THERMOREGULATION

IN THE NEONATE

Developed by - Lisa Fikac, MSN, RNC-NIC

Expiration Date - 7/1/17

This continuing education activity is provided by Cape Fear Valley Health System, Training and Development Department, which is an approved provider of Continuing Nursing Education by the North Carolina Nurses Association, an accredited approver by the American Nurses Credentialing Center’s Commission on Accreditation.

0.6 Contact hours will be awarded upon completion of the following criteria:

• Completion of the entire activity • Submission of a completed evaluation form • Completion a post-test with a grade of at least 85%.

The planning committee members and content experts have declared no financial relationships which would influence the planning of this activity.

Microsoft Office Clip Art and Creative Memories are the sources for all graphics unless otherwise noted.

The author would like to thank Stacey Cashwell for her work as original author.

• Identify the body’s methods of heat production and heat loss. • Discuss cold stress and its consequences for the neonate. • Discuss the clinical presentation of cold stress and strategies to maintain a

neutral thermal environment (NTE) for the neonate.

Homeotherms are animals that maintain a constant body temperature regardless of environmental temperatures.

• Humans fall in this category

The ability to regulate temperature is critical to the functioning of the body's enzymatic systems that regulate cellular functions.

• Normal body temperature is the optimal temperature needed to support cellular function.

To maintain a "normal" temperature, the body must balance the amount of heat produced with the amount of heat lost from the body.

• Body heat is produced through metabolic activity. • Heat gain is balanced against heat loss through -

o Conduction o Convection o Evaporation o Radiation

Since the late 19th century, health professionals caring for infants have known the importance of environmental temperature control and its relationship to the survival of infants.

• A French obstetrician named Pierre Budin reported improving the survival of premature infants by placing them in crude incubators.

• He manipulated the incubator's temperature through manual adjustments of gas flames.

• Although crudely developed, these incubators dramatically improved the survival rate of preterm infants.

• Data from this study proved that the maintenance of normal body temperature in response to changing environmental temperatures is essential for survival.

The ability to maintain a normal body temperature in response to a changing thermal environment improves with age.

• Temperature regulation in the infant differs from the older child or adult. o Most differences are due to immaturity which leads to a diminished

regulatory response. • Those infants at highest risk are -

o Preterm o Low birthweight o Small for gestational age (SGA) o Compromised infants

• Cold stress increases metabolism and oxygen consumption.

Thermoregulation is controlled by the hypothalamus.

• The hypothalamus receives thermal stimuli from the body's - o Skin receptors o Deep thermal receptors o Thermal receptors within the hypothalamus itself

• The hypothalamus - o Processes the information received o Compares it to the body's current temperature, o Alters body functions to either increase heat production or heat loss

The body accomplishes this by altering - • Metabolism • Motor tone and activity • Vasomotor activity • Sweating

Infants also have a higher metabolic rate than adults.

• This is due to energy demands for growth, as well as increased maintenance requirements related to the infant's large body surface area.

• The infant's large surface area-to-mass ratio makes it more difficult to maintain normal temperature, since there is relatively more surface for convection and conduction.

A neutral thermal environment (NTE) is the range of environmental temperature at which -

• Calorie and oxygen consumption is minimal AND • The infant's temperature remains within normal

range

Remember, the transfer of heat is always down a heat gradient - from hot to cold temperatures.

Peripheral vasoconstriction

When human beings get cold, one of the body's first mechanisms to generate heat is to constrict the blood vessels in the arms and legs.

• Vasoconstriction keeps blood in the core of the body which prevents heat loss through the skin's surface.

• Although this helps to conserve heat, prolonged periods of peripheral vasoconstriction reduce the amount of oxygen available to the tissue.

Increased muscle flexion and activity

In the adult s, heat is produced through shivering, but infants have a poor ability to shiver.

• Instead of shivering, infants increase activity through flexion and movement which generates muscular warmth.

• Flexion also helps to decrease the body surface area.

Premature and sick infants may be unable to flex in response to cold stimuli the way that mature infants can.

• The premature infant has poor muscle tone and primarily lies with his extremities extended.

• The premature infant has a very large surface area to body mass ratio. o In other words, a large area is exposed to the environment, and this can

result in rapid heat loss.

