• Postnatal physiological weight loss is approximately 5–10% in first week after birth
• Preterm babies have more total body water and may lose 10–15% of their weight in first week after birth
• Postnatal diuresis is delayed in respiratory distress syndrome and in babies who had significant intrapartum stress
• Preterm babies have limited capacity to excrete sodium in first 48 hr 
• Sodium chloride 0.9% contributes a significant chloride (Cl^–) load which can exacerbate metabolic acidosis
• Liberal sodium and water intake before onset of natural diuresis is associated with increased incidence of patent ductus arteriosus, necrotising enterocolitis and chronic lung disease
• After diuresis, a positive sodium balance is necessary for tissue growth
• Preterm babies, especially if born <29 weeks’ gestation, lose excessive sodium through immature kidneys
• Babies <28 weeks have significant transepidermal water (TEW) loss
  • TEW loss leads to hypothermia, loss of calories and dehydration, and causes excessive weight loss and hypernatraemia

Weigh

• On admission
  • Daily for intensive care babies: twice daily if fluid balance is a problem 
  • use in-line scales if available

Serum sodium

• Daily for intensive care babies
• If electrolyte problems or ≤26 weeks, measure twice daily 
  • admission electrolytes reflect maternal status: need not be acted upon but help to interpret trends 
  • serum urea not useful in monitoring fluid balance: reflects nutritional status and nitrogen load

Serum creatinine

• Daily for intensive care babies
• Reflects renal function over longer term
  • trend is most useful 
  • tends to rise over first 2–3 days 
  • gradually falls over subsequent weeks 
  • absence of postnatal drop is significant

Urine output

• Review 8-hrly for intensive care babies
  • 2–4 mL/kg/hr normal hydration
  • <1 mL/kg/hr requires investigation except in first 24 hr after birth
  • >6–7 mL/kg/hr suggests impaired concentrating ability or excess fluids

Humidification

• If <29 weeks, humidify incubator to ≥60%
• If ventilated or on CPAP ventilator, set humidifier at 39°C negative 2 to ensure maximal humidification of inspired gas

Normal fluid volume requirements

• Day 1 
  • glucose 10% 
  • if birth weight <1000 g or 1001–1500 g and baby not anticipated to reach 100 mL/kg/day enterally by day 5, start parenteral nutrition (PN) (see Parental nutritionguideline)
• Day 2
  • glucose 10% and potassium 10 mmol in 500 mL (depending on electrolyte results) or PN
  • use sodium chloride 0.45% in arterial line fluids
  • add sodium only when there is diuresis, or weight loss >6% of birth weight 
• Day 3 
  • glucose 10%, sodium chloride 0.18% and potassium 10 mmol in 500 mL or PN (with potassium 2 mmol/kg/day and sodium 4 mmol/kg/day) 
• After day 4 
  
  • glucose 10% (with maintenance electrolytes adjusted according to daily U&E) 
  • or PN
• Fluid volume requirements are a guide and can be increased faster or slower depending on serum sodium values, urine output and changes in weight
• Babies receiving phototherapy may require extra fluids depending on type of phototherapy

Response to treatment should be proportionate to degree of hyponatraemia

Causes

Excessive free water

• Reflection of maternal electrolyte status in first 24 hr
• Failure to excrete fetal extracellular fluid will lead to oedema without weight gain
• Water overload: diagnose clinically by oedema and weight gain
• Excessive IV fluids
• Inappropriate secretion of ADH in babies following major cerebral insults, or with severe lung disease
  • treatment with indometacin or ibuprofen

Excessive losses

• Prematurity (most common cause after aged 48 hr)
• Adrenal insufficiency
• GI losses
• Diuretic therapy (older babies)
• Inherited renal tubular disorders

Inadequate intake

• Preterm breastfed babies aged >7 days

Excessive IV fluids and failure to excrete fetal ECF

Management

• Reduce fluid intake to 75% of expected 

Inappropriate ADH

Clinical features

• Weight gain, oedema, poor urine output
• Serum osmolality low (<275 mOsm/kg) with urine not maximally dilute (osmolality >100 mOsm/kg)

Management

• Reduce fluid intake to 75% of expected
• Consider sodium infusion only if serum sodium <120 mmol/L

Acute renal failure

Management

• Reduce intake to match insensible losses + urine output
• Seek advice from middle grade doctor/consultant

