Objective: To compare the risk of hyponatremia using hypotonic saline (HT) versus isotonic solution (IT) and adverse effects.

Methods: Randomized clinical trial, open, non-blind, in patients admitted to Pediatric Intensive Care Unit after elective surgery, from 6 months to 14 years, weight greater than 6 kg, onset natremia between 130-150 mEq/L and signed informed consent, with no oral intake and maintenance intravenous fluid therapy. Patients with ADH secretion abnormalities are excluded. Sodium, calcium, potassium and chlorine plasma levels are measured on admission, 8, 24 and 48 hours. Patients with normonatremia (135-145 mEq/L) are analyzed.

Results: 60 patients received HT solution (glucosaline 1/3: 51 mEq/L sodium) and 70 IT solution (glucosaline 5/0.9%: 154 mEq/L sodium). The incidence of hyponatremia (sodium <135 mEq/L) at 8 h of the HT group was 38.9% and 4.6% of IT (p <0.001) (OR= 13.15; 95% CI: 3.65-47.3), and at 24 hours: 43% in HT group and 11% in IT (p= 0.021) (OR= 6.15; 95% CI: 1.43-26.39). 3 cases of hypernatremia 8 and 24 horas, all in IT (7%), without significant differences (p= 0,2), and no difference in the incidence of hyperchloremic acidosis. Mean values of potassium and calcemia were normal with low incidence of hypokalemia and ionic hypocalcemia.

Conclusions: The use of IT solution in the postoperative period of general surgery protects against hyponatraemia, especially in the first 24 hours, with no significant risk of hypernatremia or hyperchloremic acidosis. Standar commercialized solutions without potassium and calcium supplements could be used in the short-term postoperative period, if are normal at admission.

The dilutional and iatrogenic hyponatremia is an underdiagnosed and avoidable situation, that can result in a life threatening condition, such as cerebral edema. The intravenous administration of hyponatremic fluids, as a maintenance fluid therapy according to Holliday & Segar, has been related with this entity or the worsening of mild hyponatremia in acute ill pediatric patients and also postoperative pediatric patients. These patient's kidneys are unable of excrete free water because of an excess of antidiuretic hormone (ADH) effect. Best evidence at the present time says that isonatremic maintenance fluids can prevent hyponatremia and its health threatening consequences. We report a case of successful development and deeply insist on the necessary changes that paediatricians must do on using intravenous isotonic saline solutions instead of hyponatremic fluids.