ObjectivePostoperative hyponatremia is a frequent metabolic disturbance that may cause life-threatening complications. It results from both a positive electrolyte-free water (EFW) balance and an antidiuretic hormone release. During surgery, intracellular solutes may leak out of cells because of an increased membrane permeability leading to increased osmolality, cellular water shift, and redistribution hyponatremia, a concept coined the sick cell syndrome. Because of release of osmotically active solutes, plasma or urinary osmolar gap should increase. Therefore, we tested the hypothesis that postoperative hyponatremia may be related to a translocational mechanism evidenced by a postoperative increase of the osmolar gap rather than to a positive EFW balance.SettingAn anesthesiology department in a 1,200-bed university hospital.DesignA 5-month prospective observational study.SubjectsThirty-three consecutive patients undergoing elective hip arthroplasty under general anesthesia. They were divided into two groups whether the postoperative plasma sodium concentration decrease was ≥2 mmol/L (group 1) or <2 mmol/L (group 2).MeasurementsPlasma sodium concentration ([Na+]p) and plasma osmolality were measured before induction of anesthesia and at skin closure. Osmolality was calculated at the same times. Plasma osmolar gap (OGp) was calculated as the difference between measured and calculated osmolality. Postoperative urinary osmolar gap (OGu) was calculated in the same way. EFW balance was calculated as the ratio of (infused EFW − excreted urinary EFW) to total body water.ResultsIn 33 patients, a significant [Na+]pdecrease of −2.0 was observed. No relationship was demonstrated between EFW balance and perioperative [Na+]pvariation (r= .28;p= .12). A relationship was observed between perioperative OGpvariation and perioperative [Na+]pvariation (r= .74;p< .0001). In the 19 group 1 patients, [Na+]pdecreased by −3.0 mmol/L. EFW balance did not differ between group 1 and group 2 patients. No statistical relationship was observed between EFW balance and perioperative [Na+]pvariation in group 1 (r= .20;p= .40) and in group 2 (r= .43;p= .14). OGpincreased only in group 1 but not in group 2 patients, and postoperative OGuwas greater in group 1 than in group 2 patients. A relationship was observed between perioperative OGpvariation and perioperative [Na+]pvariation in group 1 (r= .53;p= .02) but not in group 2 (r= .32;p= .26).ConclusionHyponatremia after hip arthroplasty may not be related to a positive EFW balance. The postoperative increase of the OGpand the greater postoperative OGuin patients developing postoperative hyponatremia suggest the release of osmotically active solutes leading to cellular water shift from intracellular to extracellular spaces. These data may support the clinical relevance of the sick cell syndrome in the postoperative context.