Fluid maintenance and deficit replacement — formative SAQs

SAQ 1 (10)

A 4-year-old boy weighing 16 kg is admitted with gastroenteritis. He is thirsty and lethargic with sunken eyes and reduced skin turgor, normal capillary refill and a pulse of 110. His serum sodium is 138 mmol/L, potassium 4.0 mmol/L, and he has passed urine in the emergency department. He is unable to tolerate oral fluids. ^[8][2]

1. Calculate his daily maintenance water requirement and the maintenance sodium and potassium requirements, showing your working. (3) ^[1][2]
2. Write the maintenance fluid prescription you would use, naming the fluid, tonicity, dextrose and potassium content, and justify why it is isotonic. (4) ^[2][3]
3. He is assessed as 8 percent dehydrated. Calculate his fluid deficit and outline how you would replace it over the next 24 hours alongside maintenance. (3) ^[8][7]

Model answer

Maintenance calculation. By the Holliday-Segar rule, daily water is 100 mL/kg for the first 10 kg plus 50 mL/kg for the next 6 kg: 1000 plus 300 equals 1300 mL per day, which is about 54 mL per hour (the 4-2-1 rule gives 40 plus 12, or 52 mL per hour, in agreement). Maintenance sodium is 2 to 3 mmol/kg/day, so 32 to 48 mmol per day, and potassium is 1 to 2 mmol/kg/day, so 16 to 32 mmol per day. ^[1][2]

Maintenance prescription. The fluid is 0.9 percent saline with 5 percent dextrose and 20 mmol/L of potassium chloride at about 52 mL per hour, an isotonic maintenance fluid. Potassium is added because the serum potassium is normal and he has passed urine, confirming renal perfusion. The reason for isotonic is that the sick child has raised non-osmotic ADH from pain, nausea and fever, which makes the kidney retain free water; a hypotonic bag in that state causes hospital-acquired hyponatraemia and hyponatraemic encephalopathy. The McNab trial and the 2024 Amer meta-analysis confirm that isotonic maintenance fluid reduces hyponatraemia without a compensatory rise in hypernatraemia. ^[2][3][5]

Deficit replacement. The deficit is percentage dehydration times weight times ten: 8 times 16 times 10, or 1280 mL. Half is replaced over the first 8 hours (640 mL, at 80 mL per hour on top of the maintenance rate) and half over the next 16 hours (640 mL, at 40 mL per hour on top of maintenance). The combined rate over the first 8 hours is maintenance plus deficit, so about 132 mL per hour, tapering to about 92 mL per hour for the second 16 hours. He is not in shock, so no boluses are given; had he needed boluses, those volumes would be subtracted from the deficit. The sodium is monitored within 24 hours and daily, and the child is re-weighed daily. ^[8][7]