Hypothermia
is defined as the drop of central temperature below 35° Celsius.
Traditionally, hypothermia is considered as moderate above 32°,
serious between 32° and 25° and major under 25°. But
its severity depends on the patient's condition, the cause and the
rate of its onset.
The organism can schematicaly be divided in two compartments: the core where the main organs are (brain, heart, deep vessels), maintained at a steady temperature and the shell, peripheral, of which the temperature varies according to the environment. Humans keep their central temperature at 37° by balancing heat production (thermogenesis) and dissipation (thermolysis). Thermogenesis depends on energy reserves and the necessary oxygen to metabolize them. So it is decreased in exhausted, hypoxic or traumatized people. Thermolysis depends on morphotype, clothing, and environmental conditions (wind, humidity, depth of burial, running water...). In the case of traumatism, thermolysis is increased by the lack of reflex adaptation (vasodilation below a spinal fracture) or behavioural (coma).
Moreover, basal metabolism decreases to 50% of its normal level at 30° and to 20% at 20° C. This interesting feature allows prolonged survival in the case of a circulatory arrest.
All the organs decrease their metabolism at different levels, this leads to a rich overall clinical picture which varies according to the severity of the hypothermia and the haemodynamic condition. Firstly agitated and shivering (34°), then in an hypertonic coma (28°), the patient, livid and rigid, with a mydriasis and imperceptible pulse and respiration, presents all the clinical aspects of death around 20°.
Harmful effects of hypothermia are essentially cardiac; it classicaly kills by ventricular fibrillation around 28°; usually resistant to defibrillation. This VF can be the result of an excessively agressive treatment or a mechanical irritation by rough handling of the patient. As regards circulation, the peripheral vasoconstriction leads to a diversion of the fluids towards the deep venous compartment. The organism makes up this hypervolemia by an increase of diuresis. An extra-vascular leak to extra and intra cellular spaces is also seen. The magnitude of this real hypovolaemia (diuresis) and relative hypovolaemia (inter-compartments) depends on the rapidity of the onset of the hypothermia. Biological modifications essentially concern the kaliemia (hypo or hyper), the acid-base equilibrium and the haemostasis.
The pathophysiological mechanisms lead to distinguish three kinds of hypothermia, very different in their treatment. This emphasizes the importance of a precise knowledge of the circumstances.
The cold is so severe, that thermogenesis doesn't succeed in maintaining the equilibrium; the organism cools before the energy reserves are used and the blood volume modifications are set up. This kind of hypothermia occurs in injured alpinist or skiers, immobilized in snow, a crevasse or a torrent.
This is hypothermia of an unharmed alpinist stuck in the mountain. It only appears when the energy reserves are used, so its onset is more or less rapid according to the patient's condition. Thermogenesis is very reduced, spontaneous rewarming is more uncertain and cooling can continue even with a minimal heat loss. Moreover, the liquid movements have time to appear and there is a big risk of hypovolemia during rewarming.
This is found in poor urban environments, in winter. Onset is very slow, the inter-compartment liquid transferts are important, spontaneously compensated by the patient, with a big risk of edema during rewarming.
From 1988 to 1996, 117 hypothermia victims have been treated in Chamonix hospital. 40% of them were under 32°. The common causes are isolation in bad weather (48%), falls in narrow crevasses (40%) and for a lesser part avalanches (12%). Pure hypothermia only represents 22% of the cases. Associated injuries are essentially from the head (50%), spine (12%), abdomen (18%) and thorax (4%). Overall mortality is 22%, but these patients suffer from other injuries in 95% of cases. On the whole, these figures lead to the two following principles:
The inadapted response of a wounded person, specially hurt at the abdomen, the spine or the head, promotes the rapid onset of hypothermia at altitude. The temperature of a well equipped, but seriously wounded victim, can drop as low as 30° in less than one hour at 4000 m in the Mont-Blanc massif.
The organism can withstand cold for several days if he is healthy. Consequently, a hypothermic patient is a traumatized patient until proven otherwise.
Firstly, the diagnosis must be established. The most useful thermometer is the epitympanic thermometer, with its limits in the case of low cerebral blood flow or cardiac arrest. The oesophageal thermometer is more precise, but it can promote an additional stress when it is positioned.
The
only specific treatment feasible at the scene is the delivering of
warm and humid air to the conscious patient, in order to slow down
heat loss. We use a simple, compact and light device, which runs by a
chemical reaction between carbon dioxyde and soda lime.
