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37° C
Heat production or
Body
temperature
Death from
°F
Age (days)
Neutral thermal environment
35
°C
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CHAPTER 6 Heat Balance
139
Inevitable body cooling
Death from
cold
Decreasing body
oxygen consumption
temperature
Thermoregulatory
range
Summit
metabolism
Range of
thermal
neutrality
Chemical
regulation
Inevitable
body
heating
heat
Increasing
body
temperature
FIGURE 6.1 Temperature versus oxygen consumption. Effect of environmental oxygen consumption and body temperature. (From Klaus
M, Fanaroff A. Care of the High-Risk Neonate, 2nd ed. Philadelphia, PA: Saunders; 1979.)
36
96
95
94
93
Infant weighing 1 kg at birth
35
34
FIGURE 6.2 Neutral thermal environments. Range of temperature to provide neutral environmental conditions for infant lying naked on
warm mattress in draft-free surroundings of moderate humidity (50% saturation) when the mean radiant temperature is the same as air
temperature. Shaded areas show average neutral temperature range for healthy infant weighing 1 kg (dark) or 2 kg (light) at birth. Optimal
temperature probably approximates to lower limit of neutral range as defined here. Approximately 1°C (1.8°F) should be added to these
operative temperatures to derive appropriate neutral air temperature for single-walled incubator when room temperature is less than 27°C
(80°F) and more if room temperature is much less. (From Hey EN, Katz G. The optimum thermal environment for naked babies. Arch Dis
Child. 1970;45:328.)
92
91
90
89
33
32
Infant weighing 2 kg at birth
0510 15 20 25 30
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the more accurate and sensitive direct calorimetry are
used to study the production and expenditure of heat
in newborns. Factors such as ambient air temperature,
airflow velocity, relative humidity (RH), and temperature and composition of objects in direct contact with
the infant or to which heat may be radiated compose
the infant’s thermal environment.
Maintaining Heat Balance (Heat
Production vs. Losses)
When exposed to a cold environment, a neonate
senses the reduced skin surface temperature (using
sensors in the skin, primarily the face) and senses
core body temperature (using sensors along the
spinal cord and in the hypothalamus). Information
from these various sensors is processed (probably
in the posterior hypothalamus), including average
temperature, rate of temperature change, and size
of the stimulated area. Cold stress results in the
initiation of a series of reactions to increase
heat production and decrease heat loss. In adults,
the most significant involuntary method of heat
production is shivering. Neonates rarely shiver
and must rely on nonshivering, or chemical,
thermogenesis to produce the needed heat. This
process is initiated in the hypothalamus and transmitted through the sympathetic nervous system,
leading to the release of norepinephrine at the site
of brown fat. Brown fat, found mostly in the nape
of the neck, axillae, and between the scapulae
of newborns, is a specialized type of fat. It is
unique in that it contains thermogenin, which is the
key enzyme regulating nonshivering thermogenesis.
Norepinephrine causes the release of free fatty acids,
which with thermogenesis undergo combustion in
the mitochondria of brown fat cells, releasing heat.
Lipoprotein lipase also provides further triglyceride
substrate for heat production.
Oxygen and glucose also are consumed during
nonshivering thermogenesis. Thus an infant who
already has low oxygen or glucose levels may become
hypoxemic or hypoglycemic if added thermal stress
occurs. Preterm babies do not possess sufficient
brown fat stores to mount a significant heat
production response to compensate for even
minimal cold stress.36 When servocontrol is used,
the thermistor must not be placed over an area
of brown fat (such as in the axilla), which may
directly heat the overlying skin, causing a decrease
in servocontrolled heat output.
3
Heat generated within the body is transferred by
conduction through tissues along a gradient from
warmer to cooler areas such as the skin surface. An
initial response to a cold environment is to constrict superficial blood vessels to minimize the
transfer of heat from the core to the surface of
the body. Superficial vasoconstriction, which gives
the skin a mottled appearance in response to cold
stimulus, results in a lower skin temperature reading
to the thermocontroller and consequently causes an
increase in the incubator temperature. The smaller
the body size, the less effective vasoconstriction is
in conserving heat.
Compared with adults, newborns have a very
large surface area to body mass ratio and therefore have a relatively large area exposed to the
environment from which heat can be lost. More
mature infants may try to minimize their surface area
by changing positions to decrease exposed surface
area when faced with cold stimulus, but immature
infants cannot flex the trunk and extremities effectively. They also have little subcutaneous fat tissue
(which acts as insulation) to help prevent heat conduction to the body’s surface, where the heat would
be lost.
Heat is transferred from the infant’s body to the
environment (i.e., everything in proximity to the baby)
along a temperature gradient from warmest to coolest.
