Neonatal fluid, electrolyte and nutritional management — formative SAQs

SAQ 1 (10)

A 1000-gram infant is born at 28 weeks' gestation. On day 2 of life the infant is on 80 mL/kg/day of parenteral fluid with a serum potassium of 7.0 mmol/L and peaked T waves on the ECG, despite a urine output of 3 mL/kg/h. [4] [3]

1. Outline your initial fluid prescription for this infant from day 1 through the first week, including the expected weight trend. (3) [3]
2. Explain the mechanism of the hyperkalaemia and your immediate management of the ECG changes. (4) [4]
3. Describe your nutritional strategy for the first two weeks, including the rationale for early parenteral nutrition. (3) [2] [1]

Model answer

1. 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 weight, intake/output and serum sodium. The infant is expected to lose 5-15% of body weight in the first days as the extracellular fluid contracts (the physiological diuresis) — this weight loss is expected and is not treated with extra fluid, because liberal fluid increases the risk of PDA and NEC. Sodium is withheld until diuresis begins (day 2-3); potassium is added only once urine output is established. [3]
2. This is non-oliguric hyperkalaemia of prematurity — a developmental phenomenon caused by a shift of potassium out of cells due to immature Na+/K+-ATPase activity and rapid cell turnover, occurring with good urine output rather than renal failure. The peaked T waves and K+ above 7.0 demand immediate management: cardiac monitoring, calcium gluconate (0.5 mL/kg of 10% slowly with ECG monitoring) to stabilise the myocardium, then measures to shift potassium into cells (insulin-dextrose, a beta-agonist) and remove it. [4]
3. Start parenteral nutrition on day 0 with amino acids at 1.5-2 g/kg/day to prevent catabolism and negative nitrogen balance during the period of highest brain growth, advance lipid toward 3-3.5 g/kg/day, and provide the energy (110-135 kcal/kg/day), glucose at a glucose infusion rate of 4-6 (advancing toward 10-12) mg/kg/min, calcium, phosphate and micronutrients. Begin minimal enteral feeds of expressed breast milk (10-20 mL/kg/day) to prime the gut, advance cautiously toward full feeds with fortification, and wean parenteral nutrition as enteral feeds build — tracking growth on the Fenton chart as the judge. [2] [1]

SAQ 2 (10)

A 5-day-old exclusively breastfed term infant has lost 12% of birthweight. The infant is lethargic with a sunken fontanelle. Serum sodium is 158 mmol/L. [5]

1. What is the diagnosis, and what is the underlying mechanism? (3) [5]
2. Outline your fluid and electrolyte management, including the rate of sodium correction and why. (4) [5]
3. Discuss the prevention of this condition and the role of postnatal weight monitoring and lactation support. (3) [6]

Model answer

1. Hypernatraemic dehydration of the exclusively breastfed newborn, caused by inadequate breast milk intake from poor lactation. The sodium is elevated (above 150 mmol/L) with weight loss exceeding 10%, producing a hyperosmolar state; the free water deficit exceeds sodium loss. The lethargy and sunken fontanelle reflect moderate-to-severe dehydration. [5]
2. Rehydrate slowly — correct the sodium no faster than 0.5 mmol/L/hour or 10-12 mmol/L per day — because rapid correction causes cerebral oedema and seizures as water shifts rapidly into brain cells. Use oral or intravenous fluid (the latter if severely dehydrated or unable to feed), monitor weight and sodium frequently, support lactation with expression and supplementation, and treat the cause (poor intake). Avoid hypertonic solutions. [5]
3. Prevention centres on early and effective lactation support — timely initiation of feeding, assessment of latch and milk transfer, and monitoring of weight, wet nappies and feeding behaviour in the first days, especially in first-born infants of primiparous mothers. Scheduled weight checks at day 3-5 catch excessive loss early. Where weight loss exceeds 10% or the infant is unwell, prompt assessment and supplementation prevent progression to hypernatraemia. [6]