Neonatal fluid, electrolyte and nutritional management — viva

Stem

The examiner hands you the chart of a 1000-gram, 28-week infant on day 2 of life: on 80 mL/kg/day of parenteral fluid with parenteral nutrition, a serum potassium of 7.0 mmol/L with peaked T waves on the ECG, urine output 3 mL/kg/h, and a weight 6% below birthweight. [4]

Examiner: Walk me through your assessment of this infant's fluid and electrolyte status. [4]

Strong answer: This infant is following the expected physiological trajectory — the 6% weight loss is the contracting extracellular fluid compartment (the diuresis), expected and not to be treated with extra fluid, and the fluid of 80 mL/kg/day on day 2 is appropriate. The concern is the potassium: at 7.0 mmol/L with peaked T waves, this is non-oliguric hyperkalaemia of prematurity — a developmental shift of potassium out of cells from immature Na+/K+-ATPase activity, not renal failure, given the good urine output. The ECG changes make this an emergency: I would institute cardiac monitoring, give calcium gluconate to stabilise the myocardium, and shift and remove potassium. [4]

Branch 1 — The fluid prescription

Examiner: How do you construct the fluid prescription for this infant over the first week? [2]

Strong answer: I start at 60 mL/kg/day on day 1 and advance by about 20 mL/kg/day toward 130-150 mL/kg/day, titrated to the weight trend, intake/output and the serum sodium. The infant is meant to lose 5-15% of weight in the first days as the extracellular fluid contracts — I expect and allow that loss, because giving extra fluid risks a patent ductus and necrotising enterocolitis, as the Cochrane review of restricted versus liberal water intake confirmed. Sodium is withheld until diuresis begins (day 2-3) — the Hartnoll trial showed delayed sodium gives better body composition. [2] [5]

Examiner probe: Why does the very preterm infant tolerate fluid so poorly? [2]

Strong answer: Total body water is highest at the lowest gestation (about 90% at 24 weeks), the kidney has a low glomerular filtration rate that cannot excrete a water load, and the surface-area-to-mass ratio is enormous — so overload becomes oedema and a symptomatic ductus, while inadequate intake becomes dehydration and hypernatraemia rapidly. [2]

Branch 2 — The hyperkalaemia

Examiner: Manage the hyperkalaemia for me. [4]

Strong answer: With peaked T waves and a K+ of 7.0, I treat immediately. First, cardiac monitoring and an ECG. I give calcium gluconate, 0.5 mL/kg of 10% slowly with ECG monitoring, to stabilise the myocardium. Then I shift potassium into cells — insulin and dextrose, or a nebulised or intravenous beta-agonist. I remove potassium with the kidney (furosemide if there is output) or the gut (a potassium-binding resin). I check for and correct acidosis, review the potassium in the parenteral fluid, and recheck the level. Because this is the developmental non-oliguric form, it usually resolves as the tubule matures — but I treat the ECG changes aggressively. [4]

Examiner probe: What would change your mind about the cause? [4]

Strong answer: Oliguria, a rising creatinine, haemolysis or tissue breakdown (NEC), or severe acidosis would point to renal failure or a catabolic cause rather than the benign developmental form — and a persistent or worsening hyperkalaemia beyond the first week would make me reconsider. [4]

Branch 3 — The nutrition strategy

Examiner: This infant is on parenteral nutrition from day 0. Why, and how do you advance? [3] [1]

Strong answer: I start parenteral amino acids at 1.5-2 g/kg/day from birth because withholding them leads to negative nitrogen balance within hours — the preterm brain is growing and myelinating at its fastest rate and has negligible protein stores, so early catabolism drives extra-uterine growth restriction and poorer neurodevelopment. I advance lipid toward 3-3.5 g/kg/day, supply glucose at a glucose infusion rate of 4-6 (advancing toward 10-12) mg/kg/min, and provide the energy (110-135 kcal/kg/day), calcium, phosphate and micronutrients. In parallel I begin minimal enteral feeds of expressed breast milk (10-20 mL/kg/day) to prime the gut, advance cautiously toward full feeds with fortification for the preterm, and wean parenteral nutrition as the gut takes over. I track growth on the Fenton chart — 10-15 g/kg/day with head growth — because growth is the judge of the whole prescription. [3] [1]

Examiner probe: How do you minimise the risk of necrotising enterocolitis while advancing feeds? [1]

Strong answer: Use expressed breast milk (the single most protective factor), start with minimal enteral feeds and advance at a moderate rate (around 20-30 mL/kg/day) in the stable infant, slower if there are risk factors (growth restriction, absent or reversed end-diastolic flow, indomethacin), and watch for feeding intolerance — residuals, abdominal distension, blood in the stool — as a signal to hold and reassess. [1]