Fluid maintenance and deficit replacement — viva

Opening (must-hit)

"This child has hospital-acquired hyponatraemia, almost certainly from hypotonic maintenance fluid in the postoperative state. The 0.45 percent saline is hypotonic, and after surgery her ADH is raised by the stress response, so her kidney is retaining free water and her sodium has fallen to 131. Her drowsiness is the early signal of hyponatraemic encephalopathy. My immediate plan is to stop the hypotonic fluid, change to an isotonic maintenance fluid — 0.9 percent saline with 5 percent dextrose and potassium chloride — reassess her neurological state, and check the sodium again. I would escalate to a higher-acuity bed because she is symptomatic." [2][7]

Examiner: "Walk me through her correct maintenance prescription."

"By Holliday-Segar, a 20 kg child needs 100 mL/kg for the first 10 kg plus 50 mL/kg for the next 10 kg, which is 1000 plus 500, or 1500 mL per day, around 63 mL per hour. The 4-2-1 rule gives 40 plus 20, or 60 mL per hour, which agrees. Her maintenance sodium is 2 to 3 mmol per kg per day and potassium 1 to 2 mmol per kg per day. The prescription is 0.9 percent saline with 5 percent dextrose and 20 mmol per litre of potassium chloride at about 60 mL per hour. I confirm the potassium is normal and she is passing urine before the potassium goes in." [1][2]

Examiner: "Why is the hypotonic bag wrong here, specifically?"

"Because the postoperative child has raised non-osmotic ADH. The stress of surgery, pain, nausea and anaesthesia all drive vasopressin, so the collecting duct stays open to water and the kidney cannot excrete a dilute load. Hang a hypotonic bag in that state and the free water is retained, the serum sodium falls, and water moves into the brain. Roberts and colleagues confirmed that perioperative water homeostasis in children shifts toward retention, which is exactly the mechanism here. The defence is tonicity: an isotonic fluid prevents net free-water movement even when ADH holds the volume." [11][7]

Examiner: "What evidence underpins the switch to isotonic?"

"The AAP 2018 clinical practice guideline recommends isotonic solutions with appropriate potassium chloride and dextrose for maintenance fluid in children aged one month and older. The McNab trial summarised in JAMA showed that isotonic maintenance fluid substantially reduces hyponatraemia compared with hypotonic fluid in hospitalised children, the Cochrane review reached the same conclusion, and the 2024 Amer meta-analysis confirmed the reduction without a compensatory rise in hypernatraemia or fluid overload. So the evidence is mature and consistent." [2][3][5]

Examiner: "Now turn it around — a child with hypernatraemic dehydration. How does your approach change?"

"Hypernatraemic dehydration, sodium above 145, is the scenario where rapid correction kills. The brain has accumulated idiogenic osmoles, so if I drop the sodium too fast, water rushes into brain cells and causes cerebral oedema, seizures and brainstem herniation. First I resuscitate shock with isotonic boluses if needed, because the circulation takes precedence. Then I correct the deficit slowly over 48 hours, not 24, and I hold the sodium fall to no faster than 0.5 mmol per litre per hour and no more than 10 to 12 mmol per litre in 24 hours. I check the sodium every 4 to 6 hours and slow down if it is falling too quickly." [7]

Examiner: "Finally — the deficit math. A 10 kg child at 8 percent dehydration."

"The deficit is percentage times weight times ten: 8 times 10 times 10 is 800 mL. I replace half over the first 8 hours and half over the next 16 hours, on top of maintenance, and I subtract any bolus volumes already given. So 400 mL over 8 hours is 50 mL per hour added to maintenance for the first phase, then 25 mL per hour added to maintenance for the second. I monitor the sodium within 24 hours, daily thereafter, and re-weigh the child every morning." [2]

Closing summary

"In summary: the postoperative child with a falling sodium has hospital-acquired hyponatraemia from hypotonic fluid and raised ADH — switch to isotonic with dextrose and potassium and escalate. Maintenance is Holliday-Segar 100/50/20. Deficit is percent times weight times ten, half over 8 and half over 16. Hypernatraemia is corrected slowly, no faster than 0.5 mmol per litre per hour. The single most reliable bedside marker is the daily weight, and the single most reliable cause of harm is the prescription that was never reviewed." [2][7]