[Sodium balance in premature infants]

The understanding of water and electrolytes metabolism is essential in providing an adequate therapy in the treatment of low birth weight infants. In the first days of life sodium balance is negative [10, 11], since sodium renal loss is rather big and sodium peroral intake is inadequate [12]. It is not recommended to add sodium in the first 24-48 hours of life to extremely immature babies (Usher) [13]. The daily requirements of sodium in preterm infants range from 2 to 3 mmol/kg. Sodium intake should be adjusted to each patient, considering the gestational age, the severity of illness, plasma sodium concentration, sodium excretion by urine, which depends on morphological maturity and reabsorbitional capacity of the proximal tubule.

Aim of the study was to investigate the relation between sodium balance and body weight gain in the first 10 days of life in preterm infants on different feeding regimens.

Twenty-one preterm infants, gestational age from 28 to 36 weeks, eutrophic, postnatal age from 1 to 10 days, treated at the Institute for Preterm Infants in Belgrade, were included in the study. All infants were divided into three groups: the first group, eight babies, fed by mothers' milk, were additionally given 10% glucose with physiological solution of sodium chloride; the second group, six infants, also fed by mothers' milk, were additionally fed by amino acids, and 10% glucose solution and physiological sodium chloride solution; the third group, seven infants, were on total parenteral nutrition (10% glucose solution, 0.9% sodium chloride solution, amino acis and fatty emulsions). We organised a prospective balance study over the period from 20.01, to 01.11.1986 during which we calculated sodium retention by measuring sodium intake and urine sodium excretion. All infants had the same fluid intake from 70 to 150 ml/kg/day, both enteral and parenteral. Sodium intake varied from 1 to 3 mmol/kg/day. Sodium excretion was measured on the fifth and tenth day of life in a 24-hour-urine collection and was calculated by the following formulas: Osmolal index was calculated as urine osmolality-serum osmolality ratio. Osmolal clearance was calculated: Water balance was calculated on the basis of total fluid intake in ml/kg/day and diuresis in ml/kg/day.

The initial body weight loss was within physiological limits, 7-8% of the birth weight. In the study period none of the infants achieved his/her birth weight. In the third group the weight gain was 3% comparing to the birth weight, which was statistically significant (p < 0.05) (Table 2). The sodium intake was within expected levels-from 1.32 to 2.03 mmol/kg/day. Sodium intake was statistically higher in the third group (2.03 mmol/kg/day) than in the first and second groups (p < 0.05). We found negative sodium balance in three infants in the first group and two in the second group, and in all infants of the third group sodium balance was positive on the fifth day of life. We found no statistically significant differences among the groups when testing their sodium balances by hi-square test (Graph 2). When studying serum and urine osmolality and calculating osmolality index and osmolar clearance, we found that these levels were between normal values, without statistically significant differences among the groups (Graph 3). Sodium, protein, urea and creatinine levels were also normal, without statistically significant differences among the groups (Table 3).

On the basis of our study we can emphasize the following findings regarding the relation between weight gain and sodium balance. In the first group three babies started with weight gain from 6th to 10th day of life. In the second group six babies started with weight gain in the same period-from 6th to 10th day. Gain weight of babies in the third group was by 3% greater in the same period compared to the birth weight, what makes a significant difference (p < 0. (ABSTRACT TRUNCAT