The management of severe accidental hypothermia is not exclusive of rescue or urgent pre-hospital medicine but can also be extended to Critical Care Medicine and even heart surgery and extracorporeal circulation programs. In the event of accidental hypothermia it is essential to correctly select the victims who might benefit from transfer to a higher level hospital center. Wrong decisions at the time of first aid can subject the patient to long and unnecessary transfer to a hospital that may not have the technology best suited for the management of severe hypothermia. The International Commission for Mountain Emergency Medicine (ICAR MEDDCOM), the International Society for Mountain Medicine and Medical Commission and the International Mountaineering and Climbing Federation (UIAA MEDDCOM) have published consensus guides for the classification and pre-hospital management of severe hypothermia victims secondary to mountaineering accidents.1 The basic principles for the classification, resuscitation and management of accidental hypothermia victims are the same in the maritime, mountaineering and urban settings.2

Hypothermia is defined as a decrease in core body temperature (CBT) to below 35°C. The condition is classified as mild when CBT is between 35 and 32°C, moderate when between 32 and 30°C, and severe when under 30°C. Accidental hypothermia is turn refers to a spontaneous, unintended decrease in CBT generally occurring in a cold environment, associated to an acute problem of some kind, and without prior damage of the hypothalamus, which is where the body thermostat is located.

Cold is the clear and fundamental cause of accidental hypothermia, though its pathogenic action depends on the intensity of cold, the duration of exposure, and the environmental conditions. The usual causal situation is immobilizing injury in a cold scenario, cold exposure without adequate protection, or immersion in cold water (Table 1).

Thermoregulation is the balance between heat production (thermogenesis) and heat elimination (thermolysis). This active equilibrium, which keeps body temperature as close as possible to 37°C, allows the enzyme systems to function within a narrow optimum metabolic window or margin. In situations of mild hypothermia, the thermoregulatory mechanisms operate at a maximum in an attempt to combat heat loss, with trembling (shivering), cutaneous vasoconstriction, diminished peripheral perfusion, increased cerebral blood flow, increased diuresis (cold diuresis), increased heart rate, increased respiratory rate, increased cardiac output and increased blood pressure. If the condition of the victim worsens and the CBT drops to below 30–32°C, enzymatic activity is slowed, the capacity to generate heat decreases, the thermoregulatory system becomes exhausted, the functions of different body organs progressively decrease, and death ultimately results due to cardiorespiratory failure.3–7

Of the physiopathological alterations reflected in Table 1, four are particularly relevant:

• (1)

  Ventricular fibrillation (VF) and asystolia may occur below a CBT of 28°C. However, some authors consider that asystolia does not usually manifest above 23°C, unless there is some other concomitant cause, and that asystolia presenting before the temperature drops to 23°C implies a poorer prognosis.8

• (2)

  Inadequate patient management, e.g., involving sudden postural changes, can trigger ventricular fibrillation.

• (3)

  At a temperature of 18°C the brain can tolerate periods of cardiac arrest 10 times longer than at 37°C.4

• (4)

  Oxygen consumption decreases by 6% for every 1°C drop in CBT, and this reduction also affects the central nervous system (CNS) (brain and spinal cord). The decrease in oxygen consumption causes hypothermia to exert a preventive effect upon brain and spinal cord hypoxia, allowing complete neurological recovery after prolonged immersion in icy water, in serious head and spinal cord injury victims, or in cases of cardiorespiratory arrest.4,9,10

Until well into the 1980s, the “lethal triad” of hypothermia, acidosis and coagulopathy was considered to be the main cause of mortality in critical trauma patients. Even in urban settings, and logically conditioned to the climate and temperature, when the duration of transport to the nearest hospital is about 15min, close to 50% of all penetrating injury victims suffer hypothermia upon admission to the emergency room.11 According to different studies in trauma victims, the mortality rate among hypothermic versus normothermal patients increases up to 50%.12 Those particularly vulnerable are burn victims, patients with serious head injuries, or individuals with upper spinal cord injuries affecting the sympathetic chain. Age also exerts an influence, and in this sense vulnerability is greatest among the very young and the very old, due to their scant capacity to respond to body heat loss. Preventive measures to minimize heat loss and, where indicated, the start of active rewarming, are two key elements for good trauma victim management.13

