Core body temperature is the most standardized physiological parameter for diagnosing and classifying the severity of HT. Mild HT is defined as CBT between 32 and 35°C, while moderate HT is defined as CBT between 28 and 32°C, and severe HT is defined as CBT<28°C.15,16 Furthermore, some experts also include deep hypothermia (CBT 20–24°C)7,15 or modify mild HT (temperature 36°C in the presence of trauma).17

The Swiss system for the pre-hospital classification of HT, developed to help rescue teams, estimates CBT from the observation of clinical signs and allows the stratification of severity without having to actually measure CBT (such measurement often being difficult to obtain in the rescue zone17). Based on observation of the level of consciousness and the patient vital signs, 5 grades of HT are identified, with the corresponding correlation to estimated CBT15:

• •

  Mild (grade I or HT I): conscious and shivering victim (35–32°C).

• •

  Moderate (grade II or HT II): drowsy and non-shivering victim (32–28°C).

• •

  Grave (grade III or HT III): unconscious victim, but with positive vital signs (28–24°C).

• •

  Deep (grade IV or HT IV): apparent death, due to absence of vital signs (13.7–24°C).

• •

  Irreversible (grade V or HT V): irreversible death due to HT: totally frozen body (<9–13°C).

The main limitation of the Swiss system is the individual physiological variability in relation to HT. Because of this variability, the estimation of CBT based only on the clinical signs represents an approximation. For example, in some victims shivering may be present at temperatures of under 30°C, or the vital signs may be present at under 24°C – a temperature at which the risk of ventricular fibrillation (VF) is high.18,19 An analysis of the published cases found a strong correlation between CBT and the grade of HT, though in other cases CBT was seen to be overestimated. A revision of this classification, with the analysis of the mentioned cases, corrected CBT to levels lower than those observed. Thus, CBT for HT I would be 28.1°C, versus 22.1°C for HT II and 19.3°C for HT III.18,20

On the other hand, shivering and level of consciousness can be affected by factors such as age, gender, body composition, race, associated conditions such as trauma, central nervous system diseases and intoxication caused by toxic agents, etc., or the administration of drugs such as sedatives, muscle relaxants and opiate analgesics.11

The body measurement zones, in decreasing order of invasiveness, are the pulmonary artery, esophagus, bladder, rectum, eardrum (epitympanic measurement), mouth and skin. The most reliable measurement is that obtained from the pulmonary artery, but it is not advised because of its invasiveness and the risk of triggering arrhythmias.

Esophageal temperature measurement is minimally invasive, correlates well to pulmonary artery temperature, and is very useful in the monitoring and treatment of victims with diminished level of consciousness.18,19 The esophageal catheter should be placed in the lower third of the esophagus in order to avoid overestimating CBT when rewarming is made with warm and humidified oxygen.21

Epitympanic temperature recording correlates well to carotid temperature. It is useful in victims that breathe spontaneously, but is categorically discarded (due to measurement error) in the presence of very low external temperatures, water or snow within the external auditory canal, hemodynamic instability or cardiac arrest (CA).22,23

Rectal and bladder temperature measurements are not recommended outside warm environments or in the rewarming of unconscious victims. Measurement during rewarming may result in underestimated (delay of up to 1h with the real CBT value) or overestimated readings (if performed with peritoneal or colon lavage).11,18,19 Oral cavity or skin temperature measurement is only useful in normothermia.11

The so-called temporal artery thermometers, which measure at the skin surface, do not afford accurate recordings in situations of HT.24

The heat flux thermometer is a novel and noninvasive method which in the Intensive Care Unit (ICU) offers an accurate CBT value.25 If such accuracy is confirmed in the pre-hospital setting, it could prove useful for evaluating and monitoring the treatment of HT.

Core body temperature is a standardized parameter for classifying HT.14–16 However, a drop in CBT is accompanied by great individual clinical variability referred mainly to level of consciousness, the intensity of shivering or hemodynamic stability.26 Based on this variability, and since it is not always possible to measure CBT in the pre-hospital setting, or many rescue teams lack the necessary measuring device,17,27–29 the recommended key factors guiding treatment are the level of consciousness, the intensity of shivering and hemodynamic stability based on blood pressure and heart rate (Fig. 1).11,18,19 The measurement of CBT affords useful additional information.18

Figure 1.

