Thermal Care of the Newborn – Bidding a Warm Welcome!
Thermoregulation is a key physiological challenge for all newborns during their foetal to neonatal transition. Neonatal hypothermia is a worldwide problem known to be associated with increased morbidity and mortality. Inherent physiological limitations coupled with behavioural and environmental factors of the delivery room place every newborn at risk of hypothermia; however, with simple interventions, hypothermia can be prevented in most instances. The cornerstone in management is maintaining the WARM CHAIN, a series of 10 interrelated processes designed to minimize heat loss from the newborn by strengthening thermal barriers and providing additional exogenous warmth. This review article describes the pathophysiology and global burden of neonatal hypothermia, discusses key principles of thermal management, and recommends useful strategies for temperature maintenance in the delivery room and the neonatal intensive care unit (NICU).
Thermoregulation is the ability to balance heat production and loss in order to maintain core body temperature within a normal range. It is a key physiological challenge for all newborns during their foetal to neonatal transition. In the intrauterine environment, the foetus is not required to thermoregulate. The highly efficient placental bed dissipates the foetal energy of metabolism into the maternal milieu within a tight gradient of 0.5°C. However, at birth, sudden exposure to a relatively cold external environment triggers primitive thermogenic responses in the newborn to independently regulate its body temperature.1
In term infants soon after birth, body heat is lost through different mechanisms: evaporation of the amniotic fluid from wet skin, conduction to surrounding surfaces in contact with the baby’s skin, convection draughts from overhead air conditioners or open doors, and radiation to the surrounding environment. Evaporative losses contribute the most to thermal instability at birth and during the first week of life (see Figure 1).
Apart from the intrinsic physiologic characteristics of newborn infants, other factors that influence the extent of heat loss include environmental factors in the delivery room (eg, ambient air temperature, humidity, and temperature of surrounding surfaces) and regional cultural practices such as the immediate removal of vernix. In low-resource settings with temperate climates, it has been shown that infants lost more heat in the winter than summer months.
Cold stress response after birth consists of increased involuntary muscular activity, vasoconstriction, and non-shivering thermogenesis. After birth, healthy full-term infants mount a surge of norepinephrine and thyroid-stimulating hormones that mediate fatty acid oxidation in the brown fat stores to meet the increased metabolic demand of thermoregulation. Brown fat is a special tissue laid down in the neck, chest, and abdomen of the foetus only during the third trimester of pregnancy. Subcutaneous fat, hepatic glycogen reserves, and milk feeds also contribute to a lesser degree to produce heat.
Infants who are born preterm or with intrauterine growth restriction are at an even greater risk than term infants for hypothermia owing to the disproportionate body mass – surface area ratio, exposed body posture, decreased energy reserves, immature epidermal barrier, and poor vascular tone.2 The immature stratum corneum of premature infants allows for transepidermal water loss (TEWL) of as high as 200 mL/kg/day and for each gram of water lost by evaporation from the body surface approximately 580 calories (0.58 kcal) of energy is consumed.3
In the absence of a thermally protective environment, core body temperatures decrease at a rate of 0.2–1.0°C per minute in the first 30 minutes of the newborn’s life.4 Therefore, thermal homeostasis is essential for survival, and uncontrolled losses place the newborn infants at risk for significant morbidity and mortality.
DEFINITION AND CLASSIFICATION OF NEWBORN HYPOTHERMIA
Normal temperature range in newborns is not clearly defined and depends on the site of measurement (rectal, axillary, skin) and the type of thermometer used (mercury, digital, infrared). The American Academy of Pediatrics (AAP) and World Health Organization (WHO) define the range of normal newborn core temperature measured as an axillary temperature of 36.5–37.5°C.5 They further classified the severity of hypothermia into mild (36.0–36.4°C), moderate (32.0–35.9°C), and severe (<32.0°C) based on core body temperature (see Table 1).
The Acute Care of at-Risk Newborns (ACoRN) Neonatal Society recommends a temperature of 36.3–37.2°C to be clinically relevant to avoid adverse neonatal outcomes.8 Hyperthermia is hence defined as axillary temperature of >37.3°C8 or 37.5°C5.