In first care it is essential to determine CBT using an epitympanic (ear) or esophageal thermometer allowing low temperature measurements.1 Esophageal temperature measurement is the most adequate and reliable option, however. Epitympanic measurements can give rise to false readings in certain circumstances such as very low external temperatures, blocking of the auditory canal with snow or water, and the absence of carotid flow (as in cardiac arrest). Epitympanic measurement can be useful in spontaneously breathing patients, but is categorically discarded in deceased subjects.3

The ICAR MEDDCOM and the UIAA MEDDCOM proposed a practical method based on the clinical signs and their relation to CBT. This classification, which could also be used by minimally trained, non-medical personnel, divides hypothermia into 5 grades1:

• •

  Grade I: The victim is conscious and trembling (35–32°C).

• •

  Grade II: The victim is drowsy and is not trembling (32–28°C).

• •

  Grade III: The victim is unconscious but with positive vital signs (28–24°C).

• •

  Grade IV: Absence of vital signs; apparent death (24–13.7°C?).

• •

  Grade V: Death secondary to irreversible hypothermia (CBT<13°C).

Since there are severe hypothermia victims who have successfully recovered after several hours of asystolia10; since the lowest CBT resulting in recovery without neurological sequelae has been 13.7°C16; and since the rescue medical teams working “on the ground” may have doubts as to whether they are dealing with a grade IV victim with associated lesions but in a potentially reversible condition, or whether they are dealing with a grade V victim implying death due to irreversible hypothermia, the ICAR MEDDCOM and UIAA MEDDCOM consensus guide on accidental hypothermia contemplates a series of clinical data that allow us to differentiate between these two hypothermia grades and thus avoid futile resuscitation attempts. These data are summarized below1:

• (1)

  The chest and abdomen are not compressible.

• (2)

  The heart is permanently in asystolia (in grade IV there may be asystolia or VF).

• (3)

  The core temperature is <13.7°C (?), this being the lowest CBT resulting in successful resuscitation recorded to date.16

• (4)

  Serum potassium is >12mEq/l. This latter criterion is only applied in cases of hypothermia caused by asphyxia, as in avalanche or water immersion victims. In other etiologies of hypothermia with the associated possibility of hemolysis or rhabdomyolysis, as in polytraumatized patients for example, further studies are needed.

The general objectives of pre-hospital care are withdrawal and insulation from the cold environment, the prevention of a posterior drop in CBT (afterdrop), careful management of the victim and transfer to a hospital with the technological means required for treating this grade of hypothermia. As protection and insulation measures, and in addition to isolating the victim from the floor and affording protection from rain, wind or snow, it is useful to change wet clothes to dry clothes, cover the victim with a cap or hat, gloves, woolen blankets or sleeping or survival (bivouac) bag. The recommendations for management of the different grades of hypothermia are detailed below1,17 (Fig. 1).

Figure 1.

Pre-hospital care algorithm for severe accidental hypothermia severe victims (up to grade IV). ECC, extracorporeal circulation; CPR, cardiopulmonary resuscitation; ALS, advanced life support; ICU, Intensive Care Unit.

(0.42MB).

Modified from: Peek et al.49

In the mountain regions of Europe and North America, the practice of winter sports causes an average of 150 deaths each year due to snow avalanches. Such accidents tend to affect a number of victims and typically take place in areas that are difficult to reach by the rescue teams. Death is a result of asphyxia, trauma or accidental hypothermia. The above described classification of hypothermia and its treatment is closely related to people who have been buried by an avalanche and are in a situation of cardiorespiratory arrest.22

Three factors are implicated in patient classification and survival in cardiac arrest resulting from burial in a snow avalanche: the duration of burial in the snow, the presence or absence of a snow-free airway, and the CBT of the victim. Based on these three factors, and knowing that the cooling rate of an avalanche victim with a free airway is 3°C/h (after 90min buried in snow the CBT has dropped to 32°C), there are a number of individuals who cannot survive in the presence of cardiac arrest:

• (1)

  Those buried for more than 35min and with airway obstruction at the time of extraction.

• (2)

  Those buried for more than 35min, with a snow-obstructed airway and a CBT of ≤32°C.

• (3)

  Buried victims with airway obstruction and initial serum potassium >12mmol/l.

