Neurohormonal of Thermoregulation

Dr. Dini Sri Damayanti,MKes

Body TemperatureShell temperature:

Temperature closer to skinOral temperature

36.6o-37.0oC (97.9o-98.6oF)

Core temperature:Most important temperatureTemperature of “core” (organs in cranial, thoracic and abdominal cavities)Rectal temperature

37.2o-37.6oC (99.0o-99.7oF)

Heat ProductionExergonic reactions:

Oxidation and ATP use.

Most heat generated by brain, heart, liver and glands at rest.Skeletal muscles 20-30% at rest. Can increase 30-40 times during exercise.

Thermoregulatory Center

Hypothalamus:Preoptic area neurons: hypothalamic thermostat:

Heat-losing centerHeat-promoting center

Monitors temperature of blood and receives signals from peripheral thermoreceptors.Negative feedback loops

Thermoregulatory Center

Heat-losing center (hipotalamus anterior):Activates heat losing mechanisms:

Dilation of dermal arterioles: increase blood flow to skin.Sweating.Increased respiration through mouth.Behavioral: remove clothing.

Inhibits heat-promoting center.

Thermoregulatory Center

Heat-promoting center ( hipotalamus posterior ):

Activates heat generating mechanisms:SNS:

Vasoconstriction of dermal arterioles: decrease blood flow to skin Stimulates arrector pili muscles: hair stands on endShivering thermogenesis: spinal reflex of alternating contractions in antagonistic muscles

Nonshivering thermogenesis:Long-term mechanism stimulating thyroid hormone release T3 and T4.

Inhibits heat-loss center.

Nonshivering ThermogenesisTemperature sensors are in the skin (in newborns particularly the face), the spinal cord and the hypothalamus.Temperature information is processed in the hypothalamus. Norepinephrine (NE) is released in response to cold stress.Result: Vasoconstriction and increased metabolic activity.

Nonshivering Thermogenesis

Vasoconstriction also occurs in infants but the primary response is increasing heat production from brown fat metabolism.

NE stimulates receptors on brown fat cells; activates lipase which releases intracellular fatty acids.Conversion of T4 to T3 inside brown fat cells. T3 cellular metabolic rate

Brown fat mechanism is known as nonshivering thermogenesis

Uncoupling protein uncouples mitochondrial oxidative phosphorylation.H+ gradient heat rather than ATP Result: Large increase in heat production and O2 consumption.In case of hypoxia, temperature because of O2

Mechanisms of Heat Transfer

Radiation:Infrared radiation.

Conduction:Direct transfer of energy through physical contact.

Convection:Heat loss to air around the human body.

Evaporation:Energy change in water molecule from liquid to vapor.

Thermoreceptors

Return to

37°C

Muscles of skin

arteriole walls relaxCore

body temperat

ure >37°C

HypothalamusSweat glands increas

e secreti

on

nerves

Muscles

reduce activity

Thermoreceptors

NEGATIVE

FEEDBACK

Blood temperature

Body

loses

heat

© 2008 Paul Billiet ODWS

Thermoreceptors

Return to

37°C

NEGATIVE

FEEDBACK

Blood temperature

Body

loses

less heatBod

y gain

s heat

Muscles of skin

arteriole walls

constrictCore

body temperat

ure <37°C

Sweat glands decreas

e secreti

on

nerves

Muscles

shivering

Thermoreceptors Hypothalamus

nerves

© 2008 Paul Billiet ODWS

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Goal of Thermoregulation

Maintain correct body temperature range in order to:

maximize metabolic efficiencyreduce oxygen useprotect enzyme function

reduce calorie expenditure

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THERMOREGULATION IN THE THERMOREGULATION IN THE NEONATENEONATE

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Challenges of thermoregulation in

Neonatal carePrior to delivery infants do not maintain A temperature independentlyInfant’s in-utero temp is generally 0.5˚C higher than mother’s tempRapid cooling occurs after delivery

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Neurologic adaptation: Thermoregulation

Maintenance of body temp is a major task Skin is thin & blood vessels are close to the surfaceHave little subcutaneous fat to serve as barrier to heat lossTerm Infants have 3x the surface to body mass of an adultPreterm infants and SGA infants have 4x the surface mass to body mass of an adultPreterm infants are especially susceptible to heat loss due to poor tone, fat and thinner skin than term infants

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Neutral Thermal Neutral Thermal TemperatureTemperature

A neutral thermal temperature isthe body temperature at which anindividual's oxygen use and energyexpenditure are minimized.

