Chapter Contents
Introduction 263
Body temperature control mechanisms 264
Central nervous thermoreceptive mechanisms 264
Brown adipose tissue 264
Response to a cold environment 264
Response to a warm environment 264
Normal heat transfer 264
Measurement of temperature 267
Rectal temperature 267
Axillary temperature 267
Skin temperature 267
Other estimates of core temperature 267
Body temperature and thermal neutrality 267
Thermal neutral range 267
‘Normal’ temperature 267
Practical management of the thermal environment 268
At delivery 268
Nursing care of the newborn infant 268
Incubator care 269
Care under radiant warmers 270
The heated water filled mattress 272
Thermal environment during transport 272
Surgery 272
Disorders of temperature control 272
The baby with a low body temperature (below 36°C) 272
The baby with a raised body temperature (above 38°C) 273
Therapeutic hypothermia 274
Introduction
The ability of the newborn to maintain a normal temperature is critical to his or her survival yet, even in today’s neonatal intensive care units, thermal care is often given a low priority ( ; ). demonstrated a link between poor temperature control and increased neonatal mortality and morbidity over 50 years ago. The 1995 EPICure cohort demonstrated that a low admission temperature was an independent risk factor for increased mortality in newborn infants delivered at less than 26 weeks’ gestation ( ). In this cohort mortality for newborn infants who were admitted to the neonatal intensive care unit at 23, 24 and 25 weeks’ gestation was 58%, 43% and 30% respectively if the admission temperature was less than 35°C. In addition the Confidential Enquiry into Stillbirths and Deaths in Infancy (CESDI) Project 27/28 report of care for newborn infants at 27 and 28 weeks’ gestation showed that mortality was 73% in babies with an admission temperature below 36°C compared with 59% in warmer babies ( ).
The newborn baby has many thermal homeostatic mechanisms, but a large surface area to body weight ratio and being born wet into a draughty, cold environment contribute to significant cold stress.
Body temperature control mechanisms
The production of heat energy is central to the homeostatic repertoire of the newborn ( ). Heat is generated in most tissues from the inefficiency of mitochondrial adenosine triphosphate production and utilisation. This can occur from an increase in heart rate, shivering in skeletal muscle and nonshivering thermogenesis. The increase in heart rate response is mediated by the sympathetic nervous system. When a skeletal muscle shivers or the heart rate increases then heat is generated. However shivering is not an important feature of the newborn’s cold response. Instead, nonshivering thermogenesis in brown adipose tissue (BAT) plays a critical role in temperature homeostasis in the newborn infant.
Central nervous thermoreceptive mechanisms
Environmental temperature has a direct and more rapid effect on skin temperature than on core body temperature ( ). Thermal afferents from the skin to the preoptic hypothalamus promote rapid cold defence responses before core body temperature is affected. At a molecular level the transient receptor potential family of cation channels mediates sensation across a broad range of cold and warm physiological temperatures.
Central nervous system thermoregulatory pathways to and from the preoptic hypothalamus can stimulate heat production in response to a cold environment, fall in body temperature or the presence of cytokines in BAT, the heart or skeletal muscle. Cutaneous cold afferents to the preoptic hypothalamus then generate α motor neuron activity and shivering. In addition to cutaneous receptors, the preoptic hypothalamus receives afferents from core body structures such as the brain, spinal cord and abdomen via the spinothalamocortical pathway. Cold and warm receptors are included in the splanchnic and vagus nerve abdominal afferents. Deep body structures are not as susceptible to changes in environmental temperature as the skin but play an indirect role in temperature regulation.
Brown adipose tissue
BAT has a specific heat-generating function and is found in characteristic areas of the baby, such as interscapular, axillary, perirenal, and thoracic regions and between neck muscles ( ). In the term newborn, BAT accounts for 2–7% of the infant’s weight. BAT cells begin to differentiate at 26–30 weeks’ gestation, continue to develop until approximately 5 weeks after delivery and over this timeframe can increase by up to 150% ( ). BAT metabolism is controlled by the sympathetic nervous system, noradrenaline (norepinephrine) release and β 3 -adrenergic receptor binding to brown adipocytes ( ). Heat is generated by a facilitated proton leak across the mitochondrial membranes of brown adipocytes that uncouples oxidative phosphorylation. This occurs because of the high expression of uncoupling protein 1 (UCP1) in BAT mitochondria (Budge et al. 2003; ). and demonstrated that oxygen consumption almost doubled (4.8 versus 8.2 ml/kg/min) and plasma glycerol increased at an environmental temperature of 25–26°C compared with 34–35°C. These results strongly suggested that heat production occurred in BAT when the human newborn was exposed to cold. Although not specific to BAT, MRI studies have demonstrated that the preterm infant has significantly less adipose tissue and an altered deposition pattern compared with term newborn infants ( ). This might suggest that the preterm infant has limited ability to generate heat through nonshivering thermogenesis from both a lack of BAT and lower UCP1 expression.