Excessive renal sodium losses

Management

• Check urinary electrolytes
• Calculate fractional excretion of sodium (FE Na⁺ %):
  • FE Na⁺ = [(urine Na x plasma creatinine)/(urine creatinine x plasma Na)] x 100 
  • normally <1% but in sick preterm babies can be up to 10%
  • affected by sodium intake: increased intake leads to increased fractional clearance
  • if >1%, give sodium supplements
• Calculate sodium deficit
  • = (135 − plasma sodium) x 0.6 x weight in kg 
  • replace over 24 hr unless sodium <120 mmol/L or symptomatic (apnoea, fits, irritability)
  • initial treatment should bring serum sodium up to approximately 125 mmol/L
• Use sodium chloride 30% (5 mmol/mL) diluted in maintenance fluids. Ensure bag is mixed well before administration
• See Renal failureguideline

Adrenal insufficiency

Clinical features

• Hyperkalaemia
• Excessive weight loss 
• Virilisation of females
• Increased pigmentation of both sexes
• Ambiguous genitalia

Management

• Seek consultant advice

Inadequate intake

Clinical features

• Poor weight gain and decreased urinary sodium

Management

• Give increased sodium supplementation
• If receiving diuretics, stop or reduce dose

Excessive sodium intake leading to water retention

Clinical features

• Inappropriate weight gain

Management

• Reduce sodium intake

Treatment of acute symptomatic hyponatraemia with seizures

• Do not manage hyponatraemic encephalopathy using fluid restriction alone
• Give sodium chloride 2.7% 2 mL/kg IV via a central line over 10–15 min
• If symptoms still present, repeat 
• Measure serum sodium hourly until symptoms resolve
  • when symptoms resolved, ensure serum sodium does not increase by >12 mmol/L/24 hr

Prevention

• Prevent high TEW loss
  • use plastic wrap to cover babies of <32 weeks’ gestation at birth
  • nurse in high ambient humidity >80%
  • use bubble wrap
  • minimise interventions
  • humidify ventilator gases

Causes

• Water loss (most commonly)
  • TEW
  • glycosuria
• Excessive sodium intake
  • sodium bicarbonate 
  • repeated boluses of sodium chloride 
  • congenital hyperaldosteronism/diabetes insipidus (very rare)

Hypernatraemia resulting from water loss

Clinical features

• Leads to weight loss with hypernatraemia

Management

• Increase fluid intake and monitor serum sodium

Osmotic diuresis

Management

• Treat hyperglycaemia with an insulin infusion (see Hyperglycaemia guideline)
• Rehydrate with sodium chloride 0.9%

Hypernatraemia resulting from excessive intake

Management

• If acidosis requires treatment, use THAM (trometamol) instead of sodium bicarbonate 
• Reduce sodium intake
• Change arterial line fluid to sodium chloride 0.45% 
• Minimise number and volume of flushes of IA and IV lines

• Do not leave bag of fluid connected (blood components excepted)
• Nurse to check hourly:
  • infusion rate
  • infusion equipment
  • site of infusion
• Before removing giving set, close all clamps and switch off pump 

Useful information

• Percentage solution = grams in 100 mL (e.g. glucose 10% = 10 g in 100 mL)
• 1 millimole = molecular weight in milligrams

Compositions of commonly available solutions

Useful figures

• Sodium chloride 30% = 5.13 mmol/mL each of Na and Cl
• Sodium chloride 0.9% = 0.154 mmol/mL each of Na and Cl
• Potassium chloride 15% = 2 mmol/mL each of K and Cl
• Calcium gluconate 10% = 0.225 mmol/mL of Ca
• Sodium bicarbonate 8.4% = 1 mmol/mL each of Na and bicarbonate
• Sodium chloride 0.9% 1 mL/hr = 3.7 mmol Na in 24 hr

Osmolality

• Serum osmolality = 2(Na + K) + glucose + urea (normally 285–295 mOsmol/kg)
• Anion gap = (Na⁺ + K⁺) - (Cl^– + HCO₃^–) normally 7–17 mmol/L
• Normal urine: osmolality 100–300 mOsmol/kg, specific gravity 1004–1015
• Babies can dilute urine up to 100 mOsmol/kg, but can concentrate only up to 700 mOsmol/kg