Accidental hypothermia is an environmental condition with basic principles of classification and resuscitation that apply to mountain, sea or urban scenarios. Along with coagulopathy and acidosis, hypothermia belongs to the lethal triad of trauma victims requiring critical care. A customized healthcare chain is involved in its management, extending from on site assistance to intensive care, cardiac surgery and/or the extracorporeal circulation protocols. A good classification of the degree of hypothermia preceding admission contributes to improve management and avoids inappropriate referrals between hospitals. The most important issue is to admit hypothermia victims in asystolia or ventricular fibrillation to those hospitals equipped with the medical technology which these special clinical scenarios require. This study attempts to establish the foundations for optimum management of accidental hypothermia from first emergency care on site to treatment in hospital including, resuscitation and rewarming with extracorporeal circulation. Classification of hypothermia Cardiopulmonary resuscitation La hipotermia accidental es una patología ambiental con unos principios básicos de clasificación y reanimación que sirven tanto para el medio montañoso, marítimo o urbano. Esta patología ha formado parte, junto a la acidosis y la coagulopatía, de la famosa «tríada letal» de las víctimas traumáticas en situación crítica. En su manejo y asistencia está implicada toda una cadena asistencial que se extiende desde la medicina de urgencia prehospitalaria hasta la medicina intensiva, llegando incluso hasta la cirugía cardiaca y/o a los programas de circulación extracorpórea. Una buena clasificación prehospitalaria del grado de hipotermia facilitará su manejo inicial y evitará traslados interhospitalarios o secundarios innecesarios. Lo fundamental es trasladar, con la mayor urgencia posible, a las víctimas hipotérmicas en asistolia o fibrilación ventricular hasta aquellos hospitales que tengan la capacidad tecnológica adecuada para el tratamiento de estas especiales situaciones clínicas. Este artículo, trata de sentar las bases que faciliten un manejo adecuado de la hipotermia accidental desde la primera asistencia prehospitalaria hasta tratamiento final hospitalario, incluyendo la reanimación y el recalentamiento con circulación extracorpórea. Reanimacion cardiopulmonar
Introduction 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 Definition 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. Physiopathological alterations (Table 1) 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:
Consequences of hypothermia in trauma victims 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 Etiological classification of hypothermiaAcute hypothermia In acute hypothermia the exposure to cold is so great and sudden that the capacity of the body to resist cold is overwhelmed even though heat production is maximum or close to maximum. This situation is characteristic of people buried in snow avalanches or in cold water immersion victims. In those individuals who initially survive with effective thermoregulatory mechanisms, hypothermia takes about 30min in becoming established.14 The duration of survival depends on the balance among the efficacy of the thermoregulatory response, the insulation possibilities, clothing, and the environmental or water temperature.15 Subacute hypothermia In this case the critical factor is exhaustion and depletion of the body energy reserves leading to a drop in CBT. This situation is typically seen in hikers and mountain climbers. Since the condition is usually accompanied by hypovolemia secondary to fluid transfer among the different body compartments, rewarming measures should be associated to intravenous fluid administration. Subchronic hypothermia Subchronic hypothermia occurs when there is prolonged exposure to slight cold aggression and a thermoregulatory response that is insufficient to counter such aggression. The onset tends to be insidious and slow. The classical example is that of an elderly person lying immobile on the floor after a fall at home, with femoral neck fracture. Resuscitation is complex and implies important mortality, due to the existing hemodynamic instability, the typical comorbidities found in elderly people, and frequent complications in the form of respiratory tract infections and myocardial ischemia or ischemia of the extremities. Rewarming must be slow and careful in such weak victims. Measurement of CBT 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 Classification of hypothermia in first care and according to the clinical situation 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:
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:
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. Guides on pre-hospital resuscitation according to the grade of hypothermia (Fig. 1)17 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). Grade I (35–32°C) Since in this scenario the patient is conscious and has mobility, we not only provide thermal protection and insulation but also encourage physical exercise, which warms faster than shivering alone (caution: this may produce CBT afterdrop). Hot beverages with sugar can be administered. Transfer to hospital is indicated only in the presence of comorbidity or suspected occult injuries. Grade II (32–28°C) In this case shivering has been abolished and the patient is vulnerable to fatal arrhythmias (VF/ventricular tachycardia (VT) and asystolia), particularly when management is moreover inadequate (e.g., wet clothing should be removed by cutting, not pulling). If the level of consciousness is low, the patient should be placed in the horizontal position to avoid CBT afterdrop or peri-rescue collapse (see below). Rewarming should be started with the means available: warm and humidified air or oxygen therapy, warm water bags or chemically heated packs applied to the truncal arterial regions (neck, armpits, groin)—though always avoiding direct contact with the skin. If the nausea reflex is preserved and there is no risk of bronchoaspiration, hot beverages with sugar can be administered. Evacuation to hospital is indicated, with the possibility of intensive care. Grade III (28–24°C) The victim shows diminished consciousness, with a high probability of fatal arrhythmias. Initial management should be very careful, with thermal protection and insulation, humidified and warm oxygen therapy (40–45°C), and monitorization of