In these three cases the death of the victim is certified at the site of the accident, in the same way as in the presence of fatal injuries. However, there are two additional situations where ALS maneuvering should be started, according to the current international recommendations, with transfer to a hospital center equipped with the means needed for intensive care and, where applicable, the start of rewarming1,17:

• (1)

  Victims extracted within 35min or less. If no response is obtained following 20min of resuscitation, death as a result of asphyxia is certified.

• (2)

  Victims buried for over 35min, with a CBT of ≤32°C (grade II or higher), and doubts as to obstruction or no obstruction of the airway.

The considerations referred to the rescue of snow avalanche victims are summarized in Table 2,23 and the assessment and treatment algorithm is shown in Fig. 2.22,24

Figure 2.

Algorithm for the assessment and treatment of avalanche victims at the site of the accident.

(0.38MB).

Modified from: Brugger et al.24

Since the vital signs may be absent or undetectable in severe hypothermia (grade IV), they are of little use in certifying patient death. In these cases, where the pulse and breathing can be practically imperceptible and it is easy to make mistakes, most authors are of the opinion that the pulse is irrelevant in hypothermia,24 though some authors recommend large artery pulse palpation for at least 40s.25 Before concluding that cardiac arrest has occurred, it is advisable to monitor the ECG tracing for at least 1min. Cardiac monitorization is also essential to detect fatal arrhythmias. When the skin of the victim is extremely cold or damp, it may prove difficult to obtain an ECG tracing with adhesive electrodes. In such situations needle electrodes are advised.24 The current portable echocardiography or Doppler devices can be very useful for assessing the presence of cardiac output.

If cardiac arrest has been confirmed, or if there is no pulse, immediate CPR should be started, with a compression/ventilation ratio identical to that used in the resuscitation of normothermal patients, and provided the continuity of CPR can be guaranteed until reaching the hospital—particularly considering that the absence of a pulse does not necessarily imply cardiac arrest, and that CPR itself may trigger VF, as a result of which interruption would prove fatal for the patient. If necessary, orotracheal intubation is to be carried out, with mechanical ventilation adopting a protective ventilatory strategy [low positive end-expiratory pressure (PEEP) and tidal volume], or the administration of warm, lactate-free fluid therapy, since in the context of hypothermia the liver metabolization of lactic acid may be affected, thereby causing lactic acidosis.26 It has been shown that adrenalin improves coronary perfusion in asystolia due to hypothermia, though it was not found to improve survival in experimental studies in pigs.27,28 When the CBT does not exceed 30°C, adrenalin and other vasoactive drugs should be administered with great caution, since the adrenergic receptors respond poorly to low temperatures, and the reduction of vasoactive drug metabolism may give rise to potentially toxic concentrations when administered on a repeated basis. Amiodarone poses the same problems.29 When the CBT with rewarming exceeds 30°C, the intervals between successive drug doses should be doubled until the CBT is again close to normal. From this point onwards, the doses used can be those usually administered in resuscitation in general.

The rhythm disorders that appear in hypothermia, with the exception of fatal arrhythmias, tend to resolve spontaneously as the core temperature returns to normal. Sinus bradycardia can be regarded as physiological, and no pacemaker is needed unless the condition persists after rewarming. Although patient response to defibrillation is usually not seen until the CBT has exceeded 30°C, it can be attempted at lower temperatures and before reaching hospital. If defibrillation proves ineffective, no more than three discharges should be applied, and the technique should only be attempted again once the CBT has exceeded 30°C.30,31 In Spain, a case of VF reversion has recently been described with a CBT of 26°C.10

Temperature afterdrop refers to the additional decrease in CBT occurring once the patient has already been protected from the cold and rewarming has been started. This serious complication, which can trigger ventricular fibrillation,32 is related to peripheral vasodilatation, with the return of cold blood from the extremities towards the core blood circulation compartment,33–35 and active external rewarming. Other authors consider afterdrop to be a physical phenomenon not necessarily related to peripheral vasodilatation and attributable to the existing thermal gradient once cold exposure has ceased, between the peripheral compartment (skin and extremities), which is still cold, and the internal or core compartment (internal organs) which is warmer and transmits heat to the peripheral compartment.36 Rewarming shock or collapse is likewise observed during rewarming and related to the afterdrop effect; according to some authors, it is also related to active external rewarming and peripheral vasodilatation,37 while other investigators consider both phenomena to be independent of the form of rewarming involved.36

