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The present study was performed to investigate simultaneously total brain water, T1 and T2 relaxation times, and hyaluronan (HA) in fetal and neonatal rabbits. Attempts were also made to establish the relationship of HA to total brain water and to T2-derived motionally distinct water fractions, since HA is known to bind water and to limit tissue water mobility. Experiments were carried out in fetal Pannon white rabbit pups at gestational ages of 25, 27, 29, and 31 days and at a postnatal age of 4 days. The brain tissue water content (desiccation method), T1 and T2 relaxation times (H1-NMR method), and HA concentration (radioassay HA 50) were measured, and free and bound water fractions were calculated by using multicomponent fits of the T2 relaxation curves. Compared with values in newborn pups, water and HA contents were found to be highly elevated in the preterm brain and decreased markedly during early postnatal life. The trends and time courses of T1 and T2 relaxation times proved to be similar, but the postnatal decrease in T2 was preceded by a significant decline in late gestation. Maturity-related changes occurred in the T2 relaxation derived bound water fraction which amounted to 4-19% of brain water. The bound water fraction appeared to be independent of total brain water and HA concentration, and HA is, therefore, unlikely to be the only factor controlling brain water mobility. The clear dissociation of bound water fraction from total water suggests restructuring of brain water during the perinatal period.

The present study was performed to investigate simultaneously total lung water, T(1) and T(2) relaxation times, and hyaluronan (HA) in preterm and term rabbits. Attempts were also made to establish the relationship of HA to total lung water and to T(2)-derived motionally distinct water fractions. Experiments were performed in fetal Pannon white rabbit pups at gestational ages of 25, 27, 29, and 31 d and at a postnatal age of 4 d. Lung tissue water content (desiccation method), T(1) and T(2) relaxation times (H(1)-NMR method), and HA concentration (radioassay) were measured, and free and bound water fractions were calculated by using multicomponent fits of the T(2) relaxation curves. Lung water content and T(1) and T(2) relaxation times were highest at a gestational age of 27 d and then declined steadily during the whole study period. Similar trends and time courses were seen for the fast and slow components of the T(2) relaxation curve. The T(2)-derived free water fraction remained unchanged at a gestational age of 25-29 d ( approximately 67%), but increased progressively to a value of 78.5 +/- 7.9% at 31 d (p < 0. 001) and to 83.4 +/- 9.4% at the postnatal age of 4 d (p < 0.01). Opposite changes occurred in the bound water fraction. Lung HA concentration decreased with advancing gestation from 870.8 +/- 205.2 microg/g dry weight at 25 d to 162.6 +/- 32.4 microg/g dry weight at 31 d (p < 0.001), but it was increased 2-fold postnatally. HA correlated positively with total lung water (r = 0.39; p < 0.001) but not with the bound water fraction. It is suggested that the physiologic lung dehydration is associated with macromolecule-related reorganization of lung water and that the role of HA in this process needs to be further investigated.

Rabbit pups were delivered by cesarean section 1 or 2 d before term, or vaginally around term, and then reared in room air or exposed to intermittent or continuous hyperoxia (> 85%) for up to 9 d. Pups were killed at different ages, and lung hyaluronan (HA; microgram/g of dry lung weight) and lung water content, measured as wet/dry lung weight, were determined. Compared with the day of birth, the lung HA concentration did not change significantly on succeeding days in pups kept in air delivered 2 d (-2 d) or 1 d (-1 d) before term, whereas the water content decreased significantly. Continuous exposure to hyperoxia resulted in a significantly raised lung HA concentration 6 d postterm in both -2 d and -1 d pups, and intermittent exposure to hyperoxia resulted in a significantly raised HA concentration 6 d postterm in -1 d pups, compared with the groups exposed to room air. These increases were accompanied by significantly elevated wet/dry lung weight ratios. Microscopic examination revealed significantly increased HA staining scores in alveoli, arterioles, and bronchioli in both hyperoxia-exposed groups of -2 d pups 6 d postterm, and nonsignificantly higher scores in -1 d and vaginally delivered pups of comparable age, compared with the scores at birth. The results indicate that oxygen exposure neonatally may result in an increase in lung HA accompanied by an increase in lung water content. The increase in lung HA concentration in our study may be an effect of oxygen free radicals or of oxygen-induced stimulation of inflammatory mediators.