Brown fat metabolism

Brown fat metabolism is initiated in the hypothalamus by way of the sympathetic nervous system.

• Brown fat is located around the - o Kidneys o Adrenal glands o Mediastinum o Subscapula o Axilla

• Storage of brown fat occurs primarily during the third trimester of pregnancy. • Therefore brown fat is may be very limited in preterm babies.

o So remember - the more premature the infant the less brown fat is stored.

The release of norepinephrine is triggered by the hypothalamus as a result of cold stress.

• When norepinephrine is released, it causes the release of free fatty acids (FFA), which undergo combustion in the mitochondria of the brown fat cells.

o This process releases HEAT. o One consequence due to the release of FFA is that FFA

competes with bilirubin for albumin binding sites. This increases the risk of hyperbilirubinemia and possibly kernicterus.

Of the three methods of heat production, brown fat metabolism is the one most utilized by the newborn.

Conduction is the transfer of heat between two solid objects that are in direct contact.

• For example, between the skin and a cooler object like the surface of a bed.

Convection is the transfer of heat between a solid surface, such as the infant, and either moving air or liquid around the infant.

• For example, air that enters through an open isolette porthole and moves around the infant.

Evaporation is the loss of heat through the energy used in the conversion of water to its gaseous state.

• For example, cooling that occurs immediately after delivery due to wetness.

Radiation is the transfer of heat between solid surfaces that are not in direct contact.

• For example, between isolette and outside window.

The Term Newborn's Physiologic Response to Cold Stress

When the term newborn becomes cold, a sequence of physiologic responses begins to occur. When cold stress is detected by the body's peripheral and core temperature sensors, signals are sent to the hypothalamus. This activates the release of norepinephrine. In response, norepinephrine causes -

• Pulmonary vasoconstriction o Leads to a greater possibility of right-to-left shunting.

This leads to hypoxemia. • Peripheral vasoconstriction

o When prolonged and coupled with increased oxygen consumption, this leads to hypoxia.

• Increased metabolic rate, which leads to - o Increased oxygen consumption o Increased glucose consumption

The Preterm Newborn's Physiologic Response to Cold Stress

The preterm newborn is at a much greater risk for developing hypothermia for many reasons. Reasons include -

• Larger surface area to body mass ratio • Decreased amounts of insulating fat • Thinner, immature skin • Poor muscle tone • Little or no brown fat

In addition the preterm newborn has limited -

• Pulmonary vasoconstriction • Peripheral vasoconstriction • Glycogen stores

Therefore, the detrimental effects of cold stress may occur much more rapidly and with greater severity in the preterm infant.

Hypoglycemia

When the infant is cold stressed, the metabolic rate increases to produce heat.

• When the metabolic rate increases, glycogen stores and blood glucose are used up much faster.

• This puts the infant at risk for hypoglycemia. • Remember, the body needs glucose to meet its energy needs.

Acidosis

When the infant is cold stressed, brown fat is converted to heat and free fatty acids.

• Fatty acids and lactic acid are formed during muscular activity by the breakdown of glycogen.

• They are then dumped into the bloodstream, causing a lower blood pH and metabolic acidosis.

• Acidosis also causes vasoconstriction of the pulmonary blood vessels, which leads to hypoxemia.

Hypoxia

When an infant is cold, extra oxygen is needed to produce heat.

• A cold infant with normal lungs increases his respiratory rate in order to get extra oxygen.

• If there is lung disease present, he may not be able to get enough oxygen to maintain brain and body functions.

• Infants show signs of respiratory distress by - o Grunting o Retracting o Nasal flaring o Tachypnea.

Infants do not have to be severely cold stressed to develop these consequences.

• Some infants demonstrate these consequences if their temperature drops only one or two degrees.

REMEMBER

Cold stress results in ->

• The release of norepinephrine leads to vasoconstriction which - o Reduces heat loss o Initiates heat production

• Increasing metabolic rates cause an increase in utilization of glucose stores.

