Objective To compare the difference in plasma sodium at 16–18 h following major surgery in children who were prescribed either Hartmann's and 5% dextrose or 0.45% saline and 5% dextrose.

Design A prospective, randomised, open label study.

Setting The paediatric intensive care unit (650 admissions per annum) in a tertiary children's hospital in Brisbane, Australia.

Patients The study group comprised 82 children undergoing spinal instrumentation, craniotomy for brain tumour resection, or cranial vault remodelling.

Interventions Patients received either Hartmann's and 5% dextrose at full maintenance rate or 0.45% saline and 5% dextrose at two-thirds maintenance rate.

Main outcomes measures Primary outcome measure: plasma sodium at 16–18 h postoperatively; secondary outcome measure: number of fluid boluses administered.

Results Mean postoperative plasma sodium levels of children receiving 0.45% saline and 5% dextrose were 1.4 mmol/l (95% CI 0.4 to 2.5) lower than those receiving Hartmann's and 5% dextrose (p=0.008). In the 0.45% saline group, seven patients (18%) became hyponatraemic (Na <135 mmol/l) at 16–18 h postoperatively; in the Hartmann's group no patient became hyponatraemic (p=0.01). No child in either fluid group became hypernatraemic.

Conclusions The postoperative fall in plasma sodium was smaller in children who received Hartmann's and 5% dextrose compared to those who received 0.45% saline and 5% dextrose. It is suggested that Hartmann's and 5% dextrose should be administered at full maintenance rate postoperatively to children who have undergone major surgery in preference to hypotonic fluids.