Newborn fluid and electrolyte balance

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.