In practice, the victim must be carefully extricated; insulated, immobilized, possibly sedated, and smoothly transported to the hospital. In the case of a cardiac arrest, cardiac massage and ventilation must be done at a normal rhythm; their output is sufficient for the reduced metabolic demands of the hypothermic organism for a long time.
Every change in the blood volume distribution caused by mobilization or the beginning of rewarming can set off ventricular fibrillation. Transport must be as gentle and short as possible, which is often difficult given the patient's agitation.
The patient rewarms passively with his own heat production. He (or she) is placed in a normally heated room and one must be cautious to not rewarm the shell before the core. The rewarming rate depends on energy reserves. It is usually 1°/h but can reach 5° per hour.
Danger comes from different stimuli, either physical (over rapid external rewarming with hypovolemia, rough handling...), hormonal (releasing of catecholamin during resuscitation manoeuvers without analgaesia) or biological (hyperkaliemia and acidosis). Blood volume can be checked by a deep veinous cathether if it remains far from cardiac tissue. Obviously, the balancing of fluids requires heated liquids.
The rewarming record for an accidental hypothermia is currently 14.4°. In induced hypothermia, it is far below, at 9°C. As for the duration of a cardiorespiratory arrest in hypothermia, it can be longer than an hour. Consequently, if no asphyxia occured during the onset of hypothemia, it is very difficult to confirm the death as long as the patient is hypothermic. One must try to resuscitate all hypothermic patients, but not all dead people whose body has cooled. The best biological clue is kaliemia. When it is over 10 mmol/l at the time of admission, the chances of resuscitation are nil. This indication can be serve as a criterion of death.
Active external rewarming is the oldest described method. Besides a great risk of burning (slow down of the cutaneous and sub-cutaneous flows), it presents the disadvantage of rewarming the periphery before the core, increasing in the shell the oxygen consumption, promoting in it a metabolic acidosis, and causing a peripheral vasodilation which increases hypovolemia. One can't reasonably advise this technique, except in the case of mild hypothermia which we rewarm with an air blanket.
In fact, in the case of a circulatory failure or when the patient doesn't have sufficient energy resources, the physiological phenomenoms oblige us to rewarm the core before the shell, even by irrigation of the internal cavities (peritoneal, mediastinal, pleural), or by extra-corporeal circuit.
This is the reference treatment of hypothermia with cardiac arrest. Several teams have reported remarkable success. Meanwhile, rewarming cardiopulmonary bypass is not the panacea: it demands a trained and available team and a heavy technical structure, often distant from the scene of the accident. Heparin coated circuits must be prefered, but even so, this method increases the usual hemostasis disorders of hypothermia. So it is not viable in the case of multiple trauma.
Several methods are described: rewarming by an arterio-venous shunt or by haemofiltration and dialysis, especially veno-venous. This last method is interesting because it is usable in the majority of the hospitals where an intensive care unit is available. It allows the hydro-electrolytical rebalancing; the combination of dialysis and haemofiltration allows a supplementary input of heated fluids. Finally, it doesn't preclude cardiac massage and can possibly function without heparin.
In 1984 Osborne has reported the case of a man at 25°C, with a circulatory arrest caused by ventricular fibrillation, resuscitated after a four hour cardiac massage, peritoneal and stomach irrigation. This proves that simple techniques can compensate the lack of cardiopulmonary bypass, which is the case in most primary hospitals that receive mountain victims. Different methods have been used with success: gastric, peritoneal, mediastinal and pleural irrigations. In this last technique we put two drains in the same pleural cavity (one in the arm pit, the other under the clavicle) through which circulates reheated saline (40°). This gives a rewarming rate of about 4° per hour. As it doesn't detour the blood by an extracorporal circuit, it doesn't increase bleeding disorders. Associated with peritoneal dialysis, It is currently the reference method in our service.
Prognosis is good in the case of pure hypothermia (only 1 death at 25° in our series). As in other studies, mortality is high (56%) in the case of associated traumatisms. All the difficulty lies in the impossibility of rewarming these patients with uncontrolled haemorrhages and anaesthesia in this context is very difficult. Moreover, some lesions, hidden by the decreased metabolism, appear during the rewarming phase, specially traumatic cerebral edema, which poses the question of a possible maintainance of these patients in hypothermia.
(c) DMTM CHAMONIX 1998