Paediatric Dehydration, Hyponatremia, and the Wrong Fluid
A 3-year-old boy weighing 15 kg is brought with 3 days of watery diarrhoea and vomiting. He is lethargic, has sunken eyes, dry mucous membranes, capillary refill 4 seconds, heart rate 142/min, respiratory rate 42/min, and BP 82/50 mmHg. His weight before illness was 16.5 kg. Laboratory: sodium 128 mmol/L, potassium 3.1 mmol/L, chloride 98 mmol/L, bicarbonate 10 mmol/L, pH 7.22, PaCO2 25 mmHg, glucose 68 mg/dL, creatinine 0.4 mg/dL, and urine output 0.3 mL/kg/hour. The mother reports he has passed no urine for 8 hours. He has no fever, neck stiffness, or rash. The emergency physician wants to give 500 mL of 0.45% saline over 1 hour and then continue with 0.45% saline with 20 mEq/L KCl. A senior nurse questions this plan.
1. Assess the severity and type of dehydration and calculate the initial fluid bolus, deficit, and maintenance requirements.
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This child has severe dehydration with shock: lethargy, sunken eyes, capillary refill of 4 seconds, tachycardia, hypotension and minimal urine output. The pre-illness weight of 16.5 kg versus 15 kg indicates a 1.5 kg loss, approximately 9–10% dehydration. The sodium of 128 mmol/L indicates hypotonic hyponatremia; the history of diarrhoea and vomiting with poor intake explains both sodium and free-water loss.
Give an initial bolus of 0.9% sodium chloride 20 mL/kg = 300 mL over 20–30 minutes and reassess. Repeat the bolus if shock persists. The estimated deficit is 1,500 mL. After resuscitation, replace the deficit plus maintenance over 24–48 hours, subtracting boluses already given. Maintenance by the 4-2-1 rule is 50 mL/hour. Use isotonic fluid and aim for a sodium rise of no more than 8–10 mmol/L in 24 hours to reduce the risk of cerebral oedema. Check glucose and calcium and monitor urine output.
2. Critique the proposed plan of 0.45% saline with 20 mEq/L KCl.
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The plan is unsafe for two main reasons. First, 0.45% saline is hypotonic. In a child with symptomatic hyponatremia and possible cerebral oedema, hypotonic fluid can lower serum sodium further and precipitate seizures, worsening neurological injury. Isotonic 0.9% saline or Ringer’s lactate is preferred for initial resuscitation and deficit replacement. Second, adding 20 mEq/L KCl before adequate urine output is established risks hyperkalemia, especially in a child with prerenal acute kidney injury. Potassium should be added only after urine output is documented and potassium is checked, usually at 20–40 mEq/L in maintenance fluid.
The rate of 500 mL over 1 hour is also too rapid for routine maintenance and could cause volume overload. The correct approach is an isotonic bolus, reassessment, then isotonic deficit replacement with careful sodium monitoring. Hypertonic 3% saline is reserved for seizures or severe symptomatic hyponatremia, not for this child at present.
3. Interpret the acid-base status and explain the respiratory compensation.
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The pH of 7.22 indicates acidemia. Bicarbonate is low at 10 mmol/L, so the primary disorder is metabolic acidosis. The PaCO2 of 25 mmHg is low, indicating respiratory compensation. Using Winter’s formula, expected PaCO2 = 1.5 × HCO3 + 8 ± 2 = 1.5 × 10 + 8 = 23 ± 2 mmHg; the measured 25 mmHg is appropriate.
The anion gap is Na − (Cl + HCO3) = 128 − (98 + 10) = 20 mmol/L, which is elevated. This suggests a mixed acidosis: diarrhoea causes a normal-anion-gap hyperchloremic acidosis, while dehydration, poor perfusion, and ketosis add a high-anion-gap component. Glucose is 68 mg/dL, so hypoglycemia is not the main driver. Treatment is circulatory resuscitation with isotonic fluid; bicarbonate is not routinely indicated unless the pH is below 7.1 with severe symptoms or there is a specific indication. Recheck gases after fluids.