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The purpose of this
course is to prepare the healthcare professional to care for neonatal
thermoregulation.
At the completion of
this module the learner will be able to:
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1. |
identify methods of heat
production in infants, |
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2. |
identify ways in which heat
loss occurs in infants, |
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3. |
discuss the consequences of
cold stress and
hyperthermia, |
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4. |
describe neutral thermal
environment, and |
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5. |
describe the methods of
providing temperature
support. |
Cold stress and hyperthermia may
have serious consequences for all
newborns. In small for gestational
age and preterm infants (<2500 g)
these consequences may be
devastating and may increase both
morbidity and mortality rates. The
more premature an infant, the more
susceptible to even the slightest
alteration in environmental
temperature. Yet all infants need to
maintain specific thermal control in
order to survive. Several factors
lead to increased heat losses in the
newborn infant. The neonate has a
large skin surface area-to-body
weight ratio, which increases heat
and fluid loss. The fluid loss from
the skin results in massive heat
loss. The thin skin with blood
vessels that are near the surface
provides poor insulation, leading to
further heat loss. Careful attention
must be paid to the thermal
environment from the moment of birth
to the time they are capable of
temperature regulation.
Prior to delivery infants are in an
environment that maintains a stable
body temperature. At the time of
delivery, body temperature changes
rapidly. The infant’s temperature
may drop several degrees as he
passes from a warm, protected
uterine environment to that of a
cool delivery room. Nursing or
medical care for the infant may
inhibit or delay his being warmed.
Maternal analgesia that is
transferred across the placenta
prior to delivery may slow the
infant’s metabolism enough to
inhibit his ability to generate heat
during the first few days of life.
Simple procedures such as vital
signs, assessment, and diaper
changes may place infants at risk
for losing body heat. Frequent or
prolonged heat-losing episodes in
infants who have limited heat
producing and conserving resources
may lead to cold stress. Cold stress
in turn may lead to physiologic
changes that can compromise the
infant. Protecting babies against
heat loss improves survival and
decreases metabolic demands (calorie
expenditures).
Maintenance of an optimal thermal
environment is considered one of the
most important aspects of effective
neonatal care and is basic to daily
practice. The American Academy of
Pediatrics (AAP) recognizes the
importance of thermoregulation in
neonatal resuscitation and includes
it as part of the process of
resuscitation.
Temperature measurement instruments
include single-use paper
thermometers, glass and mercury
thermometers, and a variety of
electronic thermometers. Each method
is satisfactory for accurate
temperature measurement when used
correctly. Rectal temperature
historically has been considered the
most accurate measurement of core
body temperature. The core
temperature does not decline until
the infant has lost the ability to
produce heat. Axillary or skin
surface temperatures may be lower
than rectal temperatures by as much
as 1ºC, but there is generally a
difference of less than 0.4C. Skin
temperature readings taken over the
site of large brown fat stores may
yield a falsely high reading,
because these skin areas tend to
remain warmer. High evaporative
losses may produce falsely high
readings in abdominal skin
temperature measurements. Inguinal
site temperature measurement may be
more closely aligned to rectal
temperature and provide a less
traumatic site for core temperature
measurement. Infrared tympanic
thermometers are used because the
ear canal is a highly vascular
region whose blood perfusion is the
same as that which perfuses the
hypothalamic region, the area
responsible for temperature control.
Temperature readings are obtained by
placing a small probe into the ear
canal, which should approximate the
core temperature. When the tympanic
thermometer is used correctly, there
is only about a 0.3ºC to 0.5ºC
difference compared with an axillary
temperature, which is lower.
A change in measured temperature may
not occur until the infant has lost
the ability to generate heat. The
infant may display subtle signs of
distress:
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Tachycardia |
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Tachypnea |
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Short-term response: changes
in behavior and response |
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Long-term responses: poor
growth patterns and
behavioral changes |
In cold stress, Tachypnea results
from an increased need for oxygen as
the result of an increase in
metabolism. In heat stress, the
infant becomes tachypneic to
increase expiratory heat losses.
Assessing temperature fluctuations
over a period of time, using the
same method of assessment is more
important in evaluating
thermoregulation than the actual
temperature value at one point in
time. Assessing the infant for other
factors, such as growth, oxygen
needs, and feeding tolerance also
contributes to the determination of
appropriate thermal control. This
assessment is done by reviewing
growth charts, FiO2 needs, feedings,
and emesis. Changes in growth
patterns have frequently been
overlooked as indicators of
temperature instability. Energy
demands for temperature control take
precedence over the demands for
growth. The infant who is
experiencing slow weight gain or
erratic growth patterns may be
experiencing poor thermal control.