Heat losses occur by four principal mechanisms:
radiation, conduction, evaporation, and convection. Fig.
6.3 illustrates these four mechanisms and identifies
interventions to minimize their effects.
Much less frequently, a newborn must call on physiologic responses to an environment that is too warm,
and these responses are somewhat limited. As skin
temperature rises, superficial blood vessels dilate,
increasing the transfer of core body temperature to the surface. Increasing the temperature
gradient between the skin and the environment
increases heat loss from the body. When exposed
to elevated environmental temperatures, preterm
babies generally cannot generate sweat to eliminate
heat by evaporation. Maturing babies develop this
eccrine gland function first on the forehead, followed
by the chest, upper arms, and more caudal areas.
Thermoregulation requires energy (caloric)
expenditure:
• Basal metabolic rate: 50 kcal/kg/day
• Thermoregulation: 10 kcal/kg/day
• Thermic effects of feeding: 8 kcal/kg/day6 (see
Chapter 17)
Cold
RADIATION EVAPORATION
CONDUCTION CONVECTION
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windows
Away from
cold
windows
Cold scales
Cold
walls
Double-
wall
Isolette
Wet skin
and
blanket
CHAPTER 6 Heat Balance
Dried
skin and
blanket
Labor
and delivery
141
Away
from
Warm scales
draft
FIGURE 6.3 Radiation, or heat loss in the form of electromagnetic photons, occurs from warm skin surfaces to a cooler object not in
contact with the newborn (e.g., inside the incubator wall, nursery wall, window). Radiant heat loss is independent of ambient air temperature
and is the main source of heat loss because of the infant’s large exposed body surface area. Conduction is the loss of heat to a cooler object
in direct contact with the newborn (e.g., cold scale, unwarmed bed, stethoscope, examiner’s hand). Convection is the loss of heat to moving
air at the skin surface and depends on the air’s velocity and temperature. Evaporation of water from the skin and mucous membranes also
causes heat loss, especially in the delivery room. The thinner stratum corneum layer of skin of very-low-birth-weight infants makes evaporative
heat and water loss and fluid management ongoing problems. (Courtesy Lynn Jones, RN.)
Methods of Measuring Temperature
because the sigmoid colon makes a right-angle turn
approximately 3 cm from the anal opening. Esophageal
A neonate’s temperature can be determined by various
methods.
123
Deep body (core) temperature may be
measured in the rectum or esophagus and on the tympanic membrane. Rectal thermistors are thin, flexible
probes that must be inserted at least 5 cm to obtain an
accurate reading. Insertion of a rigid rectal thermometer to this depth increases the risk for perforation,
and tympanic readings are difficult to obtain and usually impractical.
Continuous monitoring of abdominal skin temperature with the newborn lying supine is a noninvasive method that has good correlation with rectal
temperatures in preterm35 and very-low-birth-weight
(VLBW) preterm infants.
Cold draft
131
Only abdominal skin
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BOX
6.1
• Abdominal skin temperature of 36.5°C (97.7°F) results in minimal
• Abdominal skin temperature of 35.9°C (96.6°F) results in an in-
• Abdominal skin temperature of 37.2°C (98.9°F) results in an in-
ALTERATIONS IN OXYGEN CONSUMPTION
WITH CHANGES IN ABDOMINAL SKIN
TEMPERATURE: RESEARCH BASIS
oxygen consumption.
crease in oxygen consumption of 10%.
crease in oxygen consumption of 6%.
118,119
temperature has been shown to be an effective monitor of neutral thermal environment. Box 6.1 clearly
illustrates that even a slight change (less than 1°C) in
abdominal skin temperatures affects oxygen consumption and basal metabolic rates. Therefore the range of
thermal neutrality illustrated in Fig. 6.1 is a very
narrow range—less than 1°C on either side of
abdominal skin temperature of 36.5°C (97.7°F).
Because of the risks involved, rectal tempera-
tures should not be taken on a routine basis in
neonates. Axillary temperatures require disturbing
and handling, minimal exposure due to undressing
(which may lower temperature), and result in crying
and restlessness in some neonates.
38,74
Axillary temperatures are measured by using glass, electronic, or
disposable thermometers. The tip of the thermometer
should be held firmly in the midaxillary area for at least
3 minutes in preterm infants and 5 minutes in term
infants. When taken properly, axillary temperatures
provide readings as accurate as rectal and core
temperature methods.45 Approximating rectal temperatures from axillary temperatures is clinically
possible by adding 0.3°C (difference in axillary and
rectal values in preterm neonates) to 0.4°C (difference in axillary and rectal values in term neonates)
to the axillary value.68 In term infants, axillary
temperatures should be maintained at 36.5°C to
37.5°C (97.7°F to 99.5°F). For preterm infants,
the normal axillary temperature ranges between
36.3°C and 36.9°C (97.3°F and 98.4°F).