Minimal metabolic rateMinimal metabolic rate

Minimal oxygen consumptionMinimal oxygen consumption

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Body temperature in the newborn infant

Classification of hypothermia is based on core temperature

NORMAL – 36.5 to 37.3˚C (97.7 –99.2˚F)Cold Stress 36.0 to 36.4˚C (96.8 – 97.6 ˚F)

Cause for concern

Moderate hypothermia 32 –35.9˚C (89.6-96.6˚F)

Danger, warm infant

Severe hypothermia – below 32˚C (89.6 ˚F)Outlook grave, skilled care urgently needed

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Neutral Thermal Environment (NTE)

The air temperature surrounding the babysupports maintenanceof a neutral thermalbody temperature.

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ThermoneutralityThermoneutralityWhen the air temperatureis in the correct range andthe infant’s body maintainsa neutral thermaltemperature, we haveachieved thermoneutrality

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Why Are Infants At Greater Risk for

Thermoregulation Problems?

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Thermoregulation Risk Factors

PrematureSGANeuro problemsEndocrineCardiac / respiratory problemsLarge open areas in the skinSedated InfantsDrug exposure

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Why are they at Risk???Brown Adipose TissueBody surface areaSQ FatGlycogen storesBody water contentPosture

HypoxiaHypoglycemiaAnomaliesCNS Sedation

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What Do We Know?

Infants have more skin surface perpound of body weight than olderchildren or adultsMore skin means more radiant heat and more insensible water loss.

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• less brown fat and glycogen stores

• decreased ability to maintain flexion

• increased body surface area compared toweight

Risk factors for Preterm Infants

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What Do We Know?

The majority of an infant’s

thermal receptors are found

in the face, neck, and shoulder

area.

Stimulation of these

receptors will result in chilling

and calorie expenditure

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What Do We Know?Shivering, which is themain way in which olderchildren and adultsgenerate heat, isimpossible or not effective in infants.Neonates and younginfants generate heat byburning brown fat.

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Production of HeatMetabolic ProcessesVoluntary Muscle ActivityPeripheral VasoconstrictionNonshivering Thermogenesis

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Metabolic ProcessHeat Generation by

Oxidative metabolismGlucoseFatsProtein

Metabolic Energy BrainHeartAdrenal Gland

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Voluntary Muscle Activity

Postural changesRestless movementsLimited use to Newborn

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Peripheral Vasoconstriction

Reduces skin blood flowDecreases loss of heat from the body

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Nonshivering Thermogenesis

Metabolism of brown adipose tissueInitiated in hypothalamusSympathetic nervous systemNorepinephrine release at the site of brown fat

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What is Brown Fat?Brown fat is an energy source for infantsIt can be found:

Near Kidneys and adrenalsNeck, mediastinum, scapular,

and the axilla areas. Can not be replaced once used

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Brown FatBrown FatIn full term infants brown fat is 4 % -10% of adipose deposits.

In preterm infants, brown fat will not be found until 26-30 weeks gestation, and then only in small amounts.

Brown fat generally disappears 3-6 months after birth, except in cold stressed infants (where it will disappear sooner.)

Hypoxia causes impairment of brown fat metabolism

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So What?So What?When the air temperature

around the baby is cool,thermoreceptors in theskin are stimulated. Non-shivering thermogenesisis initiated and brown fat isburned for energy to keepthe body temperature stable.

This is the infant’s initial response.

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What Next?Conversion of brown fat

uses oxygen and glucose, therefore, the cold stressed

infant will become hypoxicand hypoglycemic.

Blood gas and glucose levels

are affected. Growth is affected as calories

are used to stay warmrather than grow.

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What are the Signs and Symptoms of Thermal Instability?

