An understanding of the thermal requirements of the high-risk infant was slow to develop. Pierre Budin, historically the first neonatologist, had perhaps the earliest insight into the clinical importance of the thermal environment. In 1907 in his book, The Nursling, he emphasized the need for temperature control after noting a markedly increased survival rate when the infant’s rectal temperature was maintained ( Table 6-1 ). He recommended an air temperature of 30° C (86° F) for the small (1 kg), fully clothed infant. Sadly, his observations were neither fully understood nor appreciated in the first 50 years of the 20th century. In addition, during this period, the clinical value of two variables (temperature and humidity) was confused.
| Temperature | Survival Rate |
|---|---|
| 32.5° C to 33.5° C | 10% |
| 36.0° C to 37.0° C | 77% |
The modern era of neonatology was heralded by Silverman et al. in three sequential analyses, whereby they resolved the importance and relationships between the incubator temperature and relative humidity. In the first study, the researchers compared high and low humidity in two groups of infants. Infants in the high humidity group had a lower mortality rate but higher rectal temperatures. In the second study, to end the confusion caused by two variables, they controlled the humidity and examined the effect of varying only environmental temperature. They noted a striking difference in survival rates. With only a 4° F increase in incubator temperature (from 85° F to 89° F), they observed a 15% increase in survival rate at the higher temperature (68.1% versus 83.5%), with the biggest difference affecting the smallest infants. In a further study, controlling environmental temperatures but varying humidity caused no difference in survival.
Hill, in part, clarified the profound effects of environmental temperature on survival observed by Silverman et al. working with kittens and guinea pigs; she found that in 20% oxygen, oxygen consumption and rectal temperatures varied with the environmental temperature ( Fig. 6-1 ). She noted a set of thermal conditions at which heat production (measured as oxygen consumption) is minimal, yet core temperature is within the normal range (neutral thermal environment). When the animals were cooled while breathing room air, their oxygen consumption markedly increased and body temperature was maintained. However, when they were given 12% oxygen and cooled, oxygen consumption did not increase, and the animals’ body temperatures decreased. This, as well as the work of Bruck and others, has emphasized that the human infant is a homeotherm and not a poikilotherm, as is a turtle. If a nonhypoxic infant is cooled, the infant maintains body temperature by increasing the consumption of calories and oxygen to produce additional heat. Homeotherms possess mechanisms that enable them to maintain body temperature at a constant level, more or less accurately, despite changes in the environmental temperature. In contrast, the body temperature of a turtle decreases if it is placed in a cool environment.
The increased survival rate in the warmer environment observed by Budin and Silverman presumably resulted from the decreased oxygen consumption and carbon dioxide production as environmental conditions approximated the neutral thermal environment. An immature infant with a minimal ability to transfer oxygen and excrete carbon dioxide across his or her lungs has the least chance of becoming hypoxic or developing a respiratory acidosis—increased Pa co 2 —if maintained in an environment that minimizes oxygen consumption or metabolic rate.
Maintaining the neutral thermal environment became the first and foremost foundation of the modern era of neonatal intensive care. Recently, controlled cooling has been introduced to reduce the metabolic requirements of the brain and so to reduce injury in full-term infants with moderate to severe hypoxic-ischemic encephalopathy.
This chapter will summarize key elements of thermal regulation in addition to other important factors in the physical environment as they relate to the sick newborn.
Physiologic Considerations
Heat Production
The heat production within the body is a byproduct of metabolic processes and must equal the heat that flows from the surface of the infant’s body and the warm air from the lungs over a given period if the mean body temperature is to remain constant. A characteristic of the homeothermic infant is the ability to produce extra heat in a cool environment. In the adult, additional heat production can come from (1) voluntary muscle activity, (2) involuntary tonic or rhythmic muscle activity (at high intensities, characterized by a visible tremor known as “shivering”), and (3) nonshivering thermogenesis. The latter is a cold-induced increase in oxygen consumption and heat production, which is not blocked by curare, a drug that prevents muscle movements and shivering. In the adult, shivering is quantitatively the most significant involuntary mechanism of regulating heat production, whereas in the infant, nonshivering thermogenesis is probably most important. From animal and human studies, it can be inferred that, in the human infant, the thermogenic effector organ—brown fat—contributes the largest percentage of nonshivering thermogenesis.
