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Objectives: To test the hypothesis that the level of relative humidity (RH) in which preterm infants are nursed might influence their postnatal skin maturation., Study Design: In 22 preterm infants (GA 23-27 weeks), transepidermal water loss (TEWL) was determined at postnatal ages (PNA) of 0, 3, 7, 14, and 28 days. At a PNA of 7 days, the infants were randomized to care at either 50% or 75% RH., Results: TEWL decreased at a slower rate in infants nursed at the higher RH. At a PNA of 28 days, TEWL was about twice as high in infants nursed at 75% RH (22 +/- 2 g/m2 h) than in those nursed at 50% RH (13 +/- 1 g/m2 h; P < .001)., Conclusions: The results indicate that the level of RH influences skin barrier development, with more rapid barrier formation in infants nursed at a lower RH. The findings have an impact on strategies for promoting skin barrier integrity in extremely preterm infants.

Maintaining fluid and heat balance is of vital importance to the newborn infant. At birth, the infant is exposed to a cold and dry environment, and preterm neonates in particular, are then at risk of dehydration and hypothermia. These conditions may have serious consequences and significantly influence mortality and morbidity. Preterm neonates have a high rate of water and heat loss mainly as a consequence of their immature skin. The care environment influences the magnitude of water and heat exchange and needs to be individually tailored on the basis of the infant's clinical status, maturity at birth and postnatal age. This paper reviews data obtained from series of studies on neonatal water and heat exchange using non-invasive measurements of insensible water loss and calculations of different modes of heat exchange. These studies have influenced the way in which newborn infants are being nursed today.

The rate of evaporation of water from the skin of 13 infants born at 24 (n=3) and 25 (n=10) weeks of gestation was measured on the first day after birth and at postnatal ages of 1, 3, 7 and 28 d, using the gradient method. Transepidermal water loss was estimated from this rate and corrected to an ambient relative humidity (RH) of 50%. Transepidermal water loss, corrected to 50% RH, was high on the first day after birth (58.4+/-14.8 g m(-2) h(-1)) and remained at the same level during the second day (59.3+/-17.6 g m(-2)h(-1)). It then decreased significantly to 43.8+/-9.5 at a postnatal age of 3 d, 36.1+/-12.6 at 7 d and 24.2+/-7.7 g m(-2) h(-1) at 28 d (p < 0.001). Within the group investigated, there was no significant correlation between transepidermal water loss and body or skin temperature, birth weight, gender, mode of delivery or gestational age. Transepidermal water loss on the first day after birth was somewhat lower than the highest losses previously found in infants born at 25 weeks of gestation, and of the same magnitude as previously reported for infants born at 25-27 weeks. Transepidermal water loss at postnatal ages of 1, 3, 7 and 28 d in the present study was higher than that previously found in the group of infants born at 25-27 weeks. In conclusion, in infants born at 24-25 completed weeks of gestation transepidermal water loss was high immediately after birth and decreased with increasing postnatal age, but at a slower rate than previously reported for slightly more mature infants.

To study the effect of intubation on respiratory water loss (RWL) during heat stress, 10 young nonsedated lambs were exposed to radiative heat stress both when intubated and when not. RWL, oxygen consumption (VO2), and carbon dioxide production were monitored continuously by using a flow-through system with a mass spectrometer for gas analysis. When the lambs were not intubated, heat stress caused RWL to increase by 218%, whereas VO2 and body temperature remained unchanged. When the lambs were intubated, heat stress caused RWL to increase by 131% and VO2 to increase by 36%. On extubation during heat stress, RWL increased by 117 +/- 48% (standard error of the estimate) of the preextubation value and body temperature started to fall. This study shows that intubation reduces the ability of the lamb to increase RWL and heat loss during heat stress in a warm environment, possibly as an effect of exclusion of the nose and a reduction in dead space. After extubation, RWL increases markedly, a finding that might also be valid for intubated infants.

