Introduction

Cold injury is uncommon in Australia due to its warm climates. However, its significance is more important in children, as they have:

When cold injuries do occur, they can be subdivided into generalised injury (namely accidental or environmental hypothermia) and localised injury. Cold injury is also a common problem potentially complicating any other severe illness (especially trauma) and must be prevented.²

Normal physiology: a review

Heat production is derived from basal metabolism, digestion, and muscular activity, which may be voluntary (exercise) or involuntary (shivering). Emotional factors and hormonal fluctuations influence heat production. The main mechanisms by which the body compensates for low core body temperature are by increasing its metabolic rate, primarily through shivering, and by shunting blood away from non-essential organs to preserve vital organs. The capacity to shiver is dependent on local glycogen stores and the rate of change of core and external temperature.^1,3

Neonates are the patients most prone to hypothermia. They are unable to shiver and have limited stores of energy. Because of this, newborn children utilise catabolism of brown fat to generate heat. This is an inefficient process that consumes oxygen, thus exacerbating hypoxia. In addition, the large surface area to weight ratio, due to a relatively large head, contributes to heat loss. At birth, neonates are covered in amniotic fluid, and evaporative losses are significant. An overhead radiant heater is not adequate to compensate for this evaporative loss.^4,5

Heat loss from the human body is by four methods:

Radiation occurs when heat energy leaves the skin at the speed of light. Patients with more fat become more hypothermic than thinner patients, due to the former’s larger surface area for radiation heat loss. In children, who have a higher surface area to weight ratio, it accounts for up to 50% of all heat loss; indeed, up to 75% in neonates. This higher number in neonates is due to a proportionally larger head increasing the surface area:weight ratio.^1,4 Radiation losses decrease when a patient is clothed.

Conduction of heat is poor in air and therefore does not contribute much to hypothermia in normal circumstances. However, water-conductive heat loss is 24 times more than that of air. It is this method by which patients suffering from water immersion become profoundly hypothermic, and how patients in wet clothes become hypothermic quickly. The surface on which the patient is lying also contributes to conduction heat loss. For instance, a patient lying on snow is likely to become more hypothermic than a person lying on sand.

Convection occurs as warm air next to the skin is replaced by cool air. This can contribute to 25% of total body heat loss in still air. In a wind of 63 km hr^–1, this increases by 14 times. This is described as the ‘wind chill factor’.

Evaporation from the skin accounts for only 7% of heat loss at rest. This may be increased in cold, dry conditions and by sweating. Evaporation from the respiratory tract removes another 7%. This can increase by breathing faster (such as at high altitude or during exercise).

Temperature is perceived through central and peripheral mechanisms. Heat sensors in the central hypothalamus receive input from the skin, central arteries, and viscera. It is this central thermostat that is reset, which causes fever. Skin receptors respond to a change in skin temperature but do not themselves indicate the patient’s core temperature. A result of all this input is that the body responds by those autonomic reflexes listed below to increase or decrease core body temperature.³

Hypothermia

This is defined as a core temperature of <35°C.¹ Hypothermia is classified on the basis of severity. The reason for this classification is that it influences the rewarming mechanisms that are most often deployed. It is also related to the physiological ability of the patient to compensate for hypothermia. An easy way to remember these temperature ranges is:

Tables 22.4.1 and 22.4.2 show the main consequences of hypothermia at a given temperature.^1,3,6 Much of our understanding of this pathophysiology comes from controlled hypothermia in cardiac surgery. Note that there is a huge variation of the onset of certain clinical signs based on temperature level. For instance, some patients may exhibit confusion at higher temperatures compared with others. Note that in children clinical manifestations of altered consciousness may be subtle.

Note that only during severe hypothermia does protection from hypoxia occur, due to decreased demand for oxygen by tissues, and even then only at extremely low temperatures (patients <20°C can tolerate anoxia for up to 60 minutes). Metabolic processes slow by approximately 6% for each 1°C drop in body temperature.¹ Thus at 28°C the basal metabolic rate is about 50% of normal. This leads to hypoventilation and hypoxia. However, at this temperature the decreased cellular metabolism affords some protection against hypoxia.

Cold diuresis is an initial brisk diuresis; this is due to decreased tubular reabsorption and also a decreased production of antidiuretic hormone. There is also an increased central blood circulating volume as blood is shunted away from the periphery, thus presenting the kidneys with an apparent increased blood volume for filtration.¹