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Chapter: Paediatrics: Neonatology

Newborn fluid and electrolyte balance

The newborn baby is largely water (775% term, 785% at 26/40).

Newborn fluid and electrolyte balance




   The newborn baby is largely water (775% term, 785% at 26/40).


   There is a large extracellular compartment (65% of body weight at 26/40 compared to 40% by term, 20% in adult).


   There is a rapid loss of extracellular fluid after birth.


   Decreased pulmonary vascular resistance increases blood flow to left atrium, thereby inducing increased Atrial Natriuretic Peptide release (i GFR/d Na+ reabsorbtion/ ‘fall’ rennin-angiotensin aldosterone system).


   Physiological increased urine output at 712–24hr after birth.


   Na/K ATPase activity is low at birth, but increases steadily (Na/K ATPase is responsible for reabsorbing Na+ from renal tubular lumen, in turn creating a gradient to allow reabsorption of Glucose, Na+ and amino acids. Immature infants have lower enzyme activity.


Preterm babies have


   A variable ability to excrete a sodium load.


   An excellent ability to deal with water load.


   Modulated by ADH (osmo and baro-receptors).


   A tendency to lose sodium in urine over first weeks as the increased glomerular filtration rate (GFR) exceeds ability to resorb Na+.

   A high transepidermal water loss (TEWL). Evaporation from immature skin, <28/40. To reduce nurse in incubator with 80% humidity


   Respiration related water losses (ventilated and spontaneously breathing infants) can be countered with warm-humidified gases.


   Sick infants (e.g. respiratory distress syndrome (RDS)) will have delayed dieresis;


   giving additional Na+ will further delay diuresis and may worsen outcome;


   attempts to induce diuresis (e.g. with Furosemide) unlikely to be helpful.


Postnatal weight loss


   Weight loss after birth is normal;

o up to 10 % in well term infants over the first week of life;

o greater in preterm/VLBW.


   Rising sodium suggests dehydration (term and preterm infants).


   Failure to lose weight may suggest fluid retention/overload.


   Infants with >10% weight loss require further assessment of feeding;

o risk of hypernatraemic dehydration;

o usually breast-fed infants with unrecognized poor feeding;

o weigh all babies day 3 (some suggest day 5);

o check U&E if weight loss >12%;

o support mother with breast expressing/top-up feeds;


   may require admission/NG feeds/IV fluids

Specific disturbances



Na+ < 130mmol/L.



o water overload (most common in first-week);

o maternal fluid overload;

o iatrogenic;

o sick infant (e.g. birth asphyxia, sepsis);

o excess renal loss (common ‘late’ cause in preterm infants);

o GI loss, e.g. diarrhoea, NG aspirates, high output stoma;

o drainage of ascites/CSF;

o other (e.g. hypoadrenalism of any cause, Bartter syndrome/Fanconi syndrome).


Symptoms: irritability, apnoeas, seizures.


Treatment: dependent on underlying cause (e.g. fluid restriction/ Na+ supplementation)

Take care as too rapid correction can cause neurological damage.




   Risk of seizures if Na+ >150mmol/L.


Causes: water depletion (usual), excess Na+ administration (unusual as normally retain water also).

Two major at-risk groups:


o extreme preterm infants in first days of life (excess water losses, e.g. TEWL);

o breast-fed infants with poor intake.

Treatment: increase fluid intake (caution with rapid correction).



K+  <2.5mmol/L—causes:

o excess losses (diarrhoea, vomiting, NG aspirate, stoma, renal/ diuretics);

o inadequate intake (failure to recognize daily requirement, e.g. TPN).

Correct with supplementation (IV or enteral):


o caution with enteral if GI disturbance;

o extreme caution with IV infusion as risk of heart arrythmia.



K+ >7.5mmol/L OR >6.5mmol/L and ECG changes.

Causes: Failure of K+ excretion, e.g. renal failure.



o myocardial stabilization: calcium gluconate;

o elimination: calcium resonium, dialysis;

   re-distribution: salbutamol, insulin.


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