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.
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.
To assess the various heat exchanges with the environment a multisegment, anthropometric, thermal mannequin representing a neonate with a birth weight of 900 g has been designed. The mannequin simulates not only dry heat loss (radiative+conductive+convective body heat exchanges) but also the evaporative skin water loss which can be encountered in low-birth-weight neonates. The model was placed in the supine or prone position in a closed incubator (air temperature, 33 C; relative air humidity, 50%; air velocity below 0.1 m s(-1)). Experiments were performed with the mannequin either naked or wrapped in a flexible, plastic bag (with the head exposed) used to prevent excessive body water loss at delivery and during the following hours About 30% of the model's total surface was wetted with water. Our results demonstrated that body position does not modify dry and evaporative heat losses, whatever the experimental conditions. The plastic bag acts rapidly and reduces total heat loss by 30% to 34%, primarily through a reduction in evaporative water loss (between 5.4 and 6.7 g kg(-1) h(-1)). When the bag is present, the uncovered surface of the head accounts for about 50% of the total heat loss. This simple and inexpensive solution can be used to prevent thermal stress and dehydration in very small premature neonates.
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