• The infant uses anaerobic metabolism to produce heat resulting in - o Lactic acid production o Hypoxia

• The end results— o Metabolic acidosis o Hypoglycemia o Eventually, death, if hypothermia continues

There are several factors that predispose the preterm infant to hypothermia -

• A greater body surface area to weight ratio • Limited availability of fuel for heat production due to

limited - o Brown fat stores o Glycogen stores

• Limited subcutaneous tissue due to shortened gestation

• Immature central nervous system (CNS) which delays the infant's response to cold stress

• Increased skin permeability which allows increased evaporative heat loss • Increased handling by health care workers for procedures, IV restarts, etc. • Potential birth asphyxia

o This decreases the effect of norepinephrine on non-shivering thermogenesis which causes delayed -

Metabolic response to cold stress Vasoconstriction

• Analgesics and anesthetics decrease the metabolic rate which compromises the infant's ability to generate heat.

o Those given to - Mom prior to delivery of the infant Infant for pain control or after surgery

The definition of neutral thermal environment (NTE) is the range of environmental temperature at which calorie and oxygen consumption is minimal, and the infant's temperature remains within normal range (97.8o to 98.8oF or 36.5o to 37.1oC).

Factors Affecting the NTE

• Gestational age o The lower the gestational age the less brown fat the infant will have available. o The infant has a large surface area to weight ratio. o The infant needs higher temperatures to maintain his temperature.

• Weight o Same risks as with the gestational age.

• Environment - Radiant warmer versus isolette o The radiant warmer exposes the infant to insensible water loss and drafts. o Under the radiant warmer, insensible water loss is increased by 40-50%

compared with losses in an isolette. o The isolette offers better temperature control. o The isolettes used in most NICUs are double-walled which cuts down on

insensible water loss. o Newer isolette models also provide better capability for use of humidification

(e.g. Giraffe®, Isolette® 8000). • Drafts

o Prevent infant exposure to drafts. o Position the infant's bed in the nursery away from vents, windows, doors.

• Activity o Infants who are very active (such as crying,) generate heat by increasing their

metabolic rate. • Wetness

o Infants who are wet or lying on wet diapers become chilled through evaporation.

The infant who is cold stressed may be pale with extremities that are cool to the touch in an attempt to conserve heat by vasoconstriction.

Respiratory distress and acrocyanosis can occur as oxygen consumption increases.

If the infant continues to be hypothermic the following may occur -

• Apnea • Bradycardia • Central cyanosis

The infant may be irritable initially, but then becomes lethargic as cold stress continues.

• This is due to central nervous system (CNS) depression.

Other observable changes may include -

• Hypotonia • Weak cry • Weak suck • Gastrointestinal signs such as -

o Residuals o Abdominal distention o Emesis

Chronic hypothermia can result in poor weight gain and possible death.

The hyperthermic infant feels warm to touch.

• The infant's skin may appear red as the infant attempts to increase heat loss by vasodilatation.

• An infant less than 36 weeks gestation, generally, will not sweat as a term infant may.

• The infant may - o Be irritable or lethargic o Be hypotonic o Be tachypnic or apneic o Have a weak or absent cry o Feed poorly

Though hyperthermia is a rare occurrence in the NICU, environmental temperature is a concern when attempting to keep infants warm.

Remember, the preterm infant does not shiver nor sweat. Therefore, checking the infant's temperature is the only way to know if the infant is hot or cold.

Rewarming cold infants should begin immediately in order to avoid complications.

• Slow rewarming by providing external heat should be at a rate of ½-1o F per hour. • If the infant is less than 1000 grams consider ½o F per hour since rapid rewarming

increases the risk of apnea and intraventricular hemorrhage.

Methods to Prevent Heat Loss

CONDUCTION

• Avoid placing infants on cold surfaces such as x-ray plates, scales, examining tables.

o Place a cloth diaper between the X-ray plate and the infant.

• Prewarm the admission bed before the infant arrives. • Prewarm the isolette or any environment prior to

placing the infant inside. • If the infant is stable, placing the infant skin-to-skin

with mom can help with prevention of heat loss.

CONVECTION

• Maintain the temperature in the delivery room and nursery at 74o F.

• Avoid drafts from vents and doors when possible. • Use only warmed oxygen for infants.

o Inspired oxygen/air temperatures below an infant's NTE can cool a baby.

• Transport the infant in prewarmed transporters. • Use plastic wrap over the infant when the infant is on the

radiant warmer (open bed).

o Remember - Infant must be intubated. o If the infant does not have an airway established, the plastic could obstruct his

airway if it falls down against the infant. • Transfer extremely low birth weight (ELBW) infants to a humidified isolette as soon as

possible.

