Temperature Control

in the Neonate

Introduction

Hypothermia associated w/ increased morbidity/mortality in newborns of all birth weights/ages

Now considered independent risk factor for mortality in preterm

Western philosophy of conventional care – premature baby should be

Placed under radiant warmer

Uncovered for full visualization and to allow radiant heat to reach body

More attn now focused on thermal care immediately after birth and during resuscitation

Premature Susceptibility to Heat

Loss

High surface area to volume ratio

Thin non-keratinized skin

Lack of insulating subQ fat

Lack of thermogenic brown adipose tissue

(BAT)

Inability to shiver

Poor vasomotor response

Thermoregulation

Metabolic rate of fetus per tissue wt. higher than adult

Heat also transferred from mother to fetus via placenta/uterus

Fetal temp consistently 0.3-0.5 deg C higher than mother’s (always in parallel)

Even when mother’s temp elevates (eg fever)

Despite BAT in utero, fetus cannot produce extra heat

Exposed to adenosine and prostaglandin E2 inhibitors of non-shivering thermogenesis (NST)

Metabolic adaptation for physiologically hypoxic fetus since NST requires oxygenation

Inhibition of NST allows accumulation of BAT

Thermoregulation

Heat gain/loss controlled by hypothalamus and limbic system Thermoregulatory system immature in newborns (esp

premature newborn)

In term infant, response to cold stress relies on oxidation of brown fat (NST) Development begins 20th wk until shortly after birth

(comprises 1% body wt at that time)

High concentration stored TG’s

Rich capillary network densely innervated by sympathetic nerve endings

Temperature sensors on posterior hypothalamus stimulate pituitary to produce thyroxine (T4) and adrenals to produce norepinephrine

Lipolysis stimulated energy produced in form of heat in mitochondria instead of phosphate bonds by uncoupling protein-1 (aka thermogenin)

Risk Factors

All neonates in 1st 8-12hrs of life

Prematurity

SGA

CNS problems

Prolonged resuscitation efforts

Sepsis

Adverse Consequences of

Hypothermia

High O2 consumption hypoxia, bradycardia

High glucose usage hypoglycemia / decreased glycogen stores

High energy expenditure reduced growth rate, lethargy, hypotonia, poor suck/cry

Low surfactant production RDS

Vasoconstriction poor perfusion metabolic acidosis

Delayed transition from fetal to newborn circulation

Thermal shock DIC death

Modes of Heat Loss

Conduction - direct heat transfer from skin to object (eg mattress)

Convection - heat loss through air flow

Also depends on air temp

Radiation - direct transfer by electromagnetic radiation in infrared spectrum

Heat gained by radiation from external radiant energy source

Heat lost by radiation to cooler walls of incubator

Evaporation - heat loss when water evaporates from skin and respiratory tract

Depends on maximum relative humidity of surroundings less humidity = more evaporation

Heat Loss at Birth

Hammarlund et al, 1980

Evaporative H20 loss

81-125 gm/m2/h when unwiped in ambient temp ~25.8deg C and 42% humidity

Heat loss through

Evaporation: 60-80 W/m2

Radiation: 50 W/m2

Convection: 25 W/m2

Conduction: negligible

Total heat loss = 135-155 W/m2

All babies that were >3250g - body temp decreased 0.9deg C in 15min

Heat Loss at Birth

Hammarlund et al, 1979

Naked infants <28wks need ambient temp ~40deg

C to maintain nl temp in 20% humidity

Increasing humidity to 60% halved losses

Attempt to Overcome Losses

Radiant heaters insufficient to warm preterm

baby

Esp during resuscitation

750g baby w/ surface area of ~ 0.06m2 requires at

least 9.3W to compensate for losses at birth

At mattress lvl, max of 9W absorbed by baby if

radiant heat absorbed by, at least, 50% of mattress

Thermoneutral Environment

Temp and environmental conditions at which

metabolic rate and O2 consumption are lowest

Silverman et al

Maintaining constant abdominal skin temp b/w

36.2-36.5 deg C optimal

WHO classification of hypothermia

Mild: 36-36.4deg C

Mod: 32-35.9deg C

Severe: <32deg C

Kangaroo Mother Care (KMC)

Introduced in 1983 by Rey and Martinez in Colombia

LBW infants nursed naked (wearing only cloth diaper) between mothers’ breasts

Data from other countries show infants nursed by KMC have

Fewer apneic episodes

Similar or better blood oxygenation

Lower infxn rtes

Are alert longer and cry less

Are breastfed longer and have better bonding

Improved survival in low-resource settings

KMC

Bergman et al, 2004

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

20 infants b/w 1200-2199g using KMC vs 14 controls

Excluded if C-sec, mother too ill to look after self/infant, known HIV, BW outside 1200-2199g, 5min Apgar <6, congenital malformations

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

1/20 subjects vs 3/14 controls had bl glucoses < 2.6 mmol/L (though 40mg/dL = 2.2mmol/L)

Stability of cardio-respiratory system in preterm infants (SCRIP) score was 2.88 points higher w/in 1st 6hrs in KMC group (95% CI 0.3-5.46)

