New-born fluid and electrolyte balance

New-born 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’ renin-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 the 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 baroreceptors).
• A tendency to lose sodium in urine over the first weeks as the increased glomerular filtration rate (GFR) exceeds the ability to resorb Na⁺.
• A high transepidermal water loss (TEWL). Evaporation from immature skin, <28/40. To reduce nurse in an 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 the 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 the risk of heart arrhythmia.

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.