Decision algorithm for the pre-hospital management of accidental hypothermia.11,18,19 (a) HT II (drowsiness)/HT III (unconsciousness). (b) Decapitation, full sectioning of the trunk, decomposition or totally frozen body (HT V). ECMO: extracorporeal membrane oxygenation; HT: hypothermia; CA: cardiac arrest; CPR: cardiopulmonary resuscitation; ICU: Intensive Care Unit.

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In HT, oxygen consumption decreases 6% for every 1°C drop in CBT, and this decrease also affects the central nervous system. The drop in oxygen consumption means that HT has a preventive effect in terms of cerebral and spinal cord hypoxia – thus allowing full neurological recovery after prolonged situations of CA.4,30Table 2 describes the most extreme published cases of accidental HT and their neurological recovery, which proved practically complete in all instances.31–38

Of all the factors that have been related to drowning survival rate, the only established predictive parameter has been immersion time. In this regard, an immersion of under 5min is associated to favorable outcomes, while immersion periods of over 25min invariably prove fatal.39,40

There has been much speculation about the relationship between survival and the age of the victim. Children, and especially small infants, cool much faster than adults following immersion in cold water (<6°C),41,42 and exhibit inefficient trembling,43 have less subcutaneous fat44,45 and present a larger surface area in relation to body weight compared with adults.42 No clear relationship has been evidenced to date, however.39,40

Cases of miraculous survival with good neurological recovery after immersion in cold water, particularly in children37,38,46–48 but also in adults,36,44,49,50 led to the belief that the temperature of the water prevented cerebral hypoxia.11 This relationship likewise has not been confirmed, however.39,40

Most deaths due to snow avalanches are attributable to asphyxia, trauma and HT,51 with asphyxia being the predominant mechanism: it causes 80% of all avalanche fatalities.52 When the airway is not permeable, death occurs in the first 35min of becoming buried. Any open air space around the mouth or nose increases survival.53

The cooling of a victim buried by an avalanche may be rapid, particularly if asphyxia is present.19 The average cooling rates are between 0.8°C/h and 3°C/h, though there have been reports of cooling rates of 9°C/h.19,51,52

In 2015, the European Resuscitation Council (ERC), based on prognostic factors such as the severity of trauma (lethal or non-lethal), the duration of complete burial (>60min or ≤60min), CBT (≥30°C) and serum potassium (<8mmol/l or >8mmol/l) modified its previous recommendations and established a new recommendation for the resuscitation of avalanche victims (Fig. 2).51 More recent algorithms (Fig. 3)52 propose returning to the burial times indicated in the guide of the year 200154: do not start cardiopulmonary resuscitation (CPR) if the airway is obstructed and the burial time has been >35min. They consider that CPR can only prove successful with a burial time of under 35min, regardless of airway patency.

Figure 2.

Algorithm for assessment and intervention in the case of victims that have been completely buried by a snow avalanche. Indications of ECLS51. EAWP: electrical activity without pulse; VF: ventricular fibrillation; CPR: cardiopulmonary resuscitation; ECLS: extracorporeal life support; VTWP: ventricular tachycardia without pulse.

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Figure 3.

Triage and decision making algorithm for avalanche victims under conditions of cardiac arrest. Source: Modified from Kornhall DK, Martens-Nielsen J. The prehospital management of avalanche victims. J R Army Med Corps. 2016;162:406–412. doi:10.1136/jramc-2015-000441.52

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The first priority of rescue is the safety of the intervening team. The scenario may be dangerous, and staying there might be possible only once and for a brief period of time. Under these circumstances, and except in the clear presence of fatal injuries, the victim must be moved to a safer place before any decision can be made, including CPR if the victim is in CA.18,19

