Thermal Regulation in Newborns

Dr Philip HenschkeMercy Hospital for WomenMelbourneAustralia

The Fetus• Fetus grows in a relatively stable thermal

neutral environment in mother’s uterus.• Fetal warmth not dependent on flow of

heat from mother to fetus.• Metabolic rate of fetus X 2 maternal

metabolic rate.• Maintain temperature approximately

0.3°C–0.5°C higher than mother.• Net heat loss from fetus to mother.

• Mostly trough placenta• Some through skin.

Newborn Physiology• Newborn mammals unique in that they are

unable to shiver in cold environments.• Increase heat production by non shivering

means.• Chemical thermogenesis via brown fat.

• Thermoreceptors on skin stimulated → hypothalamus → sympathetic stimulation.

• Sympathetic stimulation → noradrenaline release from brown fat → vasoconstriction & ↑ metabolism.

• Sympathetic stimulation → T4 & T3 release → thermogenin release → fatty acid oxidation & uncoupling ATP production → thermogenesis in brown fat.

Newborn Physiology• Brown fat highly vascularized (4 - 6 Xs

white fat), with ↑ mitochondria, lipid molecules, & capillaries.• Found in neck, axilla, intrascapular

regions, & mediastinum surrounding the vasculature & major organs, near kidneys & adrenal glands.

• Present from 25 weeks but 1 – 2% in preterms compared to 4% in term infants.

• Once brown fat depleted, it cannot be replaced.

Heat Loss at Birth

• Protection against heat loss in newborn reduced compared to adult.• Naked newborn in a 23ºC environment

(warmer than many delivery rooms) suffers same cold stress that naked adult would experience at 0ºC.

• Without intervention a newborn’s body temperature will drop by 0.2°C to 1.0°C per minute.

Heat Loss at Birth• Newborn at greater risk of heat loss compared

to older child or adult.• Higher skin surface area to weight (volume)

ratio than older child or adult.• Ratio dramatically increases in smaller

infants. • Shape of extremely preterm infant like 2

large spheres (head & chest/abdomen) → highest possible surface area for volume.

• Thinness of skin & ↓ subcutaneous tissue.• ↓ insulation.• ↑ transdermal water losses.

Heat Loss at Birth

(Google Images)

Premature Infants• Increased susceptibility to heat loss.

• Large head & greater skin surface to body weight.

• Less subcutaneous fat & thinner skin• More trans-epidermal water loss.

• Decreased ability to maintain flexed posture.• Reduced brown fat & glycogen sores.• Low levels of thermogenin & 5’3’

monodeiodinase,• Low surge of thyrotropin.• ↑ risk of hypoxia → impairs brown fat

metabolism.• Can only maintain core temperature in narrow

range of environmental temperatures.

Evaporative Heat Loss

(Google Images)

Neutral Thermal Environment• Human newborns are homeotherms.

• Respond to ↓ or ↑ temperature around them by attempting to maintain body temperature in normal range of 36.5ºC to 37.5ºC.

• Compensation mechanisms → ↑ oxygen & calorie consumption.

• Neutral thermal environment (NTE) reflects range of temperature over which metabolic demands are minimal.

Neutral Thermal Environment

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Neutral Thermal Environment• As environmental temperature rises above

NTE, metabolic demands rise.• Ultimately infant is unable to compensate

for elevated temperature + accelerated water losses → death can occur.

• As environmental temperature drops below NTE, metabolic demands rise.• Infant attempts to compensate by

increasing metabolism & O2 consumption.• Point at which the normal mechanisms are

overcome → infant’s temperature begins to drop → metabolic rate drop → death can occur.

Neutral Thermal Environment• Newborns able to maintain a normal body

temperature over a range of environmental temperatures outside NTE, but this requires energy.• Cannot rely on measurement of body

temperature alone to determine if infant is not being subjected to thermal stress.

• Measurement of body temperature does not indicate if infant is near point at which capacity of infant to respond & maintain normal temperature will be overwhelmed.

→ Monitor closely for signs of stress.• Range of NTE ≤ 1.5ºC when infant weighs

< 2,500g.

Symptoms of Hypothermia• Infants may be irritable but more often are

lethargic with limited movement.• Feed poorly & may vomit after eating. • Often feel strangely cold when touched.• Skin may be bright red (O2 bound to

haemoglobin fails to dissociate). • Initially ↑ resp rate then ↓ resp rate & heart rate. • In severe cases, peripheral & facial oedema

develops.• Sclerema (subcutaneous fat necrosis) →

patches of hardened, red skin & subcutaneous tissue (impending death).

Symptoms of Hypothermia• Hypothermia associated with organ

dysfunction:• Acute renal failure• Coagulopathy• Persistent pulmonary hypertension• Intraventricular haemorrhage

• Hypoglycemia occurs more commonly in infants at risk for hypothermia.• More likely cause, rather than effect.

Significance of Thermoregulation• Hypothermia is associated with increased

morbidities such as respiratory distress syndrome and increased susceptibility to late-onset sepsis.

• Estimated 28% increase in mortality for every 1°C below 36.5°C.

(Mullany et al. Arch Pediatr Adolesc Med.2010)

Prevention of Hypothermia

The Delivery Room• Greatest risk of hypothermia occurs in minutes after

birth:• Losses caused by evaporation, convection, &

radiation can be very high. • Without precautions, body temperature can

rapidly drop by 2 - 3°C in minutes.• World Health Organization recommendations

delivery site should be warmed to 25°C.• Theatre temperatures of 23°C vs 20°C improved

admission temp by 0.7°C but no difference in incidence of hypothermia. (Duryea et al 2016)

• Use of radiant warmer is helpful, reducing conductive heat loss, but does little to reduce heat loss by other routes.

