Assessment of dry heat exchanges in newborns: influence of body position and clothing in SIDS

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.

A Neonatal Phantom for Vital Signs Simulation

Neonatal intensive care units provide vital medical support for premature infants. The key aspect in neonatal care is the continuous monitoring of vital signs measured using adhesive skin sensors. Since sensors can cause irritation of the skin and lead to infections, research focuses on contact-free, camera-based methods such as infrared thermography and photoplethysmography imaging. The development of image processing algorithms requires large datasets, but recording the necessary data from studies brings tremendous effort and costs. Therefore, realistic patient phantoms would be feasible to create a comprehensive dataset and validate image-based algorithms. This work describes the realization of a neonatal phantom which can simulate physiological vital parameters such as pulse rate and thermoregulation. It mimics the outer appearance of premature infants using a 3D printed base structure coated with several layers of modified, skin-colored silicone. A distribution of red and infrared LEDs in the scaffold enables the simulation of a PPG signal by mimicking pulsative light intensity changes on the skin. Additionally, the body temperature of the phantom is individually adjustable in several regions using heating elements. In the validation process for PPG simulation, the feasibility of setting different pulse frequencies and the variation of oxygen saturation levels was obtained. Furthermore, heating tests showed region-dependent temperature variations between 0.19 ^°C and 0.81 ^°C around the setpoint. In conclusion, the proposed neonatal phantom can be used to simulate a variety of vital parameters of preterm infants and, therefore, enables the implementation of image processing algorithms for the analysis of the medical state.

Can Mathematical Models of Body Heat Exchanges Accurately Predict Thermal Stress in Premature Neonates?

Mathematical models of body heat exchanges can be used to define the thermal limits needed to protect premature neonates nursed in incubators against thermal stress–stress that can have potentially devastating impairments on neurological development and body growth. Predictive models can help caregivers to keep a neonate’s body temperature within the normal range and to solve problems that arise during intensive care, such as the risk of hyperthermia during phototherapy, the risk of hypothermia during transport from one clinical centre to another, and the use of a plastic bag to reduce skin water loss and body dehydration. Here, we review the strengths and limitations of models used to predict the risk of thermal stress, with a focus on uncertainties in the algorithms governing heat transfers between the neonate’s skin and the complicated thermal environment encountered in incubators. We describe attempts to reduce the large number of empirical assumptions and uncertainties in this field, and suggest ways of more accurately modelling optimal thermal conditions for neonates nursed in closed incubators.

Infant exposure to emissions of volatile organic compounds from crib mattresses

Infants spend most of their time sleeping and are likely to be exposed to elevated concentrations of chemicals released from their crib mattresses. Small-scale chamber experiments were conducted to determine the area-specific emission rates (SERs) of volatile organic compounds (VOCs) in a collection of twenty new and used crib mattresses. All mattress samples were found to emit VOCs and the mean values of total VOC (TVOC) SERs were 56 μg/m(2)h at 23 °C and 139 μg/m(2)h at 36 °C. TVOC SERs were greater for new mattresses compared to used ones and were influenced by the type of foam material and the presence of mattress cover layer. A variety of VOCs were identified, with polyurethane foam releasing a greater diversity of VOCs compared to polyester foam. Large-scale chamber experiments were conducted with an infant thermal manikin. TVOC concentrations sampled in the breathing zone and interior pore air of the crib mattress foam were found to be greater than the bulk room air by factors in the range of 1.8 to 2.4 and 7.5 to 21, respectively. The results suggest that crib mattresses are an important source of VOCs and infant exposure to VOCs are possibly elevated in their sleep microenvironments.

Infant sleep position: a randomized clinical trial of an educational intervention in the maternity ward in Porto Alegre, Brazil

BACKGROUND

Few studies in Brazil have been published about sudden infant death syndrome (SIDS), and none has addressed the mother's orientation about placing the infant to sleep in the supine position. The aim of this study was to evaluate the effect on mothers of an individual educational intervention in the maternity ward about infant sleep position.

METHODS

A randomized clinical trial was conducted with a study sample of 228 mother-infant pairs assigned to an intervention or a control group. The intervention consisted of an individual orientation session at the maternity ward, at which folders and an oral explanation were given to mothers at discharge about the importance of the supine position as a preventive measure for SIDS. The outcome was the sleeping position at 3 months of age assessed during a home visit. The variables with p< 0.2 at a bivariate analysis were included in a logistic regression model.

RESULTS

Among mothers in the intervention group, 42.9 percent put their infants to sleep in a supine position at the 3-month visit, compared with 24 percent of mothers in the control group (p = 0.009). In a multivariate analysis, the intervention at the hospital was the only variable that influenced maternal practices with respect to infant sleep positioning (OR 2.22; 95% CI 1.17-4.19).

CONCLUSIONS

An individual educational session in the maternity ward about infant sleep position significantly increased the prevalence of supine position for sleeping in the infant's third month. Nevertheless, the intervention was not sufficient to guarantee that most mothers would put their infants to sleep in the recommended position.

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.

