Why is it important to keep patients at rest when they are hypothermic?

Most guidelines use a standard classification of hypothermia based on core temperature. Hypothermia is classified as mild at 35 to 32°C; moderate at 32 to 28°C; or severe at <28°C.12, 13, 14 Some experts advocate a further category, profound hypothermia, at <24°C12 or <20°C.1 The chance of survival appears to be much lower in this range, probably because of a high likelihood of cardiac arrest. In cases of hypothermia secondary to cold water immersion, loss of airway protection and drowning may also contribute to causes of death. Although core temperature is used to classify hypothermia, individual variation to core temperature is wide, as is true of other physiologic parameters. Measuring core temperature is not always feasible in the out-of-hospital environment.15

The standard classification of hypothermia by core temperature correlates with the status of the thermoregulatory system. From 35 to 32°C (mild hypothermia) thermoregulatory shivering control is functional and increases as core temperature decreases.16 With further cooling, shivering generally becomes less effective, although it can still be strong at 31°C.5 Below 32°C (moderate hypothermia), thermoregulation becomes less effective and rewarming is possible only with addition of exogenous heat. As the core temperature decreases below 32°C, level of consciousness decreases. Below 28°C (profound/severe hypothermia), most patients are unconscious and not shivering, and the risk of VF or asystole is high.17

The key factors guiding hypothermia treatment are level of consciousness, alertness, shivering intensity, physical performance, and cardiovascular stability, which is based on blood pressure and cardiac rhythm (Figure 2). Core temperature can provide additional helpful information, but it is difficult to accurately obtain in the field, and the panel recommends that this should not be the sole basis for treatment (Evidence grade: 1C).

Figure 2. Recommendations for out-of-hospital evaluation and treatment of accidental hypothermia. Abbreviations: AED = automatic external defibrillator, CPR = cardiopulmonary resuscitation, ECC = extracorporeal circulation, ECG = electrocardiogram, ETCO2 = end-tidal carbon dioxide, HPMK = Hypothermia Prevention Management Kit, ICU = intensive care unit, IV = intravenous, IO = intraosseous, O2 = oxygen, PEA = pulseless electrical activity, US = ultrasound, VT = ventricular tachycardia, VF = ventricular fibrillation. From Zafren et al.2 Reprinted with permission from the Wilderness Medical Society. ©2014 Wilderness Medical Society.

Patients can be cold and shivering, but not hypothermic. Shivering is triggered by skin cooling as a mechanism for preventing hypothermia. A shivering patient with a core temperature >35°C is cold-stressed, but not hypothermic. If temperature measurement is not possible, clinical judgment may be helpful to distinguish whether a patient is hypothermic or cold-stressed. For example, a patient who was not cold before being briefly immersed in cold water may be shivering but will not be hypothermic (Figure 2, Figure 3). Many alert, shivering patients who are well nourished and exhausted are not hypothermic.

The American Heart Association (AHA) 2010 Guidelines propose an alternate classification of hypothermia: mild (>34°C); moderate (34 to 30°C); and severe (<30°C).19 Defibrillation is less likely to be successful at temperature below 30°C than above 30°C.

The “Swiss” hypothermia classification was developed to help rescuers estimate core temperature by observing clinical signs.12 Because individuals have variability in response to cold, estimating core temperature on the basis of clinical signs is only an approximation. The stages of the “Swiss” hypothermia (abbreviated “HT”) grading system with descriptions and estimated core temperature are as follows:

HT I—clear consciousness with shivering: 35 to 32°C

HT II—impaired consciousness without shivering: 32 to 28°C

HT III—unconscious: 28 to 24°C

HT IV—apparent death: 24 to 13.7°C

HT V—death due to irreversible hypothermia: <13.7°C? (<9°C?)12

A limitation of this system is that individuals vary in physiologic response to hypothermia. Shivering may be maximal at 32 to 33°C but may continue at 31°C and may not cease until core temperature drops to approximately 30°C. A shivering patient with impaired consciousness should be treated for moderate, not mild, hypothermia. Temperature ranges for hypothermia stages should not be considered absolute but rather correlated with clinical observations. An analysis of reported cases of hypothermia revealed clinically significant temperature overlap with respect to hypothermia staging. The lowest recorded temperature was 28.1°C for HT I, 22°C for HT II, and 19.3°C for HT III.20 It is advised that rescuers focus on the entire clinical picture rather than just shivering. Many case reports describe hypothermic patients with vital signs who had core temperatures below 24°C.21, 22, 23, 24, 25 Individuals with core temperatures below 24°C are very susceptible to VF. Although the Swiss HT system can help guide rescuers in certain situations, we prefer to use the terms mild, moderate, severe, and profound hypothermia.

