burns. A barrier should be used to protect the skin when using chemical or electrical heat pads and precautions should be taken against pressure injuries.
Heated humidified oxygen
Heated-humidified oxygen can prevent respiratory heat loss but has very little heat content. Heated humidified oxygen should not be used alone as a rewarming method but can add to the effectiveness of other methods. Caution should be used to prevent burns of the face.
Distal limb warming
Unlike rewarming of the whole body in a hot shower or bath, distal limb rewarming of the arms and legs to the elbows and knees in water at 42° to 45°C works by opening arteriovenous anastomosis in the hands and feet. This causes increased return flow of warmed blood directly to the central circulation, decreasing afterdrop, and providing effective rewarming. This method is the exception to the rule that peripheral rewarming is dangerous in a hypothermic patient. Distal limb rewarming is safe only for a mildly hypothermic patient. The method was developed for use on ships and is not likely to be practical in other out-of-hospital settings.
Rewarming during transport
Large chemical heat pads can provide limited rewarming during transport. Forced air warming is an effective rewarming method that can limit afterdrop compared with the afterdrop with shivering. Liquid-filled heat blankets are neither effective nor practical for rewarming during transport. The Norwegian HeatPac can be used with CO monitoring in a well-ventilated passenger compartment of a vehicle after the unit is no longer generating an initial small amount of smoke. The HeatPac should never be used in an aircraft. The ideal temperature in the patient compartment of an ambulance is 28°C, the thermoneutral temperature of humans in air. However, 28°C is too hot for normally clothed patient attendants as well as for pilots or drivers. A reasonable compromise is to maintain the temperature of the patient compartment at 24°C or above.
Resuscitation of an Hypothermic Patient
Decision to resuscitate
Hypothermic patients in cardiac arrest have survived with normal neurologic function. Fixed, dilated pupils, and apparent rigor mortis are not contraindications to resuscitation in a hypothermic patient. However, it is not true that no one is dead until they are warm and dead. Some patients are cold and dead. Rescuers should not attempt to resuscitate a patient with fatal injuries such as decapitation, open head injury with loss of brain matter, truncal transection, or incineration. Attempted resuscitation is also contraindicated when the chest wall is too stiff to allow chest compressions. Rescuers should not attempt to resuscitate an avalanche victim who has been buried for 60 minutes or longer with an airway completely obstructed by snow or ice. This indicates death from asphyxia.
Indications for cardiopulmonary resuscitation
Cardiopulmonary resuscitation (CPR) is indicated for cardiac arrest due to hypothermia. If there are signs of life, CPR should not be performed. In a hypothermic patient, signs of life are often difficult to detect. Pulses can be very slow and faint. Pulses can be difficult to find, especially when a rescuer has cold fingers. Breathing may be slow and shallow but is sometimes easier to detect than pulses. Rescuers should feel for a carotid pulse for 1 minute. If there is no pulse or other sign of life, rescuers should start CPR with ventilation. If possible, rescuers should move the patient to a warm setting, such as a ground or air ambulance in which cardiac monitoring can be used to guide resuscitation.
Cardiac end-tidal carbon dioxide and echocardiographic monitoring
Rescuers should use cardiac monitoring, if available, to diagnose cardiac arrest. If pulses are not found, an organized rhythm on the monitor could represent pulseless electrical activity (PEA) or a perfusing rhythm with very faint pulses. Lack of a waveform on end-tidal CO 2 (ETCO 2 ) monitoring indicates cardiac arrest. Point of care echocardiography can be used to diagnose PEA if there is electrical activity without cardiac contractions. Rescuers should start CPR if a patient has a nonperfusing rhythm such as ventricular tachycardia (VT), VF, or asystole. If the rhythm appears to be asystole, the gain on the cardiac monitor should be set to maximum to detect low amplitude QRS complexes. If there is an organized rhythm other than VT, CPR is only indicated if ETCO 2 monitoring shows lack of perfusion or echocardiography confirms absence of contractions.
Automated external defibrillator
An automated external defibrillator (AED) with a cardiac monitor can be used to determine cardiac rhythm. If an AED without a monitor gives the instruction shock advised, the rhythm is VT or VF. If shock is advised, the rescuer should attempt defibrillation and start CPR. If no shock is advised, the rhythm could be asystole or an organized rhythm. If there are no signs of life and no carotid pulse is found after 1 minute, the rescuer should start CPR unless echocardiography shows contractions.
Delayed, intermittent, and prolonged cardiopulmonary resuscitation
Hypothermia protects the brain from hypoxia by reducing brain activity. In severe hypothermia, the brain can tolerate circulatory arrest for over 30 minutes. Full neurologic recovery in severe hypothermia has been reported after 8 hours 40 minutes of cardiac arrest, as well as after CPR for as long as 6 hours 30 minutes. Cases of full neurologic recovery after hypothermic cardiac arrest have also been reported with delays of 15 minutes and 70 minutes before CPR was started. In another case, good neurologic outcome was reported in a hypothermic patient who was treated with alternating 1-minute periods with and without CPR while being carried in a litter. Further evidence comes from cardiac surgery with induced hypothermia. In a rescue situation, immediate, continuous CPR may not be possible. If immediate, continuous CPR is not possible, delay to CPR and interruptions to CPR should be as short as possible. A conservative recommendation is to delay CPR for no longer than 10 minutes to allow rescuers time to move a patient to a safer location. If core temperature is 20° to 28°C, or unknown, rescuers should perform CPR continuously for 5 or more minutes and limit interruptions to CPR to 5 minutes or less. If core temperature is 20°C or below, CPR should be continuous for 5 or more minutes with interruptions 10 minutes or less. There is no known limit for the duration of CPR in a severely hypothermic patient.
