Head insulation and heat loss in naked and clothed newborns using a thermal mannequin

Impact of human body shape on forced convection heat transfer

Predicting human thermal comfort and safety requires quantitative knowledge of the convective heat transfer between the body and its surrounding. So far, convective heat transfer coefficient correlations have been based only upon measurements or simulations of the average body shape of an adult. To address this knowledge gap, here we quantify the impact of adult human body shape on forced convection. To do this, we generated fifty three-dimensional human body meshes covering 1st to 99th percentile variation in height and body mass index (BMI) of the USA adult population. We developed a coupled turbulent flow and convective heat transfer simulation and benchmarked it in the 0.5 to 2.5 m·s^-1 air speed range against prior literature. We computed the overall heat transfer coefficients, h_overall, for the manikins for representative airflow with 2 m·s^-1 uniform speed and 5% turbulence intensity. We found that h_overall varied only between 19.9 and 23.2 W·m^-2 K^-1. Within this small range, the height of the manikins had negligible impact while an increase in the BMI led to a nearly linear decrease of the h_overall. Evaluation of the local coefficients revealed that those also nearly linearly decreased with BMI, which correlated to an inversely proportional local area (i.e., cross-sectional dimension) increase. Since even the most considerable difference that exists between 1st and 99th percentile BMI manikins is less than 15% of h_overall of the average manikin, it can be concluded that the impact of the human body shape on the convective heat transfer is minor.

Numerical investigation of the convective heat transfer coefficient for a sleeping infant in a ventilation room

This study aimed to investigate the influence of wind speed and direction on the convective heat transfer from a sleeping infant in different postures. A computational fluid dynamics (CFD) model of a virtual infant manikin with realistic dimensions was developed to obtain the convective heat transfer coefficient (h_c ) at the body surface and the airflow and temperature distributions. The numerical model was validated beforehand using experimental data collected from infant thermal manikin experiments. The simulation results revealed that the infant's whole-body h_c increased from 4.00 to 15.73 W/m² ·K when wind speed varied from 0.12 to 1 m/s. Infants lost heat more quickly than adults under ventilation, with about 2 W/m² ·K higher h_c than adults in still air, and the discrepancy widened as the wind speed increased. Wind from the floor generated the highest h_c , approximately 66.4% greater than the wind from the feet at 1 m/s wind speed. Considering the wind from the feet caused the most evenly distributed h_c , ventilation equipment was suggested to be placed on the side of the infant's feet to reduce local discomfort. Based on the simulation results, empirical models of h_c were developed, which lay a solid theoretical foundation for further study on the interaction between infants and environments.

Hypoxia-ischemia-mediated effects on neurodevelopmentally regulated cold-shock proteins in neonatal mice under strict temperature control

BACKGROUND

Neonates have high levels of cold-shock proteins (CSPs) in the normothermic brain for a limited period following birth. Hypoxic-ischemic (HI) insults in term infants produce neonatal encephalopathy (NE), and it remains unclear whether HI-induced pathology alters baseline CSP expression in the normothermic brain.

METHODS

Here we established a version of the Rice-Vannucci model in PND 10 mice that incorporates rigorous temperature control.

RESULTS

Common carotid artery (CCA)-ligation plus 25 min hypoxia (8% O2) in pups with targeted normothermia resulted in classic histopathological changes including increased hippocampal degeneration, astrogliosis, microgliosis, white matter changes, and cell signaling perturbations. Serial assessment of cortical, thalamic, and hippocampal RNA-binding motif 3 (RBM3), cold-inducible RNA binding protein (CIRBP), and reticulon-3 (RTN3) revealed a rapid age-dependent decrease in levels in sham and injured pups. CSPs were minimally affected by HI and the age point of lowest expression (PND 18) coincided with the timing at which heat-generating mechanisms mature in mice.

CONCLUSIONS

The findings suggest the need to determine whether optimized therapeutic hypothermia (depth and duration) can prevent the age-related decline in neuroprotective CSPs like RBM3 in the brain, and improve outcomes during critical phases of secondary injury and recovery after NE.

IMPACT

The rapid decrease in endogenous neuroprotective cold-shock proteins (CSPs) in the normothermic cortex, thalamus, and hippocampus from postnatal day (PND) 11-18, coincides with the timing of thermogenesis maturation in neonatal mice. Hypoxia-ischemia (HI) has a minor impact on the normal age-dependent decline in brain CSP levels in neonates maintained normothermic post-injury. HI robustly disrupts the expected correlation in RNA-binding motif 3 (RBM3) and reticulon-3 (RTN3). The potent neuroprotectant RBM3 is not increased 1-4 days after HI in a mouse model of neonatal encephalopathy (NE) in the term newborn and in which rigorous temperature control prevents the manifestation of endogenous post-insult hypothermia.