In addition to hypothermia, many conditions can cause altered mental status and decreased level of consciousness. Conditions such as sepsis and severe trauma can decrease physiologic reserves and may decrease or abolish shivering.26 Many drugs and medications27 suppress shivering.28

The most accurate minimally invasive method of measuring core temperature is esophageal temperature, with the probe inserted into the lower third of the esophagus.29 The degree of accuracy afforded by esophageal temperature monitoring is helpful to guide treatment of patients with moderate or severe hypothermia. Placement of an esophageal probe via the pharynx may cause vomiting and aspiration. The airway must be protected with an endotracheal tube or supraglottic device that allows passage of a gastric tube before placement of an esophageal probe. Heated humidified oxygen does not significantly raise the temperature measured by a properly inserted esophageal probe.30, 31, 32 If the esophageal probe is not inserted into the lower third of the esophagus, an average of 24 cm below the larynx in adults,1 use of heated humidified oxygen may result in a falsely elevated esophageal temperature. Esophageal probes that do not have markings can be measured visually against the patient and marked to ensure the correct depth of insertion. Field conditions will rarely allow for placement of an esophageal probe; however, transporting air or ground medical services that have this capacity should place a probe as soon as possible.

Epitympanic (ear canal) temperature, measured using a soft probe with a thermistor in proximity to the tympanic membrane, reflects carotid artery temperature.33 Epitympanic thermometers should not be confused with the more common and much less accurate infrared “tympanic” thermometers. In patients with adequate cardiac output, epitympanic temperature reflects core temperature. Epitympanic temperature can be lower than esophageal temperature during low flow (decreased cardiac output) or no-flow (cardiac arrest) states.28 In out-of-hospital settings care must be taken to insulate the ear canal from the environment. With cold ambient temperature, epitympanic temperature may be falsely low, especially if the external auditory canal is blocked by cerumen, filled with snow, or not adequately sealed and properly covered with an isolating “cap.”33 Epitympanic temperature is much lower than esophageal temperature during initial cooling of the head in normothermic subjects. After 10 min of isolated head cooling, the mean difference between epitympanic and esophageal temperatures is about 1 to 2°C.34 Epitympanic thermometers for use in the operating room are not suitable for field use; they are not designed for use in cold environments.

The use of rectal thermometers is not advisable before the patient has been removed from the cold environment because the patient must be further exposed, increasing heat loss and potentially worsening hypothermia.

Rectal and bladder temperatures lag behind core temperature changes by as much as an hour and can give the false impression that the patient is still cooling.4,9,35 Rectal and bladder temperature overestimate heart temperature during cooling and underestimate heart temperature during rewarming.

Oral temperatures are useful only to rule out hypothermia. Nonelectronic thermometers are typically unable to measure temperatures below 35.6°C. If mercury or alcohol thermometers are used to diagnose hypothermia, they must be special “low reading” thermometers.1

“Temporal artery” thermometers, used on the skin surface, do not provide accurate temperature measurements in hypothermia.36

A noninvasive heat flux or “double sensor” thermometer is currently under development.37 This technology, which combines a skin temperature sensor with a heat flux sensor, correlates well with esophageal temperature in operative and intensive care unit settings.38,39

Rescuer safety is the first priority during rescue. The scene may be unsafe to enter, or the safety officer in a rescue may allow only brief entry. Unless there are obvious fatal injuries, rescuers might need to move the patient to a safe place before deciding to resuscitate.

Core temperature afterdrop refers to continued core cooling after removal from a cold environment. Afterdrop is caused by a combination of conductive heat loss from the warmer core to cooler peripheral tissue and convective heat loss from blood due to increased flow to cooler tissue and subsequent return to the central circulation and heart.40, 41, 42 The convective component transfers more heat, and, unlike the conductive component, is affected by the method of rewarming. In a hypothermia victim, peripheral tissue is colder than the heart. Therefore, any action that increases blood flow to cold peripheral tissue (eg, hoisting or holding victim in a vertical position, allowing the victim to stand or walk, active or passive limb movement, immersion in warm water) will increase the volume of cold blood returning to the heart. This increases cardiac work and further decreases core temperature.

