Thermoregulation in the Newborn

Audra McCay Prince, MDAssistant Professor of Pediatrics

Arkansas Children’s Hospital & UAMS Neonatology

Heat Production

Voluntary muscle activityInvoluntary muscle activity (shivering)Non-shivering Thermogenesis

Brown Fat

LocationsAbundance in infancyDiffers morphologicallyDiffers metabolicallySympathetic enervation

Mechanisms for Heat Loss

ConductionConvectionRadiationEvaporation

Conductive Heat Loss

This is heat transfer from a warmer object to a cooler object that it is in contact with.

Example: the baby loosing heat to a cold mattress or scale.

Convective Heat Loss

This is heat transfer from a warmer object to the cooler air. It depends on air flow, as well as the temperature of the air.

Example: the baby on a radiant warmer can have a large amount of convective heat loss, especially in the drafty areas of the nursery.

Radiant Heat Loss

This is heat transfer from a warmer object to a cooler object that are NOT in contact with each other.

Example: heat loss to the walls of the isolette, or heat loss to a cold window next to a crib in the nursery

Transport isolettes

Evaporative Heat Loss

Cooling of the body by the evaporation of water from the skin.

In very small infants this is increased during the first few days of life due to their very thin (non-keratinized) skin, an increased body surface area/body mass ratio, and the extracellular mass is between 80-90% H2O.

Relative Role of Evaporative Heat Loss

0

10

20

30

40

50

60

.88-1.25 1.25-1.75 1.75-2.25 2.25-2.88

Total heat productionEvaporative heat loss

Birthweight (kg)

Kca

l/kg

/da

y

Environmental Temperature

Heat Production orO2 Consumption

Decreasing Body Temp

SummitMetabolism

Zone ofThermalNeutrality

IncreasingBody Temp

Death fromHeat

Mernstein G, Blackmon L 1971

BodyTemperature

37 C

InevitableBody

Cooling

ThermoregulatoryRange

Inevitable

BodyHeating

Zone ofThermal

Neutrality

Merenstein G, Blackmon L, 1971

Homeotherm

Homeotherms possess mechanisms that enable them to maintain body temperature at a constant level more or less accurately despite changes in the environmental temperature.

An infant that is cooled and not hypoxic attempts to maintain body temperature by increasing the consumption of calories and oxygen to produce additional heat.

BodyTemperature

37 C

InevitableBody

Cooling

ThermoregulatoryRange

Inevitable

BodyHeating

Zone ofThermal

Neutrality

Merenstein G, Blackmon L, 1971

Temperature Measurements

Skin temperatureServo ControlAxillary temperatureRectal temperature

Specific Situations

In UteroDelivery RoomIsoletteRadiant WarmerOpen Crib

In Utero

Heat produced in the fetus is dissipated through the placenta to the mother

Fetal temp normally 0.6 0 higher than the mother’s temp

Maternal feverEpidural anesthesia

Delivery Room

The fetus is born into a cold wet environment, and no longer has the maternal heat reservoir.

When skin is 1st exposed to the air, rapid cooling begins with body temp dropping from 0.2-1.0 c/min.

In response to cold a nor-adrenaline surge occurs. When pursued to an extreme peripheral and pulmonary vasoconstriction occur with sub-sequent decreases in oxygenation and perfusion.

Delivery Room

Careful and immediate drying of the infant’s entire body remains critical in minimizing evaporative heat loss.

Placing on radiant warmerHatsMaternal skin

Your Mother was Right

Care of the High-Risk Neonate, 4th Edition, Klaus and Fanaroff, 1993, pg 122

Isolette

The importance of double wallsTransport isolettesSkin temp v. Air tempNever cover temp probesPlacement of probe is important

Double Walled Isolettes

Care of the High-Risk Neonate, 4th Edition, Klaus and Fanaroff, 1993, pg 119

Radiant Warmer

Must use a metallic coverAir flow is importantSaran wrapDrapes

Open Crib

Remember that babies continue to have radiant heat loss to the windows, walls, etc., If they are not bundled.

Babies that are losing weight, ex. Feeders and growers may be cold with increased oxygen consumption and caloric expenditure.

Disorders of Temperature Regulation

HypothermiaNeonatal Cold InjuryHyperthermiaAsphyxia

Hypothermia

Low birth weight infantsAsphyxiaPrematuritySepsisCNS

Neonatal Cold Injury

LBW Lethargic and feed poorly Cold to touch Red baby Slow and grunting respirations with bradycardia edema and sclerema Metabolic derangements Pulmonary hemorrhage

Treatment of Cold Injury

Re-warmingOxygenGlucoseNaHCO3

NPOAntibiotics

Causes of Hyperthermia

Environmental temperatureInfectionDehydrationCerebral birth traumaDrugs

Asphyxia

ResuscitationInability to regulate tempPay attention to heat loss

Conclusion

Attentiveness to temperature in a sick newborn is extremely important.

Minimizing O2 consumption is the goal.

Recognition and treatment of infants with cold injury can be life saving.