Humans are homotherms; capable of
maintaining body temperature at a
relatively constant level despite
changes in the external environment.
The ability of infants to regulate
temperature in response to thermal
stress is limited. Infants are
unable to sweat in order to give off
excessive heat when they become
overheated.
The infant is capable of heat
production through three mechanisms
1) voluntary muscle activity, 2)
involuntary muscle activity, and 3)
metabolism. Voluntary and
involuntary muscle activity is
limited and requires a chemical
reaction utilizing large stores of
energy. Term infants are capable of
assuming a flexed position when cool
and an extended position when
overheated. The ability is limited
in the premature infant, though it
may be present to some extent.
Non-shivering thermo genesis appears
to be the most consistent method of
heat production in the neonate
regardless of gestational age or
birth weight. The major source of
heat energy in the newborn is fatty
acids. Thermo genesis is directly
dependent on tissue oxygenation to
utilize heat energy. Oxidized fatty
acids generally are believed to
derive from brown fat stores in the
neonate.
Brown fat has high vascularization
and is virtually nonexistent in
preterm infants. Term infants have
approximately 16 percent of body
tissue mass as adipose tissue, but
the preterm infant may have as
little as 3.5 percent adipose tissue
per body weight. Brown fat is
located around the mediastinal
structures, kidneys, scapulas,
axilla and nape of the neck.
Primitive brown cells first appear
at 26-30 weeks gestation and
ordinarily disappear by three to
five weeks after birth.
Upon exposure to cold, thermal
receptors in the skin (many of which
are located in the face) signal the
neonate’s central hypothalamus
resulting in sympathetic nervous
system arousal and the release of
norepinephrine. The release of
norepinephrine then stimulates the
hydrolysis or breakdown of the brown
fat. The rapid metabolism of brown
fat produces heat, which warms the
blood perfusing surrounding tissue.
This heat is then transferred via
the circulation to the rest of the
body. This process consumes a lot of
oxygen and glucose.
Asphyxia and hypoxia further
compromise the infant’s ability to
generate heat. Utilizing energy to
produce heat requires an increase in
oxygen consumption. In the hypoxic
state, two molecules of adenosine
triphosphate (ATP) are generated
from a molecule of glucose instead
of 38 molecules of ATP generated in
the normally oxygenated infant. In
order to produce heat energy in the
hypoxic state, greater glucose
stores must be utilized. Without
sufficient oxygenation, asphyxiated
or hypoxic infants have a decreased
ability to generate heat. When the
infant with already limited
resources for heat production
encounters environmental changes
that threaten his ability to
maintain an adequate temperature, a
serious condition exists.
The metabolic rate gradually
increases during the first week of
life. Heat production also improves
during the first few days of life
with the institution of feedings. It
is not clear why heat is produced.
It may be due to increased
metabolism during digestion, or it
may be that heat can be generated
when sufficient energy is provided
via ingestion. Ingestion of human
milk has been found to increase
metabolism in low birth weight
infants, leading to production of
heat. Thermoregulatory needs
gradually change as the infant
grows, matures and feeds.
There are four mechanisms by which
heat is lost 1) conduction, 2)
convection, 3) radiation, and 4)
evaporation. Conduction is the
transfer of heat from one object to
another by direct contact. The
infant may lose heat from internal
organs to the skin’s surface and
from there to cool surfaces with
which he is in contact. Immediately
after birth, an infant placed on a
surface that has not been pre-warmed
will transfer heat to that surface.
In order to prevent conductive heat
loss, a scale, or resuscitation bed
should be always have a pre-warmed
blanket between its surface and the
infant. Heat loss can continue to
occur after the infant has been
placed in a pre-warmed incubator if
he is placed on cool x-ray plates,
for example, or is changed to linens
that have not been pre-warmed.
Convection occurs as heat is
transferred from an object to the
surrounding air. If the object is
warmer than the surrounding air,
heat will be given up to the
atmosphere. If environmental
temperatures exceed the temperature
of the object, the object will gain
heat. Convective heat losses occur
as blood travels through the body
and comes to the skin’s surface. As
air currents pass over the thin skin
surface, heat is given up to the
environment. Convective heat is also
lost via the respiratory tract as
air and heat are expired. Heat loss
can occur even when the infant is
placed under a radiant warmer. A
cool room or a high degree of air
velocity may affect the efficiency
of the warming bed. Another major
source of heat loss through
convection involves the use of
oxygen. A cool gas flow over the
infant’s face and head will cause
heat losses that will not be readily
apparent in the core temperature.
