New-born fluid and electrolyte balance

2021-03-09 12:00 AM

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

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.