110
A randomized study comparing temperature values from three skin probe positions with digital
axillary temperature values was conducted in healthy
preterm infants.
115
There was no statistically sig-
nificant difference in any of the three probe site
temperature values or between the probe site values and the digital axillary temperatures.
115
The
results of this study are similar to a previous study
conducted in healthy full-term infants.
126
Because
of the small size of the sample and methodology
followed, the results of these studies cannot be generalized to all preterm infants, especially those with
extremely low birth weight (ELBW). Both research-
ers recommend consistency in temperature measurement and evaluation of trends and patterns
over time, along with evaluation of the entire
clinical picture of each individual infant.
115,126
In search of a quicker, noninvasive, less disruptive
mode of temperature taking, the accuracy of noninvasive infrared thermometers in the neonatal population
has been evaluated in studies in premature
38,74
and in
healthy late-preterm and term infants.50 These studies
found the readings between the infrared midforehead and temporal artery and axillary readings to
be very comparable. A more recent study of temporal
artery measurement found it no more accurate than
axillary or rectal temperatures in 49 neonates.
125
For
neonates in incubators the temporal artery measurement was less accurate than for neonates in cribs. As
postmenstrual age (PMA) increased, the accuracy of
temporal artery measurements of neonatal temperature
also increased.
125
More research with various populations of neonates, in various environments (cribs, incubators, radiant warmers), and using larger numbers of
study subjects is necessary.
123
During the first week of
life, infrared thermal imaging has been studied to evaluate the full body temperature and perfusion differentials
of extremely preterm infants in heated incubators.
67
In critically ill infants, the skin temperature
is usually routinely monitored in addition to
axillary temperature readings. A skin probe is
secured to the right upper quadrant of the abdomen. The temperature probe should not be placed
under the axilla or any other position except as recommended by the manufacturer. Because an infant
responds to cold stress by vasoconstriction, a
drop in skin temperature may be the first sign
of hypothermia. The core temperature may not
fall until the infant can no longer compensate. The
axillary temperature may remain normal (or even be
elevated) because of proximity to brown fat stores.
ETIOLOGY OF HEAT
IMBALANCES
The ambient temperature range in which a healthy
full-term infant maintains a stable core temperature is narrower than the temperature range in
which an adult maintains a normal temperature.
When measures are taken to provide a neutral thermal
CHAPTER 6 Heat Balance
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143
environment for the neonate, excessive heat losses
or gains are avoided and heat balance is maintained.
Recognition of infants at risk for heat imbalance is
essential in the prevention of thermal stress.
Premature infants have a limited ability to
control body temperature and are extremely susceptible to hypothermia. Factors that contribute to
temperature instability include very thin skin, large
surface area relative to body mass, limited substrate
for heat production, decreased subcutaneous tissue,
and an immature nervous system. These infants often
have multiple health problems that necessitate frequent
interventions by health care providers with consequent
disruption of the infant’s neutral thermal environment.
A premature infant’s very thin skin and larg-
er surface area to body mass ratio allow for
increased evaporative heat loss. Term infants can
reduce surface area by flexing their extremities onto
their trunk, a skill that increases with gestational age.
Unable to maintain flexion, a preterm infant lies
primarily with extremities extended. Care provid-
ers may reduce the surface area by positioning
infants in flexion and supporting them with
blankets and rolls. The shortened gestation limits
lipid supplies, brown fat, and the accumulation of
subcutaneous tissue. The immature nervous system
delays or mutes the infant’s response to thermal stress.
The premature infant is likely to experience
other complications (e.g., respiratory distress,
sepsis, intraventricular hemorrhage, hypoglycemia) that may increase basal metabolic rate and
oxygen consumption, thus interfering with the
ability to maintain thermal stability. Numerous
procedures and interventions (e.g., medication
administration, placement of intravascular catheters,
obtaining vital signs) may impede efforts to maintain a neutral thermal environment. Care providers
should routinely check the infant’s temperature
before initiating treatments. If the temperature is
low, treatment should be delayed until a more
normal temperature is obtained. If interventions
are prolonged, temperature should be monitored
frequently, an external heat source provided, and
the intervention stopped if hypothermia occurs.
Late-preterm infants are predisposed to morbidities because of their developmental immaturity
(see Chapter 5). Less adipose tissue for insulation, less
brown fat for chemical thermogenesis, more heat loss,
and a larger ratio of surface area to weight contribute to
problems with heat balance in these infants. Morbidity
associated with heat balance in the late-preterm infant
is 10% compared with 0% for term infants.