Low core temperature should not limit resuscitation
A patient with a core temperature of 13.7°C due to accidental hypothermia was successfully resuscitated. The lowest known core temperature induced for surgery was 9°C. Both patients survived neurologically intact. If there are no contraindications to CPR, rescuers should attempt to resuscitate a patient with severe hypothermia regardless of the measured core temperature.
Defibrillation in hypothermic patients
Patients with core temperatures less than 26°C have been successfully defibrillated. Rescuers should make an attempt to defibrillate a patient whose core temperature is 30°C or below. This attempt can be a single shock at maximum power or 3 shocks, depending on local protocols. Rescuers should wait until a patient has been rewarmed at least 1° to 2°C before reattempting defibrillation. Above a core temperature of 30°C, rescuers should follow usual defibrillation protocols.
Chest compressions and ventilation in hypothermic patients
Chest compressions in hypothermic patients are relatively ineffective compared with chest compressions in normothermic patients but metabolic demands are also markedly reduced. Rescuers should perform chest compressions at the same rate as in a normothermic patient. Manual chest compressions are more difficult to perform than mechanical compressions, especially during transport. High-quality CPR, using mechanical compressions, can be used as a temporizing measure for a patient with prolonged cardiac arrest who is being transported to a hospital or other facility for extracorporeal circulation (ECC) rewarming.
Ventilations in hypothermic patients are also relatively ineffective unless an advanced airway (endotracheal tube or supraglottic device) is in place. If ETCO 2 monitoring is not available and there is no advanced airway, rescuers should administer ventilations to a hypothermic patient at the same rate as for a normothermic patient. If there is an advanced airway, ventilation should be at half the usual rate. If ETCO 2 is available, ventilations should be given at a rate that keeps ETCO 2 in the normal range. This range should be adjusted for altitudes above 1200 m.
It is difficult to measure oxygen saturation in a hypothermic patient but a hypothermic patient being ventilated at sea level is not likely to be hypoxemic. Supplemental oxygen may be helpful at altitudes above 2500 m.
Management of the airway is similar in hypothermic and normothermic patients. Endotracheal intubation or use of a supraglottic airway device may be necessary for adequate ventilation and prevention of aspiration. Although VF can occur during endotracheal intubation, it is uncommon. The risk of VF should not preclude standard airway management with intubation, if indicated. Cold-induced trismus may be resistant to neuromuscular blockade. If trismus prevents laryngoscopy, placement of a supraglottic device is usually a better option than fiberoptic intubation or cricothyroidotomy. Rescuers should not overinflate the cuff of an endotracheal tube or supraglottic device with cold air. Overinflation could cause kinking of the tube or rupture of the cuff as the patient rewarms and the air in the cuff expands.
Anesthetic and neuromuscular blocking agents are likely to be ineffective below 30°C but metabolism is decreased and the effects are likely to be prolonged when the agents become effective as the patient rewarms. If drugs are used, dosages should be lowered and dosing intervals extended.
Circulatory access in hypothermia
Intravenous (IV) access in a hypothermic patient is often difficult due to peripheral vasoconstriction. Unless an IV can be placed immediately, rescuers should use intraosseous (IO) access. Placement of central venous catheters can cause VF if the catheter or guidewire comes into contact with the myocardium. Femoral catheters are safer, but are difficult to place in the field.
Volume replacement and fluid management
Blood volume is decreased in moderate and severe hypothermia. As a patient rewarms, peripheral vasoconstriction is reversed, increasing the vascular space. Normal saline should be given to replace volume to prevent hypotension and shock. Because the liver cannot metabolize lactate at cold temperatures, lactated Ringer should not be infused. Fluids should be warmed to 40° to 42°C. Commercial fluid warmers and insulated tubing should be used to avoid infusing cold fluid.
Fluids are best given rapidly as boluses to avoid cooling or freezing of lines. IV lines are ideally saline-locked when not in use but IO lines require continuous flow to remain patent. Sufficient fluid should be given to maintain a blood pressure that supports adequate perfusion. There are no existing guidelines that specify optimum blood pressures.
Administration of glucose and insulin
Either hypoglycemia or hyperglycemia can occur in a hypothermic patient. If point-of-care testing is not available, IV or IO glucose should be given to a hypothermic patient with altered mental status. Hyperglycemia has no known adverse effects in hypothermia. Administration of insulin is not necessary if a hypothermic patient is hyperglycemic and may not be effective below 30°C.
