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, marked clinically by natriuresis, diuresis and weight loss. It is uncertain how these postnatal phenomena, which suggest an interrelationship with cardiopulmonary adaptation, are brought about. The aim of this study was to evaluate the temporal relationship between alterations in circulating atrial natriuretic peptide (ANP), respiratory status, sodium excretion and extracellular fluid volume (ECFV) in preterm babies, in the first days after birth. Eighteen male infants below 34 weeks gestational age were studied longitudinally, measuring urine output, sodium balance, arterial-alveolar oxygen ratio and circulating ANP. Daily changes in ECFV were assessed by endogenous chloride 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 immediately 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 decline in ECFV after birth, in contrast to the remaining 14 infants, in whom there was an initial increase. This study demonstrates that there is a temporal relationship between improvement in respiratory function and an acute elevation 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 with regard to the clinical management of newborns with respiratory distress syndrome.