Respiratory water loss, oxygen consumption and carbon dioxide production were measured in 32 infants on their first day after birth. Gestational age was between 27 and 41 weeks. All infants were studied in incubators with 50% ambient relative humidity and an ambient temperature that allowed the infant to maintain a normal and stable body temperature. During the measurements the infants were usually asleep. Respiratory water loss was found to be highest in the most preterm infants and lower in more mature infants. Respiratory water loss per breath (mg/kg) was almost the same at all gestational ages and the higher respiratory water loss found in the most preterm as compared with the more mature infants is thus and increased with increasing gestational age. Thus, in full-term infants respiratory water loss and transepidermal water loss are of approximately equal magnitude at an ambient humidity of 50%, while respiratory water loss constitutes a smaller proportion than transepidermal water loss in very preterm infants. Respiratory water loss increases with the rate of breathing.

Respiratory water loss, oxygen consumption, carbon dioxide production and skin blood flow were measured continuously in nine full-term infants on the first day after birth. After at least 18 min of measurements with the infant asleep in an incubator, with an air temperature of 33 degrees C and a relative humidity of 50%, the temperature of the incubator air was lowered to less than 27.5 degrees C. This resulted in a significant decrease in skin temperature and peripheral skin blood flow, while the infant's core temperature remained unchanged. At the same time, mean respiratory water loss increased from 3.7 to 6.1 mg/kg.min, which can be explained partially by the decrease in ambient humidity that accompanied the decrease in air temperature. In addition, mean oxygen consumption increased from 5.3 to 7.9 ml/kg.min and mean carbon dioxide production increased from 3.8 to 5.9 ml/kg.min. There was no concomitant increase in motor activity. Thus, when the newborn infants were exposed to cool air, they reacted with an increase in respiratory water loss, oxygen consumption and carbon dioxide production before their core temperature was affected and without increasing their motor activity.

The temperature of inspired air influences respiratory water loss (RWL) in young lambs. Water loss from the airways, oxygen consumption and carbon dioxide production were measured using an open flow-through system with a mass spectrometer, specially equipped with a water channel, for gas analysis. Measurements were made in 9 newborn lambs at 3 different inspired air temperatures keeping all other environmental factors stable, including the ambient air temperature. The water content of the inspired air was also kept constant. RWL was found to be 9.9 +/- 3.9 (SD) mg/kg/min when the temperature of the inspired air was 30 degrees C and its humidity 30%. At 40 degrees C this loss increased to 11.5 +/- 3.6 mg/kg/min, and at about 60 degrees C it increased further to 26.0 +/- 8.2 mg/kg/min. The oxygen consumption was 10.0 +/- 0.8 (SD) ml/kg/min at 30 degrees C and 10.4 +/- 2.0 ml/kg/min at 60 degrees C, a change which is not significant. Thus RWL is influenced by the temperature of the inspired air, with greater loss at higher temperatures.

Respiratory water loss was measured together with oxygen consumption (VO2) and carbon dioxide production (VCO2) in 11 full-term and eight preterm infants (mean gestational age 34 weeks, range 31-36 weeks) before and during 1 h of phototherapy. The method for determination of respiratory water loss, VO2 and VCO2 was based on an open flow-through system with a mass spectrometer for measurement of gas concentrations. All infants were studied naked in an incubator with an ambient relative humidity of 50% and with a controlled environment with respect to temperature and air velocity. The infants were calm during the measurements. Before phototherapy, in term infants respiratory water loss was 4.4 (SD 0.7) mg/kg min and VO2 5.9 (0.9) ml/kg min and in preterm infants respiratory water loss was 4.7 (0.8) mg/kg min and VO2 6.1 (0.8) ml/kg min. No significant difference was found between values obtained during or after 1 h of phototherapy and those obtained before.

