Pediatric Dehydration & Fluid Replacement

A 10-kg infant with sunken fontanels and no tears has already lost a dangerous percentage of body weight — and the fluid math you choose next determines whether that child stabilizes or crashes.

Children dehydrate faster than adults because they have a higher body surface area-to-weight ratio and higher metabolic rate, meaning greater insensible water losses. Dehydration severity is classified by percentage of body weight lost: mild (3–5%), moderate (6–9%), and severe (≥10%). Assessment findings escalate accordingly — mild shows slightly dry mucous membranes and decreased urine output; moderate adds sunken fontanels (in infants), absent tears, tachycardia, and decreased skin turgor; severe presents with hypotension, mottled or cool extremities, lethargy, and markedly delayed capillary refill (>3 seconds). The primary fluid for initial resuscitation is isotonic normal saline (0.9% NaCl) or lactated Ringer's, given as a 20 mL/kg bolus over 15–20 minutes, repeated as needed up to a total of 3 boluses (60 mL/kg). Maintenance fluids are calculated using the Holliday-Segar formula: 100 mL/kg/day for the first 10 kg, 50 mL/kg/day for the next 10 kg, and 20 mL/kg/day for each kilogram above 20. Oral rehydration therapy (ORT) with solutions like Pedialyte is first-line for mild-to-moderate dehydration when the child can tolerate oral intake. Monitor urine output closely — the minimum acceptable output is age-dependent: about 2 mL/kg/hr in infants, 1 mL/kg/hr in older children, and 0.5 mL/kg/hr in adolescents. Output below 1 mL/kg/hr is frank oliguria at any age.

Key Distinctions

Don't confuse isotonic (most common in peds, equal sodium and water loss) with hypotonic or hypertonic dehydration — isotonic shows no serum sodium shift. Students often calculate boluses using maintenance formulas instead of the 20 mL/kg resuscitation dose. A sunken anterior fontanel is an infant-specific finding — it doesn't apply to children over 18 months when the fontanel has closed.

Clinical Pearl

Think 20-20-20: 20 mL/kg bolus, over 20 minutes, reassess — and repeat up to a total of 3 boluses (60 mL/kg) before escalating. That rhythm keeps you on protocol.