Homeothermic animals possess the ability to produce and dissipate heat. Heat loss occurs by one of four processes: radiation, convection, evaporation and conduction. The environment in which the patient is situated governs the relative contribution of each. The neutral thermal environment is defined as the range of ambient temperatures at which temperature regulation is achieved by non-evaporative physical processes alone.
The metabolic rate at this temperature is minimal. The temperature of such an environment is 34°C for the premature neonate, 32°C for the neonate at term and 28°C for the adult.
Heat may be produced by one of three processes: voluntary muscle activity, involuntary muscle activity and non-shivering thermogenesis. Infants under the age of 3 months do not shiver. The only method available to increase their temperature in the perioperative period is non-shivering thermogenesis. The process is mediated by specialized tissue termed brown fat. It differentiates in the human fetus between 26 and 30 weeks of gestation. It comprises between 2 and 6% of total body weight in the human fetus and is located mainly between the scapulae and in the axillae. It is also found around blood vessels in the neck, in the mediastinum and in the loins. Brown fat is made of multinucleated cells with numerous mitochondria and has an abundant blood and nerve supply. Its metabolism is mediated by catecholamines. The substrate used for heat production is mainly fatty acids.
Radiation accounts for about 60% of the heat loss from a neonate in a 34°C incubator placed in a 21°C room. If the infant were in a thermoneutral environment of 34°C, the percentage loss by radiation would decrease to about 40% of the total heat loss, and, in addition, the total heat loss in this environment would be lower. The reason for this is that heat loss by radiation is a function of skin surface area and the difference in temperature between the skin and the room. The second major source of heat loss in the neonate is convection. This is a function of skin temperature and ambient temperature. The neonate possesses minimal subcutaneous fat that may act as thermal insulation and as a barrier to evaporative loss. A neonate has a body surface area to volume ratio about 2.5 times greater than the adult; thus a neonate may become hypothermic very rapidly.
If neonates are allowed to become hypothermic during anaesthesia, unlike adults they attempt to correct this by non-shivering thermogenesis. Metabolic rate increases and oxygen consumption may double. The increase in metabolic rate puts an additional burden on the cardiorespiratory system and this may be critical in neonates with limited reserve. The release of norepinephrine in response to hypothermia causes vasoconstriction, which in turn causes a lactic acidosis. The acidosis in turn favours an increase in right-to-left shunt, which causes hypoxaemia. As a result, a vicious positive feedback loop of hypoxaemia and acidosis is set up. The protective airway reflexes of a hypothermic neonate are obtunded, thereby increasing the risks of regurgitation and aspiration of gastric contents. The action of most anaesthetic drugs is potentiated by hypothermia. This effect is particularly important with regard to neuromuscular blocking drugs. The combination of hypothermia and prolonged action of these drugs increases the chances of the neonate hypoventilating after sugery.
Many precautions should be taken to ensure that the neonate's body temperature is maintained. First, the child must be transported to theatre wrapped up and in an incubator set at the thermoneutral temperature. The theatre should be warmed up to the thermoneutral temperature, ideally a few hours before the planned start of surgery. This interval allows the walls of the theatre to warm up and this reduces the net heat loss by radiation. One must appreciate that heat loss by radiation is a two-way process. The child loses heat by radiation to the walls and it also gains heat from the walls. All body parts that are not needed for insertion of can-nulae and for monitoring should remain covered until the child has been draped with surgical towels. If the child has to be exposed, overhead radiant heaters may be used. During surgery, the child should lie on a thermostatically controlled heated blanket. Forced air warming systems are effective at maintaining the child's temperature during surgery; these work on the principle of blowing filtered warmed air into quilted blankets with perforations. This allows warmed air to come into direct contact with the patient. Simple measures such as using bonnets to reduce heat loss from the head are very effective. Intravenous fluids and fluids used to perform lavage of body cavities must be warmed. Anaesthetic gases should be humidified and warmed in order to preserve ciliary function and to reduce heat loss from the respiratory tract.
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