Afterdrop may be clinically important in victims who are at the threshold of moderate to severe hypothermia because they are susceptible to cardiovascular instability with a small further drop in core (heart) temperature. Afterdrop of as much as 5 to 6°C has been reported in hypothermic patients.21,42, 43, 44 Therefore, care should be taken to prevent increased blood flow to the limbs during and after rescue.

Circumrescue collapse refers to light-headedness, collapse, syncope, or sudden death occurring in victims of cold-water immersion just before, during, or after rescue and removal from water.45 Circumrescue collapse can be caused by mental relaxation and decreased catecholamine output causing life-threatening hypotension or by sudden onset of cardiac dysrhythmia, likely VF.4

The act of removing a victim from water decreases hydrostatic pressure,4 which is normally greatest around the legs. Removing hydrostatic pressure allows blood to pool in dependent areas, causing decreased blood return with resultant hypotension or cardiovascular collapse. A cold heart may not be able to compensate for decreasing blood pressure by increasing cardiac output. Blood that does return from dependent areas will be cooled and will contribute to core temperature afterdrop. Afterdrop is increased if the victim has to perform work to assist in rescue (eg, having to climb a ladder onto a boat).45,46 Mechanical stimulation of the heart during rescue and extrication—combined with hypotension, afterdrop, and acidosis—may precipitate fatal dysrhythmias.47

When rescue is imminent, mental relaxation in conscious patients may be associated with decreased catecholamine release, causing decreased blood pressure with loss of consciousness and subsequent drowning.47 Circumrescue collapse has also been described in terrestrial rescue situations.23,48

Keeping the patient in a horizontal position mitigates the effects of decreased hydrostatic pressure during rescue.45 Avoiding physical effort protects against afterdrop.46 Continued mental stimulus may help maintain catecholamine stimulus.

Hypothermia lowers the threshold for VF, especially at core temperatures below 28°C.1 Movement or significant warming of the extremities, as with warm water immersion, increases blood flow to colder tissues. Blood flows to the periphery, is cooled, and has the potential to cool the heart upon return to the core, increasing the risk of VF.17,49,50 Additional blood return may also cause increased load on a heart that is already pumping ineffectively.

After rescue, the next priority for care of a hypothermic patient in the out-of-hospital setting is to maintain core temperature by preventing further heat loss.

Insulation protects from heat loss. Insulating materials include extra clothing, blankets, quilts, sleeping bags, insulated pads, and bubble wrap.51,52 A sleeping bag should not be used like a blanket; rather the patient should be placed inside, and the enclosure should be completely zipped up. Multiple sleeping bags, if available, can be placed within each other to create a multilayered enclosure. Any available insulation (eg, spare parkas) should be incorporated within or outside the enclosure in such a way as to not compromise loft. Bubble wrap is an effective vapor barrier, but it provides less insulation than the other materials.51,52 A large amount of heat can be lost to the ground by conduction.53 Significant heat can also be lost from the head and neck due to necessary exposure of the face to allow breathing.53,54

Vapor barriers protect against convective and evaporative cooling (substantially reducing heat loss) and importantly also keep the insulation dry and more effective. Barriers can be made from bubble wrap, tarps, sheets of plastic, reflective blankets, or garbage bags with a hole cut out for the face. The vapor barrier(s) may be placed inside the insulation (to keep the insulation dry if the patient is packaged wet) and/or outside the insulation to protect the insulation from a wet environment.52,55

Extra insulation can compensate for the absence of a windproof layer or vapor barrier.55 A combined method using 2 vapor barriers (1 against the patient and 1 outside the insulating layers) will protect the insulation from becoming wet from all sources.52

In windy conditions, a windproof layer, ideally a vapor barrier, provides substantial protection from convective heat loss.51

Vigorous shivering can increase heat production by 5 to 6 times the resting metabolic rate and up to 50% of maximal metabolic rate (V.O2max ).5,56 Shivering can raise core temperature by 3 to 4°C·h-1,16,28,57 but it uses a large amount of energy, stresses the cardiovascular system, and causes patient discomfort.17