The rate of evaporation from the skin (g/m2/h) was measured in 12 full-term and 16 preterm infants (gestational age 25-34 wk) both during incubator care and when nursed under a radiant heater. The method for evaporation rate measurement is noninvasive and based on determination of the water vapor pressure gradient close to the skin surface. Measurements were first made with the infant nursed in an incubator with a controlled environment with respect to humidity, temperature, and air velocity. The measurements in the term infants were performed at an ambient relative humidity (RH) of 50%, and in the preterm infants first at 50% and subsequently at 30-40%. Evaporation rate was then measured with the infant nursed under a radiant heater. In term infants, mean evaporation rate was 3.3 g/m2/h during incubator care (RH 50%) and 4.4 g/m2/h during care under the radiant heater. In preterm infants, the corresponding values were 15.5 g/m2/h in the incubator at RH 50%, 16.7 g/m2/h at RH 30-40%, and 17.9 g/m2/h under the radiant heater. It is concluded that the evaporative water loss from the skin depends on the ambient water vapor pressure, irrespective of whether the infant is nursed in an incubator or under a radiant heater. The higher rate of evaporation during care under a radiant heater is due to the lower ambient water vapor pressure and not to any direct effect of the nonionizing radiation on the skin.

Factors that may influence the water transport through the skin in infants on their first day of life were studied with a method based on determination of the vapour pressure gradient in the air layer close to the skin surface. The evaporation rate from the skin was found to vary with the site of measurement, ambient humidity, temperature, activity, gestational age and nutritional status at birth. Differences related to maturity were shown to decrease with postnatal age.

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.

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.

A method for measuring water exchange through human skin has been developed. It is based on estimation of the vapour pressure gradient immediately above the surface of the skin and permits the skin area investigated to be exposed to the ambient air during the entire period of measurement. Influence on the microclimate above the skin, with respect to humidity and temperature, is thereby minimized. An instrument for measuring small amounts of water evaporated from or absorbed by a surface per unit time and area is described and its accuracy discussed. The instrument has a high degree of accuracy and better sensitivity than previously described devices used for this purpose. It has been used primarily to investigate the rate of evaporation from the skin surface in newborn infants and from the skin in patients with burns.

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.

Respiratory water loss, oxygen consumption and carbon dioxide production were measured during rest and during moderate radiant heat stress in 9 healthy unanaesthetized lambs, age 4-15 days. The method used is based on an open flow-through system with a mass spectrometer for gas analyses. Respiratory water loss was 12.7 mg/kg min when the lamb was at rest and increased to 24.7 mg/kg min during heat stress. Oxygen consumption was 13.9 ml/kg min at rest and remained unchanged during heat stress. The mean rate of breathing increased from 63 to 125 breaths/min.

The amount of water evaporated from the skin of newborn infants and the temperatures of the skin, the ambient air and the surfaces facing the infants were measured and used to calculate the evaporative, radiative and convective heat exchange between the infant and the environment in infants of varying gestational ages. Immediately after birth the evaporative heat exchange was very high but it then decreased gradually. The radiative heat exchange was very high in the delivery room but lower in the incubators. In incubators the evaporative heat exchange was high in the most pre-term infants, especially when nursed at a low ambient humidity. Different modes of heat exchange were interrelated, so that when the pre-term infants had a high evaporation rate, a high ambient temperature was needed and the heat exchange through radiation and convection decreased. Net heat exchange was lowest in very pre-term infants at an ambient humidity of 60%.