Brown Fat
More abundant in the newborn than in the adult, brown fat accounts for about 2% to 6% of total body weight in the human infant. Sheets of brown fat may be found at the nape of the neck, between the scapulae, in the mediastinum, and surrounding the kidneys and adrenals. Brown fat differs from the more abundant white fat. The cells are rich in mitochondria and contain numerous fat vacuoles (as compared with the single vacuoles in white fat). Brown fat contains a dense capillary network and is richly innervated with sympathetic nerve endings on each fat cell. The special property of brown fat is the uncoupling protein, which results in the oxidation of food to heat rather than energy-rich phosphate bonds. Its metabolism is stimulated by norepinephrine released through sympathetic innervation, resulting in triglyceride hydrolysis to free fatty acids (FFAs) and glycerol.
The initiation of nonshivering thermogenesis at birth depends on cutaneous cooling, separation from the placenta, and the euthyroid state. The acute surge in thyroid hormones at birth appears to be of limited importance with regard to the immediate control of thermogenesis, whereas the intracellular conversion of T 4 to T 3 and the effects of norepinephrine appear to be of greater significance. Stimulation of the sympathetic nervous system by cold exposure markedly increases local norepinephrine turnover within brown adipose tissue, which may not be reflected by an increase in circulating catecholamines. This results in a marked increase in oxygen consumption without any appreciable increase in physical activity.
Interesting observations made in sheep noted that cooling of the fetus results in very small increases in FFAs and a significant decrease in body temperature. Ventilation of the fetus with increasing P o 2 resulted in a slight increase in FFAs, whereas clamping the cord resulted in a sharp increase in FFAs and glycerol. These and other observations suggest that before birth there is an inhibitor to thermogenesis, probably produced by the placenta. Possible candidates for the inhibitor are adenosine or prostaglandin E 2 . The nonshivering thermogenesis occurring in the brown fat during cooling can be turned off with hypoxia (see Fig. 6-1 ), and the sensory receptors for this are most probably the carotid body afferents.
There also appears to be a relationship between feeding and brown adipose tissue (BAT) activity in rats. Initiation of feeding is mediated by a transient dip in blood glucose concentration caused by stimulated glucose utilization in BAT. Feeding continues while BAT and core temperature continue to rise. Termination is induced by the high level of core temperature brought about by the episode of stimulated BAT thermogenesis. The time between initiation and termination determines the size of the meal and depends on the balance between BAT thermogenesis and heat loss, and thus on ambient temperature. The underlying cause of the episodic stimulation of sympathetic nervous system activity is a decline in core temperature to a level recognized by the hypothalamus as needing a burst of increased heat production. Thus, BAT thermogenesis is important in control of meal size, relating it to thermoregulatory needs. The phenomenon is termed thermoregulatory feeding , to distinguish it from feeding initiated by other stimuli.
The physiologic control mechanisms of the infant may alter the internal gradient (i.e., vasomotor) to change skin blood flow. The external gradient is of a purely physical nature. The large surface-to-volume ratio of the infant (especially those weighing less than 2 kg) in relation to the adult and the thin layer of subcutaneous fat increase the heat transfer in the internal gradient.
The heat transfers from the surface of the body to the environment of water. This heat transfer is complex, and the contribution of each component involves four means of loss: (1) by radiation; (2) by conduction; (3) by convection; and (4) by evaporation—influenced by the temperature of the surroundings (air and walls), air speed, and water vapor pressure, Of special clinical importance to the pediatrician is the considerable increase in radiant heat loss from the infant’s skin to the cold walls of incubators.