Hyperthermia and Heat Stress as Risk Factors for Sudden Infant Death Syndrome: A Narrative Review

BACKGROUND AND OBJECTIVES

Heat stress and hyperthermia are common findings in sudden infant death syndrome (SIDS) victims. It has been suggested that thermal stress can increase the risk of SIDS directly via lethal hyperthermia or indirectly by altering autonomic functions. Major changes in sleep, thermoregulation, cardiovascular function, and the emergence of circadian functions occur at the age at which the risk of SIDS peaks-explaining the greater vulnerability at this stage of development. Here, we review the literature data on (i) heat stress and hyperthermia as direct risk factors for SIDS, and (ii) the indirect effects of thermal loads on vital physiological functions.

RESULTS

Various situations leading to thermal stress (i.e., outdoors temperatures, thermal insulation from clothing and bedding, the prone position, bed-sharing, and head covering) have been analyzed. Hyperthermia mainly results from excessive clothing and bedding insulation with regard to the ambient thermal conditions. The appropriate amount of clothing and bedding thermal insulation for homeothermia requires further research. The prone position and bed-sharing do not have major thermal impacts; the elevated risk of SIDS in these situations cannot be explained solely by thermal factors. Special attention should be given to brain overheating because of the head's major role in body heat losses, heat production, and autonomic functions. Thermal stress can alter cardiovascular and respiratory functions, which in turn can lead to life-threatening events (e.g., bradycardia, apnea with blood desaturation, and glottal closure). Unfortunately, thermal load impairs the responses to these challenges by reducing chemosensitivity, arousability, and autoresuscitation. As a result, thermal load (even when not lethal directly) can interact detrimentally with vital physiological functions.

CONCLUSIONS

With the exception of excessive thermal insulation (which can lead to lethal hyperthermia), the major risk factors for SIDS appears to be associated with impairments of vital physiological functions when the infant is exposed to thermal stress.

Transition to a Safe Home Sleep Environment for the NICU Patient

Of the nearly 3.8 million infants born in the United States in 2018, 8.3% had low birth weight (<2500 g [5.5 lb]) and 10% were born preterm (gestational age of <37 completed weeks). Many of these infants and others with congenital anomalies, perinatally acquired infections, and other disease require admission to a NICU. In the past decade, admission rates to NICUs have been increasing; it is estimated that between 10% and 15% of infants will spend time in a NICU, representing approximately 500 000 neonates annually. Approximately 3600 infants die annually in the United States from sleep-related deaths, including sudden infant death syndrome International Classification of Diseases, 10th Revision (R95), ill-defined deaths (R99), and accidental suffocation and strangulation in bed (W75). Preterm and low birth weight infants are particularly vulnerable, with an incidence of death 2 to 3 times greater than healthy term infants. Thus, it is important for health care professionals to prepare families to maintain their infant in a safe sleep environment, as per the recommendations of the American Academy of Pediatrics. However, infants in the NICU setting commonly require care that is inconsistent with infant sleep safety recommendations. The conflicting needs of the NICU infant with the necessity to provide a safe sleep environment before hospital discharge can create confusion for providers and distress for families. This technical report is intended to assist in the establishment of appropriate NICU protocols to achieve a consistent approach to transitioning NICU infants to a safe sleep environment as soon as medically possible, well before hospital discharge.

The newborn infant's thermal environment in the delivery room when skin-to-skin care has to be interrupted

OBJECTIVE

Newborns are prone to hypothermia immediately following birth. Hypothermia is associated with increased morbidity and mortality rates. We sought to assess the thermal environment and metabolic costs associated with exposure to various situations in the delivery room when skin-to-skin care (SSC) has to be curtailed.

METHODS

Environmental variables (air temperature: T _a; relative humidity: RH; radiative temperature: T _r; and air convection velocity) were recorded during sequences reproducing SSC, in the maternity unit's various rooms ("passive environments") and in incubators ("active environments"). Analytical calorimetry was then used to calculate the body heat loss (BHL) from these data.

RESULTS

The analysis of 1280 measurements of T _a, RH, T _r, and air convection velocity in SSC, passive and active environments revealed that (i) the thermohygrometric environment during SSC was optimal (T _a: 32.7 ± 3.2 °C; RH: 50.9 ± 5.6%), (ii) BHL rose when SSC had to be interrupted, and (iii) the use of a radiant incubator prevented hypothermia and reduced dry BHL but not humid BHL (9.4 ± 1.5 kcal/kg/h; p < .001), relative to SSC (5.8 ± 2.0 kcal/kg/h; p < .001).

CONCLUSION

The newborn infant's thermohygrometric environment is optimal during SSC in the delivery room. When SSC was interrupted, T_a and RH always decreased, and BHL increased in all passive environments.

