Chapter 4

### INTRODUCTION

Maintaining an adequate thermal environment for the premature infant is one of the most fundamental principles of newborn intensive care. The failure of the premature infant to adequately respond to a cold stress was perhaps the earliest historically recognized distinguishable characteristic (other than size) of the premature infant. This characteristic formed the rationale for the history and origins of incubators.1 From the earliest experience of Pierre-Constant Budin, a student of Etienne Tarnier in Paris around 1900, it became apparent that survival of the premature infant is dependent on providing an adequate thermal environment.2,3 In the present day, hypothermia continues to be an independent risk factor for death.4 Indeed, thermal care has become routine in the neonatal intensive care unit (NICU).

Although all NICUs have routines for providing an appropriate thermal environment, there have been numerous changes in practice that may have introduced subtle, sometimes-unrecognized, influences on the thermal environment. A thorough understanding of the physiology of thermoregulation is necessary to maintain vigilance with respect to how subtle changes in practice might put the very immature infant at thermal risk. Much of the systems physiology of thermoregulation in the human newborn infant was thoroughly investigated in the 1960s and 1970s and continues to provide a basis of our current practice. However, the care or the extremely low birth weight infant, the introduction of developmental care, a variety of incubator designs, and changes in respiratory care practices potentially affect the nature or consistency of the thermal environment. Moreover, since the early 2000s, there have been major advances in our knowledge of the neural pathways involved in the afferent and efferent limbs of the mammalian thermoregulatory control system that have led to a reexamination of our overall concepts of thermoregulatory control.

The purpose of this chapter is to review the current state of knowledge with respect to the neural control of thermoregulation and concepts of thermal control, to review the principles of systems physiology that remain applicable to current practice, and to provide a snapshot of current practice, highlighting some areas that have received little emphasis.

### PHYSIOLOGY

#### Fetus

The fetus has little control over its thermal environment. Fetuses are metabolically active, and thermal balance is achieved by heat transfer to the mother largely via the placenta. Thus, fetal temperature closely tracks maternal temperature and is on average 0.5°C higher. The fetus has limited capacity to increase heat production because nonshivering thermogenesis is largely inhibited secondary to reduced oxygen availability5 and the inability to respond to catecholamines6,7 and thyroid hormones8 and because of placental inhibitory factors, including adenosine and prostaglandin E2 (PGE2).9, 10, 11, and 12 Thus, when umbilical blood flow is interrupted, fetal temperature increases.13, 14, and 15 When body temperature is elevated just after delivery, one must therefore consider ...

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