the ECG tracing and CBT. The triggering of VF as a result of the oxygenation and airway protection measures to prevent bronchoaspiration is considerably less likely than when tracheal intubation is attempted. This technique should only be performed where indicated according to the applicable advanced life support (ALS) algorithm, and when it can be maintained during transfer to hospital. If a venous line proves necessary (made difficult as a result of the peripheral vasoconstriction), it should be performed without causing a delay of over 5min—with due evaluation of the risk of a further drop in temperature, and avoiding central catheterization with access to cardiac cavities, due to the risk of triggering VF. The above mentioned rewarming and insulation measures should be adopted. Warm fluid therapy on the ground is scantly efficient, and only saline solution is indicated. As hypothermia progresses, the sodium levels tend to decrease while potassium increases, presumably due to a reduction in enzymatic activity of the cell membrane sodium–potassium pump. Evacuation should be carried out without delay to a hospital with the means needed for extracorporeal warming. Grade IV (24–13.7°C?) In this situation the victim suffers severe hypothermia and appears to be dead. The absence of reflexes and pupil dilatation should not be regarded as signs of death: “nobody is dead without being warm and dead”. Cardiopulmonary resuscitation (CPR) is to be started immediately, with the condition that once started, CPR should not be interrupted until reaching the reference hospital center.18,19 As in the previous grades of hypothermia, the appropriate measures for avoiding posterior temperature drops should be adopted. These victims are to be transferred immediately to a third level hospital offering the possibility of rewarming with an extracorporeal circulation pump (ECP) or extracorporeal membrane oxygenation (ECMO). Initial transfer to a hospital without these options may waste crucial time for patient survival. In more isolated or remote scenarios with no third level hospital within range, use can be made of other management guides,20 or we can evaluate the possibility of evacuating the patient to the nearest hospital center with intensive care facilities and the possibility of continuous venous–venous hemofiltration and hemoperfusion (CVVHP).21 Assessment and resuscitation of snow avalanche victims 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:
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:
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 Advanced life support in hypothermia 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 Effect of rewarming afterdrop and shock 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 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. Insulation and rewarming 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:
Pre-hospital rewarming 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:
Hospital intensive care 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:
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:
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.
Principles of extracorporeal rewarming 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 Extracorporeal rewarming techniques in hypothermia victims49 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. Victims with sufficient cardiac output Venous–venous rewarming circuit This system consists of a simple circuit with a tubing element, a centrifuge pump and a heat exchanger. The fact that it is able to operate without heparin favors the rewarming of trauma victims.63 Access is via the percutaneous route, preferably the femoral vein. Continuous venous–venous hemofiltration This system is similar to that described above, but with the possibility of offering dialysis. The maximum flow capacity of 500ml/min limits the rewarming potential. Both systems can be used in victims in asystolia, provided that ALS is not interrupted. The recovery of CBT can be very slow, because during rewarming, and in order not to alter the balance between oxygen supply and tissue oxygen consumption, cardiac output cannot be higher than that obtained with external cardiac massage (approximately 20% of normal).56 Victims in asystolia or ventricular fibrillation Extracorporeal circulation pump (ECP) The ECP is the system classically used in heart surgery units, and is useful for patients weighing over 40kg. Pediatric systems are used in the event of patients of lesser body weight. The flows that ensure the best support of gas exchange and cardiac output are 2.4l/m2/min and above. The system consists of a vena cava-aorta circuit, a venous reservoir, a pump impelling the blood towards the membrane oxygenator, a heat exchanger, and an impurities filter before returning to the aorta. The system also has a suction function that returns lost blood to the vascular bed. Since zones of blood stasis are found within the circuit and full heparinization is required (3mg/kg/h), the technique is contraindicated in severe trauma patients, particularly those with head injuries. The heparin must be antagonized once the ECP has been disconnected. In addition to a perfusionist, management of the system requires a surgical team specialized in cava-aorta cannulation through the midline sternotomy approach. This technique, by allowing direct decompression of the left ventricle, often achieves spontaneous VF reversion to sinus rhythm. Sternotomy is also preferred in small children. The femoral vessels are of little use in these subjects because of their limited caliber, and jugular-carotid cannulation increases the risk of brain damage.64 In older children or in adults, femoral vessel cannulation is possible.65 Extracorporeal membrane oxygenation (ECMO) ECMO involves technology similar to that of ECP. It is also used in intensive care medicine to provide prolonged cardiorespiratory support. Its advantages over ECP in relation to rewarming are:
Since both ECP and ECMO favor rapid rewarming, if VF fails to revert with cardiac decompression, defibrillation may be attempted. The latter in turn should not be repeated until the CBT reaches 30°C. If after defibrillation at 30°C ventricular fibrillation is seen to persist, the administration of magnesium or amiodarone may facilitate cardioversion. Post-resuscitation care
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Conflict of interests The authors declare no conflicts of interest. |