Why wrapping premature neonates to prevent hypothermia can predispose to overheating

Wrapping low-birth-weight neonates in a plastic bag prevents body heat loss. A bonnet can also be used, since large amounts of heat can be lost from the head region, but may provide too much thermal insulation, thus increasing the risk of overheating. We assessed the time required to reach warning body temperature (t38 degrees C), heat stroke (t40 degrees C), or extreme value (t43 degrees C) in a mathematical model that involved calculating various local body heat losses. Simulated heat exchanges were based on body surface temperature distribution measured in preterm neonates exposed to 33 degrees C air temperature (relative air humidity: 35%; air velocity: <0.1 m/s) and covered (torso and limbs) or not with a transparent plastic bag. We also compared metabolic heat production with body heat losses when a bonnet (2 or 3.5 mm thick) covered 10%, 40%, or 100% of the head. Wrapping neonates in a bag (combined or not with a bonnet) does not induce a critical situation as long as metabolic heat production does not increase. When endogenous heat production rises, t38 degrees C ranged between 75 and 287, t40 degrees C between 185 and 549, and t43 degrees C between 287 and 702 min. When this increase was accompanied by a fall in skin temperature, overheating risk was accentuated (37<or=t38 degrees C<or=45; 99<or=t40 degrees C<or=117; 169<or=t43 degrees C<or=194 min). Thus plastic bag and bonnet may result in hyperthermia but only when metabolic heat production rises while skin temperature falls (impeding body heat losses), as can sometimes happen with fever.

A simulation study of gel pillow heat conductance

PURPOSE

The gel-filled pillow is a device used to provide a soft surface to support and cradle an infant's head. Little is known about the thermal conductive properties of this device when used in an open crib. This simulation study evaluated the use of the Squishon 2 gel pillow in an open crib to determine the potential cooling effects on a mannequin infant.

SUBJECTS

This simulation study was conducted on a thermal mannequin.

DESIGN

A descriptive comparative repeated-measures design was employed.

METHODS

A thermal mannequin with the head placed on the gel pillow was used. The energy required to keep the mannequin head at 37 degrees C in 4 conditions was measured. The 4 conditions were as follows: (1) lying in an open crib on a standard mattress (baseline), (2) lying on the gel pillow with the disposable cover from the manufacturer, (3) the head wearing a cap and lying on the gel pillow, and (4) the head without the hat lying on the gel pillow with an insulated cover over the pillow.

MAIN OUTCOME MEASURES

Univariate analysis of variance (ANOVA) revealed significant differences in energy required to maintain the mannequin head at 37 degrees C among the 4 conditions (F3 = 283.23, P = .0001). The hat on head condition was found to decrease energy utilization by an average of 6.36 kcal/d when compared with the head on mattress condition (P = .0001). Extrapolation of energy to maintain mannequin head warmth into potential kilocalories utilized revealed that a potential increase in kilocalories needed to maintain thermoneutrality would be needed.

RESULTS

The most effective way of conserving heat was in the hat on the mannequin head while lying on a gel pillow condition. The use of a gel pillow without a hat or an insulated barrier caused an increase in energy requirements and kilocalorie usage in this mannequin model.

CONCLUSION

The results of this simulation study suggest that use of the gel pillow outside of a thermally controlled environment and in an open crib environment may increase energy use to maintain thermoneutrality. The Squishon 2 gel pillow conducts heat from the mannequin head and may increase kilocalories per day consumption in the preterm infant. Furthermore, the results of the study support previous findings that a hat helps conserve energy.

[Heat exchanges and thermoregulation in the neonate]

The newborn's energy expenditure is used in order of priority for: (i) basic metabolism; (ii) body temperature regulation and (iii) body growth. Thermal regulation is an important part of energy expenditure, especially for low birth-weight infants or preterm newborns. The heat exchanges with the environment are greater in the infant than in the adult, explaining the increased risk of body hypo- or hyperthermia. The newborn infant is a homeotherm, but over a long period of time, he cannot maintain the thermal processes. Further developments are expected to improve the infant's thermal environment, with assessment of the various heat exchange mechanisms by conduction, convection, radiation and evaporation. The quantification of the respective parts of these exchanges would improve nursing care through clinical procedures or equipment used to ensure the control of the optimal thermohygrometric conditions in incubators, especially when the likelihood of excessive body cooling is high. The present review focuses on the various body heat exchange mechanisms, the thermoregulation processes of the newborn, and their implications in clinical usage and limitations in the neonatal intensive care unit.

Influence of thermal drive on central sleep apnea in the preterm neonate

BACKGROUND

The incidence of apnea in neonates depends on a number of factors, including sleep state and thermoregulation.

OBJECTIVE

To assess the role of thermal drive (body heat loss [BHL]) in the mechanisms underlying short episodes of central apnea during active and quiet sleep in neonates.