Peri-rescue collapse is generally associated to cases of hypothermia resulting from exposure to very cold water, in which the patient suffers clinical worsening after rescue and before rewarming is started (post-immersion collapse).38 Such hypovolemic shock may manifest after cessation of the hydrostatic pressure exerted by the water upon the body.39 There are examples of victims apparently stable and conscious after rescue who develop shock with symptoms ranging from syncope to VF and asystolia.40 There have been reports of deaths immediately before, during or immediately after rescue, and up to 24h afterwards.41–43 Since vertical extraction of the victim can increase this risk, due to losses and changes in the distribution of the body fluids,40 it can be prevented by keeping the patient in a horizontal position.

Once insulation and protection against cold have been secured with the aforementioned measures, and without neglecting the possibility of afterdrop, rewarming can be started in the following forms:

• -

  Endogenous rewarming: encouraging the patient to tremble or do exercise to generate warmth.

• -

  Passive external rewarming: adequate for conscious victims, with mild hypothermia and who are able to shiver. Passive external rewarming raises CBT between 0.1 and 0.7°C/h, and is best achieved in a warm environment, covering the victim with woolen blankets, aluminum foil, a cap or hat (up to 50% of body heat can be lost from the head), etc.

• -

  Active external rewarming: the most practical and easy method is to place warm water bags or chemically heated packs over the areas of the major vessels (neck, armpits, chest, abdomen and groin), avoiding direct contact with the skin (due to the risk of burns).44 Such measures are adequate for victims with moderate hypothermia, and raise the CBT between 1 and 2°C/h.

• -

  Active internal rewarming: in victims with severe hypothermia but preserving an active cardiac rhythm we can infuse warm intravenous fluids, administer warm and humidified oxygen for breathing,45,46 or use warm saline solution to irrigate the organic cavities such as the stomach, pleural space, peritoneum, colon and/or bladder cavity. This technique is able to raise the CBT between 1 and 7°C/h. In victims under cardiac arrest, rewarming should be carried out via extracorporeal circulation—this being a more aggressive and invasive method than the above techniques, and which will be commented further below. The CBT is raised between 8 and 12°C/h in this case.

Active rewarming should not be started before the victim has been rescued, insulated and protected from the cold, and has been moved to a safe place under control, provided the means are available, pre-hospital care can include endogenous rewarming and passive and/or external rewarming, with due CBT monitorization in all cases. Pre-hospital internal rewarming with warm fluid therapy and humidified gases can also be used but is not particularly efficient. In the case of pre-hospital rewarming, two specific methods should be considered:

• (1)

  The so-called “arteriovenous shunt technique”. This facilitates active external rewarming and can be applied in a base camp, mountain refuge or onboard a ship. The arms and/or legs of the victim are immersed in water at temperature of 42–45°C, resulting in rewarming at a mean rate of 6.1°C and 9.9°C/h, respectively.47,48 Such rewarming is possible because the arteriovenous anastomoses of the hands and feet act as heat exchangers.

• (2)

  As an alternative to the administration of warm and humidified air or oxygen, many mountain rescue teams are equipped with an active internal rewarming system known as a “little dragon” or “thermal parachute”. This is a cylinder weighing 2.2kg capable of warming air or oxygen taken from the exterior and administered to the patient through a connected face mask. Rewarming is achieved thanks to a chemical reaction taking place within the cylinder between carbon dioxide and medical-grade caustic soda. The system is also available in a non-caustic version. It is able to warm the air/oxygen passing through the cylinder for 5h, at a temperature of 40–50°C, and to humidify oxygen at a rate of up to 10lpm.21

The hospital management of hypothermia constitutes the third and last link in the patient care chain, which starts with first care (extraction, evaluation, protection, resuscitation), is followed by a second care step (vigilance, rewarming, resuscitation, transfer), and ends with patient arrival and admission to hospital.

Once the patient arrives in hospital, and depending on the grade of hypothermia, admission to the Emergency Department or to Intensive Care is decided (Fig. 3).49 The general measures applied to all such victims admitted to hospital are the following:

• (1)

  Initial evaluation and compilation of the case history, including the aspects most relevant to the antecedents and etiology of the accident.

