What is the recommended dose of Zofran (ondansetron) and rehydration regimen for a 7-year-old pediatric patient weighing 42 pounds?

Ondansetron Dosing and Rehydration for 7-Year-Old, 42 Pounds

Ondansetron Dosing

For a 7-year-old child weighing 42 pounds (19 kg), administer ondansetron 0.15 mg/kg per dose, which equals approximately 2.85 mg (can round to 3 mg for practical administration), given orally or intravenously depending on vomiting severity. 1

Weight-Based Calculation

• 42 pounds = 19 kg
• Standard pediatric dose: 0.15 mg/kg 1, 2
• Calculated dose: 19 kg × 0.15 mg/kg = 2.85 mg per dose
• Maximum single dose: 16 mg (not applicable here) 1

Route and Frequency for Gastroenteritis

• Oral route preferred if child can tolerate (oral disintegrating tablet works well) 3
• Give single dose initially to facilitate oral rehydration 4
• Can repeat every 8 hours for up to 5 additional doses if vomiting persists 3
• Only use after initiating rehydration, not as substitute for fluid therapy 4

Important Age Consideration

• This child at 7 years old meets the guideline threshold: ondansetron is recommended for children >4 years of age with acute gastroenteritis and vomiting 4
• Do not use in children under 6 months for most indications 1

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Rehydration Regimen

Begin with reduced osmolarity oral rehydration solution (ORS) as first-line therapy for mild to moderate dehydration, administered at 5 mL/minute initially, then maintenance fluids to replace ongoing losses. 4, 3

Initial Rehydration Phase

For Mild to Moderate Dehydration:

• Start ORS at 5 mL/minute (300 mL/hour) 3
• Continue until clinical signs of dehydration resolve (improved skin turgor, moist mucous membranes, normal pulse, adequate urine output) 4
• Typical deficit replacement: 50-100 mL/kg over 2-4 hours for mild-moderate dehydration 4

For Severe Dehydration:

• Use isotonic intravenous fluids (lactated Ringer's or normal saline) if child has shock, altered mental status, or failed ORS therapy 4
• Continue IV fluids until pulse, perfusion, and mental status normalize 4
• Switch to ORS once child is alert and can tolerate oral intake 4

Maintenance and Ongoing Loss Replacement

After rehydration is complete:

• Resume age-appropriate normal diet immediately 4
• Replace ongoing stool losses with ORS until diarrhea resolves 4
• Estimate ongoing losses: give 10 mL/kg ORS for each watery stool 4

Practical ORS Administration

• Commercial ORS preferred (Pedialyte, WHO-ORS) over homemade solutions 4
• If child refuses ORS due to taste, small frequent sips work better than large volumes 3
• Ondansetron given 15 minutes before ORS significantly improves tolerance 3
• Avoid BRAT diet during acute phase; return to normal diet once rehydrated 4, 3

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Critical Safety Points

Ondansetron Precautions

• May increase diarrhea frequency as a side effect, but this does not worsen outcomes 4, 3
• Monitor for QT prolongation risk, though rare at standard pediatric doses 1
• Dose range of 0.13-0.26 mg/kg shows similar efficacy, so no benefit to exceeding 0.15 mg/kg 5

When NOT to Use Ondansetron

• Avoid if bloody diarrhea or high fever suggesting inflammatory/invasive diarrhea (risk of toxic megacolon with antimotility effects, though ondansetron is antiemetic not antimotility) 4
• Do not use as substitute for adequate hydration 4

Rehydration Pitfalls

• Never use antimotility agents (loperamide) in children <18 years with acute diarrhea 4
• Nasogastric ORS administration is option if child refuses oral intake but has normal mental status 4
• Do not withhold solid food for 24 hours; early feeding improves outcomes 4