Once a patient is protected from further heat loss and has adequate energy reserves, the most effective means of rescue may be for the patient to walk. Allowing the patient to shiver and rewarm, while insulated and before exercise, should help minimize afterdrop.46 This time period should last 30 min but will depend upon the situation. Standing upright increases blood flow to and from the legs, worsening afterdrop and potentially decreasing blood pressure.29 Walking or other exercise generates additional heat, but if initiated immediately after rescue, may cause a greater afterdrop in core temperature than if the patient remains at rest.46 There may be situations when immediate movement is necessary to relocate a patient to a safer environment. When this is necessary, close monitoring is prudent.

Field methods of external rewarming are useful in both shivering and nonshivering patients. Active (exogenous) rewarming methods, such as large electric heat pads or blankets,58 large chemical heat pads,59,60 warm water bottles,61 and the Norwegian charcoal-burning Heat Pac,17,58,62 all provide significant external heat. In a shivering patient, the added heat attenuates shivering heat production. This results in a rate of core rewarming similar to that produced by shivering but has the advantages of increased comfort and decreased energy use with lower cardiac workload. In a nonshivering patient, added heat will warm the core, even if slowly, in a patient lacking capacity for endogenous heat production. The Heat Pac should be used with caution because it can generate potentially toxic levels of carbon monoxide (CO).32

To maximize total body net heat gain, active heating will be more effective in conjunction with insulation and vapor barrier(s) to create an effective hypothermia enclosure system. Five such systems were compared on normothermic subjects in -20°C conditions.63 The systems (all including active heating and vapor barrier) included 3 commercial systems using heavy insulation enclosures, a user-assembled system using a 3-season sleeping bag, and the Hypothermia Prevention Management Kit (HPMK).63 Initially evaluated for effectiveness between other rewarming systems,64 the HPMK was commercially developed for the Department of Defense-Joint Trauma System (JTS) as a lightweight, compact kit designed for field use that combines an oxygen-activated self-heating liner with a vapor barrier. The HPMK was part of the JTS theater-wide strategy for battlefield casualties and has been used extensively in military operations to decrease mortality from trauma-induced hypothermia.65, 66, 67 System effectiveness (net body heat gain) generally depended on the mass of the insulation enclosure.63 The 3 commercial systems were heavy and bulky and therefore only applicable at a point of care or if they could be delivered by sled or vehicle. The user-assembled and HPMK systems could both be transported by backpack; however, the HPMK was smaller and lighter and therefore more portable, but at the expense of providing less net body heat gain. Therefore, factors such as mass, volume, effectiveness, and cost will affect the type of system used in the field. Search and rescue teams, which usually consist of 2 to 4 persons, could realistically carry a more-effective user-assembled system because the items can be separated and dispersed to multiple backpacks (Figure 3). Alternatively, the HPMK should be complemented by incorporating a sleeping bag (or blankets) over the heated liner and inside the vapor barrier.

Body-to-body rewarming of a shivering patient with a warm person in a sleeping bag blunts the increase in shivering thermogenesis, resulting in rewarming rates no greater than shivering alone.57,58 Body-to-body rewarming may make the cold patient more comfortable due to decreased shivering, but at the cost of delaying evacuation.

External heat is most effective if concentrated on the axillae, chest, and back (in that order), which are the areas with the highest potential for conductive heat transfer.53 Upper torso rewarming is more effective than extremity rewarming.68 Some scenarios may. however, preclude applying heat to the chest (eg, cardiopulmonary resuscitation [CPR] in progress or treatment of chest injury). Applying heat to the head, although requiring more technique to insulate and apply the heat, has been shown to be equally effective in shivering and nonshivering subjects, thus providing an alternative warming approach in extenuating circumstances.69

Cold skin is very susceptible to injury from pressure or heat.70 There have been reports of burns associated with use of a hot water bottle with lukewarm water applied directly to hypothermic skin,71 the HPMK,64 water-perfused warming blankets, a Heat Pac, and hot pads.72 Burns have been reported both in controlled settings while researching rewarming methods and during rescue of hypothermic patients. It is important to visually inspect the heated skin at regular intervals (eg, 20 to 30 min) to observe for excess reddening or other signs of pending burns; in these cases, active heating should be stopped in the affected areas. This must be done segmentally and carefully to minimize heat loss. Heated pads should be applied with great caution to areas such as the back that are difficult to visualize or under constant pressure from body weight or immobilization systems.