RADIATION

• Avoid placing cribs, radiant warmers, and isolettes near outside walls or windows. • Use only double-walled isolettes.

EVAPORATION

• Dry the baby after delivery and remove wet blankets. o In the delivery room, use of a plastic warming

bag for the ELBW increases NICU admission temperatures.

• Do not bathe the baby until the temperature has stabilized.

o Wash only a small area of skin at a time. o Dry the area before moving to the next area.

• Use humidified heated oxygen. o Again, inspired oxygen/air temperatures below an infant's NTE can cool the

baby. • Do not allow the infant to remain in wet diapers.

Assess the following -

• Vital signs are monitored often. • Blood gases may be ordered as needed to monitor for metabolic acidosis. • Blood glucose is monitored to screen for hypoglycemia. • A complete blood count (CBC) may be ordered to assess for initial signs of infection. • Oxygen saturation is monitored since oxygen consumption is increased with

hypothermia.

Methods for Rewarming

RADIANT WARMERS

There are two modes used with the radiant warmer -

• The manual mode is a constant source of heat that allows the radiant warmer to continue to heat even if the infant has the temperature probe attached.

o This mode is used to pre-warm an admission bed or during a procedure, but caution should be used by monitoring the infant's temperature if the procedure is prolonged.

o Remember, in the manual mode, the radiant warmer continues to heat no matter how hot or cold the infant may be.

o Never leave an infant unattended when the radiant warmer is in the manual mode.

• The servo mode is used when an infant is lying under a radiant warmer. o When the skin temperature is close to 36.5oC (97.8oF) (slightly higher in the very

low birth weight infant and slightly lower in the very large infant), the infant's core temperature will be close to 37oC (98.6oF).

o Nursing care includes - Keeping clothes off the infant--use only a small diaper Securing the temperature probe to the infant's abdomen if lying in a

supine position Securing the temperature probe to the infant's lower back if lying in a

prone position Placing the probe close to the spleen or liver since they are highly vascular

areas Avoiding placement of the probe on extremities and bony surfaces. This

gives a false temperature reading, which may lead to overheating the infant.

o Cover the temperature probe tip with a reflective disc or cover. o To prevent false reading make sure the probe is not between the baby and the

mattress. • The radiant warmer will alarm.

o ALWAYS check the infant when a radiant warmer alarm sounds. o Make sure the probe is attached properly and the warmer is set on servo mode.

• It is very important to monitor the infant's temperature!!!

ISOLETTES

Isolettes provide a controlled, enclosed environment for the neonate.

• Most NICUs use only double-walled isolettes. • This reduces insensible water loss and decreases radiant heat loss which provides less

temperature fluctuation when the isolette doors are open.

The two modes of temperature control that may be used are skin temperature control and air temperature control.

• When using the skin temperature control mode - o Isolette temperature setting should be set at 36.5o-37oC. o If the probe becomes disconnected from the skin, the isolette will continue to

heat possibly overheating the infant. o If the probe becomes covered with a diaper or blanket, cooling may occur since

the probe will sense that the infant is warmer than his true body temperature. • Nursing care includes -

o Securing the temperature probe to the infant's abdomen when lying in the supine position and to the infant's lower back when lying in the prone position.

o Placing the probe close to the spleen or liver since they are highly vascular areas. o Avoiding placement of the probe on extremities and bony surfaces.

This gives a false temperature reading which may lead to overheating the infant.

ADDING CLOTHING

• Using a hat to cover the infant’s head helps to reduce heat loss since the head is the largest part of the infant's anatomy and is a large source of heat loss.

• Adding layers of clothing may also help to insulate the baby. o T-shirts, socks, and sleepers are clothes that may be easily layered.

• One drawback to adding clothes is that it decreases the infant's visibility.

THERMAL WARMING PADS

• Thermal warming pads are another tool to help maintain the baby's temperature. o Some institutions require an order from the medical team.

• Use caution when using thermal warming pads!!! o The premature baby's skin is thinner than the term newborn's skin and burns

easily. o Make sure the warming pad is well covered with a double-folded blanket before

placing the baby's skin in contact with the pad. o Chux pads are not appropriate to cover the warming pad!

There is a plastic leak prevention liner on this type of pad, and the pad can stick to the baby's skin as it becomes too warm.

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