Dry heat loss in incubator: comparison of two premature newborn sized manikins

Keeping premature newborns warm is crucial for their survival. Their ability to prevent excessive heat loss to the environment and to control their body temperature is limited. The risk of hypothermia is particularly important for low-birth-weight newborns with a large body surface area in relation to their mass of heat-producing tissues. The present study was performed to assess the body heat loss difference between small and large body-size premature newborns using two anthropomorphic thermal manikins of premature newborns of 900 g and 1,800 g (respective body surface areas of 0.086 and 0.150 m2). The dry heat loss from the six body segments of the small manikin (S) was measured and compared with that of the large manikin (L). The two manikins were exposed to five different environmental temperatures ranging between 29 and 35 degrees C in a single-walled, air-heated closed incubator. The magnitudes of heat loss decreased significantly by 20.4% between the two manikins [small manikin 110.1 (44.3) W/m2 vs large manikin 87.6 (25.8) W/m2, mean values with one standard deviation]. The results obtained from the comparison of the heat loss measures from the two manikins confirm the fact that the heat loss increases with an increase in the ratio of the body surface area to body mass. The thermal manikin appears to provide an accurate method for the assessment of thermal conditions in neonatal care.

Assessment of the efficiency of warming devices during neonatal surgery

This study assessed the relative efficiency of different warming devices (surgical sheets covering the body and a tubegauze on the head, forced-air warming, warming mattress) commonly used to prevent body hypothermia during neonatal surgery. Dry heat losses were measured from a thermal manikin, which simulated a low-birth-weight neonate of 1,800 g. The manikin's surface temperatures (35.8 degrees C) corresponded to those of neonates nursed in closed incubators. Experiments were performed in a climatic chamber at an ambient temperature of 30 degrees C, as commonly found in operating theatres. The supine manikin was naked or covered with operative sheets with a 5x5 cm aperture over the abdomen. Its head could be covered by a tube-gauze. Additional warming was provided by conduction through a warming mattress (surface temperature, 39 degrees C) and/or by convection (Bair Hugger, forced-air temperature 38 degrees C). Covering the manikin with surgical sheets decreased the dry heat loss by 10.4 W. Additional forced-air warming was more efficient than the warming mattress to reduce the total dry heat loss (6.8 W vs 2.1 W). Heat losses were reduced by 7.9 W when combining the warming mattress and Bair Hugger. The heat loss from the head of the covered manikin was reduced from 4.5 W to 3.9 W when the head was covered with the tubegauze. Our data indicate that forced-air warming is more effective than conductive warming in preventing neonatal hypothermia during abdominal operations.

Sudden infant death syndrome: a cybernetic etiology

The brain's processes, by hypothesis, involve information processing by an extraordinarily complex, highly sophisticated, self-organizing cybernetic system embedded in the central nervous system. This cybernetic system generates itself in successive stages. Breathing is, by default, an autonomous function, but breath control is learned. If there is not a smooth transfer of function at the time when a successor system (one that enables autonomous breathing to be overridden by voluntary control) takes over, breathing may cease, without any overt cause being detectable, even with a thorough postmortem examination. If conditions are such that, at that point, the infant's body lacks the strength to resume breathing again under autonomic control, Sudden Infant Death Syndrome may result. The theory explains why infants are at greater risk if they sleep face down.

Influence of head position on thermal stress in newborns: simulation using a thermal mannequin

The influence of head position on thermal stress was assessed by using a heavily clothed thermal mannequin in three different body positions [supine, face straight up (FSU); supine, face to the side (FTS); prone, FTS] and with or without the head covered by a bonnet. The mannequin was exposed to air temperatures of 29, 32, 34, and 36 degrees C. When the head is uncovered, body or head position has no impact on heat loss. When the head is covered, dry heat loss from the mannequin as a whole (and that from the head in particular) is lower (-0.35 to -0.40 W) in the FTS position than in the FSU position as a result of decreased heat loss from the surface area of the face in contact with the mattress. In the FTS position and with the head covered, there was no difference in heat loss between the prone and supine positions. The results suggest that in heavily clothed newborns whose head is covered by a bonnet, thermal stress depends on the head position.

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

In newborns, large amounts of heat are lost from the head, due to its high skin surface area. Insulating the head (for example, with a hat or bonnet) can be a simple and effective method of reducing dry heat loss. In the present study, we evaluated the safety aspects of insulating the head of low-birth-weight naked or clothed newborns by using a heated mannequin that simulates a low-birth-weight newborn. Experimental conditions (comprising a nude and three clothed setups) were performed in a closed incubator at three different air temperatures (29 degrees C, 32 degrees C, and 34 degrees C) and with and without the head being covered with a bonnet in each case, i.e., 24 experimental conditions in all. The study shows that added clothing elements and insulating the head decreases the total heat loss of the mannequin as a whole. As regards the dry heat exchange from the head, wearing a bonnet decreases the local heat loss by an average of 18.9% in all clothed and thermal conditions. This phenomenon could be at the origin of brain overheating in heavily dressed newborns, when unrestricted heat loss is limited to the face only. Our results suggest that--apart from accidental hypothermia-in order to achieve thermal equilibrium of the body, it is preferable to leave the head unprotected and to increase the level of clothing insulation over the rest of the body.