• (2)

  Exclusion of other causes of cardiac arrest such as alcohol, pharmacological or drugs of abuse overdose, and other reversible causes, using the four Hs and Ts approach of the advanced life support (ALS) algorithm.50

• (3)

  Monitorization of the main vital signs, including CBT, which is also essential for assessing the effectiveness of rewarming. Oxygen saturation measured by pulsioxymetry is usually not useful, due to the intense peripheral vasoconstriction seen in hypothermia victims.

• (4)

  Continuous ECG monitorization for the detection of arrhythmias.

• (5)

  Pertinent biochemical and hematological tests.

• (6)

  Monitorization involving catheters inserted in the right-side cavities (e.g., Swan-Ganz catheters) should be avoided, since such devices are able to induce serious arrhythmias.

• (7)

  Initial volume replacement using lactate-free crystalloids. The monitorization of urine output and central venous pressure is very useful for assessing volume replacement.

• (8)

  During rewarming, the victims require large volumes of warm fluids, since vasodilatation causes expansion of the intravascular space.

Figure 3.

Algorithm for the management of hypothermia victims in a hospital with intensive care facilities.49 ECP, extracorporeal circulation pump; ECMO, extracorporeal membrane oxygenation; CVVHP, continuous venous–venous hemofiltration and hemoperfusion.

(0.19MB).

This initial intervention and posterior patient management (Fig. 4) do not differ much from the algorithm proposed by the Steering Committee of the National CPR Plan of the SEMICYUC as a protocol for the objectives-oriented optimization of the management of post-cardiac arrest syndrome.51 During rewarming it is necessary to take the following into account:

• (1)

  Metabolic and water-electrolyte corrections should be carried out parallel to volume replacement.

• (2)

  The initial hyperglycemia does not require insulin use for correction. It usually normalizes during rewarming as the energy requirements increase and pancreatic insulin secretion normalizes. It is advisable to maintain a tolerable level of hyperglycemia, between 110 and 180mg/dl, in order to avoid hypoglycemia.

• (3)

  Hypopotassemia and hypophosphatemia can cause serious complications during rewarming.

• (4)

  The hypothalamus–hypophyseal dysfunction produced by the generalized slowing of enzyme activity requires no replacement therapy (including corticosteroids).

• (5)

  The prophylactic use of antibiotics to prevent lung infections is not indicated. The victims of avalanches or aquatic accidents that have been immersed in snow or cold water but have survived are at a high risk of suffering respiratory complications.52

• (6)

  Cardiovascular stability is only established once the CBT no longer decreases and rewarming has started. As has been commented above, vasoactive drugs and antiarrhythmic agents should not be used with low CBT values, and any such a use requires extreme caution.

• (7)

  The hypothermic heart does not easily respond to atropine, defibrillation or pacemaker implantation. Although defibrillation is usually not effective until the CBT has exceeded 30°C, there has been a report of successful defibrillation at a CBT of 20.4°C.53

• (8)

  As the CBT decreases, an increase is seen in the blood solubility of CO2 and in the pH value. Blood gas analyzers are usually programmed for a CBT of 37–38°C, and if the actual CBT values of the patient are not entered, the values obtained will not reflect the true situation. Theoretically, for every 1°C of temperature below 37°C, the pH value increases by 0.0147, PaO2 decreases 7.2% and PaCO2 decreases 4.4%.5 This means that after correction we can obtain very low PaO2 values (25mmHg).3,9 The important consideration is not so much the correction of values but the performance of serial controls during rewarming, and the comparison of results.

• (9)

  Oxygen consumption increases greatly during rewarming. In this context oxygenation is to be monitored, oxygen should be administered according to need, and mechanical ventilation should be introduced if required. Hypocapnia is frequently seen in patients subjected to mechanical ventilation.

• (10)

  The maintenance of moderate metabolic acidosis is recommended, with a pH value of about 7.25, to compensate for the left shift induced by hypothermia and facilitate oxygen uptake and release at tissue level.

• (11)

  Reductions in hematocrit should be monitored in hypothermia victims with associated traumatisms.

