How should I calculate fluid replacement (rehydration and maintenance) for a pediatric patient with dehydration, assuming no cardiac, renal, or severe electrolyte disorders?

Fluid Replacement Calculation for Pediatric Dehydration

Assess Dehydration Severity First

The first critical step is to clinically grade dehydration severity, as this determines all subsequent fluid calculations and routes of administration. 1

Clinical Classification

• Mild dehydration (3–5% fluid deficit): Increased thirst and slightly dry mucous membranes 2, 1
• Moderate dehydration (6–9% fluid deficit): Loss of skin turgor with skin tenting when pinched, dry mucous membranes, and decreased urine output 2, 1
• Severe dehydration (≥10% fluid deficit): Severe lethargy or altered consciousness, prolonged skin tenting >2 seconds, cool poorly perfused extremities with delayed capillary refill, and rapid deep breathing indicating acidosis 2, 1

The most reliable clinical predictors are capillary refill time, prolonged skin retraction time, and decreased peripheral perfusion—not sunken fontanelle or absent tears. 2, 1

Obtain an accurate body weight immediately to calculate fluid deficit and monitor response. 2, 1

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Rehydration Phase: Calculate and Replace Deficit

Mild Dehydration (3–5% deficit)

Administer 50 mL/kg of oral rehydration solution (ORS) containing 50–90 mEq/L sodium over 2–4 hours. 2, 1

• Begin with very small volumes (≈5 mL, one teaspoon) using a spoon, syringe, or medicine dropper, then gradually increase as tolerated 2, 1
• For a 10 kg child: 50 mL/kg = 500 mL over 2–4 hours 1

Moderate Dehydration (6–9% deficit)

Administer 100 mL/kg of ORS over 2–4 hours using the same small-volume technique. 2, 1, 3

• For a 10 kg child: 100 mL/kg = 1,000 mL over 2–4 hours 1, 3
• If oral intake fails, use nasogastric administration at 15 mL/kg/hour 1, 3

Severe Dehydration (≥10% deficit)

This is a medical emergency requiring immediate intravenous rehydration with 20 mL/kg boluses of Ringer's lactate or 0.9% normal saline until pulse, perfusion, and mental status normalize. 2, 1

• Administer boluses without delay; repeat as needed until circulation is restored 1
• Once stabilized, transition to ORS for remaining deficit replacement 1

Reassess hydration status after 2–4 hours of any rehydration therapy; if still dehydrated, re-estimate the deficit and continue therapy. 2, 1, 3

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Maintenance Phase: Calculate Daily Fluid Needs

After correcting the deficit, calculate maintenance fluids using the Holliday-Segar formula: 1

• First 10 kg: 100 mL/kg/day
• Second 10 kg: 50 mL/kg/day
• Each additional kg >20 kg: 20 mL/kg/day

Example for a 12 kg child: (10 kg × 100 mL) + (2 kg × 50 mL) = 1,100 mL/day (≈46 mL/hour) 1

Use 0.9% normal saline with 5% dextrose for the first 24 hours after deficit correction. 1

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Ongoing Loss Replacement: Add to Maintenance

Replace each watery stool with 10 mL/kg of ORS. 1, 3

Replace each vomiting episode with 2 mL/kg of ORS. 1, 3

Age-Specific Volumes

• Children <2 years: Give 50–100 mL of ORS after each loose stool 1, 3
• Older children: Give 100–200 mL after each stool 1, 3

For a 10 kg toddler with 5 watery stools: 10 mL/kg × 5 = 500 mL additional ORS beyond maintenance 1

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Nutritional Management During Rehydration

Resume age-appropriate diet immediately after rehydration is achieved; do not impose "bowel rest." 2, 1

• Continue breastfeeding without interruption throughout the illness 2, 1, 3
• Resume full-strength formula immediately after the 2–4 hour rehydration period 1, 3
• Offer starches, cereals, yogurt, fruits, and vegetables; avoid foods high in simple sugars and fats 2, 1
• Feed every 3–4 hours to maintain nutrition and promote intestinal recovery 1

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Complete Calculation Example: 10 kg Child with Moderate Dehydration

1. Rehydration phase (first 2–4 hours): 100 mL/kg = 1,000 mL of ORS 1, 3
2. Maintenance (after rehydration): 100 mL/kg/day = 1,000 mL/day (≈42 mL/hour) of 0.9% NaCl with 5% dextrose 1
3. Ongoing losses: Add 10 mL/kg (100 mL) per watery stool and 2 mL/kg (20 mL) per vomit 1, 3

Total first 24 hours: 1,000 mL (deficit) + 1,000 mL (maintenance) + ongoing losses = ≈2,000 mL minimum 1

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Critical Pitfalls to Avoid

Do not use hypotonic solutions for initial rehydration—use isotonic ORS (50–90 mEq/L sodium) or 0.9% normal saline to prevent iatrogenic hyponatremia. 1

Do not withhold food or delay feeding; "bowel rest" lacks evidence and delays recovery. 2, 1

Do not use homemade salt-sugar solutions, sports drinks, or juices—commercial ORS ensures proper electrolyte composition. 1

Antimotility agents (loperamide) are absolutely contraindicated in all children <18 years due to risks of respiratory depression and serious cardiac adverse reactions. 1

Do not rely solely on sunken fontanelle or absent tears for dehydration assessment—these are less reliable than skin turgor, capillary refill, and perfusion. 2, 1

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When to Escalate Care

Seek immediate medical evaluation if: 1

• Severe lethargy or altered consciousness develops
• Bloody diarrhea appears
• High fever emerges
• Persistent vomiting prevents fluid intake
• Diarrhea continues >5 days
• Signs of worsening dehydration despite oral therapy