Small chemical heat packs (eg, those used for hand and foot warming) do not provide sufficient heat to affect core temperature. In addition, the high surface temperature of small chemical heat packs creates a risk of thermal burns.

Although heated humidified oxygen prevents respiratory heat loss, the respiratory tract allows limited heat exchange. Heated humidified oxygen is not effective as a solitary rewarming method,30, 31, 32 but it can be used as an adjunct to other methods.26 Heated humidified oxygen has the potential to cause facial burns.32

A warm shower or bath markedly increases peripheral blood flow, increasing afterdrop and potentially causing hypotension.29,42 Using a warm shower or bath, even in a patient who is mildly hypothermic, may cause cardiovascular collapse. This method of warming may be considered for patients who are cold-stressed or after an initial period of rewarming for those with mild hypothermia.

Distal limb warming in 42 to 45°C water to the elbows and knees is effective for warming alert, mildly hypothermic patients.9 This method works by opening arteriovenous anastomoses in the hands and feet, causing increased return flow of warmed blood directly from the arms and legs to the core. This is an exception to the general rule that peripheral rewarming is contraindicated in hypothermic patients. Because the warmed superficial venous blood bypasses the cold arteries in the extremities, there is little countercurrent heat exchange. In the one laboratory study that used this method, the afterdrop was less than the afterdrop for shivering.9 Distal limb rewarming in water was designed for use on watercraft and is difficult to apply for other out-of-hospital transport.

Continued rewarming is challenging during transport. A randomized, controlled study of care in helicopter and ground advanced life support units showed a small increase in core temperature with using large chemical heat pads but decreased core temperature with passive rewarming, reflective blankets, warm IV fluids, and warm IV fluids plus reflective blankets.60

Forced air warming, usually with an air-filled plastic baffled blanket with continuous heated airflow through perforations in the bottom of the blanket, is an effective way to rewarm a hypothermic patient.30,73,74 In 1 study, afterdrop with forced air warming was less than with shivering.75 Forced air warming is more effective and more practical than a liquid-filled heating blanket.

The temperature in patient compartments should ideally be 28°C, which is the temperature at which unclothed resting normothermic humans will neither gain nor lose heat.1 Warming the patient compartment will protect patients from further heat loss when exposed for monitoring or other procedures. However, an air temperature of 28°C is usually uncomfortably hot for pilots, drivers, and medical providers. A slightly cooler temperature of 24°C will limit heat loss and is better tolerated by ambulance personnel.

A cold patient who is alert and shivering but who has adequate energy reserves and is not hypothermic is at low risk for afterdrop or circumrescue collapse.

Hypothermic patients have survived with normal neurologic function even after cardiac arrest.23,76, 77, 78 Many of the usual indicators of death, such as fixed, dilated pupils and apparent rigor mortis, are unreliable in hypothermic patients.76,77 Dependent lividity is an unpredictable indicator of death in hypothermia.

The dictum that “no one is dead until they are warm and dead” is based on the difficulty of diagnosing death in a hypothermic patient in the field. However, some patients really are cold and dead. General contraindications to attempted resuscitation in the field include obvious fatal injuries, such as decapitation, open head injury with loss of brain matter, truncal transection, incineration, or a chest wall that is so stiff that compressions are not possible.79

CPR is only indicated in cardiac arrest and is contraindicated if there are signs of life. In a hypothermic patient in the out-of-hospital setting, signs of life may be very difficult to detect. The heart rate can be very slow and pulses difficult to palpate. The traditional method of checking a pulse by trying to feel the pulse with a finger placed over the presumed location of an artery is limited by cold. Cold fingers have decreased sensitivity to tactile stimuli. Breathing can be very slow and shallow but may be detectible in the absence of palpable pulses.22 If cardiac monitoring is not available, the diagnosis of cardiac arrest can be difficult.

