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EXTRACELLULAR FLUID VOLUME (ECV) CHANGES IN VERY LOW BIRTH WEIGHT (VLBW) INFANTS DURING THE FIRST TWO POSTNATAL MONTHS

Postnatal natriuresis among VLBW infants may represent physiologic reduction of the ECV or pathologic renal salt wasting. We assessed Na homeostasis between birth and hospital discharge by serial measurements of ECV in 18 preterm infants(mean GA 28 wk, SD 1.8). ECV was estimated as corrected bromide space on postnatal days 1, 7, 14, 21, 42 and 63. Results were compared to changes in body weight, Na intake and serum Na concentration.Mean body weight decreased from 1027 gm (SD 272) on day 1 to 937 gm (SD 217) on day 7, then returned to birth weight by day 14. Mean ECV was 550 ml/kg (SD 116) on day 1 and decreased to 359 ml/kg (SD 66) on day 14. Thereafter body weight and ECV increased proportionally so that mean ECV/kg remained between 336 ml/kg (SD 42) and 349 ml/kg (SD 54). ECV reduction and stabilization was usually isotonic.Six infants developed hyponatremia(serum Na < 135 mEq/l) between 11 and 31 days of age. Mean ECV/kg was significantly lower in these infants compared with infants in the same age range with serum Na > 135 mEq/l (303 ml/kg, SD 36, vs. 368 ml/kg, SD 56, p <.01). Sodium intake was not different in the 2 groups, however total fluid intake was greater in infants with hyponatremia and decreased ECV.In conclusion, the ECV of the VLBW infant decreases postnatally and is regulated within a range similar to older infants. These findings suggest that the initial postnatal natriuresis in the first 2 wks of life represents physiologic reduction of the expanded ECV of the fetus. Late hyrronatremia may indicate excessive sodium loss and ECV depletion.