Pediatric Dehydration & Fluid Management
A STEP 2 CK-focused walkthrough of pediatric dehydration: estimating the deficit, grading severity, choosing oral versus IV rehydration, and managing iso-, hypo-, and hypernatremic states with the correct fluid math and next-best-step decisions.
Overview: How the boards test peds dehydration
Acute dehydration in children is most often caused by viral gastroenteritis (rotavirus, norovirus), with losses from vomiting and diarrhea. Boards test a predictable sequence: estimate the fluid deficit → grade severity → choose oral vs IV rehydration → correct any sodium disturbance. The single most reliable measure of deficit is acute weight loss (1 kg lost ≈ 1 L fluid). When a recent weight is unavailable, severity is judged clinically — the three most useful physical signs are prolonged capillary refill, abnormal skin turgor, and an abnormal (deep) respiratory pattern. Infants dehydrate faster than older children because of a higher body-surface-area-to-mass ratio, higher total-body-water fraction, and higher metabolic rate. The exam reward is knowing that oral rehydration therapy (ORT) is first-line for mild-to-moderate disease, and that IV isotonic boluses are reserved for shock or ORT failure — not given reflexively to every vomiting child.
- Severity (% body-weight loss): mild 3-5%, moderate 6-9%, severe ≥10% (infants run higher: ~5 / 10 / 15%)
- Deficit (mL) = % dehydration × weight (kg) × 10 — e.g., 10% of a 10-kg child = 1000 mL
- First-line = ORT (reduced-osmolarity WHO ORS, ~245 mOsm/L): ~50 mL/kg (mild) or ~100 mL/kg (moderate) over 4 h, plus replacement of ongoing losses
- Ondansetron (single dose) cuts vomiting and ORT failure — try it before going IV
- Severe/shock → IV bolus 20 mL/kg isotonic crystalloid (NS or LR); reassess and repeat as needed
- Maintenance = 4-2-1 rule (Holliday-Segar)
- Use an isotonic maintenance fluid (e.g., D5-NS + KCl) per AAP 2018 to prevent iatrogenic hyponatremia
- Add potassium only after the child voids (confirms renal function)
- Hypernatremia: correct SLOWLY — lower Na <0.5 mEq/L/hr (<10-12 mEq/L/day) to avoid cerebral edema/seizures
- Best individual exam signs: prolonged cap refill, poor skin turgor, abnormal breathing
Grading dehydration severity
| Feature | Mild (3-5%) | Moderate (6-9%) | Severe (≥10%) |
|---|---|---|---|
| Mental status | Alert | Irritable | Lethargic/obtunded |
| Mucous membranes | Slightly dry | Dry | Parched, cracked |
| Skin turgor | Normal | Decreased | Tenting |
| Capillary refill | <2 s | 2-3 s | >3 s |
| Eyes / fontanelle | Normal | Slightly sunken | Deeply sunken |
| Pulse / BP | Normal | Tachycardia | Tachycardia, ↓BP (late) |
| Urine output | Normal or ↓ | Decreased | Anuria |
| Tears | Present | Decreased | Absent |
| Management | ORT | ORT (± ondansetron) | IV 20 mL/kg bolus |
Vignette: An 18-month-old has 2 days of watery diarrhea and vomiting. He is irritable but consolable; HR 150, capillary refill 3 s, dry mucous membranes, decreased tears, and reduced urine output. His weight is 6% below his last clinic value.
Assessment: Moderate isotonic dehydration from viral gastroenteritis.
Next best step: Oral rehydration therapy with reduced-osmolarity ORS, ~100 mL/kg over 4 h in small, frequent aliquots, replacing ongoing losses. If vomiting limits intake, give a single dose of ondansetron to enable ORT rather than defaulting to IV. Reserve IV fluids for ORT failure or progression to shock. Continue age-appropriate feeding/breastfeeding once rehydrated. Avoid plain water, juice, and sports drinks (wrong osmolarity/electrolytes) and antidiarrheal agents.
Vignette: A 10-kg toddler with 3 days of diarrhea is lethargic with sunken eyes, tenting skin, capillary refill 5 s, HR 180, and cool, mottled extremities. He has not urinated in 12 h.
Assessment: Severe (≥10%) dehydration with hypovolemic shock.
Next best step: Rapid IV or intraosseous bolus of 20 mL/kg isotonic crystalloid (normal saline or LR); reassess perfusion and repeat boluses as needed until perfusion improves. Then calculate the remaining deficit plus maintenance — estimated deficit here ≈ 10% × 10 kg = 1 L (severe may be underestimated; titrate to reassessment). Send electrolytes and glucose, but do not delay volume resuscitation to obtain labs. Add KCl only after the child voids.
Vignette: A 3-week-old is lethargic and irritable with a high-pitched cry and 'doughy' skin. The parents ran out of formula and added extra powder to 'make it stronger.' Serum Na is 168 mEq/L.
Assessment: Hypernatremic (hypertonic) dehydration from over-concentrated formula. Water shifts out of cells into the vasculature, so classic turgor and hemodynamic signs are blunted and neurologic signs dominate.
Next best step: Restore perfusion with isotonic saline if in shock, then correct sodium slowly — lower Na by <0.5 mEq/L/hr (<10-12 mEq/L/day), replacing the free-water deficit over ≥48 h. Rapid correction drives water into brain cells → cerebral edema and seizures. Counsel parents on correct formula mixing before discharge.
Dehydration by serum sodium
| Type | Serum Na | Typical cause | Key point |
|---|---|---|---|
| Isotonic | 130-150 | Gastroenteritis (most common, ~80%) | Na and water lost proportionally; replace with isotonic fluid |
| Hypotonic (hyponatremic) | <130 | Replacing losses with plain water; excess hypotonic IV fluid | Cerebral edema risk; correct Na carefully (hypertonic saline only for symptomatic seizures) |
| Hypertonic (hypernatremic) | >150 | Over-concentrated formula, inadequate free water, insensible losses | Doughy skin, seizures; correct Na slowly over ≥48 h |
'4-2-1' rule (Holliday-Segar) — hourly maintenance:
- 4 mL/kg/hr for the first 10 kg
- 2 mL/kg/hr for the next 10 kg
- 1 mL/kg/hr for each kg above 20
Daily equivalent = '100-50-20': 100 mL/kg (first 10 kg) + 50 mL/kg (next 10 kg) + 20 mL/kg (each kg >20).
Example — 25-kg child: (4×10) + (2×10) + (1×5) = 65 mL/hr.
Remember: maintenance is added to deficit and ongoing-loss replacement — not a substitute for them. Current guidance (AAP 2018) favors an isotonic maintenance solution with dextrose (add KCl once the child is voiding).
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