The rate of evaporation from the skin was measured before and during phototherapy in 10 full-term and seven preterm infants (gestational age 29-33 weeks). The method for measurement of rate of evaporation was non-invasive and was based on determination of the water vapour pressure gradient close to the skin surface. All infants were studied naked in an incubator with an ambient relative humidity of 50% and with a controlled environment with respect to temperature and air velocity. In the term infants the mean rate of evaporation, measured from an interscapular skin area, was 3.1 g/m2h both before and after 30 min of phototherapy. In the preterm infants the corresponding value was 9.8 g/m2h before and 9.7 g/m2h after 120 min of phototherapy. Thus, in thermally stable infants, non-ionizing radiation from phototherapy equipment does not increase water loss from the skin.

A study was performed to investigate whether measurements of the evaporation rate from the skin of newborn infants by the gradient method are affected by the presence of non-ionizing radiation from phototherapy equipment or a radiant heater. The evaporation rate was measured experimentally with the measuring sensors either exposed to or protected from non-ionizing radiation. Either blue light (phototherapy) or infrared light (radiant heater) was used; in the former case the evaporation rate was measured from a beaker of water covered with a semipermeable membrane, and in the latter case from the hand of an adult subject, aluminium foil or with the measuring probe in the air. No adverse effect on the determinations of the evaporation rate was found in the presence of blue light. Infrared radiation caused an error of 0.8 g/m2h when the radiant heater was set at its highest effect level or when the ambient humidity was high. At low and moderate levels the observed evaporation rate was not affected. It is concluded that when clinical measurements are made from the skin of newborn infants nursed under a radiant heater, the evaporation rate can appropriately be determined by the gradient method.

Continuous measurements of respiratory water loss (RWL), oxygen consumption and carbon dioxide production were made in 21 fullterm infants on their first day after birth. The infants were first studied in incubators with a temperature of 32.5 degrees C and an ambient humidity of 50%. After an interval with stable conditions the incubator temperature was raised to 36.5 degrees C while the water vapour pressure was kept constant. When the rectal temperature had increased to 37.8 degrees C or when the infant had started to sweat, the relative humidity in the incubator was increased to 50%. At the start of the measurements mean RWL was 4.9 mg/kg min. On the average, RWL increased to a maximal value of 7.0 mg/kg min in the warm environment. Mean oxygen consumption only increased from 5.3 to 5.8 ml/kg min. This meant that when nursed in this warm environment the infants were able to increase their respiratory water loss by nearly 50% without a significant change in oxygen consumption.

A study was made to determine whether the high transepidermal water loss in preterm newborn infants is affected by infusion of human albumin. In 7 preterm infants, born after 25-30 weeks of gestation, 0.5 g of human albumin per kilogram body weight (b.w.) was infused through an umbilical artery catheter in 6 h on the 1st day after birth. In 2 other infants, born after 28 completed weeks of gestation, human albumin in an amount of 1.0 g/kg body weight was infused. The plasma albumin concentration increased significantly from a pre-infusion value of 27.1 to 29.5 g/l after 4 h of infusion. Simultaneously the transepidermal water loss decreased from 26.6 to 23.5 g/(m2 X h); this difference was also significant. Within 2 h after the end of the albumin infusion, both the plasma albumin concentration and the transepidermal water loss had returned to levels that were not significantly different from the pre-infusion values. Plasma osmolality and sodium concentration did not alter during the infusion, nor did arterial blood pressure.

In order to investigate whether a change in skin blood flow is associated with changes in the evaporative loss of water from the skin of fullterm appropriate for gestational age infants, we measured the evaporation rate and skin blood flow in the body temperature range 36.6 to 37.2 degrees C. Fifteen healthy, fullterm newborn infants delivered by Caesarean section were studied in a warm environment. Evaporation rate and skin blood flow were measured non-invasively without disturbing the infant. When the body temperature slowly rose because of the warm environment, the skin blood flow steadily increased. The rate of evaporation of water from the skin slowly decreased until a body temperature of 37.1 degrees C was reached. The evaporation rate then suddenly increased as the infant began to sweat. Thus, in fullterm newborn infants born by Caesarean section the skin blood flow increases in the body temperature range 36.6-37.1 degrees C but does not influence the evaporation rate from the skin.