41
Low-Birth-Weight Newborns
LBW (<2500 g) infants can be divided into two
groups: the VLBW infant (<1500 g) and the
ELBW infant (<1000 g). Preterm infants in each of
these groups have specific needs for thermoregulation.
Heated incubators, radiant warmers, and skin-to-skin
care are all methods for maintaining the temperature
and promoting weight gain of the VLBW infant.
With caregiving, VLBW infants in servocontrolled
incubators decrease their abdominal skin temperature
with incubator opening in proportion to the type and
length of the procedure being done.32 Infants weigh-
ing between 1500 and 1600 g may be weaned to
an open crib if all criteria are met.
for weaning criteria).
During the first 12 hours of life, ELBW
preterms become hypothermic with procedures
such as intubations, chest x-ray examinations,
intravenous (IV) line placement and manipulation, suctioning, repositioning, and vital signs.
Like the late-preterm and the LBW infant, ELBW
infants have even less brown and subcutaneous fat
for maintaining body temperature. Their thin skin
also contributes to increased insensible water loss
(IWL).
SGA infants, like preterm infants, have a large
surface area relative to body mass and decreased
subcutaneous tissue, brown fat, and glycogen
stores, all of which contribute to heat imbalance.
Decreased placental blood flow frequently contributes
to the small size. The relatively large surface area of an
SGA infant increases evaporative and radiant heat loss,
whereas limited brown fat stores and subcutaneous
tissue contribute to a decreased ability to produce and
conserve body heat. Some flexion of the extremities
may be present because flexion depends on gestational
age and not weight. SGA infants have a higher metabolic rate compared with infants at similar weights
who are appropriate for gestational age (AGA). This is
believed to be caused by the larger brain size relative
to body weight. Hypoxia in utero may depress the
infant’s central nervous system (CNS) and alter
the ability to regulate temperature. Increased ener-
gy requirements coupled with limited glycogen stores
may result in hypoglycemia and limited ability to produce heat. SGA infants may require numerous interventions that disrupt the neutral thermal environment.
Care providers should ensure that the infant has a
normal and stable temperature before initiation of
treatments. If treatments are prolonged, temperature should be monitored frequently, an external
6,7,84
(see Fig. 6.5
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heat source provided, and treatments stopped if
hypothermia occurs.
Infants with neurologic damage or depression
may experience difficulty maintaining a stable
temperature. Hypoxia before, during, or after deliv-
ery, neurologic defects, and exposure to drugs such as
analgesics and anesthetics may depress the infant’s neurologic response to thermal stress. Hypoxia decreases
the effect of norepinephrine on nonshivering thermogenesis, the main route of thermal regulation in the
newborn infant. Hypoxia may also reduce the oxidative capacity of the mitochondria in brown fat and
skeletal muscles, which are involved in thermogenesis.
Infants who have experienced hypoxia in utero may
have increased norepinephrine concentrations, which
result in peripheral vasoconstriction. This may cause a
delayed metabolic response to cold stress and delayed
vasodilation in response to heat stress.
Neurologic defects that affect the hypothalamus
also may interfere with heat balance. The hypothalamus coordinates temperature input from various
sensors. Drugs such as analgesics and anesthetics cause
CNS depression and reduce the infant’s ability to
respond to thermal stress. Neuromuscular blocking
agents inhibit the infant’s ability to maintain a flexed
position, increasing exposed body surface and heat loss.
Care providers must be alert to the effect of drugs
on the CNS and the infant’s ability to regulate
temperature.
Infants with sepsis may have hypothermia or
hyperthermia. In a newborn, an elevated temperature
may begin as a response to cold stress, with peripheral
vasoconstriction and thermogenesis. Heat production
continues as the infant attempts to achieve a higher
core body temperature. Exogenous and endogenous
pyrogens may enhance thermogenesis.
Initially, an infant with sepsis may feel cool to
the touch and may have a low body temperature. As
fever progresses, temperature may rise and the infant
feels warm to the touch. Infants nursed in servo-
controlled incubators may not have an elevated
temperature. The lower heater output in response
to increasing skin temperature (by manual or
servocontrol adjustment) may mask a fever by
keeping the baby’s temperature within normal
limits. The care provider should be alert to a sudden decreased need for incubator heat support in
a previously stable infant.
Hyperthermia may be iatrogenic, caused by inappropriate control of the neonate’s environmental temperature. The most common cause is the inappropriate
use of external heat sources. Dehydration may also contribute to hyperthermia. Infants nursed with the use
of external heat sources should have their temperatures monitored frequently. Phototherapy, sunlight,
and the use of excessive clothing and blankets contribute to overheating. Dehydration may be avoided
by early recognition of infants at risk for increased fluid
loss. Increased IWL occurs in preterm infants because
of increased skin permeability and the use of phototherapy and radiant warmers. Vomiting, diarrhea, gastric
suction, and ostomy drainage also increase fluid loss.