Radiant heat loss is related to the temperature of the surrounding surfaces, not air temperature. When incubators are in cool surroundings (e.g., during transfer) the inner surface temperature of the single-walled incubator declines well below that of the air temperature in the incubator. In caring for the infant, this problem is easily solved by wrapping him or her in a light covering (transparent if necessary). The surrounding radiant temperature is then close to body temperature and more under the influence of the incubator air temperature.
Because heat may be transferred by four different routes, the physical characteristics of the newer incubators and radiant heaters should be familiar to the caretakers. Devices used to maintain thermal stability in preterm infants have advanced over time so that the latest, most advanced, user friendly, and developmentally supportive microenvironment is possible for even the smallest, least mature infants. Engineers continue to design the most efficient and effective means of assisting clinicians to achieve the neutral thermal environment, and at the same time offering the clinicians clear observation and access to the infant. The current generation of combined incubator-radiant warmers improves their chances of survival (Giraffe ®, Omnibed ®, Versulet). Furthermore, as noted, meticulous attention to keeping babies warm in the delivery room has become the standard of care.
In very small infants (<1500 g), evaporative heat loss is increased in the first days of life, the result of very thin skin that is unusually permeable to water ( Fig. 6-2 ). For further discussion, see Chapter 9 .
The effect of environmental temperature on heat production (oxygen consumption) is considered in Figure 6-3 . As the environmental temperature is decreased below point A (critical temperature), oxygen consumption increases. Body temperature, however, is maintained if heat production is adequate.
If cooling is severe and body temperature drops below point B, with cold paralysis of the temperature regulation center, oxygen consumption also drops—two-to three-fold for every 10° decrease in body temperature. Homeothermy can also be abolished by sedative drugs and brain injury. Not all babies are homeotherms all the time.
Figure 6-3 shows that oxygen consumption is minimal in two areas: the neutral thermal environment and severe hypothermia. For many years, cardiac surgeons have taken advantage of the minimal metabolic rate with body cooling (temperatures below point B). More recently, cooling has been added to neuro-intensive care for term infants with hypoxic-ischemic encephalopathy. Under normal circumstances in the neonatal intensive care unit, caregivers strive to maintain the infant in a warm environment (the neutral thermal environment, or the so-called “zone of thermal comfort”). It is important clinically to note that the infant may not be in a neutral thermal environment and yet the rectal temperature may be in the normal range . As emphasized by Hey and Katz, “body temperature alone fails to indicate whether a baby is subjected to thermal stress: it can only alert us to situations in which the thermal stress has been so severe that the baby’s normal thermoregulatory mechanisms have been at least partially overpowered.” Rectal temperature drops only when the baby’s maximum effort to preserve and produce heat fails. The first mechanism to preserve heat is vasoconstriction, and this phenomenon can easily be detected by measuring skin temperature at a peripheral part of the body. A sensitive method to detect vasoconstriction is to measure both rectal and sole of the foot temperatures.
Claiming that the optimal thermal environment for sick preterm infants is unknown, Genzel-Boroviczény et al. in a random manner, compared the effect of setting the incubator temperature to an abdominal wall temperature of 36.5° C (neutral temperature [NT]) or to a minimal temperature difference (<2° C) between abdominal wall and extremities (comfort temperature [CT]). They correctly speculated that this could affect the microcirculation perfusion, which they assessed with near-infrared photo-plethysmography (NIRP) at these two target temperatures between days 1 and 4 of life in preterm infants with normal or impaired (RED group) microcirculation as determined by a clinical score. They concluded that increasing the incubator temperature to CT changes thermoregulatory flow to the extremities in preterm infants with impaired microvascular perfusion and might improve tissue flow. So the issue of how to best set the neutral thermal zone without measuring energy expenditure remains unresolved. The old-fashioned increase in toe-tummy temperature differential is still a valuable indicator of ill health and potential sepsis.