Maintaining CBT is the next priority. The body of the victim should be wrapped in insulating material during and after rescue, with the purpose of avoiding further heat loss. Most heat loss occurs as a result of exposure of the head and neck to the environmental cold, and direct contact of the victim with the ground or stretcher.11,18,19

At present, pre-hospital rewarming is considered to be safe and beneficial, and the most effective insulation systems combine insulation with a vapor barrier.53

The vapor barrier protects against convection and evaporation, and can be applied with bubble plastic material, insulating blankets, plastic sheeting, trash bags or any other material capable of affording an impermeable and sealed layer, though always leaving the face exposed in order to allow breathing.55 In victims with dry clothing, the material is applied as the outermost layer. If the clothing is wet and cannot be changed, placing the barrier between the wet clothing and the dry layers could be an effective measure.18,19,56 Although further research is required to demonstrate its true efficacy,28 not changing the wet clothing would be of great benefit, since the victim would be moved less, with less exposure to the environmental cold, and with a lesser risk of further drops in CBT.

Shivering and active movements (where indicated) remain very effective for producing warmth and for rewarming conscious victims capable of movement (HT I). Shivering, which increases energy expenditure and oxygen demand, is uncomfortable for the victim and moreover requires the administration of warm liquids and carbohydrate-rich foods. Exhaustion of the energy reserves would cause the disappearance of trembling.57 The temperature of the liquids and foods must be controlled to avoid burns.

Other still valid alternative or complementary active external rewarming techniques are the use of reflectant blankets, electric cushions, chemical heat packs of sufficient size for the axillas, chest and back, or the administration of warm and humidified oxygen.1 These measures attenuate shivering, avoid energy depletion, and afford comfort for the victim. The heat sources should never be applied directly to the skin, as this may cause burns. Rewarming of the upper trunk is safer and more effective than rewarming of the extremities. Body-to-body rewarming inside a sleeping bag with a warm person also serves to reduce shivering, but should not be allowed to delay evacuation of the victim.11,18,19

Guaranteeing safe rescue and rewarming of a victim not in a situation of CA implies the prevention of rescue cardiovascular collapse and further drops in CBT (afterdrop).

Rescue cardiovascular collapse refers to syncope or sudden death in victims immersed in cold water occurring just before rescue, during or after rescue, or even later. It is attributed to sudden hypotension or ventricular fibrillation.58 The cessation of the hydrostatic pressure exerted by the water upon the body may trigger such hypovolemic shock.1,19 It has also been related to arterial hypotension secondary to a decrease in catecholamine release, coinciding with mental relaxation, when rescue is imminent.44 In order to prevent this situation, it is advisable to move the victim carefully – as with all hypothermia victims – and keep him/her in a horizontal position.1,18,19,44 This type of collapse has also been described in terrestrial rescue operations.31,59

Afterdrop refers to the additional drop in CBT once the victim has been protected from the cold and rewarming has started. The highest incidence of afterdrop is observed during the rewarming of moderate to severe HT. This serious complication, which can trigger ventricular fibrillation, has been related to peripheral vasodilatation and the return to the heart of the cold blood with acidemia accumulated within the extremities. Exclusive rewarming of the lower extremities or applying hot showers or baths facilitates the appearance of afterdrop. There is still controversy as to whether this effect is exclusively related to active external rewarming or to any other form of rewarming.5

Experimental studies have shown that small changes in temperature (0.5–1°C) in conscious and collaborative victims (HT I) can cause this effect with minimally invasive rewarming accompanied by physical exercise. For this reason, some experts consider that the victims should remain at rest for 30min, if allowed by the circumstances and provided there is no undue risk for the rescuers.18,19 In practice, mobilization should not be impeded if the victim is conscious and can contribute to rescue.11,60

Venous access is usually difficult because of the peripheral vasoconstriction. An important alternative, due to its similar multifunctionality, is an intraosseous access.1,19 The femoral route is free of the risk of ventricular fibrillation and is therefore the only accepted central access, but should be reserved for extracorporeal life support (ECLS).

