Free water excess is not the main cause for hyponatremia in critically ill children receiving conventional maintenance fluids.

OBJECTIVE: To examine occurrence of hyponatremia in critically ill children receiving conventional maintenance fluids (0.18% saline in 5% dextrose) and its relationship with electrolyte free water (EFW), sodium intake and natriuresis.

DESIGN: Prospective observational study.

SETTING: Pediatric Intensive Care Unit of a tertiary care teaching hospital.

SUBJECTS: Thirty eight patients, 3 months-12 years, consecutively admitted to PICU over 30 days. Main outcome measure was occurrence of hyponatremia (serum sodium < 130 mEq/L). Serum and urinary sodium, and osmolality were measured, and type and volume of intravenous fluids and total urine output were recorded 12 hourly. Daily intake of sodium and EFW, urinary sodium excretion and net balance of fluid and sodium were estimated from above. Data of hyponatremic and non-hyponatremic patients was compared using ANOVA, Mann-Whitney U, and Chi-square tests.

RESULTS: Fourteen episodes of hyponatremia were recorded in 12 patients over 397 patient days (3.5 episodes/100 patient days). Their mean (SD) serum sodium dropped from 139 (9.3) at admission to 128 (1.0) mEq/L, over a median interval of 3.5 days (range 1-15 days). Net fluid and sodium balance in hyponatremic patients did not differ significantly from non-hyponatremic patients. Within the hyponatremic group, sodium intake, urinary sodium and sodium balance were similar before and after the occurrence of hyponatremia, while total fluid (P=0.009) and EFW intake (P=0.001) were lower in the days preceding hyponatremia.

CONCLUSIONS: Fluid and sodium balance, magnitude of natriuresis and EFW intake alone did not explain occurrence of hyponatremia in critically ill children; contribution of other mechanisms needs to be studied.