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Methods of heat lossPeripheral Vasodilatation

blood flowfacilitates heat transfer from periphery

to environment

Sweating evaporative heat loss postnatal age increases the ability to sweatAppears first on term newborn head

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Healthy Vs. Sick Neonate

Healthy NewbornBrown adipose tissueProduces heat and loses heat as needed

Sick or Low birth wt infants

Increased energy demandDecreased energy storeVulnerable to heat stress

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Hypothermia – Signs/symptoms

Body cool to touchMottling or pallorCentral cyanosisAcrocyanosisPoor FeedingAbdominal distensionHypotoniaHypoglycemia

gastric residualsBradycardiaTachypneaRestlessnessShallow or Irregular RespirationsApneaLethargy

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Signs and Symptoms of Hypothermia in Infants

Vasoconstriction

Peripheral vasoconstriction occurs in an effort to limit heat loss via blood vessels close to the skin surface.

Pallor and cool skin may be noted, due to poor peripheral perfusion

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Signs and Symptoms of Hypothermia in Infants

Increased Respiratory Rate

Pulmonary vasoconstriction occurs secondary to metabolic acidosis.

Increasing Respiratory Distress Related to decreased surfactantproduction, hypoxia, & acidosis

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Signs and Symptoms of Hypothermia in Infants

Restlessness Restlessness may be a type of behavioral thermoregulation used to generate heat through muscle movement. The first sign may be an alteration in sleep patterns.Restlessness also indicates a change in

mental status as cerebral blood flow diminishes, due to vasoconstriction.

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Signs and Symptoms of Hypothermia in Infants

Lethargy If thermo-instability goes unrecognized, the infant will become more lethargic, as cerebral blood flow continues to diminish and hypoxemia and hypoglycemia become more pronounced.

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Signs and Symptoms of Hypothermia cont.

Metabolic Disturbances

Metabolic acidosis

Hypoxemia

Hypoglycemia

progress due to continued metabolism of brown fat, release of fatty acids and anaerobic metabolism (lactic acid)

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Signs and Symptoms of Hypothermia

Cardiac

As central blood volume increases, initially the heart rate and blood pressure increase

Arrhythmias

May result from depressed myocardial contractility and irritability caused by hypothermia

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Signs and Symptoms of Hypothermia

Poor weight gain occurs

when:

calories consumed

brown fat stores are used to make body heat.

Poor Feeding/Weight Loss

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Even the smallestweight loss may

take days or even weeks to recover, as infants

are limited in thevolume of food theycan eat and number of calories they cantolerate.

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Consequences of Consequences of HypothermiaHypothermia

Hypoxemia from Oxygen consumptionHypoglycemia from glucose metabolismRespiratory & metabolic acidosis secondary to anaerobic metabolismInhibition of surfactant production related to acidosis

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Consequences of Hypothermia

pulmonary blood flow related to pulmonary vasoconstriction in response to body temperature pulmonary vascular resistance compromises the delivery of oxygen at the cell level risk of developing PPHN in the near term, term or post term infant

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Prevention of Hypothermia

Hypothermia can be prevented by maintaining a neutral thermal environment and reducing heat loss.A neonate is in a neutral thermal environment when the axillary temperature remains at 36.5° - 37.3° (97.7° - 99.2° F) with minimal oxygen and calorie consumption

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Prevention of Hypothermia

Reduction of heat lossConsider the four ways by which the neonate experiences heat loss and intervene appropriately.

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Prevention of Hypothermia

Prevention of hypothermia is the best treatment but if it occurs anyway, the following is a list of what you can do to relieve the cold stress.Increase ambient air temperatureApply external heat sources

Warm hatWarm blankets or diapersChemical mattress

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Prevention of Hypothermia

Avoid stressing the babyMonitor skin temperature carefully and when it normalizes remove the external heat sources one at a time to prevent rebound hypothermia

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Hyperthermia

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HyperthermiaHYPERTHERMIA also has negative consequences for the neonate.Defined as a rectal / axillary temperature greater than 37.3°c (99.2°F)

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Risk factors for Hyperthermia

Excessive environmental tempSepsisDehydrationAlterations in the hypothalamic control mechanism

Birth TraumaAnomaliesDrugs

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Signs of Hyperthermia

TachypneaApneaTachycardiaFlushingHypotensionIrritabilityPoor FeedingSkin Temp > Core Temp

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Consequences of Hyperthermia

in Metabolic rate oxygen consumptionDehydration from insensible water lossPeripheral vasodilatation/ hypotensionFluid, electrolyte abnormalitiesseizures

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There Are a Lot of Factors to Consider.How Can I Be SureMy Patient Maintains Thermoneutrality?