Response to a cold environment
As the environmental temperature falls, the baby makes physiological and behavioural responses to maintain a constant deep body temperature. These are initiated by hypothalamic and cutaneous temperature receptors.
Physiological
Nonshivering thermogenesis results from metabolic activity in BAT (see above). Nonshivering thermogenesis is impaired in all newborns in the first 12 hours, particularly in those who are ill. Peripheral vasoconstriction also occurs in response to cold, diverting blood from the surface to the core. This is well developed in term babies but limited in very immature babies in the immediate neonatal period.
Behavioural
Whereas a child or adult will wake up and become restless when cold, the newborn infant may continue to sleep. Cold term and preterm babies tend to be more active, sleep less and adopt a flexed posture in an attempt to increase heat production and decrease heat loss.
Response to a warm environment
As the environmental temperature rises, the newborn baby attempts to prevent a rise in body temperature.
Physiological
Sweating in response to a warm environment occurs in term newborns from birth ( ; ). The amount of water lost by sweating per unit area of skin is considerably lower than that lost by a heat-acclimatised adult, although the density of sweat glands is greater in the newborn. Sweating is most marked on the forehead, temple and occiput. The palms and soles only sweat in response to emotional stress. Sweating provides some measure of defence against overheating. In congenital hypohidrotic ectodermal dysplasia sweating is impaired and these babies are particularly susceptible to heat stress. Sweating is absent in babies born at less than 36 weeks’ gestation but usually appears by about 2 weeks of age. Babies of opiate-abusing mothers have a well-developed ability to sweat at any gestational age. Vasodilatation in response to heat occurs in term and preterm babies, so that their skin is warm and red when overheated ( ).
Behavioural
As the environmental temperature increases, term and preterm babies become less active, sleep more and lie in an extended, sunbathing posture ( ; ).
Normal heat transfer
The normal fetus has a temperature approximately 0.5–1°C higher than that of the mother. At birth the newborn loses heat rapidly, mainly by evaporation. This can result in a drop of 2–3°C and the triggering of the cold defence responses described above. Term newborns can increase their metabolic rate by 100% after birth but this response is blunted in a preterm or low birthweight baby ( ). In addition the altered distribution and decreased quantity of adipose tissue and a surface area to volume ratio 3–5 times greater than that of an adult put the newborn at high risk of temperature loss.
Heat balance
The law of conservation of energy demands that, under equilibrium conditions, heat loss balances heat production. If production exceeds loss, the body temperature rises and vice versa until a new equilibrium is reached. The rate of production or loss of heat is dependent on the magnitude of the gradient between the baby and the external environment and flows from warm to cold.
Heat production
The baby produces heat by metabolic activity in all body tissues. Basal metabolic rate is difficult to measure since the newborn is rarely awake, quiet and starved, but resting levels can be measured. The resting metabolic rate (usually measured indirectly as the resting oxygen consumption) describes the metabolism of a baby who is lying still, asleep, more than an hour after the previous feed, and in thermal neutral surroundings. Under these conditions, the heat production of a healthy term newborn is similar to that of an adult when expressed per unit weight, but almost half that of an adult when expressed per unit surface area. Surface area determines a subject’s heat loss and thus the relatively low heat production per unit area explains why the newborn requires a much warmer environment than an adult. Resting metabolic rate is similar in term and preterm newborn infants when expressed per unit weight, but considerably lower in preterm newborn infants when expressed per unit surface area ( Fig. 15.1 ). Preterm babies thus require higher ambient temperatures than term newborn infants ( ).
