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]

SAQ 2 (10)

A 9-month-old infant weighing 8 kg presents with a three-day history of profuse watery diarrhoea and reduced oral intake. She is lethargic with a very dry mouth, sunken fontanelle, cool peripheries and a capillary refill of 3 seconds. Her serum sodium is 152 mmol/L. She has not passed urine for 8 hours. [7][8]

1. Describe the immediate fluid management in the first hour and explain why, including the fluid, volume and rationale. (3) [8][7]
2. Outline the subsequent deficit-replacement plan, including the total deficit volume, the duration of correction, and the maximum rate of sodium fall. (4) [7][8]
3. Explain why rapid correction of the sodium is dangerous in this infant, and state the monitoring you would put in place. (3) [7]

Model answer

Immediate management. She is in hypovolaemic shock, with cool peripheries, prolonged capillary refill and oliguria, so the first action is an isotonic fluid bolus of 20 mL/kg, which is 160 mL of 0.9 percent saline (or a balanced crystalloid such as Hartmann's), given rapidly over 5 to 10 minutes and repeated as needed up to 60 mL/kg until perfusion is restored. The bolus is isotonic and free of dextrose because the goal is intravascular expansion with an effective osmole, not maintenance. Her hypernatraemia does not change the bolus fluid, because restoring the circulation takes precedence over the sodium number. [8][7]

Deficit replacement. Once perfused, the total deficit is estimated. Severe hypernatraemic dehydration is at least 10 percent: using 10 percent for an 8 kg infant gives a deficit of 800 mL, and the bolus volumes already given are subtracted from this. Because the sodium is above 145 mmol/L, correction is extended to 48 hours rather than 24, and the free-water deficit is replaced slowly. The sodium must not fall faster than 0.5 mmol per litre per hour and not more than 10 to 12 mmol per litre in 24 hours, using a fluid less hypertonic than plasma but still sodium-containing, such as 0.45 percent saline with dextrose, adjusted to the serial sodium. Maintenance continues as isotonic with dextrose and potassium once urine output is established. [7][8]

Danger of rapid correction and monitoring. In hypernatraemia the brain accumulates idiogenic osmoles to defend its volume against the hypertonic extracellular fluid. If the sodium is corrected too fast, the extracellular fluid becomes relatively hypotonic to the adapted brain, water moves into brain cells by osmosis, and cerebral oedema, seizures, brainstem herniation and death follow. Monitoring is therefore close: check the serum sodium every 4 to 6 hours during correction, measure the urine output hourly, re-weigh daily, and observe the neurological state continuously, with a low threshold to slow the rate if the sodium is falling too quickly. [7]