These infants should receive additional fluids to replace
the increased losses (see Chapter 14).
PREVENTION OF HEAT/COLD
STRESS
Management of the thermal environment is paramount for newborn well-being. Heat balance is
determined by the amount of heat lost to the baby’s
environment offset by the amount of heat generated
by the body plus the amount of heat supplied from
outside sources. Because a smaller, more immature, and
sicker baby is less able to regulate body temperature, it
is crucial that care providers understand the physical
and physiologic principles of heat balance and be
able to maintain a neutral thermal environment. Two
broad categories of interventions foster thermal
neutrality1: blocking avenues of heat loss, and
providing external heat and environmental support to maintain temperature within the normal
range of 36.5°C to 37.5°C (97.7°F to 99.5°F). The
theoretical neutral thermal environment necessary for
neonates of 1 and 2 kg at a given age is graphed in
Fig. 6.2. Newborns of less than 800 g are not ade-
quately addressed in currently available tables but
should have a starting environmental temperature
setting of 36.5°C (97.7°F).
Delivery/Birthing Room
Attention to the details of these interventions begins in
the delivery/birthing room, in which the first step is to
adjust the ambient delivery room temperature higher
than ordinary operating rooms or patient rooms. The
air temperature in newborn care areas should
be kept at 23.8°C to 26.1°C (75°F to 79°F),
and humidity should be kept at 30% to 60%.
Warming the room and placing the resuscitation table
away from doors or drafts minimizes convective heat
110
CHAPTER 6 Heat Balance
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145
loss. Raising the delivery room temperature to
24°C to 26°C, as recommended by the World
Health Organization (WHO), decreases cold stress
in preterm infants less than or equal to 32 weeks
of gestation.60 One study that increased the ambient
temperature of the operating room from 20°C to 23°C
reduced the rate of neonatal hypothermia on admission from 50% to 35%.39 A retrospective cohort study
found that raising the delivery room’s ambient temperature from 28℃ to 34℃ resulted in fewer VLBW
premature infants with hypothermia (<36.5℃) on
admission to the NICU, and an increase in hyperthermia (>37.5℃).61 Box 6.2 lists important components
of the “warm chain” advocated by the WHO.
The newborn’s skin temperature may drop by
as much as 0.3°C/min, with core temperature
dropping more slowly after delivery. At birth,
most heat loss results from evaporation of amniotic fluid from the baby’s skin surface. Drying the
infant with prewarmed towels and immediately
replacing used ones with dry, warm towels minimize evaporative heat loss. Dry towels conduct
heat poorly when contacting the neonate’s skin.
However, cold examiner hands, stethoscopes, scales,
and bare mattresses are good heat conductors and can
add significant cold stress if not warmed before coming in contact with the newborn. Early skin-to-skin
care for the first 24 hours of life decreases hypothermia for the first 48 hours of life in late preterm and
term newborns.
106
Because hypothermia on admission to the
NICU is associated with higher morbidity and
mortality,
30,83
prevention of hypothermia is one
of four evidence-based interventions (delivery in
an appropriate level of care; antenatal asteroids; surfactant within 2 hours of birth, or early nasal continuous
positive airway pressure) that improve the out-
comes of very preterm infants.
138
Recent rates
of hypothermia on admission to the NICU in VLBW
newborns were 12.9% to 53.4% in a large cohort in
11 European countries
133
; 36% in a Canadian study83;
51% in a Brazilian study30; and 79% in a single-center
US study.
107
Another study found that the incidence
of hypothermia in preterm infants between 27 to 30
weeks’ gestational age was 93% in the first 3 hours after
admission.70 A cohort study in 11 European countries
found that 88.2% of very preterm infants were born
in units with one or more hypothermia-prevention
strategy, but 50.9% of these infants were hypothermic
on admission to the NICU.
134
Admission hypothermia
rates were 73.2% in very preterm infants born in units
BOX
6.2
1. Maintain thermal care in a draft-free delivery room (ambient tem-
2. Warm resuscitation—more preterm infants resuscitated with heat-
3. Immediate drying after birth from head-to-toe, covered with dry
4. Skin-to-skin contact with the mother after delivery, during transfer
5. Breastfeeding as soon as possible after birth, preferably within the
6. Postponing bathing and weighing. In a full-term newborn, bathing
7. Clothing/Bedding. Newborns should be covered in one to two lay-
8. Rooming-in between mother and baby should be encouraged and
9. Warm transportation within the institution, between institutions, or
10. Education. All neonatal care providers must be adequately educated
Modified from The World Health Organization Protocol. Thermal Management of the Newborn;
2014. https://www.who.int/maternal_child_adolescent/documents/ws42097th/
en/. Accessed July 22, 2019.