Because the infant’s face and head
are especially sensitive to cool
air, only warmed, humidified oxygen
should be used.
Radiation is the transfer of heat
between two objects that are not in
direct contact with each other. Heat
is transferred from the warmer to
the cooler object. Even an infant in
an isolette has many opportunities
for heat loss. Cool incubator walls
are a large source of radiant loss.
Incubators placed near air
conditioner vents or cool windows or
in drafts add to the potential heat
loss. Double-walled incubators,
infant heat shields, and radiant
heaters over the incubator have
demonstrated some success at
decreasing radiant heat loss.
Evaporation losses occur as moisture
from body surfaces is lost to the
environment. At the time of
delivery, the infant should be dried
immediately to prevent rapid heat
loss. Wrapping the infant in plastic
can decrease the amount of
evaporative loss. The smaller the
infant and the lower the gestational
age, the larger the evaporative heat
losses. Evaporative losses can also
occur through the respiratory tract.
To decrease this heat loss, warmed,
humidified oxygen should be used
when supplemental oxygen is needed.
The upper airway is sensitive to
cold and may lower core body
temperature. The infant who is
tachypneic is at a greater risk of
heat loss.
Heat gaining mechanisms include
conduction, convection and
radiation. Conduction as a source of
heat gain for the infant is minimal.
Warmed surfaces may prevent heat
loss, but they are not efficient in
providing heat. This important
source of heat stabilization should
not be ignored because prevention of
heat loss is as important as heat
gain for the infant. It should be
used with caution to prevent thermal
burns at relatively low
temperatures.
Convection is one of the most common
sources of heat gain. Convection
incubators are frequently used as
the easiest and safest method of
maintaining a neutral thermal
environment. Humidity is often
provided as an adjunct to convection
warming because it decreases the
infant’s evaporative losses and
allows him to maintain his
temperature at a lower ambient
temperature. Difficulties may arise
if monitoring (using either a skin
or air servomechanism) is not
continuous. Although convection is a
fairly efficient method of providing
heat for the infant, he is unable to
regulate heat gain and may become
overheated in a short time.
Radiant warmers are a common way of
providing heat to the infant. These
are powerful, provide heat quickly
and can be controlled via skin or
rectal probe. A drawback to their
use is the increase in evaporative
losses experienced by the infant but
can be buffered by using a clear
plastic blanket. Although it
restricts the caregiver in providing
care, the plastic film allows
complete visualization of the infant
at all times. Under a radiant
warmer, access to the infant can be
maintained without reducing ambient
temperature.
The use of an incubator heated by
either convection or radiation has
been shown to be as effective as a
radiant warmer in achieving a
neutral thermal environment. The
percentage of humidity in the
environment plays a role in
determining ambient temperature in
the incubator. Evaporative losses
are minimized when humidity is kept
between 50 and 80 percent. Low
ambient humidity requires higher
ambient temperature levels to
maintain infant skin temperature
between 36 and 37ºC. When humidity
is low, radiation heat loss may be
low in comparison to evaporation
losses because the high ambient
temperatures needed to maintain skin
temperature warm the incubator
walls. Use of an incubator in
combination with high humidity can
be effective in maintaining stable
temperatures at lower ambient
temperatures.