The lowest known core temperature from which a patient with accidental hypothermia has been successfully resuscitated is 13.7°C.80 The lowest core temperature ever induced therapeutically is 9°C.81 Both patients survived neurologically intact. Induced hypothermia for cardiac or vascular surgery is usually to 18°C and—unlike accidental hypothermia—a very controlled situation. The lowest temperature from which humans with accidental hypothermia can be successfully resuscitated is not known, and reports of recovery from extremely low core temperatures make establishing a temperature cut-off for resuscitation challenging.

Electrocardiographic monitoring is the best way to diagnose cardiac arrest in the field. An organized rhythm without detectible pulses may be pulseless electrical activity or may be a perfusing rhythm with very weak pulses. In hypothermic patients, the amplitude of the QRS complexes may be decreased.10

Starting CPR in a hypothermic patient with an organized cardiac rhythm carries a risk of causing VF that would convert a perfusing rhythm to a nonperfusing rhythm. If end-tidal CO2 (ETCO2) monitoring is available, lack of waveform indicates lack of circulation or absence of metabolism.13 If ultrasound is available, echocardiography can be used to determine if cardiac contractions correspond to electrical activity.13

Cooling reduces resting oxygen consumption of most human tissue by about 6% per 1°C decrease, with a greater decrease in brain tissue. Hypothermia preferentially protects the brain from hypoxia. At a core temperature of 28°C, whole body oxygen consumption is about 50% of normal,1 while brain oxygen consumption may be reduced to about 35% of normal.8 Surgical procedures employing deep hypothermic circulatory arrest (DHCA) have demonstrated a 7% decrease in cerebral oxygen consumption for every 1°C decrease in core temperature. Sixty percent of patients with a core temperature less than 18°C demonstrate an isoelectric electroencephalogram. DHCA is a controlled, rapid, decrease of core temperature from 18 to 20°C. Patients over the age of 60 y undergoing DHCA only tolerated an estimated 25 min of cardiac arrest, based upon incidence of postprocedural cognitive injury. Children tolerate longer periods of time, but there is a dearth of information for young and middle-aged adults.82,83 There are many documented cases of full neurologic recovery, even after extended periods of cardiac arrest as long as 9 h84,85 in persons who did not have asphyxia before they became hypothermic. Severely hypothermic patients have been resuscitated with good neurologic status after as long as 6 h 30 min of CPR.23,86, 87, 88 Prolonged cardiac arrest in severely hypothermic patients does not necessarily cause brain injury as it does in normothermic patients.

In cardiac arrest, the classic teaching is that CPR must be started promptly and continued without interruption until return of spontaneous circulation (ROSC) can be established or death is confirmed. This CPR strategy may not be possible or warranted in patients with severe hypothermia. Multiple case reports describe survival with neurologic recovery when initiation of CPR was delayed and performed intermittently.85,89 In 1 case report, a hypothermic avalanche victim was successfully resuscitated with complete neurologic recovery, although CPR was not started for 15 min after a monitored cardiac arrest.23 In another case report, an avalanche victim was extricated apneic and pulseless after a 5 h burial in a crevasse. No attempt was made to resuscitate the patient, but the patient was flown to a nearby hospital where ECG showed asystole. CPR was started 70 min after rescue. The patient made a full neurologic recovery.48 A third case report described successful resuscitation with good neurologic recovery of a hypothermic patient in cardiac arrest who was treated during evacuation with CPR in a stationary litter for 1-min periods alternating with 1-min periods of being carried without CPR.86

Continuous compressions are ideal, but intermittent compressions may be necessary to successfully and safely evacuate the patient. With properly performed compressions, it takes an estimated 5 min of cerebral oxygenation to overcome the ischemic threshold.90

A hypothermic patient will have a stiff chest wall that limits the effectiveness of chest compressions and bag-valve-mask ventilation. Myocardial and pulmonary compliance are also markedly reduced in severe hypothermia. During hypothermic cardiac arrest with CPR91 in a swine model, cardiac output, cerebral blood flow, and myocardial blood flow averaged 50, 55, and 31%, respectively, of those achieved during normothermic closed-chest compressions. However, metabolic demands are also decreased. Although there are no data to support increased survival when a mechanical compression device is used,92 mechanical devices reduce the incidence of rescuer fatigue and may permit longer periods of uninterrupted compressions, especially when bridging to ECLS.86,93