Although rectal temperature is believed to reflect core temperature more accurately and was considered to be the gold standard in the past, the AAP and WHO recommend the axillary site instead for its relative ease of measurement, accuracy, and safety.6 For early detection of hypothermia and during prolonged resuscitation at birth, a dual-site (axillary-skin) measurement is useful.
It is also important to understand that there is a clear gradient of temperature measured between rectal or axillary and skin temperature (core vs peripheries) of up to 0.5–1.0°C in a full-term newborn infant. This temperature gradient is less pronounced in preterm infants, due to their limited intrinsic thermal reserves, and varies around 0.2–0.3°C under thermoneutral conditions.7
CLINICAL IMPLICATIONS OF NEWBORN HYPOTHERMIA
As an integral component of the neonatal energy triangle,1 hypothermia may delay transition from foetal to neonatal circulation at birth and is associated with increased oxygen demands, respiratory compromise, hypoglycaemia, and neonatal jaundice requiring phototherapy. Clinical manifestations include tachypnoea with grunting, chest retractions, and alar flaring at birth. When hypothermia is prolonged, the infant responds with vasoconstriction and sluggish capillary flow, resulting in metabolic acidosis and the appearance of acro-cyanosis (blueness of fingertips). Hypothermia is a well-known factor that contributes to and worsens persistent pulmonary hypertension of the newborn (PPHN).2
Hypothermia constitutes one of the independent risk factors for mortality in preterm infants.9 Lyu, et al, reported that in a multicentre study involving nearly 10,000 preterm infants of <33 weeks’ gestation, adverse neonatal outcomes such as neurological injury, severe retinopathy of prematurity (ROP), necrotizing enterocolitis (NEC), bronchopulmonary dysplasia (BPD), and nosocomial infection had a U-shaped relationship with admission temperatures, lowest rates associated with admission temperatures (36.5–37.2°C) within the range of normothermia.10 Admission temperature is an important variable utilized for calculating neonatal mortality risk assessment scores, including the Clinical Risk Index for Babies (CRIB) Score11 and Score for Neonatal Acute Physiology - Perinatal Extension (SNAPPE).12
In 2015, the International Liaison Committee on Resuscitation published a consensus statement recommending the maintenance of body temperature within 36.5–37.5°C after birth through resuscitation, stabilization, and admission.13 These guidelines have been endorsed by the Neonatal Resuscitation Program (NRP) developed by the American Heart Association (AHA) and adopted in the resuscitation guidelines for newborn infants for most parts of the world.14
GLOBAL BURDEN OF NEWBORN HYPOTHERMIA
The problem of newborn hypothermia is practically universal and does not depend on regional differences in the climate or healthcare facility. In a systematic review by Lunze, et al, the prevalence of newborn hypothermia in hospital-based studies ranged from 32% to 85%, where the wide range was attributed to differing case definitions of hypothermia across studies and an inherent selection bias when the study population was part of a high-risk cohort and not representative of the general population. Among term infants, the incidence of hypothermia was estimated to be at <10% in the developed world but much higher at >50% in the developing nations of South America (Brazil), Africa (Zambia, Ethiopia, Nigeria, Zimbabwe), Middle East (Iran), South Asia (north India, Nepal, Bangladesh), and East Asia (China).15
Even in the best-resourced countries, newborn hypothermia remains a challenge for those who were born very preterm (<32 weeks’ gestation) or very low birth weight (VLBW, <1,500 g). The UK National Neonatal Audit Program reported that among very preterm infants, 32.6% were hypothermic on admission in 2018.16 Data from the Vermont Oxford Network in 2017 showed that nearly four in 10 VLBW infants were hypothermic on admission, with the highest rates recorded in the smallest and most immature.17 In the Asian context, Singapore reported a rate of 79.4%,18 while Malaysia reported a rate of 65% for moderate hypothermia.19
For every 1°C decrease in admission temperature among the low birth weight infants, the odds of in‐hospital mortality increases by 28% and the odds of late‐onset sepsis increases by 11%.20 Among 8,782 VLBW infants, Miller, et al, reported a hypothermia incidence of 56.2% and found that the risk factors included low birth weight, Caesarean delivery, and low Apgar scores. The composite outcome of death and severe intraventricular haemorrhage was significantly associated with moderate-to-severe hypothermia in this study (22% vs 11%).21
PRINCIPLES OF MANAGEMENT – THE WARM CHAIN
The goal of thermal care is to keep the newborn in a ‘thermoneutral zone’ that is defined as a narrow range of ambient temperature at which caloric expenditure needed to maintain normothermia is lowest, beginning from birth and during the early neonatal transition period.5 Basic principles for thermal stability is mainly achieved by enhancing thermal barriers to heat loss and providing exogenous heat without compromising accessibility during resuscitation (where required). The “Warm Chain” is a set of 10 interlinked procedures recommended by WHO as standard thermal care for newborns in both hospital and community-based settings (see Table 2) that can be adopted in all healthcare settings.