Respiratory water loss (RWL), oxygen consumption and carbon dioxide production were measured in ten fullterm infants on their first day after birth, using an open flow-through system with a mass spectrometer, specially equipped with a water channel, for analysis of gas concentrations. Measurements were made both with the infant asleep and during different levels of motor activity. The ambient temperature was maintained at approximately 32.5 degrees C and the ambient relative humidity at 50%. RWL increased from 4.2 +/- 0.7 (SD) mg/kg min when the infant was asleep to 6.3 +/- 1.0 mg/kg min when he or she was awake but calm; with increasing activity there was a further increase in RWL. The oxygen consumption increased from 5.4 +/- 0.9 (SD) ml/kg min during sleep to 6.9 +/- 0.8 (SD) ml/kg min when awake, and also increased further with increasing activity.

Measurement of transepidermal water loss without disturbing the microclimate in the air layers above the skin has not been possible with previously available methods. The introduction of a new instrument, the Evaporimeter, for measuring the vapor pressure gradient above the skin surface meant that the evaporation of water from the skin could be determined. Each measurement is of short duration, and measurements can be made even in the most preterm infants under the conditions in which they are normally nursed. Studies of the evaporation rate and transepidermal water loss by this technique have increased our knowledge considerably during the last ten years. It is now well established that the loss of water from the skin surface is related to the ambient humidity, gestational age at birth, and postnatal age, and to weight, activity, and body temperature, and that it differs between appropriate- and small-for-gestational-age infants. Conclusions of special clinical importance drawn from these studies are that the most preterm infants may lose as much as 13 per cent of their body weight as TEWL during their first day after birth even at an ambient humidity of 50 per cent, and much more at a lower ambient humidity, and that their losses gradually diminish with age but are of significance even four weeks after birth. Small-for-gestational-age infants have a lower TEWL early after birth than appropriate-for-gestational-age infants. For heat exchange between the infant and its environment, TEWL plays an important role. In preterm infants, losses of heat through evaporation are very high early after birth. On the other hand, their heat losses through other routes are smaller because of the high ambient temperature in which they have be nursed in order to maintain a stable and normal body temperature. Simultaneous measurements of evaporation rate and skin blood flow have shown that the skin blood flow increases at a lower body temperature than the evaporation rate and that not all full-term infants sweat in a warm environment even when their body temperature becomes higher than 37.2 degrees C, which may indicate a delay in the postnatal adaptation of heat regulation. In addition, this regulation can easily be disturbed by a central cold-stimulation in infants who react with sweating in a warm environment.

The amount of water evaporated from the skin was studied in 10 healthy newborn infants from their first minute of life, while being taken care of in the delivery room, and in 11 infants treated in incubators from their 30th min of life. The heat lost by evaporation, radiation and convection was calculated. Evaporation from the skin was very high during the first minutes after birth and was the main cause of heat loss during the first 15-30 min of life. Thereafter the amount of heat lost depended on the conditions under which the infant was nursed. Higher convective and radiative heat losses were found in delivery rooms than in incubators.

A flow-through system for measurement of respiratory water loss, providing data on oxygen consumption and carbon dioxide production, is described. The system uses a mass spectrometer, specially equipped with a water channel, for analysis of gas concentrations. To avoid condensation of water vapour, the tubing in the flow-through system is heated. The method was tested by evaporating known amounts of water, and was found to be very accurate with a correlation coefficient of 0.999. Measurements were made in 10 fullterm lambs of varying postnatal age (4-15 days) and a mean respiratory water loss (RWL) of 11.7 mg kg-1 min-1 was found at rest. The RWL decreased with increasing postnatal age, as did oxygen consumption. The RWL at rest was influenced by the ambient relative humidity, with higher values at lower humidities.