Resting metabolic rate rises in the immediate newborn period ( ; ). The maximum rate of heat production is approximately 50 kcal/m 2 /h reached by the age of 3–6 months and then remains constant into adult life. Many factors influence a newborn baby’s heat production ( Table 15.1 ), which is higher in rapid eye movement sleep than deep sleep, suggesting that the brain is metabolically highly active. The baby is also able to increase heat production in response to cold stress but there is considerable individual variation (see below).
| HEAT PRODUCTION IS INCREASED | HEAT PRODUCTION IS DECREASED |
|---|---|
| Awake | Deep sleep |
| Active | Ill (e.g. asphyxia or hypoxia) |
| Postfeeding | Starvation |
| Rapid growth | Malnutrition |
| Neonatal thyrotoxicosis | Hypothyroidism |
| Left-to-right cardiac shunt | Cyanotic congenital heart disease |
| Drugs (e.g. theophylline) | Drugs (e.g. chlorpromazine, caffeine) |
Sources of heat loss
Conduction
Conductive heat loss involves transfer of heat from the body core to the body surface and then to objects that are in contact with it (mattress, scales). Conductive loss can be minimised by insulation, skin to skin contact and mattresses in incubators.
Convection
Heat is lost by convection from the exposed surface of the baby to the surrounding air, and is largely determined by the difference in temperature between the two. If the ambient temperature exceeds the surface temperature of the baby, heat will be gained by convection. Convective heat loss also depends on the air speed. If it is rapid, the insulating effect of still air close to the baby’s surface is lost (forced convection) and convective heat loss increases (wind chill factor). Convection is a major source of heat loss when newborn babies are exposed in cool, draughty rooms. Convective heat loss can be minimised by swaddling, using hats, warming oxygen and minimising draughts.
Radiation
Heat is lost by radiation from the exposed skin of the baby to the surrounding surfaces. This loss is proportional to the difference between these surface temperatures but independent of the temperature and speed of the intervening air. Radiation loss is an important channel of heat loss when babies are naked in a delivery room or a single-walled incubator, but a source of heat gain when a baby is nursed under a radiant warmer.
Evaporation of water
As water evaporates from a baby’s skin, heat is lost (each millilitre of water that evaporates removes 560 calories of heat). Under normal conditions in a term baby, evaporative heat loss amounts to about a quarter of the resting heat production ( ). About a quarter of this loss is by evaporation of water from the respiratory tract, the remainder occurring by passive diffusion of water through the epidermis (transepidermal water loss (TEWL)). Nevertheless evaporative heat loss is not a major source of heat loss in term babies, except at delivery, when the skin is wet with amniotic fluid. Mature babies have the ability to increase evaporative heat loss in response to a warm environment by sweating ( ; ).
However, preterm babies have high evaporative heat losses compared with term babies ( ; ). This is the result of a high TEWL, which is up to six times higher per unit surface area in a newborn baby of 26 weeks’ gestation than in a term baby ( Fig. 15.2 ) ( ; ; ; ). The high TEWL occurs because the immature baby’s skin has a thin, poorly keratinised stratum corneum that offers little resistance to the diffusion of water. Postnatal existence rapidly hastens the development of an effective epidermal barrier ( ), so that by about 2–3 weeks of age even the most prematurely born baby has a TEWL approaching that of a term newborn infant ( ; ; ; ). The high TEWL of preterm babies is further increased by trauma to the skin ( ). However the epidermal barrier is not influenced by use of antenatal corticosteroids ( ).
Use of a radiant warmer increases evaporative heat loss by a factor of about 0.5–2.0; conventional phototherapy also increases TEWL, although it is claimed that light emitting diode phototherapy does not have this effect.
The high evaporative heat loss of babies of less than 30 weeks’ gestation in the first week or so of life complicates management of their fluid balance; for advice on this topic, see Chapter 18 . Reduction of this high evaporative loss can be achieved in a number of ways:
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Increasing the ambient humidity – evaporative heat loss decreases linearly as humidity rises ( Chapter 18 of the 4th edition; Fig. 18.4 ), so that losses at high humidity are very low ( ; ; ).
- •
Draught exclusion – for example, an incubator with low air speeds can be used ( ; ) or the baby can be nursed under a acrylic shield closed at one end ( ).
- •
Waterproofing the baby – plastic bubble blankets or clear plastic film draped over the preterm baby will reduce insensible water loss by 75% ( ). Topical emollients such as soft paraffin, Aquaphor and vegetable oil have a place in waterproofing the newborn infant so that TEWL is decreased ( ). However a Cochrane review suggests that their routine use should be avoided because of the added risk of coagulase-negative staphylococcal infection (risk ratio (95% confidence interval (CI)) 1.31 (1.02, 1.7) and any nosocomial infection 1.20 (1.00, 1.43) ( ).