The Delivery Room• Radiant warmer.• Pre-warm resuscitaire.• Dry infant with warm blankets.

• Remove wet blankets.• Hat.• Wrap in warm blankets.• Skin to skin contact with light blanket over

top.

• Vermont Oxford Network (2010) ≈ 50% of infants born weighing < 1,500 g have NICU admission temperatures < 36.5°C.• ≈ 50% of these < 36°C.

• Act as barrier to heat loss.• Reduce heat loss via evaporation & convection.

• Infant placed in plastic bag or wrapped prior to drying, overhead using radiant heater.• Reduces evaporative & convection heat loss.

• Cochrane review: (McCall et al 2018)• Improved admission temperatures.• No statistical reduction in mortality.• Possible reduction in risk of pulmonary

haemorrhage.• Incidence of hyperthermia not increased.• Current Neonatal Resuscitation Program (NRP)

guidelines recommend use in all infants < 1500 gm.

Polyethylene Wraps & Bags

Slideshare.net Google Images

Thermostable Gel Mattresses

• Warming mattresses used to provide conductive heat gain.

• Have been used in various manners including in resource poor settings as substitute for overhead heater or incubator in delivery area.• Variable results depending on clinical setting.

• Most literature indicates warming mattresses are effective in preventing hypothermia in preterm infants. (Libert et al. Biomed J Sci & Tech Res 2017)

• Not effective when used alone – limited conductive heat transfer

Thermostable Gel Mattresses

• When used in conjunction with polyethylene bag or wrap concern about temperature management.• McCarthy & O’Donnell (2011) – similar

admission temperatures but ↑ hypothermia & hyperthermia.

• Singh et al (2010) – improved admission temperatures but risk of hyperthermia.

• Chawla et al (2011) – admission temperatures significantly higher, no adverse effects.

• Excluded infants born to mothers with temperature > 38.2°C.

Thermostable Gel Mattresses • Temperature of chemical gel mattress at time of

activation affects plateau temperature when in use. • Starting temperatures of 19.2–28.3°C required to

achieve an effective and safe mattress plateau temperature of 38–42°C.

(Carmichael et al Arch Dis Child Fetal Neonatal Ed 2007)

Neonatal Intensive Care Unit• All forms of heat loss occur in NICU but

evaporative heat losses less compared to delivery room.

• Radiant warmers reduce radiant & conduction heat loss but allow ongoing large evaporative and convective heat losses. • Increase trans-epidermal fluid losses.

• Typically used to stabilise newborn and for procedures.

Neonatal Intensive Care Unit• Once infant stabilised double walled

incubator using servo control to maintain abdominal skin temperature at 36.5°C (maintain thermal neutrality & minimize oxygen use) typically used.• Double wall reduces radiant heat loss

because internal wall same temperature as interior of incubator.

• BUT warming air temperature inside incubator ↓ humidity inside incubator.

• accelerates trans-epidermal water losses.

Neonatal Intensive Care Unit• Relationship between trans-epidermal water

loss & humidity is an inversely linear one.• Evaporative losses can be reduced with

provision of high-level humidity (relative humidity > 80%).

• Relative humidity of 80 - 90% can reduce water loss in premature infant to 10% of the water loss that occurs in 50% humidity. (Hammarlund et al. Acta Paediatrica Scandinavia 1983)

• Typical NICU incubator capable of humidification of ≥ 80%.

Neonatal Intensive Care Unit• Humidification.

• Trans-epidermal fluid losses high in ELBW infants in first week of life.

• Use of high humidity reduces fluid losses & reduces required incubator.

• Continued use of high humidity beyond day 7 slows normal maturation of skin barrier.

Day of Life Humidity1 - 7 80

8 759 7010 6511 6012 5513 5014 4515 Cease

Humidity < 31 weeks

(Royal Children’s Hospital Melbourne Australia)

Neonatal Intensive Care Unit• Humidified hybrid incubator.

• Kim et al (Pediatrics 2010) compared use of radiant warmers with humidified hybrid incubator.

• Demonstrated use of a humidified hybrid incubator improved care for ELBW infants by making it possible to decrease fluid intake, improve electrolyte balance and growth outcomes without a disturbance of body temperature.

Alternatives to Incubators• In 1978 Edgar Rey, a Colombian

paediatrician concerned with the problems arising from a shortage of incubators and the impact of separating women from newborns in neonatal care units, developed Kangaroo Mother Care (KMC).

(Google Images)

• Randomized controlled trial comparing KMC to pre-warmed servo-controlled closed incubator after birth.

(Bergman et al, Acta Paediatr 2004)

• Compared 20 infants using KMC to 14 control infants (incubator care) with birth weights 1200-2199g.

• 1/20 KMC subjects vs 8/14 controls had initial temps < 35.5°C (P = 0.006)

• Associated with improved stability of cardio-respiratory system in preterm infants in 1st 6hrs in KMC group.

• A 12 month follow up study demonstrated no significant differences in physical growth patterns or in the rates of cerebral palsy, failure to thrive, visual problems, deafness, or hip dysplasia.

Kangaroo Mother Care

Kangaroo Mother Care

• A systematic meta- analysis of several randomisedcontrol trials found that KMC is associated with a 51% reduction in neonatal mortality for stable babies weighing < 2,000 g if started in the first week, compared to incubator care.• Improved thermal control, less apnoea, reduced

nosocomial infection.• Considered facility-based KMC practice where

feeding support was available. Lawn et al Int J Epidemiology 2010

• An updated Cochrane review also reported a 40% reduction in risk of post-discharge mortality, about a 60% reduction in neonatal infections and an almost 80% reduction in hypothermia.

Conde-Agudelo et al Cochrane Database of Systematic Reviews 2011.

Thank You for Your Attention

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