Hypothermia and hyperthermia develop more rapidly in the neonate than in the adult. The infant has a lower capacity for heat storage because of the higher temperature of the body shell in relation to the environment and the larger surface-to-volume ratio. Thus, the thermoregulatory system of the homeothermic infant adjusts and balances heat production, skin blood flow, sweating, and respiration in such a way that the body temperature remains constant within a control range of environmental temperatures. The control range refers to the range of environmental temperatures at which body temperature can be kept constant by means of regulation. The control range of the infant is more limited than that of the adult because of less insulation. For the nude human adult, the lower limit of the control range is 0° C (32° F), whereas for the full-term infant it is 20° C to 23° C (68° F to 73.4° F).
The insufficient stability of body temperature in the small premature infant does not indicate an immaturity of temperature regulation because the system is intact. As pointed out by Bruck, the insufficient stability “seems to be due to the discrepancy between efficiency of the effector systems and body size.” The newborn infant has a well-developed temperature regulation but a narrower control range than the adult.
As emphasized by the late Albert Okken, the body surface-to-mass ratio of very immature and very low birth rate infants is about five times higher than in adults. The disadvantage of this relatively very large surface area of the premature infant observed at both ends of the thermal spectrum. There is rapid heat loss in a cool environment and rapid overheating in a heat-gaining environment.
In Utero
While the fetus is in utero, the heat produced is dissipated through the placenta to the mother. If complete placental separation occurs in utero, the temperature of the fetus increases rapidly. Normally the temperature of the fetus is 0.6° C above the mother’s temperature. When the mother’s temperature increases secondary to infection or move commonly with the use of an epidural analgesia for labor, the fetal temperature increases to about 0.6° C higher than the mother’s. Approximately 30% to 40% of women receiving an epidural anesthetic in early labor are noted to have a fever in late labor, the cause of which is unknown. The thermoregulatory system works well for the fetus except during periods when the mother has an increasing body temperature.
Schouten et al. studied the temperatures of women during labor and observed that the mean temperature increased from 37.1° C at the beginning of labor to 37.4° C after 22 hours. Circadian temperature patterns were not observed during labor. Epidural analgesia is associated with maternal fever due to inability of the mother to dissipate heat. Nulliparity and dysfunctional labor are also significant cofactors in the fever attributed to epidural analgesia. Lieberman noted intrapartum fever higher than 100.4° F in 14.5% of women receiving an epidural, but in only 1.0% of women not receiving an epidural (adjusted odds ratio [OR] = 14.5, 95% CI = 6.3, 33.2). Neonates whose mothers received epidurals were more often evaluated for sepsis and treated with antibiotics. Although 63% of women received epidurals, 96.2% of intrapartum fevers, 85.6% of neonatal sepsis evaluations, and 87.5% of neonatal antibiotic treatment occurred in the epidural group. Compared with continuous infusion, intermittent epidural injections appear to protect against intrapartum fever in the first 4 hours of labor analgesia. This may be due to intermittent partial recovery of heat loss mechanisms between injections.
As noted above, the association between the use of epidural analgesia for pain relief in labor and intrapartum maternal fever has been established in both observational and randomized trials. There has been a suggestion, too, of an increase in adverse neonatal outcomes with intrapartum maternal fever. Greenwell et al. ∗
∗ Greenwell E et al: Intrapartum temperature elevation, epidural use, and adverse outcomes in term infants, Pediatrics 129:e447, 2012.
confirm that maternal temperature above 100.4° F developed during labor in 19.2% (535/2784) of women receiving epidural compared with 2.4% (10/425) not receiving epidural. Furthermore, maternal temperature above 99.5° F was associated with adverse neonatal outcomes. The rate of adverse outcomes (hypotonia, assisted ventilation, 1-and 5-min Apgar scores below 7, and early-onset seizures) increased two- to six-fold with maternal temperature above 101° F. Without temperature elevation, epidural use was not associated with adverse neonatal outcomes.After Birth
Hypothermia is a major cause of morbidity and mortality in infants; therefore, maintaining normal body temperatures in the delivery room is crucial. An understanding of how infants produce heat and what can be done to maintain normal body temperatures in full-term and preterm infants is essential for the preservation of thermal stability in this population.
