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“Put up some ‘four and a fifth’ at maintenance rates!” was the standard instruction to me as a young junior doctor, 25 years ago, when commencing maintenance fluids for sick children on the inpatient unit. Classically, the evidence for using 4% dextrose and 0.18% saline came from a widely quoted 1957 paper by Holliday and Segar, 1 where the maintenance ‘fluid and salt requirement calculation’ originated:For weight ranging from 0 to 10 kg, the caloric expenditure is 100 cal/kg/day; from 10 to 20 kg the caloric expenditure is 1000 cal plus 50 cal/kg for each kg of body weight more than 10; over 20 kg the caloric expenditure is 1500 cal plus 20 cal/kg for each kilogram more than 20maintenance requirements of sodium, chloride and potassium are 3.0, 2.0 and 2.0 mEq/100 cal/day respectively.

Thus, the hypotonic fluid of 4% dextrose and 0.18% saline was used for maintenance fluids, which would deliver at about 60 mOsm/L with a sodium chloride (NaCl) content of 1.8 g/L (about 30 mmol/L of both sodium (Na) and chloride (Cl)) (see table 1). It is interesting to note that this paper was based initially on well, breastfed infants, and the authors did recommend caution where specific clinical situations dictate alterations.1

Since the 1950s, those specific clinical situations have become clearer. Sick children on paediatric intensive care units (PICU), post surgery or with common general paediatric acute illness such as pneumonia, bronchiolitis, meningitis and fever are at risk of hyponatraemia.2–4 This is attributable to two main physiological processes. First, dilution of extracellular fluid volume by a number of mechanisms: increased antidiuretic hormone (ADH) secretion, increased sensitivity of the renal tubules to ADH, increased intake of free water and iatrogenic administration of free water in the form of intravenous hypotonic fluids. Second, there is increased urinary Na …