The effects of cold stress can be
detected in all aspects of body
functioning. An infant responds to
cold stimulus with increased oxygen
consumption, glucose utilization,
and acid production. The prevention
of cold stress is essential in
protecting the infant from
multisystem stress. The
cardiovascular and respiratory
systems manifest the most obvious
symptoms:
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Peripheral vasoconstriction
occurs to conserve heat. |
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As central blood volume
increases to compensate,
pulse and blood pressure
increase. |
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Once central cooling has
occurred, diuresis may
result with a decline in
pulse and blood pressure
leading to decreased cardiac
output. |
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Arrhythmias may occur
secondary to acidosis as
fatty acids break down to
generate heat. |
The CNS can be affected by cold
stress secondary to cardiovascular
changes. With peripheral
vasoconstriction the following
occur:
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Cerebral blood flow
diminishes |
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Metabolic activity is
compromised |
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Electroencephalographic
activity may decline |
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Peripheral nerve conduction
may also be delayed |
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Pupils may become fixed and
dilated |
Metabolic response to cold stress
encompasses fluid, electrolyte, and
glucose aberrations:
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In the early stages,
diuresis occurs |
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If cold stress continues,
glomerular filtration
declines along with the
reabsorption of sodium,
water, and glucose |
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Hypoglycemia occurs |
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Metabolic rate rises |
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Unstable glucose levels can
lead to further acidosis and
neurologic changes |
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As the release of
nonesterified fatty acid
increases, the liver slows
metabolism of glucose,
inhibiting thermo genesis |
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As liver function declines,
drugs are metabolized and
excreted more slowly |
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Acidosis develops as tissue
perfusion declines, lactic,
pyruvic, and organic acids
build |
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Enzymatic action within the
kidneys is blocked,
preventing acid-base
regulation through a
diminished excretion of
hydrogen ions |
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Fluid balance is further
complicated by poor
gastrointestinal absorption
and decreased peristalsis |
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Acidosis continues with an
increase in dissociation of
the indirect bilirubin from
albumin-binding sites |
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An increase in nonesterified
fatty acids is caused by
their high affinity for the
albumin-binding sites |
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In the presence of high
levels of nonesterified
fatty acids, kernicterus can
occur with relatively low
bilirubin levels |
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There is a risk for bleeding
and thrombocytopenia because
clotting factors may be
altered |
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An increase in hematocrit
and viscosity of the blood,
secondary to fluid shifts
away from vascular space may
be noted |
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Lethargy may occur |
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Refusal to eat may be noted |
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Respirations become slow and
shallow |
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Cry is weak |
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As the condition continues,
edema or sclerema may occur |
Hypothermia presents when the
infant’s temperature drops below
36.3ºC in term infants and 36.5ºC in
preterm infants. This occurs when
the infant’s own attempts of trying
to generate their own heat have
failed. Severe cold stress can
present with respiratory distress,
hypotension, and hypoxia.
Maintenance of temperature stability
should be focused on preventative
measures. At delivery the baby
should be dried thoroughly with
pre-warmed blankets and those wet
blankets removed. A cap should be
placed on the baby’s head and the
baby should be placed in skin to
skin contact with mother and cover
both with warm blankets. If a
radiant warmer is used it should be
pre-heated. Scales should be covered
with warm cloth or diaper. For
healthy term newborns, warm hands
and stethoscopes prior to contact
with baby. Use pre-warm beds,
linens, and examining tables when
possible. Position beds away from
outside walls, windows, and drafts.
Delay the initial bath until the
body temperature has stabilized
(minimum 3 normal temperatures on
hour apart). Then a tub bath rather
than sponge bath and dry quickly
should be considered to reduce heat
loss. Low birth weight infants
(micro premies) should not be bathed
for several weeks but may need to be
wiped immediately after birth if
maternally transferred infection is
suspected to reduce risk to
healthcare workers.
A gradual increase in the infant’s
temperature is recommended to keep
oxygen consumption to a minimum
during rewarming. This is
accomplished by the following:
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Incubator temperature should
be adjusted 1ºC to 1.5ºC
higher than the infant’s
temperature. |
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Hourly, the incubator
temperature may be adjusted
upwardly by 1ºC until the
infant’s temperature has
been stabilized. |
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Caps, plastic warp, and heat
shields should be removed to
prevent them from
interfering with heat gain. |
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Feedings should always be
warmed before being given to
a cold stressed infant. |
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Intravenous fluids may also
be warmed by using
blood-warming devices or by
placing an extra length of
tubing inside the incubator
to allow the warmed
environment to warm the
fluids. |
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Slow rewarming is
recommended (it is not an
exact science) |
If the infant has been severely cold
stressed, the temperature may
continue to decline during the early
stages of rewarming. Remember that
the skin probe will read 36.5
degrees C before the core
temperature is normal.
An infant who has a body temperature
greater than 37– 37.5ºC may be
considered abnormally warm. Heat
stress, excluding the febrile state,
should never occur in the neonate.
When it does it is generally caused
by improper use or monitoring of
equipment to warm infants.
When core temperatures are elevated
in febrile conditions, the skin
temperature of the distal
extremities remains cool in
comparison to the skin temperature
of the trunk. Hyperthermia can also
be a sign of hyper metabolism when
an infant is septic. The usual first
step to approach treating
hyperthermic infants is to remove
external heating sources and by
removing anything blocking heat loss
(i.e. clothing).
