Anders, M. Fluids, Don't Forget the Bubbles, 2013. Available at:
Anion gap = Na+ + K+ – (Cl– + HCO3–). normal 8-12 mEq/l.
Total body water (TBW) = intracellular fluid (ICF) plus extracellular fluid (ECF). [Weight x 600 ml in adults (500 ml in female), Weight x 650 ml in paeds, Weight x 700 ml in neonates]
ECF = intravascular fluid (plasma and lymph in the vessels) plus interstitial fluid (between cells)
Osmolality = 2 x Na+ + K+ + Glucose (mmol/l) + Urea (mmol/l).
Osmotic gap = measured osmolality – calculated osmolality
Na+ deficit [mmol/l] = (Na+Target – Na+Current) x TBW / 1000
Cl– deficit [mmol] = (Cl–Target – Cl–Current) x 0.2 x Weight
Water deficit = 4 ml x Weight x (Na+Target – Na+Current)
Maximum change in osmolality in hyper- or hypoosmolaric: 1 mmosmol/l per hour. Cave central pontine myelinolysis
Body water and blood volume composition with age
Adult bodies are 60% water (20% ECF, 40% ICF). Blood volume 70 ml/kg.
Term neonate bodies are 75% water (40% ECF, 35% ICF), and term neonates usually lose 5-10% of their weight in the first week of life, almost all of which is water loss. Blood volume 80 ml/kg.
Preterm neonates have more water (at 23 weeks gestation, 90% water composed of 60% ECF and 30% ICF), and they may lose 10-15% of their weight in the first week of life.
Small for gestational age (SGA) preterm infants may have a higher proportional body water content (90% for SGA infants vs 84% for appropriate for gestational age [AGA] infants at 25-30 weeks gestation).
|Maintenance Fluid [ml/hr] for active Children > 2 days|
|≤ 10 kg:||11 kg & < 20 kg||≥ 21 kg|
|4 ml/kg/hr||40 ml/hr +
|60 ml/hr +
|Day 1||Day 2||Day 3|
|Maintenance Fluid [ml/hr] active Neonates ≤ 3 days|
|2 ml/kg/hr||3 ml/kg/hr||4 ml/kg/hr|
|Na+ Requirements for active Neonates ≤ 3 days|
|1 – 3 mmol/kg/d||3-5 mmolk/kg/d||2-4 mmol/kg/d|
|K+ Requirements for active Neonates ≤ 3 days|
|1 – 2 mmol/kg/d||2-3 mmol/kg/d||1-2 mmol/kg/d|
|Normal maintenance fluid:|
|NaCl 0.9 % or NaCl 0.9 % in 5 % Dextrose or NaCl 0.9 % in 2.5 % Dextrose or Ringer Lactate or Hartmann Solution|
References: Pediatrics, 1957, May;19(5):823-32: Holliday at all: The maintenance need for water in parenteral fluid therapy.  Kidney Int., 2005, Jan;67(1):380-8: Friedman: Pediatric hydration therapy: historical review and a new approach.  Pediatr Nephrol. 2005, Dec;20(12):1687-700: Moritz ML at all: Preventing neurological complications from dysnatremias in children.  Arch Dis Child, 2006, 91(3):226-32: Neville at all: Isotonic is better than hypotonic saline for intravenous rehydration of children with gastroenteritis: a prospective randomised study.  N Engl J Med 2011;364:2483-95: Maitland et al: Mortality after Fluid Bolus in African Children with Severe Infection