MATERIAL AND METHOD

Twenty-two neonates (postconceptional age: 36.3 +/- 0.9 weeks) were exposed at thermoneutral (incubator temperature: 32.5 degrees C), warm (34.2 degrees C), and cool (30.4 degrees C) conditions during 3 consecutive morning naps. Oxygen consumption (VO2), skin and rectal temperatures, and central apnea were scored during active sleep and quiet sleep. The thermal drive was expressed as BHL calculated using indirect partitional calorimetry.

RESULTS

As expected, apnea occurred more frequently in active sleep than in quiet sleep (P < 0.001). The frequency of apnea in active sleep was higher in the warm condition (P < 0.05). In contrast, apnea episodes were less frequent (P < 0.05) and shorter (P < 0.05) for cool exposure, during which VO2 and rectal temperature increased. The frequency (P < 0.001, r2 = 0.31), mean (P < 0.05, r2 = 0.06), and maximum (P < 0.001, r2 = 0.19) durations of apnea were correlated with the BHL: the greater the BHL (body cooling), the less frequent and the shorter the apnea episodes. In contrast, no relationship between apnea and mean skin or rectal temperature was observed.

CONCLUSION

Apneic events were more closely related to BHL than to body temperatures. In cool exposure, the decreases in the duration and frequency of apneic episodes suggest that these events depend on the metabolic drive (which is proportional to energy expenditure).

Proposed Guidelines for Skin-to-Skin Treatment of Neonatal Hypothermia

Hypothermia is not uncommon in full-term, low-risk newborns during the first days of life. Standard care for treating hypothermia often involves the separation of the mother and the newborn while the infant is placed under a warmer and observed in the nursery. Because one important role of the postpartum nurse is to promote mother-infant attachment by encouraging the mother to spend time "getting to know" her infant, this separation can be problematic. This article proposes that skin-to-skin (STS) care, also called kangaroo care, an intervention in which the unclothed, diapered infant is placed on the mother's bare chest, be used to promote thermoregulation instead of using separation and a warmer. STS care has been recognized as a way to facilitate maternal-infant attachment and promote thermoregulation. The literature review here provides an analysis of the available evidence and the author's conclusion that the research supports the use of STS care as an alternative to traditional rewarming. The evidence was graded and organized into an evidence-based practice guideline for the use of STS care in the treatment of mild hypothermia in the low-risk infant. Suggestions for further research and outcomes measurement are included.

Assessment of whole body and regional evaporative heat loss coefficients in very premature infants using a thermal mannequin: Influence of air velocity

In human adults, experimental assessment of the evaporative heat loss coefficient (h(e)) requires a fully wetted skin surface area implying exposure to severe heat stress. For ethical reasons, this type of experimental situation is impossible to perform on neonates. The aim of the present study was to assess h(e) values in clinical situations for the body as a whole and for the different body segments, in particular, in natural and forced convection and using an anthropomorphic, sweating, thermal mannequin to represent a very small premature neonate (body mass 900 g). Skin hydration (i.e., simulated sweating) was performed by two electronic pumping systems, providing a steady adjustable flow of water to the mannequin surface. Experiments were carried out in a closed-incubator heated to air temperatures of 33 degrees C and 36 degrees C, with air velocities (Va) ranging from 0.01 to 0.7 m s(-1), and with four levels of air relative humidity (40, 50, 60, and 80%). For the body as a whole, h(e)=7 W m(-2) mb(-1) in natural convection, whereas in forced convection h(e) was 11.7, 12.4, and 14.1 W m(-2) mb(-1) for air velocities of 0.2, 0.4, and 0.7 m s(-1), respectively. As far as local h(e) is concerned, our results showed that the relative values of regional water loss in forced convection differ greatly from those observed under still air conditions. Thus, increasing air velocity enhances the heterogeneity in regional skin cooling, which may contribute to the neonate's thermal discomfort.

The effect of wool vs. cotton head covering and length of stay with the mother following delivery on infant temperature

Rectal temperature of 126 term infants was measured on admission to the nursery following variable periods of stay with the mother in the delivery room. Fifty-nine infants who wore woolen hats after delivery had higher rectal temperature than 67 infants whose head was covered by loosely applied cotton diapers (36.5+/-0.5 degrees C vs. 36.3+/-0.5 degrees C, respectively; p=0.03). Among them there were fewer infants who were admitted with rectal temperature 36 degrees C (12 vs. 26, respectively; p=0.03). In multiple regression analyses accounting for head covering with woolen hats, birth weight, gender, delivery room temperature and length of stay with the mother, only birth weight and head covering with woolen hats were significantly associated with rectal temperature at arrival in the nursery (p=0.002 and 0.03, respectively), and only head covering with cotton diapers was significantly associated with rectal temperature 36 degrees C (p=0.03). Our data imply that covering heads of term newborns with simple woolen hats may reduce or prevent heat loss following delivery, and that adequate warming of infants is achieved during prolonged stay with the mother.