Overheating can lead to a variety of
responses many of which are similar
to hypothermia with the exception of
skin color. The skin is usually
flushed or ruddy (plethoric) as
apposed to pale and mottled. Other
signs include:
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Hypo activity |
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Restlessness |
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Irritability |
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Extended posture |
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Flaccidity |
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Tachycardia |
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Tachypnea |
Neutral thermal environment is the
range of thermal environment in
which the body temperature is
normal, oxygen and caloric
consumption is minimal and the least
amount of metabolic energy is
expended. This varies based on the
infant’s age and weight. It is our
goal to maintain an environment in
which an infant does not expend a
lot of energy to maintain a
temperature within a normal range.
Nurses can do this by providing
external heat sources such as
radiant warmers, isolettes, K-pads
etc.
Servo-control mode is available with
radiant warmers and isolettes. The
infant’s skin temperature regulates
the heater output. The nurse sets a
control temperature where they wish
the baby’s skin temperature to be. A
skin probe is placed on the infant’s
skin using an insulated probe cover.
Some facilities use foam probe
covers others use gel probe covers.
Both of the covers have a reflective
side to avoid falsely picking up the
warmer temperature rather than the
skin temperature. The warmer
increases and decreases the heater
output to maintain the set control
temperature.
The critical factor is placement of
the probe. In some facilities the
practice is to place the skin
temperature probe under the infant’s
arm. When this is done there is a
constant readout of the infant’s
axillary temperature. This
temperature is very close to the
axillary temperature obtained during
vital signs. There is a problem with
this practice though when you
consider the infants physiologic
response to cold stresses and how
the warmer is designed to function.
When an infant begins to become
cooler the first area that drops in
temperature is the skin. One of the
first things an infant does in
response to cold is to vasoconstrict
the periphery (skin) to conserve
what heat is on board. When an
infant begins to respond to the cold
the first response then is for
slight decreases in skin
temperature. When the probe is place
on the skin of the exposed area, the
warmer output responds to the subtle
changes in skin temperature.
When the cold stress continues the
axillary temperature drops. When the
probe is placed under the infants
arm the warmer still responds to a
drop in temperature and maintains
the infant at a normal temperature
but the warmer output responds much
later because by the time the
axillary temperature has dropped the
infant has had to use some of his
energy to try and compensate before
the warmer has kicked in to help.
The appropriate placement is on the
skin of the upper right quadrant of
the abdomen or back (exposed skin,
not lying on probe) and should be
repositioned every time that the
infant is repositioned. The probe
should be attached firmly to the
skin visible for inspection at all
times.
Manual-control of radiant warmers
should only be used in the immediate
post delivery period and during
resuscitation, to avoid over
heating. Manual-control most
frequently is used in isolettes. The
control temperature is set for where
you want to keep the environmental
temperature (ambient temperature).
The temperature probe can still
provide information on the infant’s
skin temperature but has no effect
on the isolettes heater output.
The manual mode is used during the
process of weaning infant’s to open
cribs or for longer use when an
infant no longer requires the
specificity of the servo mode but is
too small to wean to an open crib.
In manual mode you can dress and
bundle infants as long as their
temperature is within normal
parameters.
Other manual methods of assisting
with temperature control include:
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Hats are useful for
thermo-sensitive infants. A
hat can cover 12-20 % of the
infant’s body surface area. |
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Plastic wrap over the
intubated patient, under a
radiant warmer, is used to
minimize evaporative and
convective heat loss. It is
used for small premature
infants. |
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Heating pads (K-pads) can be
used to decrease conductive
heat loss. |
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Warming diapers and linen
prior to use by placing
under warmer or in isolette
can also decrease conductive
loss. Avoid placing infants
directly on cold surfaces. |
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Remove wet linens
frequently. |
Blackburn, S., et al. (2001).
Neonatal Thermal Care, Part III: The
Effect of Infant Position and
Temperature Probe Placement;
Neonatal Network Vol. 20 No. 3,
April, pp. 25-30.
Bissinger, R. (2004). Neonatal
Resuscitation – Thermoregulation;
emedicine.com.
British Columbia Reproductive Care
Program Policy Manual (2003) Newborn
Guidelines: Neonatal
Thermoregulation.
Weber, Roberta; (2004) Neonatal
Thermoregulation; Lecture Content |
