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

Abstract

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.

Effect of posture on the thermal efficiency of a plastic bag wrapping in neonate: Assessment using a thermal “sweating” mannequin

“…Cotton was chosen because water spreads well in this material and no dripping water can be observed unless the fabric is wetted thoroughly. This method has been previously validated 21 in a study which pointed out that there was no over-or underestimation of the evaporative cooling effect in relation to the mass of water supplied to the mannequin surface.…”

Failing to meet relative humidity targets for incubated neonates causes higher heat loss and metabolic costs in the first week of life

“…Evaporative heat loss was related to several factors: the air velocity inside the incubator and primarily the water partial pressure difference between the preterm infant, in relation to transepidermal water loss , and the incubator, related to the air temperature and the RH inside the incubator . Thus, the prolonged hygrometric stress that we observed in the incubator was probably associated with high transepidermal water loss from the preterm infant over the first few days after birth and high evaporative heat losses . The difference between measured RH and target RH fell over time.…”

Conception d’un logiciel de calcul de la thermoneutralité dans les incubateurs fermés pour nouveau-nés prématurés (projet Pretherm®)

“…En effet, la difficulté majeure de cette conception réside dans le fait que le calcul des différents échanges de chaleur intervenant dans le bilan thermique de l'enfant prématuré reposait jusqu'à présent sur des incertitudes notamment en ce qui concernaient les coefficients d'échanges de chaleur et la température moyenne de rayonnement généralement calculés sur l'adulte. La conception dans notre laboratoire de mannequins calorimétriques ayant les formes anatomiques de nouveau-nés prématurés [15,17,18] a permis de résoudre ces problèmes.…”

“…A c = surface cutanée qui échange par convection, h c = coefficient de transfert de chaleur par convection (W/m 2 / • C) déterminé à partir des données de Belghazi et al [15], T inc = température de l'air circulant dans l'incubateur.…”

“…Les pertes évaporatoires par les voies respiratoire (E resp ) et transcutanée (E) sont : E = h e ωA c (P sH20 − P aH20 ) F pcl (7) h e = coefficient de transfert de chaleur par évaporation (W/mb/m 2 ) calculé par la relation de Lewis (h e = 1,67 h c ), = mouillure de la peau (sans dimension) égale à 0,35 (dans les premiers jours de vie afin de tenir compte de la grande perméabilité de la peau de l'enfant) puis 0,08 car l'activité sudorale est nulle [16], P sH20 − P aH20 = différence de pression partielle entre la peau (pression saturante pour la température cutanée moyenne) et l'air de l'incubateur (mb), F pcl = coefficient de réduction des échanges de chaleur latente due à la vêture (sans dimension). Le facteur F pcl agissant sur l'évaporation est déter-miné avec le mannequin dont la surface est totalement mouillée ( = 1) à partir du rapport entre l'évaporation mesurée sur le modèle vêtu et le modèle nu [15,17].…”