At birth, the infant’s core temperature decreases rapidly, owing mainly to evaporation from his or her moist body. The infant’s small amount of subcutaneous tissue and large surface area-to-mass ratio compared with the adult, together with the cold air and walls of the delivery room, also result in large radiant and convective heat losses. Oxygen should always be warmed. Thus, under the usual delivery room conditions, deep body temperature of human newborns can decrease 2° C to 3° C unless special precautions are taken.
Although moderate to severe cooling may result in metabolic acidosis, a lower arterial oxygen level, and hypoglycemia in the newborn infant, very slight cooling of the infant may be beneficial in his or her adaptation to extrauterine life. Cooling of the skin receptors may play a significant role in initiating respiration and stimulating thyroid function. The vasoconstriction and peripheral resistance observed with mild cooling also alters systemic vascular resistance, thereby reducing the right-to-left shunting of blood through the ductus arteriosus. With severe cooling, a vicious circle can result in severe hypoxia and even death ( Fig. 6-4 ). The neonatologist chooses to keep the infant warm following delivery to prevent metabolic acidosis and possibly dangerous reflex responses to cooling.
The body temperature of preterm babies can drop precipitously after delivery. Reports of hypothermia in babies of all birth weights, on admission to neonatal units have come from all over the world. Recent reports that showed that hypothermia on admission to neonatal units is an independent risk factor for mortality in preterm babies have refocused attention on the need for meticulous thermal care immediately after birth and during resuscitation. Their data lend weight to the view that conventional approaches to thermal care of the very preterm and low-birth-weight baby are outmoded. Bathing newborn babies shortly after birth increases the risk of hypothermia despite the use of warm water and skin-to-skin (SSC) care for thermal protection of the newborn. During the first 12 hours of life, the temperature in extremely low-birth-weight infants may also decrease with procedures such as umbilical line insertion, intubation, obtaining chest x-rays, manipulating intravenous lines, repositioning, suctioning, and even taking vital signs. Precautions should be undertaken to prevent heat loss during these procedures.
Keeping vulnerable preterm infants warm is problematic even when recommended routine thermal care guidelines are followed in the delivery suite. McCall et al. performed a Cochrane review to assess efficacy and safety of interventions designed for prevention of hypothermia in preterm and/or low-birth-weight infants applied within 10 minutes after birth in the delivery suite compared with routine thermal care. Barriers to heat loss such as plastic wraps or bags were effective in reducing heat losses in infants less than 28 weeks’ gestation but not in infants 28 to 31 weeks’ gestation. Plastic caps were effective in reducing heat losses in infants less than 29 weeks’ gestation. There was insufficient evidence to suggest that either plastic wraps or plastic caps reduce the risk of death during the hospital stay. There was no evidence of significant differences in other clinical outcomes for either the plastic wrap/bag or the plastic cap comparisons. Stockinet caps were not effective in reducing heat losses.
External heat sources, including transwarmer mattresses and SSC, were shown to be effective in reducing the risk of hypothermia when compared to conventional incubator care. The authors concluded that plastic wraps or bags, plastic caps, skin-to-skin care (SSC), and transwarmer mattresses all keep preterm infants warmer, leading to higher temperatures on admission to neonatal units and less hypothermia. However, the small numbers of infants and studies and the absence of long-term follow-up mean that firm recommendations for clinical practice cannot be given.
Simon et al. compared thermal mattresses (sodium acetate) with a plastic wrap for extremely low gestational age newborns (ELGANs) between 24 and 28 weeks’ gestation with a birth weight less than 1250 grams. Although the mattress was superior to the plastic wrap and both improved the thermal status of ELGANs, they concluded that all current interventions fall short of fully protecting all these vulnerable infants from thermal stress.
