A 18-month-old boy from rural Maharashtra presents with a 3-day history of watery diarrhea (8–10 stools/day) and vomiting. On examination: sunken eyes, skin turgor returns after 2 seconds, mucous membranes are dry, and urine output is reduced. Capillary refill time is 2 seconds. Weight before illness was 12 kg; current weight is 11.2 kg. Serum sodium is 128 mEq/L, potassium 3.2 mEq/L, chloride 98 mEq/L, and HCO₃⁻ is 12 mEq/L. What is the primary acid–base disturbance and the most appropriate initial rehydration strategy?

Explanation

## Clinical Assessment **Dehydration Classification:** - Weight loss: (12 − 11.2) / 12 × 100 = 6.7% → **moderate dehydration** - Sunken eyes + slow skin turgor (2 sec) + dry mucous membranes + reduced urine output confirm moderate-to-severe dehydration - Capillary refill 2 seconds suggests borderline shock **Electrolyte & Acid–Base Analysis:** - Na⁺ 128 mEq/L → **hyponatremia** (normal 135–145) - K⁺ 3.2 mEq/L → **hypokalemia** (normal 3.5–5.0) - HCO₃⁻ 12 mEq/L → **metabolic acidosis** (normal 22–26) - Chloride 98 mEq/L → **hypochloremia** This pattern is typical of **cholera or secretory diarrhea** with hypotonic fluid losses (water loss > electrolyte loss), leading to hyponatremic dehydration with metabolic acidosis. ## Rehydration Strategy **Key Point:** In hyponatremic dehydration, rapid correction of sodium (>10 mEq/L in 24 hours) risks osmotic demyelination syndrome. Gradual sodium repletion over 48 hours is safer. **High-Yield:** WHO-recommended approach for moderate dehydration with hyponatremia: 1. **Initial bolus:** 0.9% saline (isotonic) 30 mL/kg IV over 1 hour to restore intravascular volume and prevent shock 2. **Maintenance + ongoing losses:** 0.45% saline (hypotonic) to allow gradual sodium correction 3. **Potassium:** Add 20 mEq/L once urine output confirmed (avoid hyperkalemia in acute phase) **Clinical Pearl:** 0.9% saline is used first because it is isotonic and does not worsen hyponatremia acutely; switching to 0.45% saline after initial stabilization allows gradual correction of the sodium deficit. ## Why This Answer Option 1 is incorrect because 0.9% saline at 100 mL/kg/hour is excessive and risks fluid overload; hypernatremic dehydration is not present here (Na⁺ is LOW, not high). Option 3 (Ringer's lactate) is suboptimal for hyponatremic dehydration; it contains only 130 mEq/L sodium, which is lower than 0.9% saline (154 mEq/L) and does not correct hyponatremia as effectively. Option 4 misidentifies the acid–base disorder (metabolic alkalosis is not present; HCO₃⁻ is 12, not elevated) and suggests 5% dextrose, which is hypotonic and would worsen hyponatremia.