Plasma Osmolality in the Early and Late Neonatal Period with Particular Reference to Low Birth Weight Infants

Abstract

Data on postnatal changes of plasma osmolality were collected on two groups of newborn infants. Group I consisted of 114 normal full-term infants in whom measurements were performed in cord blood and peripheral venous blood at 12, 24 and 48 h after birth. It has been found that plasma osmolality does not change within the first 48 h, but is maintained at the level of cord plasma. The infants comprising group II were admitted to a referral unit at different postnatal ages for observation or treatment of transient neonatal disturbances. The majority of the infants and particularly those followed through the late neonatal period were low birth weight infants. Neither of the infants studied required intravenous therapy known to affect plasma osmolality. Blood samples were taken at admission and at different time intervals thereafter. Time at which blood was withdrawn varied in the individual infant. In 95 plasma samples, besides osmolality, sodium concentration was also measured. In this group of infants, after the first 3 days, plasma osmolality steadily fell with postnatal age reaching a minimum between 18–21 days. The fall in plasma osmolality was accompanied by a parallel decline of plasma sodium. The two variables were significantly correlated. It is concluded that the decrease in plasma osmolality of low birth weight infants represents profound adaptive changes of osmolal homeostasis in the late neonatal period. The possible mechanisms underlying the total osmotic pressure changes, such as expansion of extracellular volume, sodium loss, immaturity of kidney function are discussed.