Using a method described earlier, the evaporation rate (ER) from the skin was studied at different ambient humidities in 14 full-term and 10 pre-term small for gestational age (SGA) infants. Transepidermal water loss (TEWL) was estimated in 25 SGA infants born after 30-40 weeks of gestation. Comparisons were made with infants appropriate for gestational age (AGA). A linear relationship was found between ER and ambient humidity in full-term SGA infants, but with lower ER values than in AGA infants. Lower ER values were also found in moderately pre-term SGA infants at different ambient humidities. ER was higher at lower ambient humidities in both SGA and AGA infants. In full-term and moderately pre-term SGA infants TEWL was lower than in corresponding AGA infants.

In order to investigate whether sweating and cutaneous vasodilatation. induced by a warm environment, is inhibited by central cold-stimulation, the evaporation rate and skin blood flow were studied by non-invasive techniques before and after the feeding of cold glucose to 17 fullterm appropriate for gestational age infants, born by elective Caesarean section, on their first day after birth. In newborn infants who were visibly sweating and had an increased skin blood flow at a body temperature above 37.1 degrees C, such central cold-stimulation quickly inhibited sweating and decreased skin blood flow. In infants with almost the same body temperature but with no sweating, this stimulation caused no change in evaporation rate or skin blood flow. The regulation of body temperature in fullterm newborn infants can thus be disturbed by central cold-stimulation, resulting in a decreased heat loss to the environment even though the body temperature is elevated.

The amount of water evaporated from the skin of newborn infants and the temperatures of the skin, of the ambient air, and of the surfaces facing the infants were measured and used as a basis for calculation of the evaporative, radiative and convective heat exchange between the infant and the environment. The infants were of varying gestational ages, from 25 to 39 completed weeks of gestation. Evaporative heat exchange was high in the most preterm infants when nursed at a low ambient humidity, while the high ambient humidity needed to maintain these infants at a stable body temperature led to a low loss of heat through radiation and convection or even a heat gain. In the more mature infants evaporative heat exchange was lower, while radiative and convective heat exchange was higher.

The amount of water lost from the skin and the relation of the water loss to environmental factors, activity, body temperature, gestational age, nutritional status at birth and post-natal age were studied in full-term and pre-term newborn infants. The method we used was non-invasive and based on determination of the water vapour pressure gradient immediately above the skin surface. We found that the transepidermal water loss per unit time and area (TEWL; g/m2 h) could be calculated by using values from only three measurement sites. TEWL increased with activity and when body temperature was elevated above 37.1 degrees C. On the first day after birth TEWL in appropriate for gestational age (AGA) infants was exponentially related to gestational age, with much higher values in the most pre-term infants than in full-term infants. In full-term small for gestational age (SGA) infants TEWL was significantly lower than in full-term AGA infants. In pre-term and full-term AGA infants and in full-term SGA infants we found an inverse linear relationship between ambient humidity and evaporation rate (ER; g/m2 h) from the skin. The level of ER and the slope of the relationship depended on the gestational age of the infant, with higher ER in the more pre-term infants. Analysis of the relationship between ambient humidity and ER indicated that there is a gradual change in the permeability of the skin with gestational age. In pre-term infants TEWL gradually decreased during the first weeks of life. In full-term AGA infants TEWL was almost unchanged during these first weeks.

Using a method described in a previous article the transepidermal water loss (TEWL) was studied in 10 healthy newborn infants at rest and during activity. On the average TEWL was 37% higher during activity than during rest although no sweating was observed. In 9 infants placed in incubators with an ambient temperature slightly above the thermoneutral range measurements were made as the body temperature rose. TEWL was almost constant until a temperature of 37.1 degrees C was reached whereupon the water loss suddenly increased as the infant started sweating.

