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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

 

Normal

 

   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

 

Hyponatraemia

Na+ < 130mmol/L.

 

Causes:

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.

 

Hypernatraemia

 

   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).

 

Hypokalaemia

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.

 

Hyperkalaemia

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

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

Treatments:

 

o myocardial stabilization: calcium gluconate;

o elimination: calcium resonium, dialysis;

   re-distribution: salbutamol, insulin.

 

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Paediatrics: Neonatology


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