SCRIP Score

SCRIP 2 1 0

HR Regular Decel to 80-100 Rte <80 or

>200 bpm

RR Regular Apnea <10s or

periodic

breathing

Apnea >10s or

tachypnea >80

O2 sat >89% 80-89% <80%

Barriers to Heat Loss Cochrane database review

4 studies compared barriers to heat loss vs. no barriers

2 comparison subgroups

Plastic wrap/bag vs routine care

Stockinet cap vs routine care

Plastic wrap/bag vs routine care

3 studies involving 200 infants all <36wks

All placed under radiant warmer, wrapped to shoulders while still wet, heads dried and resuscitated according to guidelines

GA <28wks: wrap group had temps 0.76deg C higher than controls (95% CI 0.49-1.03)

GA 28-31wks: no statistical difference

Barriers to Heat Loss

Plastic wrap/bag vs routine care (cont)

1hr after admission for GA <28wks, no statistical difference

(though direction was in favor of intervention)

Plastic wrap significantly reduced risk of hypothermia (core

temp <36.5deg C) on admission to NICU

RR 0.63 (95% CI 0.42-0.93)

NNT found to be 4 (95% CI 3-17) - so 4 infants would need to be

wrapped in plastic to prevent 1 from becoming hypothermic

No significant differences found in duration of O2 therapy,

major brain injury, duration of hospitalization, or death

Barriers to Heat Loss

Stockinet cap vs routine care

1 study involving 40 AGA infants w/ GA’s 32-36wks

Exclusion critera: 5min Apgar <7, SSx CNS defect, sepsis, or maternal temp >37.8deg C during labor

Cap group had caps placed ASAP after drying under radiant warmer and infants <2500g were transported in incubator

BW <2000g: Cap group had core temps 0.7deg C higher than control (95% CI -0.01-1.41) - borderline statistical difference

BW >/= 2000g: no sig dif

No sig dif in preventing hypothermia

External Heat Sources

Cochrane database review

2 studies compared external heat sources to

routine care

2 comparison subgroups

Skin-to-skin vs routine care (already mentioned)

Transwarmer mattress vs routine care

External Heat Sources

Brennan et al, 1996

24 infants w/ BW </= 1500g

Transport Mattress (TM) - made of sodium acetate - activated to ~40deg C when delivery imminent

Infant placed upon blankets covering mattress, dried, then placed on TM directly

Control group = same intervention but w/o TM

Both groups resuscitated according to guidelines then transferred to NICU on radiant warmer surface

External Heat Sources

Brennan et al, cont

Increase of 1.6deg C in TM group (95% CI 0.83-2.37)

Evidence suggests that TM significantly reduces risk of hypothermia w/ RR 0.3 (95% CI 0.11-0.83)

NNT = 2 (95% CI 1-4)

No adverse occurrences reported in this study, though other studies have had infants sustain 3rd deg burns

In Conclusion Plastic barriers effective in reducing heat loss in

newborns <28wks

No evidence yet to suggest plastic barriers decrease duration of O2 therapy, hospitalization, or incidence of major brain injury/death

Stockinet caps effective in reducing hypothermia in newborns <2000g, but not >/= 2000g

KMC shown to be effective in stable newborns down to 1200g in reducing risk of hypothermia

TM decreases incidence of hypothermia </= 1500g

In the end, the smaller the baby, the more likely any intervention will be of benefit

Areas of Further Study

Need more studies w/ larger population bases

Short- and long-term outcomes need to be

studied further (especially w/

neurdevelopmental F/U)

Secondary outcomes that need further study:

Hypoglycemia RDS Intubation/ve-

ntilation

Length of stay

Metabolic acidosis ARF Growth Adverse events

Neonatal Energy Triangle

References

Laroia, N. “Double wall versus single wall incubator for reducing heat loss in very low birth weight infants in incubators.” Cochrane Database of Systematic Reviews. Vol (3) 2007.

Fienady, V. “Radiant warmers versus incubators for regulating body temperature in newborn infants” Cochrane Database of Systematic Reviews. Vol (3) 2007.

Asakura, H. “Fetal and Neonatal Thermoregulation.” Journal of Nippon Medical School. Vol. 71 (2004) , No. 6.

Ibe, O.E. “A comparison of kangaroo mother care and conventional incubator care for thermal regulation of infants <200 g in Nigeria using continuous ambulatory temperature monitoring.” Annals of Tropical Paediatrics (2004) 24, 245-251.

Bergman, N.J. “Randomized controlled trial of skin-to-skin contract from birth versus conventional incubator for physiological stabilization in 1200- to 2199-gram newborns.” Acta Paediatrica (2004) 93: 779-785.

McCall, E.M. “Interventions to prevent hypothermia at birth in preterm and/or low birthweight babies.” Cochrane Database of Systematic Reviews. Vol (3), 2007.

Watkinson, M.A. “Temperature Control of Premature Infants in the Delivery Room.” Clin Perinaol 33 (2006) 43-53.

“Knobel, R.B. “Heat Loss Prevention for Preterm Infants in the Delivery Room.” J Perinaol 25 (2005) 304-308.

The neonatal energy triangle Part 2: Thermoregulatory and respiratory adaptation.” Paediatric Nursing. Sept. Vol 18 no 7.