If an automated external defibrillator (AED) with a cardiac monitor is available, it can be used for cardiac monitoring. An AED without a cardiac monitor can also be used for diagnosis. The cardiac rhythms that may benefit from cardioversion or defibrillation (shockable rhythms) are VT and VF. VT is rare during moderate or severe hypothermia. The instruction “shock is advised” means that the rhythm is VT or VF. The instruction “no shock advised” on an AED without monitoring capability can mean that the rhythm is asystole or an organized rhythm, which may be pulseless electrical activity. Current AHA guidelines recommend a single shock.19

Defibrillation is only indicated for a shockable rhythm (pulseless VT or VF). An AED will only advise a shock if the rhythm is VT or VF. Current resuscitation guidelines recommend a single shock at maximum power for a patient whose core temperature is below 30°C.13,17,94

Patients have been successfully defibrillated at core temperatures below 26°C.95, 96, 97, 98 If defibrillation below 30°C is unsuccessful, delay further shocks until the temperature is greater than 30°C; below 30°C, defibrillation is less likely to be successful. Defibrillation in a patient whose core temperature has reached 30°C should follow guidelines for normothermic patients.13

The principles of airway management are the same in a hypothermic patient as in a normothermic patient. In a patient who is not breathing spontaneously or who is breathing spontaneously but not protecting their airway owing to a decreased level of consciousness, advanced airway management with endotracheal intubation or a supraglottic airway device is indicated to provide adequate ventilation and to protect against aspiration.13,99 Although there are case reports of VF occurring during endotracheal intubation of a hypothermic patient,21,50,100,101 this is an uncommon complication. In a multicenter study, 117 hypothermic patients were intubated endotracheally after preoxygenation with 100% oxygen with no induced dysrhythmias.102

Hyperventilation has many potentially adverse effects in hypothermia, including decreased cerebral blood flow. As shown in the swine model, ventilation without an advanced airway is limited by decreased thoracic compliance.91,103 If available, ETCO2 monitoring can be used to prevent hyperventilation.

If the patient is intubated or has a supraglottic device, ventilation is more effective than in a patient without advanced airway management.

ETCO2 monitoring can be used to keep ETCO2 in the normal range. This range depends on altitude.

At low core temperatures, drug metabolism is decreased; anesthesia and neuromuscular blockade are prolonged.104, 105, 106

Oxygen extraction is not a limiting factor in survival in hypothermia at sea level.107

Obtaining intravenous (IV) access is often difficult in hypothermic patients. Intraosseous (IO) access is fast and reliable. Because the myocardium is irritable in hypothermia, catheters that contact the heart may cause dysrhythmias. Internal jugular or subclavian central lines that extend into the right atrium are contraindicated unless a short catheter is inserted. There is a risk of causing VF if the wire used during placement of a central venous catheter using the Seldinger technique is advanced into the heart. The femoral vein approach allows central venous access without the danger of inducing dysrhythmias, but it may be difficult in the field. Unsuccessful attempts often cause hematomas.

Circulating blood volume in moderate and severe hypothermia is reduced.9,107 During rewarming, vasoconstriction that previously limited the vascular space is abolished. Volume should be replaced to avoid severe volume depletion with resultant shock, while avoiding administration of fluid sufficient to cause volume overload. To prevent further core temperature cooling, IV/IO fluid should be warmed to at least 40°C and preferably to 42°C. In the field, IV/IO bags and tubing should be insulated. Fluid warmers, preferably commercial products that have been proven effective, should be used. Because the effective perfused mass (thermal core) is decreased in hypothermia as a result of intense peripheral vasoconstriction,9 administration of fluid warmed to 40 to 42°C may help increase core temperature. Because metabolism is depressed, glucose-containing fluid is not essential. The fluid of choice for volume replacement is normal saline. Lactated Ringer’s solution should not be used in a hypothermic patient because the cold liver cannot metabolize lactate.1 Some clinicians use a mixture of crystalloid and colloid.1

Giving fluids in boluses, as rapidly as possible, rather than by continuous infusion will alleviate problems with cooling of fluid or freezing of lines, which can occur even if lines are insulated. The ideal method is to saline lock the line when there will be a long pause after a bolus. Boluses of 500 mL can be titrated to maintain adequate systolic blood pressure, depending on the degree of hypothermia. There is no available evidence to quantify a target systolic blood pressure.