THE WARM CHAIN
perature in delivery room at least 25°C to 28°C; prewarm all linens
and surfaces that will be in contact with the infant’s skin). Turn
radiant warmer on 20 to 30 minutes before birth, on manual mode
with 100% heater output.
ed, humidified gas were normothermic on admission to the NICU
than those who received cold, dry gas
towel/blanket and hat placed on head.
and in the postpartum area to prevent hypothermia and to treat cold
stress.
first hour. Provides caloric intake for heat generation.
should be postponed until the next day. Weighing should be postponed till baby is adequately covered and making a zero correction
for clothing.
ers of clothes, a hat, and hands covered. Swaddling, a custom of
wrapping bands should be avoided.
facilitated so that frequent skin-to-skin contact and breastfeeding
occur.
discharge home must be provided to protect thermal stability. Stable babies (including preterm and LBW infants) should be transported well-wrapped and skin-to-skin with their mothers. VLBW and/
or unstable admitted babies should be transported in a prewarmed
incubator. Temperature should be monitored before and after transport. In utero transport to a regional center is the preferred mode of
transport.
and informed about the principles of the warm chain.
135
94
without systematic hypothermia-prevention strategies.
The most recent study from 18 centers in the NICHD
Neonatal Research Network found that low and high
admission temperatures were more common in the
extremely preterm than in the moderately preterm
infants69 (see Box 6.3). The probability of being
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BOX
6.3
Digitalizing
Schedule
ADMISSION TEMPERATURES OF EXTREMELY
AND MODERATELY PRETERM NEWBORNS
Extremely preterm
newborns (< 29 weeks’
gestation)
Moderately
preterm
newborns
(29–33 weeks’
gestation)
(2012–2013) (2002–2003) (2012–2013)
< 36.5°C 40.9 % 32% 38.6%
36.5°C–37.5°C 56% 6.2% 57.3%
> 37.5°C 52.9% 2% 4.2%
Data from: Laptook AR, Bell EF, Shankaran S, et al and the Generic and Moderate
Preterm Subcommittee of the NICHD Neonatal Research Network: Admission temperature and associated mortality and morbidity among moderately and extremely preterm
infants. J Pediatr. 2018;192:53.
hypothermic on admission to the NICU increased
with decreasing gestational age
69,134
and in-hospital
mortality is inversely related to admission temperature.69 Higher admission temperatures in preterm
infants after delayed cord clamping (see Chapter 4)
have been demonstrated in two studies.
7,43
The benefit of using polyethylene plastic bags
and wraps for babies born at 26 to 30 weeks of
gestation to preserve body heat and prevent hypothermia at admission has been shown in numerous
studies.* This type of warming is ideal for a preterm
infant (at birth and the immediate hours following)
awaiting transportation to a tertiary care facility or
indeed a baby born in a tertiary care facility. The baby
is placed on a warm towel (but not dried) and
placed under a radiant warming heating device.
The baby (excluding his or her head) is placed
fully in the polyethylene bag or is wrapped in the
polyethylene sheet. The baby should remain under
the radiant warmer, as the heat, acting through
the covering on the baby’s moist skin, creates a
warm thermal environment. Cutting an appropriate-size hole through the covering over the area
of insertion can facilitate the introduction of any
catheter or cannula. Polyethylene bags for warmth
have been adopted by the Neonatal Resuscitation
Program (NRP) (see Chapter 4).
NRP guidelines emphasize how hypothermia
may reduce the extent of brain injury after hypoxia and that hyperthermia may worsen the extent
of brain injury during reperfusion after hypoxic
events. The recommended goal is to maintain
normothermia for the infant and avoid iatrogenic hyperthermia in resuscitated newborns,
especially those late preterm and term neonates
who meet the criteria for neonatal cooling for
hypoxic-ischemic encephalopathy (HIE) (see
Chapter 26, Box 26.7).
Resuscitation should take place on a preheated
radiant warmer so that the adverse consequences of hypothermia are avoided. Use of heated,
humidified oxygen (rather than cold, dry oxygen) given from birth through NICU admission
results in normothermia of preterm newborns
on admission to the NICU.
90,94
In an attempt to
maintain heat balance, the neonate increases cellular
metabolism and oxygen consumption, which increases the risk for hypoxia, cardiorespiratory problems,
and acidosis. Hypoglycemia is also a risk factor,
because the infant must consume more glucose
for heat production. Other complications include
clotting disorders, neurologic problems, hyperbilirubinemia, and even death if the untreated hypothermia progresses.
