Postoperative decrease in plasma sodium concentration after infusion of hypotonic intravenous solutions in neonatal surgery

Background. Hypotonic i.v. solutions can cause hyponatraemia in the context of paediatric surgery. However, this has not been demonstrated in neonatal surgery. The goal of this study was to define the relationship between infused perioperative free water and plasma sodium in neonates. Methods. Newborns up to 7 days old undergoing abdominal or thoracic surgery were included in this prospective, observational study. Collected data included type and duration of surgery, calculated i.v. free water intake, and pre- and postoperative plasma sodium. Statistical analyses were performed using the Pearson correlation, Mann-Whitney test, and receiver operating characteristic analysis with a 1000 time bootstrap procedure. Results. Thirty-four subjects were included. Postoperative hyponatraemia occurred in four subjects (11.9%). The difference between preoperative and postoperative plasma sodium measurements (Delta NaP) correlated with calculated free water intake during surgery (r=0.37, P=0.03), but not with preoperative free water intake. Calculated operative free water intake exceeding 6.5 ml kg(-1) h(-1) was associated with Delta NaP >= 4 mM with a sensitivity and specificity [median (95% confidence interval)] of 0.7 (0.9-1) and 0.5 (0.3-0.7), respectively. Conclusions. Hypotonic solutions and i.v. free water intake of more than 6.5 ml kg(-1) h(-1) are associated with reductions in postoperative plasma sodium measurements >= 4 mM. In the context of neonatal surgery, close monitoring of plasma sodium is mandatory. Routine use of hypotonic i.v. solutions during neonatal surgery should be questioned as they are likely to reduce plasma sodium.