Cardiovascular drugs
Most of the evidence regarding drug effects in cardiac arrest due to hypothermia comes from animal studies. Animal studies with the vasopressors epinephrine and vasopressin have shown some potential benefits, including improved cardiac perfusion pressure, ROSC, and increased survival. There is 1 human case report in which a patient had ROSC after vasopressin was given but the patient later succumbed to multiple organ system failure. The antidysrhythmic agents amiodarone and bretylium have been studied for treatment of ventricular dysrhythmias in hypothermic animals with mixed results. There are 2 human case reports of ROSC after administration of bretylium for VF.
There are insufficient human data to justify recommendations for cardiovascular drugs in hypothermia below 30°C. Metabolism of drugs is decreased and protein binding increased in hypothermia. When a patient rewarms, drugs that were given during hypothermia may become bioavailable at toxic levels. It is prudent not to give vasoactive drugs to a patient with core temperature less than 30°C. Between 30° and 35°C drugs should be given at the normal dose but with twice the usual interval between doses.
Atrial dysrhythmias during rewarming
Atrial dysrhythmias often occur during rewarming and should not be treated in a patient who is hemodynamically stable. Atrial dysrhythmias generally resolve during rewarming.
Transcutaneous pacing
Transcutaneous pacing may be indicated in a bradycardic patient whose blood pressure is too low to allow arteriovenous rewarming. Transvenous pacing is contraindicated in hypothermia due to cardiac irritability.
Transport and Triage
An alert patient who is shivering can be treated in the field unless the patient has an injury or medical condition requiring evacuation. Any patient who had asphyxia from drowning or avalanche burial is at risk for delayed complications and should be evacuated to a facility capable of managing acute respiratory distress syndrome.
A patient with severe trauma is at risk for the lethal triad of acidosis, coagulopathy, and hypothermia. Severely injured patients are unable to shiver and become hypothermic quickly, even during mild weather. A severely injured patient should be treated as soon as possible with active rewarming to prevent hypothermia. Injuries should be stabilized and the patient transported ideally to a trauma center.
A cold-stressed or hypothermic patient with a decreased level of consciousness should be treated with active rewarming even if shivering. A patient with moderate or severe hypothermia who is hemodynamically stable should be transported to the nearest hospital or rural clinic with rewarming capabilities. A hemodynamically unstable patient should be transferred to a hospital that can perform ECC using extracorporeal membrane oxygenation or, less desirably, cardiopulmonary bypass. If transport to a hospital with ECC capabilities is not feasible, due to distance or bad weather, the patient should be transferred to the closest facility capable of rewarming. Hemodynamically unstable patients and patients in cardiac arrest have made full recoveries after resuscitation without ECC.
Use of serum potassium to determine viability
High serum potassium is a marker of cell lysis and death. The highest known serum potassium in a patient who was successfully resuscitated was 11.8 mmol/L in a 31-month-old child. The true value of serum potassium may have been lower based on a rapid reported drop to 4.8 mmol/L in 25 minutes without specific intervention. A more recent case involved successful resuscitation of a 7-year-old girl who had profound hypothermia, with a nasopharyngeal temperature of 13.8°C, and a serum potassium of 11.3 mmol/L after drowning and submersion for an estimated 83 minutes. There is no report of survival of a hypothermia patient with serum potassium greater than 12 mmol/L. If a hypothermic patient in cardiac arrest has serum potassium greater than 12 mmol/L, CPR should be discontinued and the patient can be declared dead without further rewarming.
Avalanche Burial
The most common cause of death in avalanche burial is asphyxia. A victim who survives an avalanche initially may die from asphyxiation due to hypercapnia and hypoxemia, airway obstruction due to snow or other debris, trauma, or constriction of the chest preventing inspiration. A rapid decrease in survival occurs during the initial 35-minute asphyxial phase. If the airway is not patent, death will occur within 35 minutes. In a victim with a patent airway, an open space in front of the mouth or nose, referred to as an air pocket, can slow asphyxiation, prolonging survival.
Cooling of a victim of avalanche burial can be rapid, especially if the victim is asphyxiated. The average cooling rate after avalanche burial is reported be about 3°C per hour. Previous recommendations suggested using standard resuscitation for a victim buried less than 35 minutes or with a core temperature 32°C or above. In 2015, recommendations by the European Resuscitation Council (ERC) were changed to be consistent with other hypothermia recommendations. The current recommendation is to use standard resuscitation if burial was 60 minutes or less or core temperature is 30°C or above. If there is ROSC, the victim should be evacuated to the closest hospital that is able to manage associated injuries.
A victim with vital signs who was buried longer than 60 minutes or with core temperature less than 30°C should be transferred to the nearest facility capable of providing active rewarming. If the victim is in cardiac arrest and has a patent airway, CPR should be started. If the victim is in cardiac arrest and does not have a patent airway, the decision to withhold CPR and resuscitation is reasonable.
A victim of avalanche burial is most likely to be asphyxiated before becoming hypothermic. The highest serum potassium documented in an avalanche victim who was successfully resuscitated was 6.4 mmol/L. In 2015, the ERC proposed serum potassium of 8 mmol/L as the upper limit for attempting resuscitation of an avalanche victim. Resuscitation should be discontinued if serum potassium is 8 mmol/L or more.
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