Measurement of temperature
Rectal, axilla and between skin and mattress temperatures correlate well with core temperature as recorded by an electronic thermometer inserted 5 cm beyond the anus ( ) but in general terms the peripheral temperature will underestimate the core reading. However, for most purposes the measurement of axillary temperature should be used, except during therapeutic hypothermia or when there is concern that the baby is significantly hypothermic. The normal range of rectal temperature in the newborn is 36.5–37.5°C. Although axillary temperature is generally lower than rectal temperature by about 0.5°C, the current American Academy of Pediatrics guidelines recommend adopting a similar normal range, 36.5–37.4°C ( ). The National Institute for Health and Clinical Excellence guideline defines normal newborn axillary temperature as ‘around 37°C’ (NICE clinical guideline 37, Routine postnatal care of women and their babies. Issue date June 2006). More information on temperature monitoring can also be found in Chapter 19 . The World Health Organization ( ) defines mild hypothermia as 36–36.5°C, moderate hypothermia as 32–36°C and severe hypothermia as <32°C.
The goal of good thermal care is to achieve a core temperature between 36.8°C and 37.3°C with a core–peripheral difference of less than 1°C.
Rectal temperature
This can be measured using a flexible thermocouple or thermistor inserted 5 cm from the anal margin. Shallower insertion gives a falsely low recording because blood from the surface of the legs may return via venous plexuses around the anus. Rectal temperature measurement is contraindicated in necrotising enterocolitis, and carries a risk of damage to the mucosa.
Axillary temperature
Because of the risk associated with rectal temperature measurement, axillary measurement is preferred. This is the current recommendation of the American Academy of Pediatrics. In this way temperature is usually measured by an electronic thermometer with the probe placed firmly in the roof of the axilla with the baby’s upper arm held against the side of the chest wall. The axillary temperature can be up to 1°C lower than rectal temperature, but the normal range is still usually defined as 36.5–37.4°C (see above).
Skin temperature
A thermocouple or thermistor lightly taped to the newborn infant’s skin can be left for repeated or continuous measurements. The upper abdomen is a convenient site, as changes with environmental temperature are not as great as at a peripheral site and the skin is conveniently flat. The normal range depends on the tissue insulation, the newborn infant’s size and the time after delivery. The newborn infant below 1 kg birthweight has an abdominal skin temperature close to the rectal or axilla temperature. If the temperature probe is placed between the newborn infant’s back and the underlying mattress and then allowed to equilibrate, the final measured temperature is similar to core, rectal or axillary measurements ( ).
Other estimates of core temperature
Oesophageal temperature at the level of the heart and nasopharyngeal temperature at the brim of the internal acoustic meatus are feasible and good approximations of brain or central temperature but are invasive ( ; ). demonstrated that an insulated transcutaneous core temperature sensor using a method that relies on the principle of zero heat flow provides a reliable estimate of rectal temperature in preterm infants. More recently a novel earphone type infrared tympanic thermometer has been used in adult surgical patients and was within a mean (± sd ) of 0.08 (0.34)°C and 0.11 (0.55)°C of respectively oesophageal or rectal temperature ( ).
Body temperature and thermal neutrality
Thermal neutral range
This is defined as the range of environmental temperatures at which a baby’s heat production and oxygen consumption (as a proxy measure of metabolic rate) are minimal and there is no sweating ( Chapter 18 of the 4th edition; Fig. 18.4 ). In the term naked newborn the range is wider and lower than in the preterm baby. An exact definition of the thermal neutral range is complicated as some of the elaborate original work was done in an era when the addition of incubator humidity was not the norm ( ; ; ). However Hey studied 123 healthy naked newborns weighing between 960 and 4760 g in an incubator with 50% relative humidity and 30–35°C temperature. The thermal neutral range at birth and 10 days of age was 34–34.5°C and 35–35.5°C, then 32.5–33.5°C and 34–34.7°C for babies with a birthweight of 2000 g or 1000 g respectively ( ). A later study based on gestation rather than birthweight determined similar ranges of the thermal neutral zone ( ). In clothed cot nursed babies an environmental temperature of 23–27°C was necessary for thermal neutrality ( ; ) compared with an environmental temperature of 29°C for those with birthweight <1.5 kg ( ). Newborn babies are best nursed at an environmental temperature close to this range ( ; ).