In a hospital setting, routine thermal care includes providing a warm delivery room at a minimum of 25°C (which is rarely achieved), drying the infant thoroughly soon after birth, removing any wet blankets, wrapping in a prewarmed blanket, prewarming any contact surfaces, eliminating draughts, and close proximity to outside walls and placing the infant under a prewarmed radiant heater.5
Initiating skin-to-skin contact (SSC) between the mother and infant soon after delivery, as well as early breastfeeding, ideally within an hour after delivery, should be encouraged if possible, as long as not contraindicated. During SSC, it is imperative that the mother-newborn dyad should receive vital sign monitoring by qualified healthcare personnel. Early bathing and removal of the protective vernix caseosa before the period of neonatal adaptation is not recommended in modern neonatal care.22 In a previous randomized trial by Gabriel, et al, high thermal stability (regarded as an average skin temperature rise of 0.07°C) and breastfeeding success rates were observed in the SSC group.23
THERMAL MANAGEMENT OF THE HIGH-RISK INFANT AT BIRTH
In high-risk infants, such as those born very preterm or with VLBW, additional measures will be required for optimal thermal care that may be tailored according to the local disease prevalence, healthcare setting, and economic feasibility. Several combinations of interventions in addition to routine thermal care can maintain a preterm infant’s temperature, although there are no definite recommendations on the most effective.
Polyethylene wrap placed on VLBW infants before drying in the delivery room may produce a micro-humidified environment and improve TEWL by up to 70%. Once an infant has been wrapped at the delivery room, it is essential to maintain the wrapping in order to preserve the achieved thermal stability. Health care providers can devise techniques for effective wrapping such that unwrapping of the high-risk infant is deemed unnecessary, even during procedures requiring access, such as placement of umbilical catheters.2 A Cochrane systematic review of eight studies involving 1,171 neonates demonstrated that the use of plastic wraps or bags compared with routine care led to higher temperatures on admission to NICUs with less hypothermia, particularly for extremely preterm infants (mean difference, 0.65°C, 95% confidence interval [CI], 0.52–0.79).24
Maintaining a higher temperature of >26.0°C in the delivery room helped, but was insufficient in preventing hypothermia in most newborns who were born at <29 weeks’ gestation without the adjunctive use of polyurethane bags.25 There are currently no studies addressing the benefit of a warmer delivery room temperature in addition to polythene bag wrapping and use of a radiant warmer.1
Evidence is emerging on the efficacy of exogenous thermal mattress used in combination with polythene wrap and routine thermal care. The use of a thermal mattress was significantly associated with keeping VLBW infants warmer (mean difference, 0.65°C, 95% CI, 0.36–0.94) and reducing the incidence of hypothermia on admission to the NICU, with no adverse effects such as hyperthermia.24 Transwarmer®, an exothermic mattress containing sodium acetate gel (Draeger Medical, SE Asia Pte Ltd, Singapore), has been widely used in tertiary neonatal units, in combination with polythene bag and radiant warmer with excellent results. The mattress emits latent heat of crystallization when activated and provides a conductive heat source during resuscitation and subsequent transport to the NICU. This strategy can be safely recommended for preterm infants, provided that the skin temperature is closely monitored to avoid inadvertent hyperthermia. Previous studies by Leslie, et al, and McCarthy, et al, reported that the combination of plastic bagging and thermal mattress was beneficial as compared with polyurethane bag alone immediately after birth (mean difference, 0.37°C, 95% CI, 0.09–0.66).25
Newer strategies proposed to maintain preterm infants’ thermoneutrality at birth include the use of heated humidified gas (HHG) for respiratory support during resuscitation. In previous randomized controlled trials by Meyer, et al,26 and McGrory, et al,27 a significant reduction in admission hypothermia was reported without an overall increased risk of hyperthermia. Although the use of HHG appeared safe and possibly beneficial for infants at <28 weeks’ gestation, there is a need to balance the benefits, costs, and risks of introducing an additional piece of delivery room equipment, considering that other lower-cost interventions (polyurethane bags/wraps or use of exothermic mattress) are equally effective.