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It Is Important to Review and

Understand theFour Methods

of HeatTransfer

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Possible Sources of Heat Loss

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CONVECTIVE HEAT LOSS can be prevented by:

Providing warm ambient air temperaturePlacing infants less than 1500 grams in incubatorsKeeping portholes of the incubator closedWarming all inspired oxygenOn open warmers keeping sides up and covering infant if possibleUsing Infant Servo Temperature Control

Strategies to prevent heat loss:

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Strategies to prevent heat loss:

RADIANT HEAT LOSS can be prevented by:

Avoiding placement of incubators, warming tables and bassinets near cold windows, walls, air conditioners, etc..Placing a knit hat on the infant’s headWrapping tiny babies in saran or “bubble” wrap environmental temperature

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Strategies to prevent heat loss:

CONDUCTIVE HEAT LOSS can be prevented by:

Placing a warm diaper or blanket between the neonate and cold surfacesPlacing infant on pre-warmed table at time of deliveryWarming all objects that come in contact with the neonateAdmitting infant to a pre-warmed Skin to skin contact

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Strategies to prevent heat loss:

EVAPORATIVE HEAT LOSS can be prevented by:

Keeping the neonate and his/her environment dry.Drying the baby immediately after delivery.Placing preterm or SGA infant in occlusive wrap/bag at deliveryDelay bath until temperature is stable

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Interventions for at Risk Infants

Pre-warmed radiant warmer bedPre-warmed incubatorDo not leave a warmer bed or incubator in the manual modeServo mode allows the baby to control the heat output of the warmer unitsHeated water padHeat lampWarm and humidify inspired gasesOcclusive wrap/bag at delivery

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Interventions for at Risk Infants

Open incubator portholes and doors only when necessaryBlanket over incubatorCluster care

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Interventions to ConsiderCover thermoreceptor-rich areas, such as the head.

Dry well after baths, especially the head and neck area.

Dress and cover infants, when in cribs, to prevent them from chilling.

Warm fluids prior to dressing changes

Rewarm slowly to prevent a potential subsequent drop in blood pressure.

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Rewarming the Hypothermic Infant

Always be prepared to intervene Rewarm slowly (0.5˚C per hour)Monitor closely (vital signs every 15 – 30min)

Core tempSkin temp will be higher than axillaryBlood pressure

Rewarming may lead to vasodilation - hypotension

Heart rate and rhythm Bradycardia & arrhythmias common with hypothermia

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Rewarming Monitor Respiratory rate and effort

Increased distressApnea

Oxygen saturationsHypoxemia / desaturationsBe prepared for need for respiratory supportMonitor acid/base status

Blood glucoseMonitor- infant at increase risk for hypoglycemia

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Guidelines for RewarmingIncubator better control than warmerSet temp 1 – 1.5˚C above core tempAssess infant temp every 15-30 minutesAs infants core temp reaches set temp and infant is not showing signs of deterioration increase set temp again.Continue process until temp within normal range

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Signs of Deterioration during rewarming

Tachycardia – due to in cardiac outputCardiac arrhythmiaHypotensionHypoxemia / DesaturationsWorsening respiratory distressWorsening acidosis

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Cooling the overheated neonate

Extended position- not flexedExpose skin- remove clothingKeep active temp reduction methods to a minimum to prevent dramatic heat lossMonitor temperature and VS every 15 – 30minutesBe prepared to intervene

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ConclusionsHypothermia in the newborn is due more to a lack of knowledge than to lack of equipment.Hypothermia is a preventable condition that has well documented impact on morbidity and mortality. Therefore, assisting the infant to maintain a normal body temperature and preventing hypothermia during stabilization is critical