Not too hot and not too cold: temperature regulation in premature babies. (Part 2)

The ins & outs of keeping a  premature  baby warm

We will discuss the difficult problem of heat regulation in the preterm human baby because the principles illustrated will help us understand heat regulation in small & large animals in hot & cold environments.

Before birth the ideal environment for growth and development of the baby is regulated by the mother. She controls the optimal input and output of energy and nutrients to the foetus, with the fine  adjustments being performed by the placenta. After birth the baby, with the continuing help of the mother, has to take over many of these processes e.g. absorption of nutrients via the intestine, excretion of waste products via the kidneys. A large energy supply is essential for rapid growth and development to continue after birth. Thus wasting of energy to keep warm must be avoided. Maintenance of an optimal energy balance is particularly important in the preterm (premature) baby. He or she is born early and has much lower glucose and fat reserves. Provision of an adequate intake of milk is not easy because the baby may have breathing problems &/or the immature intestine does not absorb milk optimally for the first weeks of postnatal life. Thus steps must be taken to prevent unnecessary energy wastage i.e. use of energy sources to keep warm instead of grow.

Heat is produced by basal metabolism as energy rich compounds e.g. glucose & fats are broken down (analogy of car engine heating up when petrol burns). This endogenous production (thermogenesis) can be increased when adults are cold by muscle metabolism: voluntary or involuntary movement (shivering). Babies, however, cannot shiver and produce extra heat by “non-shivering thermogenesis”. This heat is produced by brown fat in the back i.e. between the scapula & around the kidneys. This fat has been referred to as an “electric blanket” while normal white fat has been called a “woolen blanket” as it does not produce heat but prevents heat loss by insulation. The preterm baby has had less time in-utero to develop brown and white fat reserves & so is limited in this ability to increase heat input (shown by dotted endogenous input arrow in daigram) and decrease heat output. Thus in nursing the preterm baby exogenous heat sources are provided by using overhead radiant heaters or incubators (thick exogenous input arrow). More recently, based on research in developing countries  “Kangaroo care” has been widely adopted:  the baby is nursed on the mother’s or father’s chest in direct contact with the skin, and warmed by conductive heat exchange (see diagram).

The ins & outs of keeping a preterm baby warm (click to enlarge. Note “<”  means “decrease”)

It is also important to decrease heat loss from babies who have a relatively large surface area (see diagram showing  “<”  i.e. decreased  evaporation, convection, conduction, and radiation losses using thin dotted arrows). Unlike flexed term babies, preterm babies have poor muscle tone & lie stretched out, and this enlarged surface area increases heat loss. The baby is nursed on a rubber mattress to prevent conductive losses. The relatively large head is dressed with a padded gauze bonnet, and if the baby must be observed it is covered in plastic foil to prevent evaporative loss and/or under a transparent rigid plastic tunnel to prevent radiant heat losses. Silver foil may be used to reflect radiant heat loss when babies are transported from home to intensive care units. Modern incubators and respiratory ventilators have fine servo-control systems for regulation of humidity to prevent evaporative heat loss from the permeable skin and lung surfaces respectively. Note that immature babies cannot sweat but their initially poorly keratinized skin is very permeable and large amounts of so called “insensible skin losses” of water occur in early life.

As the preterm baby matures he is able to absorb more energy from the milk and the heat losses decrease as the skin keratin thickens and insensible water loss decreased. The baby is then placed in a cot and dressed.

We have discussed the ins and outs of heat regulation in man, and in the next blogs we will use these principles to discuss how large and small animals maintain their temperature in very hot and very cold climates.

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