Radiant Warmer Power and Body Size as Determinants of Insensible Water Loss in the Critically Ill Neonate

Pediatr. Res. 15: 1495-1499 (198 1) Radiant Warmer Power and Body Size as Determinants of Insensible Water Loss in the Critically I11 Neonate STEPHEN BAUMGART,'"' WILLIAM D. ENGLE, WILLIAM W. FOX, and RICHARD A. POLlN Division of Neonatology of The Children's Hospital of Philadelphia and the Department of Pediatrics. University of Pennsylvania School of Medicine, Philadelphia, Pennsylvania, USA Summarv 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. 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 influIced 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 number of serious com~lications.Underestimation of fluid needs 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. MATERIALS A N D METHODS Heat delivered by a radiant warmer (radiant power density), and insensible water loss were studied in 12 critically ill newborn infants (Table 1). Gestational ages of these infants ranged 25-36 wk, and weights ranged 0.67-2.10 kg. Subjects were from I to 7 days of age at the time of study. Infants were nursed unclothed on radiant warmer beds (Air Shields, Infant Care System) servocontrolled by the infants' skin temperature to maintain axillary temperature within 36.5 to 37.2OC. The ambient temperature of the intensive care nursery is maintained at 25°C and 40 to 45% relative humidity by a double walled construction and environmental control. Convective currents in the nursery occur as a result of movement of personnel, and from air ventilation ports located in the ceiling. No attempt was made to alter these conditions in order to study the environmental ambience as experienced by the infant. All infants were endotracheally intubated and were receiving a mixture of humidified oxygen and compressed air. Ten of the 12 infants were mechanically ventilated and two were receiving continuous positive airway pressure. Nine infants had respiratory distress syndrome and three were ventilated for apnea of prematurity. None of the infants was asphyxiated and all were stable at the time of study. Informed parental consent was obtained for all infants included in the study. To determine the relationships between radiant power density, body weight, and insensible water loss, heat received by these infants was monitored by a radiant power monitoring technique previously described (I). As reported, a wattmeter (Ohio Semitronics, WM 1000) was connected between the radiant warmer 1496 BAUMGART ET AL. Table 1. Patient data ' Patient no. Wt (kg) Calculated body surface area (cm2) Estimated gestational age (wk) Age at time of study (days) FIO, PIP/CPAP (cm/H20) IMV (breaths/min) I 0.67 0.8 1 0.84 1.18 1.25 1.29 1.41 1.42 1.50 1 .SO 1.97 2.10 700 800 900 1 100 1 100 1200 1200 1250 1300 1300 1600 1600 27 28 25 32 30 30 32 32 32 32 33 36 7 4 2 2 I I 2 4 4 4 3 4 0.38 0.39 0.34 0.30.45 0.55-0.60 0.55 1.00 0.25 0.25 0.38-0.48 0.30 0.32 12/3 -/5 19/4 18/4 20-25/6 25/5 28/7 l5/4 12/2 5- 18/4 -/3 16/5 25 2 3 4 5 6 7 8 9 10 II 12 60 15-25 60 30 5 (...truncated)