Vulnerability of the preterm baby

The temperature of the fetus is between 0.5°C and 1°C higher than that of the mother.13

Although some heat loss after delivery may be important in stimulating metabolic adaptation it is important to avoid a continuing drop in body temperature.14 This can be achieved in the healthy term baby by drying and wrapping, applying a hat, or using skin-to-skin care. The preterm baby though often needs a period of stabilisation and has traditionally been placed naked on a resuscitaire.

The baby exchanges heat with its environment through radiation, convection, conduction and evaporation. The immature baby is at high risk of net heat loss because of a high surface area to volume ratio and increased evaporation of fluid from the skin. Mechanisms of heat production are limited as is the ability to reduce heat loss by peripheral vasoconstriction.15 Given these physiological limitations, the goal of care of the preterm baby is to maintain a normal body temperature and minimise thermal stress, particularly cold stress.

A number of strategies are used to try and achieve a thermoneutral state for the baby. However, in order to evaluate their effectiveness it is important to understand temperature measurement techniques.

Temperature measurement

There are two problems with temperature measurement. First is a problem in trying to define a ‘normal’ temperature as this will depend very much on where, and possibly how, it is being measured. There is no such thing as a single central temperature, with tissues varying depending on their metabolic rate.16 Second, a normal central temperature does not mean the baby is in a thermoneutral state. A baby may be expending a vast amount of energy to maintain a normal temperature and yet be stressed thermally.17 In addition, the very low birth weight baby may demonstrate characteristics of a poikilotherm and show a temperature that drifts up and down, matching changes in the ambient temperature.15

Normal temperature ranges for newborns have not been clearly established. A published range for rectal and axillary temperatures is 36.5–37.5°C and 35.6–37.3°C, respectively, for both term and preterm infants; the skin temperature is 35.5–36.5°C in term babies and 36.2–37.2°C in preterm babies.18 Sauer et al6 suggest that conditions for thermoneutrality are met in very low birth weight infants when at rest their core temperature is between 36.7°C and 37.3°C and the core and mean skin temperatures are changing less than 0.2–0.3°C/h, respectively.

Modes of measurement

Various techniques and body sites have been used to provide an estimate of body core temperature.

Electronic thermometers may be used in manual and predictive mode. In predictive mode they give a rapid reading but the neonatal algorithms used have never been validated in the preterm population in large pragmatic trials.

Infrared technology is being used in the development of ear and skin thermometers. Tympanic temperature measurement has gained popularity in children, but in the newborn it reflects central temperature poorly due to technical difficulties related to the size of the ear canal.19 The literature on forehead and temporal artery thermometry in the newborn is limited.20 21

The usual sites used to the determine temperature of the newborn are the axilla, rectum and skin. Axillary temperature is usually measured intermittently and has little associated risk. Error in measurement is observed depending on the placement of the probe, adequate closure of the axillary pit, blood flow to the axillary region and possibly activation of non-shivering thermogenesis.22,–,24

Rectal temperature provides an approximation of core temperature but this mode has a risk of trauma. Reproducibility is uncertain; possible causes of inaccurate rectal readings include the value depending on the depth of insertion, dwell time, whether the baby has just passed stool and on the flow and temperature of the blood returning from the lower limbs.25 26

Probes attached to the skin are used for continuous temperature monitoring. A close approximation to deep body temperature can be achieved using the ‘zero heat flux’ technique.27 If a probe is placed over an area of skin from which no heat can be lost then, with heat flow down a temperature gradient from the centre of the body to the periphery, this area of skin will equilibrate with the deep body temperature. This can be achieved with a probe placed between the scapulae and a non-conducting mattress.

The intermittent measurement of a single temperature tells how well the baby is maintaining that temperature, but nothing about the energy being used to achieve thermal balance. Just because the measured temperature is acceptable does not mean that the environment is adequate, with the baby using energy to overcome the effects of thermal stress. The continuous measurement, and display, of central (abdominal, axilla or zero heat flux) and peripheral (sole of the foot) temperatures detects thermal stress by showing a change in the central–peripheral difference that occurs before any alteration in central temperature. The preterm baby who appears to be comfortable in its environment has a central temperature in the normal range for whichever site is being used and a central–peripheral temperature difference of 0.5–1°C. An increasing central–peripheral temperature difference, particularly above 2°C, is usually caused by cold stress and occurs before any fall in central temperature.28 A high central temperature, particularly if unstable, along with a wide central–peripheral gap is seen in septic babies.29

The sick immature baby should have continuous dual temperature monitoring. In the stable, growing preterm infant it is traditional to measure temperature, often axillary, at regular intervals. This will give limited information about the thermal state but may be important when there are concerns about poor growth or other signs of illness.

Options for achieving these goals

Ongoing care in the neonatal unit

As cold stress soon after delivery is associated with increased mortality there is no reason to assume that it is of any less importance during the immature baby's ongoing transition during the early days of life. Beyond the period of initial illness, failure to maintain an adequate thermal environment may increase apnoeic episodes36 and have an adverse effect on growth.37

The preterm baby, even if well, may be unable to maintain an adequate temperature without some additional source of heat. There is a variety of ways of achieving this including incubators, radiant heaters and heated mattresses. The heated gel or water-filled mattress is very effective in helping maintain the temperature of the well preterm baby while allowing easy access for the parents and staff. Clothes act as a significant thermal barrier to heat loss. Skin-to-skin contact is an effective way of maintaining body temperature, even in very preterm infants.38

Evaporative fluid and heat losses remain a challenge in the management of the preterm baby. The more immature the baby the higher the TEWL; subsequent evaporation is a major cause of fluid and heat loss. The skin matures rapidly after birth and in practical terms babies reach close to adult water losses from the skin at approximately 10–14 days of life.3 4

Evaporative water loss from the skin depends on the ambient water vapour pressure, irrespective of how the baby is being nursed. Various measures have been used to try and reduce these losses, with the most effective being an increase in the levels of humidity in the immediate environment of the baby. Careful attention to maintaining skin integrity will reduce ongoing transepidermal fluid losses. Preventing damage to the skin, for example from tape, is important. The use of semipermeable non-adhesive dressings lowers TEWL and reduces the bacteria numbers in the covered skin, but no study has shown any effect on fluid requirements.8 Emollients on the skin will prevent fluid loss and reduce drying, skin cracking and fissuring.9 They do not cause burns when exposed to radiant heat or phototherapy. A Cochrane review39 indicated that there is an increased risk of sepsis with the use of emollients, but studies conducted in low-resource countries suggest that their use can significantly reduce mortality.40

Warming devices

Control of the thermal environment is important for all babies, but it is the unwell, immature baby that presents the greatest challenge. They are usually nursed either in an incubator or on an open platform, under a radiant heater. A Cochrane review concluded that no study has shown any significant difference in outcome for babies nursed using either of these devices.41