Hypo- and hyperglycemia have been reported in hypothermia.96,108 Point-of-care glucose testing is routine in patients with an altered level of consciousness but may not be available in an out-of-hospital setting. Hyperglycemia has not been shown to be deleterious in hypothermic patients.96

There is limited evidence regarding drug effects in hypothermic cardiac arrest in humans. Most of the evidence comes from animal studies.109 The cold heart has long been considered to be unresponsive to vasopressor or antiarrhythmic medications, although some animal studies have suggested otherwise. In a study of hypothermic dogs, epinephrine increased coronary perfusion pressure and ROSC after defibrillation.110 In a hypothermic pig study, vasopressin increased coronary perfusion pressure with active compression-decompression CPR using an impedance threshold valve, but not with standard CPR.111 Vasopressin improved ROSC and 1-h survival after defibrillation in a study of hypothermic pigs.112 There is a case report of ROSC with vasopressin following unsuccessful use of epinephrine (2 mg) in a hypothermic patient, but the patient subsequently died OF multisystem failure.113

The ideal pharmacologic approach to ventricular dysrhythmias remains unresolved. Class III agents, such as bretylium and amiodarone, are theoretically ideal because they act directly against fibrillation. Amiodarone is less effective in hypothermia than in normothermia and carries a risk of inducing torsades des pointes.114 The safety of amiodarone in hypothermia is not known. In a study of hypothermic dogs, the combination of epinephrine and amiodarone increased ROSC after defibrillation following administration of epinephrine alone.110 Bretylium failed to increase the incidence of ROSC in a study of hypothermic dogs.115 In another dog study, neither amiodarone nor bretylium improved ROSC.116 There are 2 clinical reports of resolution of VF following infusion of bretylium.117,118

In hypothermia, drug metabolism is decreased and protein binding is increased.11 Drugs given have little activity while the patient is hypothermic but may reach toxic levels with rewarming.

Two case reports suggest that transcutaneous pacing may be beneficial in the hypothermic patient.119 In both cases, transcutaneous pacing was instituted to increase blood pressure to facilitate arteriovenous rewarming rather than to control heart rate.

Atrial dysrhythmias in hypothermic patients during rewarming are common and resolve spontaneously once the patient has been sufficiently rewarmed.120

Core temperatures <35°C are associated with decreased survival in patients with severe trauma.121,122 Severe trauma can cause acidosis and coagulopathy. In trauma patients with hemorrhagic shock, the “lethal triad” of acidosis, coagulopathy, and hypothermia is associated with multiorgan system dysfunction123 and extremely high mortality.124

Stabilization of injuries for transport is the same in a hypothermic patient as in a normothermic patient.

Alert patients with mild hypothermia can be treated in the field.

Profoundly hypothermic patients (<28°C) and those with hemodynamic instability and witnessed out-of-hospital cardiac arrest may benefit from transport to centers capable of ECLS. ECLS includes the techniques of extracorporeal circulation, extracorporeal membrane oxygenation (ECMO), and coronary bypass. ECLS provides both oxygenation and hemodynamic support for unstable patients while allowing for controlled rewarming.

Patients who are not profoundly hypothermic or hemodynamically unstable should be transported to the nearest facility.87,126, 127, 128, 129, 130, 131

Hemodynamically unstable patients require critical care and may benefit from ECLS with ECMO or CPB. ECMO is preferred over CPB,127 but both have been used successfully to rewarm severely hypothermic patients. Profoundly hypothermic patients with witnessed cardiac arrest, regardless of return to spontaneous circulation in the field, have a greater chance of survival if transferred to a center where ECLS can be initiated.87,126, 127, 128, 129, 130, 131

Many geographic areas do not have a hospital capable of ECLS. Bad weather or other factors may prevent transfer of a patient to a hospital with ECLS. Hemodynamically unstable hypothermic patients, including hypothermic patients in cardiac arrest, have been successfully resuscitated with complete neurologic recovery without using ECLS.88,132, 133, 134

Hypothermic patients with injuries or other medical comorbidities should be transferred to a facility able to appropriately manage the patient. Comorbidities can alter the clinical presentation of hypothermia and could potentially delay recognition of severe hypothermia.135 Asphyxiated patients (from avalanche or drowning) may appear stable but are at risk for delayed complications and are likely to require a higher level of care.127