Acute gastroenteritis is one of the common illnesses affecting infants and children throughout the world. Correction of dehydration is the mainstay of treatment, which is given as oral rehydration therapy (preferably) or intravenous fluids, depending on the severity of dehydration and various other factors. It is imperative to correctly choose the type of fluid to avoid electrolyte disturbances that might lead to dangerous complications.

Among children in the United States, acute gastroenteritis remains a major cause of morbidity and hospitalization, accounting for >1.5 million outpatient visits, 200,000 hospitalizations, and approximately 300 deaths per year.

The volume of fluid lost through stools can vary from 5 mL/kg body weight per day, which is approximately normal, to ≥200 mL/kg body weight per day, which is severe diarrhea. Dehydration and electrolyte losses associated with untreated diarrhea are the main reason for morbidity in acute gastroenteritis. History, physical examination, and dehydration assessment are the main features in the assessment of diarrhea. Treatment usually includes two phases: rehydration and maintenance. The rehydration phase involves quick replacement of fluids usually within 3 to 4 hours. The maintenance phase is the phase in which maintenance calories and fluids are administered. IV fluids are the mainstay of management in cases of severe dehydration.

IV fluid administration to maintain water and electrolyte balance in an individual was first described by Holliday and Segar in 1957. They had based the free water requirement on evidence that it equated the energy expenditure in healthy children. However, sodium and potassium requirements (3.0 and 2.0 mEq/100 kcal/24 hour, respectively) were rationalized based on intake of electrolytes by infants receiving breast and cow milk. Use of hypotonic IV fluids was based on the above recommendations. Although these recommendations may be appropriate for the healthy child, they do not necessarily apply in acute illness, where energy expenditure and electrolyte
requirements deviate significantly from that of a normal healthy individual. The calculation based on this recommendation could overestimate the hypotonic solution needs in patients.

Hyponatremia occurs when the water to sodium ratio is increased, which means that the plasma sodium concentration is <136 mM. This can be either due to decrease in sodium or an increase in the water. This could occur when there is a positive balance of electrolyte free water in the body due to increased input of water than excretion of water. A decrease in the water output usually occurs secondary to antidiuretic hormone (ADH) secretion. In normal individuals, ADH is controlled by osmotic stimuli and hence water diuresis occurs only when the plasma sodium level falls below 136 mM (due to ADH suppression). However, hospitalized pediatric patients have multiple causes for nonosmotic stimuli for ADH secretion, and hence when the plasma sodium level falls below 136 mM, water diuresis does not take place due to the presence of ADH (nonosmotic stimuli) and this increases the available electrolyte free water in the body. Administration of hypotonic solution would further contribute to the electrolyte free water, leading to hyponatremia in these patients. It has been shown that patients admitted with gastroenteritis have obligate urinary sodium losses irrespective of initial serum sodium. The urinary tonicity at presentation of these patients is approximately equal to normal saline. Therefore, infusion of a hypotonic solution, which is lower in tonicity than that of urine passed, is predictive of a decrease in subsequent serum sodium.