• (12)

  Coagulation alterations are to be monitored: thrombocytopenia, platelet dysfunction, prolonged clotting time due to alteration of the extrinsic coagulation pathway, and disseminated intravascular coagulation.6,7

• (13)

  Neither the method nor the rate of rewarming is dependent upon the patient CBT. The decision should be based on the presence or absence of a cardiac rhythm and its influence upon cardiac output and peripheral perfusion.

• (14)

  After rewarming with peritoneal lavage, there have been reports of fatal mesenteric ischemia caused by the difference in gradients between the cold blood of the mesenteric circulation and the warm saline of the peritoneal lavage. Such situations have caused the technique to be questioned. However, these problems have also been described as middle-term complications of the extracorporeal circulation rewarming of severe hypothermia patients.54

• (15)

  In the presence of severe hypothermia, where the priority concern is to restore an effective cardiac rhythm as quickly as possible, the techniques guaranteeing faster rewarming are the extracorporeal approaches such as ECP and ECMO. A hospital lacking these techniques but with a critical care unit could resort to continuous venous–venous hemofiltration and hemoperfusion (CVVHP).55,56 Promoting the latter technique in second level hospitals or centers with critical care units would offer an alternative in the organization of the management of accidental hypothermia victims and could obviate the need for transfer to a third level hospital equipped with ECP and/or ECMO, which possibly would take longer and prove more costly. The two additional advantages of CVVHP are the following:

Figure 4.

Algorithm for the management of accidental hypothermia in an Intensive Care Unit.

(0.39MB).

The technique is more accessible, less costly and very probably closer to the accident site than ECP or ECMO found in third level centers—though according to some authors CVVHP has not been as effective in victims with asystolia or ventricular fibrillation.

• •

  This system has the added advantage that the circuit can be operated without heparin; as a result, it can be useful in application to victims with traumatisms contraindicating rewarming with ECP or ECMO.

Extracorporeal rewarming is the technique affording the fastest recovery of patient CBT (up to 10°C/h). According to many authors, it is the best option for rewarming patients under cardiac arrest.57–60 Extracorporeal rewarming can resuscitate hypothermia victims with complete neurological recovery and no ulterior sequelae, even after 2h of asystolia—though such full recovery has also been possible in longer cardiac arrests without such invasive techniques.10 According to these authors, the use of such an invasive technique implies an important delay in the start of treatment. One of the largest studies published to date found the delay associated with these rewarming procedures to be 141±50min.57 The survival rate among those patients without asphyxia prior to hypothermia can reach 64%, though the prognosis is much poorer when asphyxia effectively precedes hypothermia, as in immersion or avalanche victims.61

The rewarming rate depends on the relationship between the temperature of the blood and the flow allowed by the extracorporeal pump. High temperature gradients between the blood within the circuit and the patient CBT are related to a poorer neurological prognosis.62 Normally the pump is operated with a circuit/patient temperature gradient of between 5 and 10°C, which allows internal and external rewarming and at the same time minimizes the afterdrop effect. Temperatures in excess of 40°C can cause denaturalization of the blood cellular and humoral components.

As has been commented, during the rewarming of victims subjected to mechanical ventilation, respiratory alkalosis due to hypocapnia is observed, more or less altered by other variables such as previous metabolic acidosis. There is ongoing debate over the best way to manage pH and patient ventilation under these circumstances. Management in “pH-stat” form keeps the pH value stable by supplying extra CO2 to ventilation of the patient. Management in “alpha-stat” form in turn is based on lowering ventilation to keep the pH stable at 7.40. The experts recommend this latter approach.57–61

The indications of extracorporeal rewarming techniques can be applied to: (1) victims with positive and sufficient cardiac output; and (2) victims with inexistent or insufficient cardiac output.

• -

  Hyperthermia should be avoided during and after rewarming.

• -

  Post-resuscitation care should be continued after rewarming has been achieved, including the maintenance of mild hypothermia if necessary.51

• 1.

  International Commission for Mountain Emergency Medicine: www.ikar-cisa.org. Access confirmed in November 2011.

• 2.

  International Society for Mountain Medicine: http://www.ismmed.org. Access confirmed in November 2011.

• 3.

  Medical Commission of the International Mountaineering and Climbing Federation: http://www.theuiaa.org/medical.html. Access confirmed in November 2011.

The authors declare no conflicts of interest.