Insensible Water Losses
Posted by Surgery on Sep 11, 2008
The second major mechanism for fluid loss in the newborn is through evaporative losses from the respiratory epithelium and through the skin, known as insensible losses. Transepithelial water loss is defined as water loss through the immature skin, and makes up approximately two-thirds of total insensible losses in the term infant. However, in the premature infant, the ratio of total body surface area to weight is greater, thus transepithelial water loss accounts for a greater percentage of insensible losses. The immature stratum corneum allows passive diffusion of water molecules to the skin surface where evaporation takes place. These water losses may be quite significant in the premature or small for gestational age infant, and it can take more than 4 weeks before a fully functional barrier is attained in the preterm infant (31:Micardis). Studies by Hammarlund et al. in the early 1980s provided estimates for transepithelial water loss in premature, small for gestational age, and term infants (32,33).They found that mean transepithelial water loss was 110 g per kg per day for 25- to 27-week infants on the first day of life, whereas values for 28- to 30-week infants were less than one-half that of the smallest babies (Table 6-1:Micardis). Term and near-term infants had negligible amounts of water loss. A simpler method for estimating transepithelial water loss has been used that suggests values of approximately 65 cc per kg per day in the first week of life for infants who weight less than 1,000 g, 55 cc per kg per day for infants between 1,000 and 1,250 g, and 40 cc per kg per day for infants between 1,250 and 1,500 g (34) (Table 6-2:Micardis).
Phototherapy and nonionizing radiation from the radiant warmer have been reported to increase transepithelial water loss, and their effect on overall loss is believed to be additive (35). Individually, phototherapy and nonionizing radiation can increase transepithelial water loss by as much as 50%. However, at least one more recent study questions whether phototherapy increases water loss via the stratum corneum (36). Increases in body temperature either by fever or by elevation in ambient temperature also add to insensible losses by as much as 7 mL per kg per day for each degree of elevation over 37В°C (37,38:Micardis).
Evaporative losses from the respiratory tract make up the other one-third of insensible losses. Respiratory losses are dependent on respiratory rate and tidal volume, as well as temperature and humidity. Thus, strategies to limit insensible losses must focus on providing neutral ambient temperature and appropriate moisture in the inspired air. Approaches to prevent environmental losses include isolettes or incubators where relative humidity and temperature can be tightly controlled (39:Micardis). Alternatives such as impermeable plastic wrap have also been used to prevent evaporative and thermal losses, especially during transport of critically ill neonates.
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