The purpose of this study is to determine if isotonic solutions reduce the risk of hospital acquired hyponatremia compared with hypotonic solutions.

Multicenter, Randomized, Controlled clinical trial.

The purpose of this study is to determine if isotonic solutions reduce the risk of hospital acquired hyponatremia compared with hypotonic solutions

This study will include 150 patients in 2 groups according to these type of pathology: group 1: medical conditions which includes respiratory problems (pneumonia, bronchiolitis, acute asthma attack) and acute gastrointestinal problems (bacterial or viral gastroenteritis) and group 2: surgical conditions (pre, peri or postoperative patients will be included). Each group will be randomized to receive 3 types of interventions: 0.3% Saline (hypotonic solution), 0.45% Saline (hypotonic solution) or 0.9% Saline (isotonic solution). In non-dehydrated patients the daily total volume of liquid infused will be determined by the volumetric Holliday- Segar* formula and the daily total volume will be calculated by adding the maintenance requirements (using the Holliday Segar formula) to the fluid deficit (according to percent of estimated weight**) in dehydrated patients.

The glucose concentrations in the first group will be 3.3%, and in group 2 and 3 will be 5%. The potassium concentration will be 20mEq/L in all groups. Blood samples will be collected before, 8 hours and 24 hours after the start of intravenous fluid, for the analysis of sodium, potassium, glucose, urea and creatinine. The adverse clinical outcomes at 8 and 24 hours of the start of intravenous fluid will also be recorded. The sodium plasma level, the incidence of hyponatremia and the adverse clinical outcomes will be compared.

*0-10 kilogram (kg): 100ml/kg per day; 10-20 kg: 1000 ml + 50 ml/kg/day over 10 kg; >20 kg: 1500 ml + 20ml/ kg over 20kg

** In patients under 10 kg correspond to a loss of 5% (50 mL/kg) in mild dehydration and 10% in moderate dehydration. In 10 kg or more: 3% (30 mL/kg) in mild dehydration and 6% (60 mL/kg) in moderate dehydration.

Inclusion Criteria:

• Children between 3 months to 15 years.
• Need for intravenous fluids for 8 hours or more.
• Normal serum sodium at the beginning of the study of the study (135-145 mEQ/L).
• Acute respiratory problems (acute asthma attack, bronchiolitis or pneumonia).
• Acute gastrointestinal disorders (viral o bacterial gastroenteritis).
• Elective surgical procedures.
• Pre, peri o post operative patients (acute appendicitis, intussusception, intestinal perforation, ileus).

Exclusion Criteria:

• Severe hyponatremia (serum sodium < 125 mEq/L).
• Severe hypernatremia (serum sodium >155 mEq/L).
• Need for intravenous fluids for less than 8 hours according with their treating physician.
• Severe dehydration or Shock.
• Preexisting chronic diseases (renal diseases, heart diseases or endocrine disorders).
• Neurologic diseases.
• Head trauma.
• Cerebral edema or Intracranial hypertension.
• Diabetic ketoacidosis.
• Use of diuretics one week or less before the study.
• Need for admission to the pediatric critical intensive care unit