Fluid maintenance and deficit replacement

Fluid therapy is where physiology meets arithmetic, and both have to be right. A maintenance bag is not a generic drip. It is a calculated prescription of water, sodium, potassium and glucose, matched to a child's weight and to whatever the body is doing with ADH that day. The two classic errors are the opposite of each other: giving too little water, so the child slides into hypernatraemia, or giving too much free water, so the sodium falls and the brain swells. The Holliday-Segar rule solves the first problem, and the move to isotonic maintenance fluids solves the second. [1] [2]

This page treats prescription as a sequence of four questions. How much water does the child need each day? What tonicity should it be? How much deficit must be added back, and over how long? What ongoing losses must be matched? Answer all four and the fluid chart writes itself. The evidence base for the isotonic shift is now large and consistent, built on the Holliday and Segar observation, the McNab trial, the Cochrane and meta-analytic syntheses, and the American Academy of Pediatrics 2018 clinical practice guideline that made isotonic maintenance fluid the default for hospitalised children. [2] [3] [5]

Overview & Definition

Maintenance fluid is the daily intake of water and electrolytes needed to balance the obligatory losses a healthy child makes every day: urine, stool, sweat, and insensible loss from the skin and respiratory tract. Holliday and Segar derived it in 1957 from the metabolic observation that water need tracks energy expenditure, and their numbers have survived because they are simple and they work. [1]

The daily water requirement is 100 mL/kg for the first 10 kg of body weight, 50 mL/kg for the next 10 kg, and 20 mL/kg for every kilogram above 20. A 25 kg child therefore needs 1000 plus 500 plus 100, which is 1600 mL per day, or about 67 mL per hour. Expressed as the 4-2-1 hourly rule, the same child needs 4 mL/kg/hour for the first 10 kg, 2 mL/kg/hour for the next 10 kg, and 1 mL/kg/hour for each kilogram above 20 — giving 40 plus 20 plus 5, or 65 mL per hour. The two methods agree to within rounding. [1] [2]

Maintenance electrolytes ride along with the water. The daily sodium requirement is 2 to 3 mmol/kg/day, and the daily potassium requirement is 1 to 2 mmol/kg/day. Dissolved into the Holliday-Segar water volume, this lands the potassium at roughly 20 mmol per litre, which is why a standard maintenance bag adds 20 mmol/L of potassium chloride to its dextrose-saline base. [2]

Deficit is the fluid already lost before the child arrives, and it is separate from maintenance. It is estimated from the percentage dehydration: a clinically mild deficit is about 5 percent of body weight, a moderate deficit is 5 to 10 percent, and a severe deficit is at least 10 percent. Each 1 percent dehydration equals 10 mL/kg of lost fluid, so a 10 kg child at 10 percent dehydration has lost 1000 mL. [8]