Because a significant amount of heat is lost
through the surface area of the head, with its abundant blood supply and the brain’s high heat production, covering the infant’s head with some
insulating material conserves heat during transfer
to the nursery or NICU and afterward. Stockinet
material is relatively ineffective for this purpose and
provides poor insulation. The best material is thick,
maintains its shape with use, and has a high percentage of air volume trapped in the fibers. Knitted wool
caps, plastic caps, or Thinsulate material may provide
the best results.88 Combining polyethylene body
wraps and polyethylene caps results in better temperatures in preterm infants than use of polyethylene
wraps and a cotton cap.
116
Occlusive plastic wrap alone is not totally
effective in preventing hypothermia after birth in
the very preterm infant. Several recent studies using
plastic wrap and self-heating gel mattresses together
to prevent heat loss in preterm infants less than 31
weeks of gestation have been conducted.
58,89,112,120,122
These studies have found significant reduction in
the incidence of hypothermia with the use of
gel mattresses compared with the incidence of * References 20, 22, 34, 47, 57, 64, 73, 76, 78, 89, 112, 113, 120, 124
CHAPTER 6 Heat Balance
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147
hypothermia on admission in very preterm infants
who were born before the use of gel mattresses
(3.3% vs. 22.6%).58 Several of these studies have
also noticed a higher incidence of hyperthermia
on admission.
58,89,112
Three nursing interventions
(occlusive wrap, occlusive wrap and chemical
mattress, and increasing delivery room temperature) were studied to determine if they normalized
admission temperatures in ELBW (<1000 g) and
LBW (<1500 g) preterms.
12,76
Each intervention
resulted in a normal admission temperature without the risk of hyperthermia.
12,76
Several other
studies using occlusive wrap have elevated the initial
temperature of VLBW infants without producing
hyperthermia.
34,112,113
Quality improvement programs
using bundled interventions have been successful in
reducing the incidence of hypothermia in VLBW
preterm infants,
19,29,52,108,111,136
as well as late preterm
and LBW infants in mother-baby units.6 Monitoring
admission temperatures is recommended as a resuscitation quality indicator by the Australian and New
Zealand Committee on Resuscitation Guidelines.
79
There are a variety of ways to maintain thermal
neutrality. Accessibility, IWL, servocontrol versus manual control of temperature, and safety are major considerations when determining the method to use for
an individual neonate.
Incubators
Incubators provide a controlled, enclosed environment that is heated convectively with warm air.
The temperature in an incubator may be servocontrolled to maintain a desired skin temperature
or air temperature. As the temperature varies
from the desired “set point,” proportional control
units gradually increase or decrease heat output to
maintain a constant temperature (without the wider
temperature fluctuations seen with simple on-off
controllers). Incubators controlled by abdominal
skin servocontrol have been found to reduce
neonatal death rates in LBW, and especially in
VLBW, neonates.
trolled incubator to the desired skin temperature, the sensor should be attached to the right
upper quadrant of the abdomen with insulated
temperature patches. The sensor should not be
placed over areas of brown fat deposits, because
the higher-than-expected temperature information to the controlling unit will result in a
lower-than-desired heat output.
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In setting the servocon-
3
Inadvertent cooling may take place if the sensor
is covered with clothes or a blanket or if the baby
lies on it. Lying prone on the abdominal skin probe
results in warmer temperatures than those recorded
from probes not entrapped between the skin and mattress, resulting in a cooler incubator than intended.14
If the sensor becomes disconnected from the skin,
unwanted heating may occur because an erroneously
low temperature reading causes an unwanted increase
in heat output.
101
One must also consider that when
an insulated patch is used to cover the thermistor,
skin temperature is sensed as being higher than if
tape covers the thermistor, resulting in decreased heat
output by the warming device. The desired skin
temperature used for skin servocontrol is generally
36.0°C to 36.5°C (96.8°F to 97.7°F).85 Modern
incubators also can be servocontrolled to a desired air
temperature. This mode has been shown to provide a
more stable thermal environment and less temperature
variation compared with skin servocontrol.
Air servocontrol maintains a constant ambient air
temperature when other factors such as phototherapy,
external radiant heat, unstable room temperature, or
direct sunlight are not confounding variables. Infants
who were managed with skin servocontrol had
more variable but higher air temperatures and
spent more time in a neutral thermal environment.
Babies managed with air servocontrol had less
variability in air temperatures but more variability
in infant body temperature. A review of published
trials concluded that VLBW babies whose skin
servocontrol is set at 36°C had a lower mortality
rate than those managed with air servocontrol at
31.8°C.
121
A newer study of air temperature control
versus skin servocontrol in preterm neonates less than
32 weeks gestational age found that a body temperature of 37°C was associated with lower energy costs
and greater weight gain in the first 11 days of life.31
The question of air versus skin servocontrol or manual
control is still debatable for any given situation, and
probably neither is the perfect solution for all babies.