During the first weeks after birth the transepidermal water loss was determined repeatedly in 7 infants born after 25-27 weeks of gestation, in 13 infants born after 28-30 weeks and in 14 full-term infants. All infants were appropriate for gestational age. The very pre-term infants had high evaporative water losses from their skin on their first days after birth. A gradual decrease was then observed, but after four weeks the values were still higher than those in full-term infants. The transepidermal water loss in full-term infants was unchanged during the first two weeks of life, but increased during the subsequent two weeks. The relation between evaporation rate from an interscapular skin area and ambient humidity was studied repeatedly during the first week of life in two very pre-term infants. The magnitude of evaporation decreased with increasing post-natal age, but was still dependent on the ambient relative humidity.

Using a method described earlier, the evaporation rate (ER) was studied at different humidities in 12 newborn infants born after 25 to 30 weeks of gestation and 10 infants born after 32 to 35 weeks. Transepidermal water loss (TEWL) was estimated in 32 infants born after 25 to 39 weeks of gestation. The ER values were highest in the infants with the lowest gestational age and the susceptibility to changes in ambient humidity was also greater at lower gestational ages. An exponential relationship was found between TEWL and gestational age. TEWL being 15 times higher in infants born after 25 weeks of gestation than in full-term infants.

Measurements of water loss from the airways, oxygen consumption and carbon dioxide production were made in 12 fullterm, newborn infants on their first day after birth, using an open flow-through system. The system includes a mass spectrometer, specially equipped with a water channel, for analysis of gas concentrations. To avoid condensation of water vapour, the tubing in the flow-through system is heated. Respiratory water loss was 4.9 +/- 1.2 (SD) mg/kg min, which meant an insensible water loss from the respiratory tract of 25.4 +/- 6.9 (SD) g per infant and 24 h at rest, at an ambient temperature of 32.2 degrees C and with an ambient humidity of 50%. Oxygen consumption was 6.0 +/- 0.8 (SD) ml/kg min. An inverse relationship was found between respiratory water loss and ambient humidity, with higher losses at a low than at a high humidity. Oxygen consumption did not change very much with ambient humidity.

Insensible water loss (IWL) is an important factor in the thermoregulation and water balance of the newborn infant. A method for direct measurement of the rate of evaporation from the skin surface has been developed. The method, which is based on determination of the vapour pressure gradient close to the skin surface, allows free evaporation. From measurements performed on 19 newborns placed in incubators, a linear relation was found between the evaporation rate (ER) and the humidity of the environment at a constant ambient temperature. A 40% lower ER was recorded at a high relative humidity (60%) than at a low one (20%) in the incubator. At measurements on different sites of the body, a high ER was observed on the face and peripheral parts of the extremities, while ER at other sites was relatively low. By determining ER from different parts of the body and calculating the areas of the corresponding surfaces, the total cutaneous insensible water loss for the infant in question could be obtained. The transepidermal water loss (TEWL) for the whole body surface area was calculated to be 8.1 g/m2h. On the basis of measurements performed it was found that the total cutaneous insensible water loss can be estimated with a reasonable degree of accuracy by recording ER from only three easily accessible measurement points.

In 68 appropriate for gestational age (AGA) and 33 small for gestational age (SGA) infants, transepidermal water loss was studied during the first four weeks after birth. The method used to measure evaporation rate is based on measurement of the water vapour pressure gradient close to the skin surface. All measurements were made at an ambient humidity of 50% and with the infants calm and quiet. At all the investigated post-natal ages in both AGA and SGA infants, an exponential relationship was found between transepidermal water loss (g/m2 h) and gestational age, this loss being much higher in the pre-term infants than in those born at term. The transepidermal water loss was generally lower in SGA than in AGA infants during the first week of post-natal life, irrespective of gestational age at birth. There was a gradual decrease in transepidermal water loss with increasing post-natal age in both pre-term AGA and pre-term SGA infants. Three weeks after birth this water loss tended to be higher in pre-term SGA infants than in pre-term AGA infants of corresponding gestational age.