Trevisanuto et al. conducted a randomized trial and concluded that for very preterm infants, polyethylene caps are comparable with polyethylene occlusive skin wrapping to prevent heat loss after delivery. Both these methods are more effective than conventional treatment; however, many babies are still admitted with low temperatures.
Temperature Control in the Very Low-birth-Weight Infant
Even though infants with a birth weight less than 1250 g make up less than 1% of the total babies born annually in the United States, they frequently constitute a significant percentage of the babies in the intensive care nursery. Very low-birth-weight (VLBW) infants’ limited ability to produce heat, their increased evaporative water loss at birth secondary to extremely thin skin, as well as their small heat capacity (the result of their large surface-to-volume ratio) make them unusually susceptible to cold stress.
Because one of the first responses to thermal stress in these infants is a change in peripheral vasomotor tone with vasodilation when overheated and vasoconstriction with cooling, a continuous assessment of central and peripheral temperatures and their difference is clinically helpful in promptly interpreting the effect of the thermal environment on the infant.
In a study of the first 5 days of life in 79 infants weighing less than 1000 g and 71 infants weighing 1000 to 1500 g, central temperature (Tc) was measured with an abdominal skin probe over the liver and peripheral temperature (Tp) on the sole of the foot to calculate the central-peripheral temperature difference (Td). The nursing care attempted to keep the abdominal skin temperature between 36.8° C and 37.2° C to maintain a Td of less than 1° C. The infants were nursed in a double-walled incubator with 80% humidity, and the nurses altered the air temperature. In the heavier babies, the Tc had a constant median value of 36.7° C and increased to 36.9° C for the next 4 days. During the first day, the Tc was lower than the sole of the foot nearly 20% of the time, suggesting a slow vasomotor response to cold stress in these very immature infants. To prevent cold stress in this group, Tc was greater than 37.5° C 12% of the time. The normal pattern of Tc greater than Tp was seen more commonly after the first day. In infants weighing less than 1000 g, a Td greater than 2° C can be caused by poor perfusion resulting from hypovolemic shock. In these infants, there was other evidence of hypovolemia 11% of the time (such as increasing heart rate or decreasing blood pressure). With hyperthermia (Td less than 1° C) heart rate increased. If the Tc was greater than 38° C with Td greater than 1° C, the infant was investigated to rule out hypovolemia and sepsis.
For appropriate for gestational age infants weighing less than 1500 g, it is recommended that the infants be nursed in double-walled incubators, with low air velocity and additional humidity for the first week of life. Td should be maintained at less than 1° C. Modern incubators provide such an environment.
Nutrition and Temperature
Because of the relationship between metabolic rate and body temperature, both fluid and nutritional requirements for growth are intimately linked with temperature regulation. This is especially important to the small premature infant maintained in a slightly cool environment. Caloric intake is limited by the small capacity of his or her stomach. Fewer calories would be required for maintenance of body temperature if the infant was in a warmer environment; thus, in the neutral thermal environment, caloric intake can be more effectively used for growth.
The insensible loss of water parallels the metabolic rate, with 25% of total heat produced being dissipated in this manner. Thus, an elevated metabolic rate results in elevated fluid losses and, hence, increased fluid requirements. The neutral thermal temperature allows for small feedings and reduced caloric requirements for growth.
Glass et al. were able to quantitate the effect of temperature control on growth, comparing 12 matched, healthy, small infants aged 1 week -who weighed between 1 and 2 kg. These infants were divided into a “warm” group (abdominal skin temperature maintained at 36.5° C [97.7° F]) and a “standard” group (abdominal skin temperature maintained at 35° C [95° F]). Both groups received 120 kcal/kg/d. Those in the warm group showed a significantly more rapid increase in body weight and length; however, their cold resistance (ability to prevent a decrease in deep body temperature in a cool environment) was diminished. Identical growth rates could be obtained by increasing caloric input intake the standard group.
