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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] = (ClTarget – ClCurrent) 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 +
2 ml/kg/hr
60 ml/hr +
1 ml/kg/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
NaRequirements 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


[1] Pediatrics, 1957, May;19(5):823-32: Holliday at all: The maintenance need for water in parenteral fluid therapy.

[2] Kidney Int., 2005, Jan;67(1):380-8: Friedman: Pediatric hydration therapy: historical review and a new approach.

[3] Pediatr Nephrol. 2005, Dec;20(12):1687-700: Moritz ML at all: Preventing neurological complications from dysnatremias in children.

[4] 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.

[5] N Engl J Med 2011;364:2483-95: Maitland et al: Mortality after Fluid Bolus in African Children with Severe Infection

 All Marc’s PICU cardiology FOAM can be found on PICU Doctor and can be downloaded as a handy app for free on iPhone or AndroidA list of contributors can be seen here.

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