The circulatory volume in moderate to severe HT is reduced. This situation begins to manifest during rewarming as peripheral vasoconstriction gradually subsides. In order to secure hemodynamic stability, it is advisable to administer the required amount (with control of overload) of physiological saline solution warmed to 41–42°C. In order to maintain the temperature, the most practical approach is to administer it in bolus form rather than as a continuous infusion.7

However, fluid therapy as a measure to warm the victim is not very effective. Danzl and Huecker calculated that each liter administered at 42°C provides 14kcal to a 70-kg victim with a CBT of 28°C, and elevates CBT by only 0.33°C.64 Enormous warm fluid volumes therefore would be needed to substantially elevate CBT.

Future investigations are needed to evaluate the effectiveness of recently developed battery-equipped liquid warming devices.53

Most studies of these drugs have been carried out in animals. Because of the limited available evidence, they should not be administered to patients with CBT<30°C.65

In the presence of bradycardia with hypotension, the use of a percutaneous pacemaker should be considered (intravenous pacemakers are contraindicated due to the risk of arrhythmias).19

Electrocardiographic monitoring is the best method for diagnosing lethal arrhythmias or CA. Lethal arrhythmias such as ventricular tachycardia or ventricular fibrillation usually do not respond to defibrillation until CBT has risen to reach 30°C – though there have been reports of successful defibrillation with CBT 24.2°C.66 It is advisable to attempt three cycles of adrenalin/defibrillation (at maximum discharge) in situations below 30°C. If unsuccessful, CPR should be continued, and repetition should not be made until CBT rises 1–2°C or until 30°C is reached.52 Once the latter temperature has been reached, we should continue with the recommendations of the current CPR guides.51,67

The lowest CBT successfully resuscitated in an accidental HT victim was 13.7°C.36 The lowest therapeutically induced CBT was 9°C.68 In both cases recovery was without neurological sequelae. Since cardiac surgery uses hypothermia induction to as low as 10°C, this temperature has been suggested as the lowest temperature for starting resuscitation.18 Nevertheless, although with the existing technology it is unlikely that successful resuscitation can be achieved with temperatures under 10°C, there are no convincing data for setting a lower limit for successful resuscitation. The recommendation is that in the absence of contraindications to CPR, we should attempt resuscitation in all deep HT victims, regardless of CBT.18,19

In the absence of vital signs for 1min (including carotid pulse), CPR should be started unless echocardiography (if available) evidences cardiac contractions.19

Hypothermia victims often require prolonged high-quality CPR.31–35 Manual CPR might prove technically impossible during rescue or evacuation. Mechanical thoracic compression devices help maintain continuous and quality CPR during evacuation and transport and, in severe HT, facilitate sufficient oxygen supply to the vital organs.11 However, maintaining continuous CPR also might not be possible. In these situations, and conditioned to the CBT, we could delay CPR or apply it manually and intermittently (Fig. 4).69

Figure 4.

Manual and intermittent CPR algorithm for hypothermic victims when continuous CPR is not possible.69 CA: cardiac arrest; CPR: cardiopulmonary resuscitation; CBT: core body temperature.

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Echocardiography or capnography can be used to monitor the efficacy of CPR or check the recovery of spontaneous heart rate.63

Hyperpotassemia is a cell lysis and death marker. The highest known potassium level where resuscitation proved successful was 11.8mmol/l.70 The experts and guides recommend suspending CPR and declare patient death if the serum potassium concentration exceeds 12mmol/l.11,18,19

In avalanche victims, the highest concentration where resuscitation proved successful in the absence of associated trauma was 6.4mmol/l.71 According to the European Resuscitation Council (ERC), the potassium cut-off point for not starting or for suspending CPR should be over 8mmol/l (Fig. 2).51 Recent algorithms (Fig. 3) propose a revision in which the potassium levels are not taken into acount.52

Since the introduction of ECLS as a rewarming technique, routine practice and the literature question the validity of potassium as a viability marker.13 Recently, Pasquier et al. have developed a scoring system with multiple independent variables (the HOPE score) that is superior to dichotomic triage based on the potassium concentration, for predicting which victims stand to benefit from ECLS. Further studies are needed to validate this tool, however.72