THERMAL MANAGEMENT IN THE NEONATAL INTENSIVE CARE UNIT
Preterm infants born at <32 weeks’ gestation or weighing <1,500 g are ideally placed inside a closed pre-warmed humidified incubator following admission to the NICU. A skin temperature probe is placed on the infant when using the servo control to assist the incubator in adjusting heat output based on the ‘set’ skin temperature and the ‘actual’ skin temperature. The axillary temperature must be measured on admission and recorded hourly until it is stable.
TEWL can be reduced efficiently by altering relative humidity (RH) during the first 2 weeks of life while the epidermal barrier matures. An RH of 90% can reduce TEWL by 45% for VLBW infants.2 An RH of 80% is recommended for infants born at <27 weeks’ gestation. Humidity and temperature settings should be individually assessed for each infant according to the particular infant’s gestational age, weight, serum sodium levels, fluid balance, and skin condition, and weaned accordingly.28 The appearance of ‘rain-out’ (due to condensate) on the inside wall of the incubator usually means a mismatch between the set humidity and incubator temperature, and must be avoided. Plastic wrap and thermal mattress used at delivery should be kept in situ until incubator humidity commences and the incubator temperature is within the thermoneutral range.
BIDDING A WARM WELCOME
Every neonatal unit should address the issue of newborn hypothermia by increasing staff awareness and compliance to the existing thermal care practices. A multidisciplinary team approach that builds or revamps work processes for providing optimal thermal care by implementing evidence-based practices and best practice recommendations is key to improve short- and long-term outcomes, especially in VLBW infants.29 Delivery room checklists30 and customized thermal care bundles31 can be formulated and incorporated using simulation-based training within staff in-service education sessions.
Newborn hypothermia is a strong predictor of neonatal outcomes at all gestations and should be recorded as a quality indicator of neonatal care. Hypothermia is more often due to lack of awareness rather than a lack of equipment. To prevent hypothermia, neonatal care teams must know how to reliably apply the basic principles of thermodynamics while preparing and supporting thermally labile newborns. NICUs that engage in collaborative quality improvement projects and external benchmarking can achieve and sustain the heights of thermoregulatory performance. Most often, simple interventions coupled with a culture for continuous improvement at the frontlines of care are sufficient for the greatest impact on preserving neonatal well-being.
About the authors
Dr Priyantha Ebenezer Edison is Senior Staff Registrar at the Department of Neonatal and Developmental Medicine, Singapore General Hospital, Singapore and Clinical Lecturer at National University of Singapore-Yong Loo Lin School of Medicine and Nanyang Technological University, LKC School of Medicine, Singapore. Conflict of interest: none.
Clinical Associate Professor Daisy Kwai-Lin Chan is Senior Consultant in the Department of Neonatal & Developmental Medicine, Singapore General Hospital, Singapore and Clinical Associate Professor at the National University of Singapore-Yong Loo Lin School of Medicine and Nanyang Technological University, LKC School of Medicine, Singapore. Conflict of interest: none.