Fig. 6.4 depicts a research-based algorithm for weaning
from servocontrol to air control in an incubator.
Radiant heat loss to cooler incubator walls, especially in single-walled incubators, is a significant
source of heat loss. The use of double-walled incu-
bators (with the inner wall warmed to the ambient
air temperature inside the incubator) results in less
radiant heat loss from the baby. With a skin-set
servocontrol temperature, the decreased radiant heat
loss (because of warmer incubator walls) is offset by
UNIT TWO Support of the Neonate148
Remove the temperature probe and heat-reflecting disk
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Criteria for Weaning to Air Control
1. Infant is medically stable and in a condition that
permits weaning.
2. Infant requires minimal heat output from servo control set at 36.5° to 37° C (97.7° to 98.6° F).
3. Infant is gaining weight adequately: 15 to 20
g/kg/day, based on gestational age and
chronologic age.
1. Determine infant’s age and weight.
2. Determine appropriate incubator temperature range
(see Table 6-1).
using soap and water or mineral oil.
Obtain the infant’s axillary temperature to establish a
baseline temperature. Temperature should be at least
36.5° C (97.7° F).
Switch the heat from servocontrol to air control on the
incubator and set the incubator control temperature
(see Table 6-1).
1. Obtain the infant’s axillary temperature every 30
minutes to 1 hour.
2. Increase or decrease the temperature of the air
control no more than 0.5 degree per 30 minutes or 1
degree per hour to maintain the infant’s temperature.
FIGURE 6.4 Research-based algorithm for weaning from servocontrol to air
control mode in an incubator. (Courtesy Vivian Brown, RN.)
increased convective heat loss (because the ambient air
temperature necessary for the desired skin temperature is lower). Consequently, there is no net change
in the mean environmental temperature. Double-
walled incubators provide less temperature fluctuation when doors are open, thus providing a more
stable caregiving environment. Evaporative heat
loss is not appreciably different with single- and double-walled incubators. One may increase the humidity
in incubators to decrease the infant’s metabolic rate
only if a neutral thermal environment cannot be
achieved by increasing the ambient temperature.
In one study, addition of a double-walled roof
to a single-walled incubator resulted in less radiant
heat loss but more convective and evaporative skin
heat loss that was off-set by increasing the incubator’s ambient temperature by 0.15 to 0.20℃.
The tiniest neonate has a large evaporative heat
loss, and maximum air temperature is limited by the
incubator controls, thus making it difficult to reach
an air temperature high enough for thermal support.
In such cases, hypothermia can be avoided by
increasing the ambient humidity within the incubator by using the water reservoir or supplying
warmed humidified air into the incubator with
respiratory humidifiers. Humidification has been
shown to decrease fluid requirements and decrease
the incidence of electrolyte imbalances in babies
weighing less than 1000 g.46 Careful attention
should be given to preventing bacterial growth in the
humidification system (see Chapter 23). Incubator
temperatures may also be manually controlled by estimating the appropriate temperature for the baby’s age
and weight and setting the incubator to that temperature (see Table 6.1).
Regardless of whether one is using skin or air
servocontrol or manual temperature adjustments,
the baby’s temperature and the air temperature
must be monitored and recorded regularly. The
incubator should be kept away from air conditioning
ducts, direct sunlight, and cool windows that may cool
or warm the incubator. Room temperature should
be kept at 23.8°C to 26.1°C (75°F to 79°F), and
humidity should be maintained at 30% to 60%.4
Seasonal mapping of one NICU showed seasonal
variation in humidity level and evaporative temperature, both influences on thermal environment.
127
The
researchers recommend periodic assessment of air,
evaporative, and radiant temperatures, as well as humidity, in multiroom NICUs.
127
Alarms for both high and
low temperature levels always should be turned on.
The principal disadvantage of maintaining sick
newborns in incubators is the limited access to
them when extensive procedures are necessary.
Incubators also may be perceived by mothers
as a barrier between them and their infants and
prolong feelings of fear and insecurity, compared
with heating methods that provide easier access to
the baby. Holding the baby in skin-to-skin con-
tact (kangaroo care) helps promote bonding and
relieves some of maternal and paternal fears (see
later). Stable preterm infants dressed in a diaper, shirt,
and cap and wrapped in two blankets can also maintain a normal temperature when held close to their
parent’s body. Keeping the skin probe attached to
the infant and plugged into the incubator allows
33
frequent monitoring of the infant’s temperature.
We also now have an increasing awareness of and
concern about the high noise levels within incubators.
Such noise poses a potential deleterious effect on the
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