Body fluid compartment changes following neonatal surgery. 
The neonate is born with an excess of total body water (TBW) in the range of 75% to 85% of body weight (v 60% in the adult), which is due to a large extracellular fluid volume (ECF) of 40% to 50% of body weight (v 20% in the adult).  In an attempt to define the changes that occur in TBW and ECF following neonatal surgery, the following prospective study was carried out.  Twenty newborns with major congenital anomalies (gastroschisis, 10; esophageal atresia, 4; ileal atresia, 3; omphalocele, 2; and malrotation, 1) were studied weekly for 4 weeks following surgery while being maintained on a standard protocol of total parenteral nutrition (TPN).  Total body water was measured using deuterium oxide, and extracellular fluid volume was assayed with sodium bromide; both were expressed as percent body weight (BW).  Weight gain or maintenance was observed in all infants.  TBW remained unchanged (from 85.4% to 83.0%), and ECF decreased from 51.2% to 36.7% during the observation period.  The TBW in the ten gastroschisis infants decreased from 87.3% to 78.0% v the ten other newborns, in whom there was no change (84.0% to 85.0%).  The ECF changes were more dramatic, decreasing from 51.6% to 32.3% in the gastroschisis infants, v 50.8% to 45.5% in the other neonates (P = .0156).  There were no differences between the two groups in the intake and output of fluids.  The following conclusions were drawn: (1) Following major surgery, newborns show a steady decrease in ECF and minimal change in TBW in spite of adequate weight gain while receiving TPN.(ABSTRACT TRUNCATED AT 250 WORDS)