It is, therefore, difficult to decide whether the premature infant, after the early neonatal period, should be maintained in the neutral thermal environment for optimal growth or be prepared for some of the rigors of a cold apartment or house.
Longitudinal data on resting energy expenditure (REE) in extremely immature infants and full-term neonates are scarce, but are necessary to understand the energy requirements in neonatal nutrition during the first weeks of life. REE values increased in all gestational age groups from the first week to 5 to 6 weeks of postnatal age, with the most pronounced increase in the smallest infants (+140%) and the smallest increase in the full-term neonates (+47%). Knowledge of the energy requirements is critical to meet the goals and ensure growth. Furthermore, the ability to modify energy expenditure (EE) is extremely helpful when energy intake is limited, which is why careful attention to the thermal environment is so important for very immature babies. Music such as Mozart may help, too. Lubetsky et al. present evidence that music generally may help premature infants by lowering stress hormone levels, leading to enhanced weight gain and growth.
Practical Applications
Delivery Room
The temperature of the delivery room is frequently set for the comfort of the medical staff rather than for the comfort of the newborn. Careful and immediate drying of the infant’s entire body remains critical in minimizing evaporative heat loss. Many pieces of equipment are available to warm the infant—in particular, incubators and radiant warmers. However, the warm body of the mother is well-suited to meet this need. Christensson compared body temperatures over the first 90 minutes of life in healthy full-term neonates cared for with skin-to-skin with their mothers. Infants were thoroughly dried immediately after birth and placed either on their mother’s chest and abdomen and covered with a light blanket or wrapped in cotton blankets and placed in a cot. The infants placed skin-to-skin warmed significantly faster than those in the cot. Oxygen consumption measurement while skin-to-skin revealed that they were in a neutral thermal environment. Thus, for the normal full-term infant, skin-to-skin on the mother’s chest is an ideal location for the first 2 hours of life. Also, this would allow the infant to crawl to the mother’s breast and begin to suckle on his or her own.
Skin-to-skin care has been practiced in primitive and high technology cultures for body temperature preservation in neonates. Karlsson measured regional skin temperature and heat flow in moderately hypothermic term neonates (mean rectal temperature of 36.3° C) and observed that the mean rectal temperature increased by 0.7° C when placed skin-to-skin on their mothers’ chests. Caution must be exercised when attempting this in very immature infants. Bauer noted no significant changes in temperature or oxygen consumption in the first postnatal week for infants between 28 and 30 weeks’ gestation ; however, infants of 25 to 27 weeks of gestational age lost heat during skin-to-skin contact. They recommended postponing skin-to-skin care for these infants until week 2 of life, when their body temperature remains stable and they are calmer during skin-to-skin contact than in the incubator.
Incubators
In the United States, most intensive care units have double-walled incubators in which the temperature of the inner wall of the incubator is not affected by a cooler room temperature. However, because single-walled incubators are still found in the United States and many countries throughout the world and the temperature of the single walls cannot be controlled, it should be emphasized that the radiant heat loss of the infant to the wall of these incubators varies. Figure 6-5 indicates how the temperature of the inner wall of the incubator decreases with cooler room temperatures—a major disadvantage when nursing a sick infant. If the nursery is cool (23.8° C to 15.6° C [75° F to 60° F]) or if the incubator is placed near a cool window or wall, it is difficult—usually impossible—to locate and maintain the neutral thermal environment. The infant loses heat to the cold incubator wall and needlessly increases oxygen and caloric consumption in his or her efforts to stay warm. The magnitude of this loss can be predicted if room temperature is known. Hey and Katz found that operative temperature (true environmental temperature, taking into account radiation and convection) decreased 1° C below incubator temperature for every 7° C that incubator air exceeded room temperature. Unless the incubator, room air, and radiant surfaces have similar temperatures, innumerable thermal conditions can exist.
