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NICU guideline identifier

Fluids - Electrolyte and Arterial line fluids

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Summary Electrolyte Requirements

Day 1 No routine electrolytes 
Day 2 onwards Add electrolytes: 
    Na+ 3 mmol/kg/day (very preterm infants may need more) 
    K+ 2 mmol/kg/day 
    Ca2+ 1 mmol/kg/day
Do not add Ca2+ routinely in relatively well infants only likely to be on short term IV fluids.
This is most babies as the sicker, longer term babies will be on IVN)

See also the fluids calculator 

Additive Concentration Requirement   Formula 
Na+ 4 Molar
3 mmol/kg/day
500 x weight (kg) x 0.75 
24 hour total fluids (ml) 
K+ 1 Molar
2 mmol/kg/day
 500 x weight (kg) x 2 
24 hour total fluids (ml)
Ca2+ 10% Ca gluconate
1 mmol/kg/day
500 x weight (kg) x 4.5 
24 hour total fluids (ml)
  • Assumes that additives are added to a 500ml bag of fluid. 
  • Add Heparin 250units/500ml to central venous line (including long line) fluids. 
  • If daily requirements are increased or decreased, these formulations need to be recalculated. 
  • This system does not take into account the amount of sodium in arterial fluids or other infusions and calculations need to be adjusted accordingly.

Sodium Requirements

In general, fluid management in the first few days is adjusted primarily on serum sodium values and changes in weight.

  • High serum sodium values usually indicate that the infant requires more fluids.
  • Low serum sodium values may indicate that the infant requires less fluid, or that the infant has high sodium losses. Infants with hyponatraemia may grow poorly.


Serum Na+ <130
Commonest Causes   
• Prematurity. Renal Na+ loss from a high fractional excretion of Na+.
• Inadequate Na+ intake.
• Excessive water intake. Excessive maternal fluid intake during labour/delivery can lead to neonatal hyponatraemia.
• Diuretic therapy, especially loop diuretics (e.g. furosemide).
• Acute tubular necrosis (tubular Na+ loss) and other causes of renal failure.
Indomethacin. Reduces free water clearance and fractional excretion of sodium, with the lower free water clearance leading to hyponatraemia.
• SIADH. ADH has a limited ability to concentrate the urine in the newborn, and acts primarily as a vasopressor.
• Excess Na+ loss. Diarrhoea, Gastric, pleural, CSF, 17OH progesterone deficiency.
• An isolated low Na+ should not be treated.  Repeat the sample.
  There is an error of measurement within the analyser of ±4mmol/L.
• Urinary Na+ if high Na+ requirements. This may allow for an estimate of sodium replacement.
• Treatment will depend on the underlying cause and the severity of the hyponatraemia.
  It can sometimes be difficult to determine whether the main cause of hyponatraemia is excessive water, inadequate body sodium,
  or a combination of the two.
• Excessive water: reduce the water (fluid) intake.
• Reduced body sodium (from inadequate intake or increased excretion):
  Oral sodium supplements
  -  2 Molar NaCl supplements (1ml = 2mmol NaCl).
  -  Usually start at 3mmol/kg/day additional NaCl.
  -  Occasional infants will require ≥12mmol/kg/day NaCl.
  Intravenous sodium infusion
  Refer to the sodium infusion calculator.


Serum Na+ >150mmol/L. In NICU, management is usually altered for serum Na+>145mmol/L.
Commonest Causes   
• Excessive water loss: Very preterm insensible water loss, Diarrhoea, Polyuria.
• Excess Na+ intake. Relatively common with sodium bicarbonate infusions.
  Other medications and infusions may contain large quantities of sodium. For example, an arterial line containing 0.9% NaCl and
  running at 1ml/hour will give 3.6mmol/day of NaCl.
• Interpret high Na+ values in clinical context. Is the baby dehydrated? Are there ongoing fluid losses? Is the baby receiving medications
  or infusions that contain large amounts of Na+?
• Increase fluid intake.
• Reduce Na+ intake.

Potassium Requirements


Serum K+ <3.5mmol/L
Capillary K+ values are generally higher than arterial K+ values so a low K+ on a capillary sample indicates significantly lower arterial K+ values.
Commonest Causes   
• Inadequate intake
• Alkalosis (particularly infants receiving sodium bicarbonate infusions.).
  Alkalosis may be secondary to bicarbonate treatment, over-ventilation, or loss of acid from gastric secretions.
• Renal causes
• Diarrhoea
• Medications (including diuretic therapy, sodium bicarbonate infusions, salbutamol, and insulin)
• Evaluate baby for potential causes of hypokalaemia (as above).
• Treatment will depend on the underlying cause and the severity of the hypokalaemia.
  CAUTION: Consider delayed treatment or monitor carefully if urine output is low or renal function is abnormal.
  Oral potassium supplements
  -  2 Molar KCl supplements (1ml = 2mmol KCl).
  -  Usually start at 2mmol/kg/day additional KCl.
  -  Monitor the serum K+ carefully and adjust dose accordingly.
  Intravenous potassium infusion 
  Refer to the potassium drug protocol.


Refer to the hyperkalaemia guideline.

Arterial Line Fluids

Add Heparin 250units/500ml to arterial fluids.

<1000gms Initially 0.5ml/hr of 0.45% NaCl (=0.9 mmol/day Na+). 
Change to 0.9% NaCl (=1.8mmol/day Na+) as soon as serum Na+ is stable. 
>1000 gms Initially 0.5 ml/hr of 0.9% NaCl (=1.8mmol/day Na+). 
Increase to 1 ml/hr (=3.6mmol/day Na+) as fluid intake increases. 

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Document Control

  • Date last published: 31 January 2007
  • Document type: Clinical Guideline
  • Services responsible: Neonatology
  • Owner: Newborn Services Clinical Practice Committee
  • Editor: Sarah Bellhouse
  • Review frequency: 2 years