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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.