Comparison of the incidences of hyponatremia in adult postoperative critically ill patients receiving intravenous maintenance fluids with 140 mmol/L or 35 mmol/L of sodium: retrospective before/after observational study.

Purpose: The purpose of this study was to compare the incidences of hyponatremia in adult postoperative critically ill patients receiving isotonic and hypotonic maintenance fluids.

Methods: In this single-center retrospective before/after observational study, we included patients who had undergone an elective operation for esophageal cancer or for head and neck cancer and who received postoperative intensive care for >48 h from August 2014 to July 2016. In those patients, sodium-poor solution (35 mmol/L of sodium; Na) had been administered as maintenance fluid until July 2015. From August 2015, the protocol for postoperative maintenance fluid was revised to the use of isotonic fluid (140 mmol/L of sodium; Na). The primary outcome was the incidence of hyponatremia (<135 mmol/L) until the morning of postoperative day (POD) 2.

Results: We included 179 patients (Na: 87 patients, Na: 92 patients) in the current study. The mean volume of fluid received from ICU admission to POD 2 was not significantly different between the two groups (3291 vs 3337 mL, p = 0.84). The incidence of postoperative hyponatremia was 16.3% (15/92) in the Na cohort, which was significantly lower than that of 52.9% (46/87) in the Na group (odds ratio = 0.17, 95% confidence interval 0.09-0.35, p < 0.001]. The incidences of hypernatremia, defined as serum sodium concentration >145 mmol/L, were not significantly different between the two groups.

Conclusion: In this study, the use of intravenous maintenance fluid with 35 mmol/L of sodium was significantly associated with an increased risk of hyponatremia compared to that with 140 mmol/L of sodium in adult postoperative critically ill patients.