Radiant Warmer Power and Body Size as Determinants of Insensible Water Loss in the Critically Ill Neonate
Summarv number of serious com~lications. Underestimation of fluid needs Twelve critically ill neonates mechanically ventilated for respiratory failure (mean weight 1.33 kg, mean gestation 31 wk) were studied to quantitate the effects of radiant power from a radiant warming device, body weight, and body surface area on insensible water loss. Radiant power density (Mw/cm2) was measured using a wattmeter and thermopile transducer. Insensible water loss was measured using a Potter Baby Scale.
... r Baby Scale. Weight correlated inversely with insensible water loss, (r = -0.86, P < 0.001). Radiant power density correlated inversely to weight, (r = -0.71, P < 0.001). There was a significant increase in insensible water loss as radiant power density increased, (r = 0.54, P < 0.05). Net radiant power received (W/kg) by infants over their exposed surface area, correlated directly to insensible water loss, (r = 0.67, P < 0.01) irrespective of body weight. Critically ill neonates ventilated for respiratory failure and nursed under radiant warmers incurred greater insensible water losses than p-eviously reported for well infants. The magnitude of this increaotd insensible water loss is inversely related to body size and is determined directly by the radiant power density required to maintain body temperature. Speculation Quantitative measurement of radiant power density delivered to critically ill newborn infants nursed under servocontrolled radiant warmers facilitates estimation of insensible water loss. Used in conjunction with body mass and surface geometry, quantitative radiant power assessment is clinically apilicable-to monitoring insensible water loss. Calculation of sarenteral fluid reauirements might be enhanced using this techniiue. Estimation of parenteral fluid requirements is an essential part of managing the critically ill newborn infant (7, 24, 28). The current method of estimating fluid needs is based on replacement of measured urine volume plus insensible water loss (7, 24). Numerous factors are known to influence the magnitude of insensible water loss (28). Some of these factors include the infant's mvironment (15, 19), metabolic rate (4, 18, 22, 25), respiratory .ate (20, 21) gestational maturity (13), body size (31). and surface srea and proportion of body composition as water (13). Because f the multiplicity of these factors, calculation of parenteral fluid :quirements is often a complex problem. Another more recent concern in estimating parenteral fluid -quirements is the reevaluation of insensible water loss as influ-Iced by advances in life support technology. The use of radiant Irmers and phototherapy may produce profound changes in id balance (12, 17, 23, 28, 31) . Moreover the advent of this ,hnology has allowed smaller and less mature infants to survive =7). The physiology of fluid homeostasis in these very low birth :ight, critically ill infants is essentially unknown (24). Inaccurate assessment of fluid requirements may result in a may lead to dehydration, hypotension, poor perfusion with acidosis, hypernatremia, and cardiovascular collapse (6, 9, 30). Administration of excessive fluid has been implicated in the pathogenesis of pulmonary edema, congestive heart failure, opening of ductal shunts, and bronchopulmonary dsyplasia (5, 26). Recent investigations have assessed insensible water loss in the newborn infant to provide better guidelines for parenteral fluid replacement. Bell et al. (3) and Marks er al. (19) have suggested that radiant energy received by these infants from radiant warmers and phototherapy may be one of several factors determining the magnitude of insensible water loss. Engle et al. (1 1) have quantitated radiant power delivered to critically ill infants and correlated this power to decreased urine output and increased urine concentration. However, there are little data on insensible water loss in very low birth weight infants ventilated for respiratory disease. It is this group of infants that is frequently nursed under radiant warmers and therefore is at highest risk for increased insensible water loss. The purpose of this investigation is to demonstrate the degree to which insensible water loss is affected by body size and radiant warmer power delivery in low birth weighi, critically ill neonates.