‘Normal’ temperature
The concept of a single ‘normal’ temperature for a newborn infant is erroneous. Clearly, a baby with a normal body temperature can be sweating or be markedly cold stressed, so body temperature is an insensitive guide to the suitability of the thermal environment. Doctors, nurses and parents commonly assume that if a baby has a normal body temperature the ambient temperature conditions must be satisfactory. In doing so, they fail to distinguish between being cold and feeling cold. A careful observer can recognise that a baby is feeling cold before the baby actually becomes cold, by assessing the baby’s posture, activity, skin colour and peripheral skin temperature. However, the very low birthweight baby is an exception. A baby of less than 1 kg has such a low heat production per surface area, limited metabolic response to cold and poor tissue insulation that he or she appears poikilothermic (body temperature drifts up and down with the ambient temperature). Measurement of body temperature in such a baby is a convenient and reliable guide to the suitability of the thermal environment.
The recommendations for thermal protection in the newborn infant do not specify a gestational age, but the broad recommendations are that a newborn infant should maintain a temperature of 36.5–37.5°C to avoid cold stress with a delivery room temperature of 25°C. A 10-step ‘warm chain’ prevents heat loss in the newborn infant and has been successfully field-tested in eight developing countries.
studied the range of temperature in 200 term Chinese infants over the first 72 hours after delivery. The mean ( sd ) rectal temperature at delivery was 36.9°C (0.28), which dropped by a mean of 0.4°C (0.34) over the first 30 minutes, then rose by a mean of 0.24°C (0.26) over 15 hours. The core temperature stabilised at a mean of 36.7°C by 48–72 hours. Skin to skin care was the only intervention necessary to restore normal temperature. In addition there is a recognised drop in core temperature of 0.5–0.6°C in the first 2 hours of nighttime sleep that develops over the first 2–4 months of age ( ; ).
Practical management of the thermal environment
At delivery
The ideal temperature of a delivery room or operating theatre is about 25°C, comfortable for the mother, her attendants and the baby ( ). However most delivery rooms are cooler than this, and the wet newborn infant is delivered into a draughty room. Thus, delivery itself usually presents a significant cold stress.
The newborn baby should be given to the mother to establish bonding and breastfeeding. There is evidence that skin to skin contact facilitates breastfeeding and hence this should be encouraged, although not at the cost of thermal stress; the baby should be dried and covered if the room is not warm enough. Weighing, bathing and application of name bands can all be deferred. The baby can also be clothed appropriately later. Babies who need resuscitation should be dried (if term) and placed under the radiant warmer of a resuscitaire. Preterm babies of less than 32 weeks’ gestation should be placed into a polyethylene film or bag straight from delivery and without prior drying. This has been shown to be an effective way of avoiding a fall in rectal temperature in the immediate postnatal period ( ; ; ). Wrapped babies had significantly higher admission temperatures than unwrapped infants (weighted mean difference 0.63°C, 95% CI 0.38, 0.87) although there was no significant difference in mortality. The baby should remain in the polyethylene film or bag until he or she has been weighed, transferred into a humid incubator and has a temperature of >36.5°C.
In addition phase change gel mattresses and skin to skin care in combination with polyethylene film or bag are associated with a rise in admission temperature and a reduction in hypothermia of premature newborn infants on admission to neonatal intensive care ( ; ; ; ).
Nursing care of the newborn infant
Parents prefer to see their baby dressed. A clothed baby who is placed in a cot covered by blankets and nursed in a warm room is usually in a neutral thermal environment and generally appears content. Clothing more than doubles the insulation, and bedding (a sheet and two blankets) further increases it, so that the resistance to heat loss of a clothed, wrapped baby is three times greater than that of a naked one ( Table 15.2 ) ( ; ). The head is a large part of the total surface area of the baby, has a higher surface temperature and consequently loses much heat. A hat is an effective method of increasing thermal insulation, and is especially useful in low birthweight babies, who have relatively larger heads and whose trunk may need to be exposed.
| RESISTANCE DUE TO | NAKED | HAT WRAPPED IN ONE SHEET | CLOTHED IN A COT UNDER BLANKETS |
|---|---|---|---|
| 1 flannelette sheet and 2 blankets around a clothed baby | 0.61 | ||
| 1 flannelette swaddling sheet around an unclothed baby | 0.81 | ||
| Thick gauze hat | 0.22 | 0.22 | |
| Vest, napkin and long nightdress | 1.25 | ||
| Boundary layer of still air around the skin | 0.78 | 0.78 | 0.78 |
| Vasoconstricted body tissues | 0.29 | 0.29 | 0.29 |
| Total resistance | 1.07 | 2.10 | 3.15 |
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