Postnatal weight loss and contraction of the extracellular compartment is triggered by atrial natriuretic peptide

Following birth there is a contraction in the extracellular compartment, ma rked clinically by natriuresis, diuresis and weight loss. It is uncertain h ow these postnatal phenomena, which suggest an interrelationship with cardi opulmonary adaptation, are brought about. The aim of this study was to eval uate the temporal relationship between alterations in circulating atrial na triuretic peptide (ANP), respiratory status, sodium excretion and extracell ular fluid volume (ECFV) in preterm babies, in the first days after birth. Eighteen male infants below 34 weeks gestational age were studied longitudi nally, measuring urine output, sodium balance, arterial-alveolar oxygen rat io and circulating ANP. Daily changes in ECFV were assessed by endogenous c hloride balance, following baseline measurement of bromide space. There was a clear period of improvement in respiratory function in 15 babies and in these there was a highly significant elevation in circulating ANP, either i mmediately prior to, or during, the period of improvement. In three infants there was no definable period of respiratory improvement. In four babies, two of whom had very mild respiratory distress, there was an immediate decl ine in ECFV after birth, in contrast to the remaining 14 infants, in whom t here was an initial increase. This study demonstrates that there is a tempo ral relationship between improvement in respiratory function and an acute e levation in circulating ANP. Babies with respiratory distress syndrome are at risk of initial expansion of the extracellular compartment after birth. This is likely to increase morbidity. These observations are of relevance w ith regard to the clinical management of newborns with respiratory distress syndrome. (C) 2000 Elsevier Science Ireland Ltd. All rights reserved.