What does Harrison's Internal Medicine recommend as first‑line therapy and fluid replacement regimens for acute gastroenteritis‑related dehydration, including ORS dosing for mild and moderate dehydration and IV crystalloid bolus for severe dehydration?

First-Line Therapy for Acute Gastroenteritis-Related Dehydration

Reduced-osmolarity oral rehydration solution (ORS) is the first-line therapy for mild-to-moderate dehydration, administered at 100 mL/kg over 2–4 hours; isotonic intravenous crystalloids (lactated Ringer's or normal saline) are reserved exclusively for severe dehydration, shock, altered mental status, or ORS failure. 1, 2

Oral Rehydration Solution (ORS) Dosing

Mild-to-Moderate Dehydration (3–9% fluid deficit)

• Administer reduced-osmolarity ORS (<250 mmol/L) at 100 mL/kg over 2–4 hours 2
• Commercial formulations include Pedialyte, CeraLyte, or Enfalac Lytren 3
• The WHO-recommended formulation contains approximately Na 90 mM, K 20 mM, Cl 80 mM, HCO₃ 30 mM, and glucose 111 mM 2
• Do not use apple juice, Gatorade, or soft drinks—their incorrect osmolarity worsens electrolyte imbalances 3

Replacement of Ongoing Losses

• Replace each watery stool with 10 mL/kg of ORS 2
• Replace each vomiting episode with 2 mL/kg of ORS 2
• Continue ORS until diarrhea and vomiting resolve 1, 2

Alternative Administration Route

• Nasogastric delivery of ORS may be used for patients with moderate dehydration who cannot tolerate oral intake, or children who are weak or refuse to drink but have normal mental status 1, 2

Intravenous Crystalloid Therapy

Severe Dehydration (≥10% fluid deficit)

• Immediately administer isotonic IV fluids (lactated Ringer's or normal saline) 1, 2
• Severe dehydration is characterized by altered mental status, shock, poor perfusion, inability to tolerate oral intake, or ileus 2, 3

IV Fluid Administration Protocol

• Continue IV rehydration until pulse, perfusion, and mental status normalize 1, 2
• Once stabilized, transition to ORS to replace the remaining fluid deficit 1, 2
• In patients with ketonemia, an initial IV fluid bolus may be required before oral rehydration can be tolerated 1, 2

Rapid IV Rehydration Approach

• Administer 20–30 mL/kg of crystalloid solution intravenously over 2 hours for moderate dehydration that has failed ORS 4
• This approach effectively resolves vomiting and reduces admission rates 4

Nutritional Management During Rehydration

• Resume age-appropriate regular diet immediately during or after rehydration—do not withhold food 1, 2, 3
• Continue breastfeeding throughout the diarrheal episode in infants 1, 2, 3
• Early refeeding prevents malnutrition and may reduce stool output 2

Adjunctive Therapies to Facilitate Oral Rehydration

Antiemetics

• Ondansetron may be given to children >4 years and adults with vomiting to facilitate ORS tolerance 1, 2, 3
• This improves oral rehydration success and decreases need for IV fluids and hospitalization 5

Antimotility Agents (Critical Contraindications)

• Loperamide is absolutely contraindicated in all children <18 years with acute diarrhea 1, 2, 3
• Loperamide must never be given to any patient with bloody diarrhea, fever, or suspected inflammatory diarrhea due to risk of toxic megacolon 1, 2, 3
• In immunocompetent adults with acute watery diarrhea, loperamide may be used only after adequate hydration is achieved 1, 2, 3

Common Pitfalls to Avoid

• Do not withhold ORS in favor of IV fluids for mild-to-moderate dehydration—ORS is equally effective and avoids IV complications such as phlebitis and infection 3
• Do not use sports drinks or fruit juices for rehydration—their inappropriate osmolarity can worsen electrolyte imbalances 3
• Do not give loperamide before confirming adequate hydration, as it can mask worsening dehydration 3
• Reassess hydration status 2–4 hours after initiating rehydration therapy 2

When IV Therapy is Mandatory

IV fluids are required when any of the following are present: 2

• Severe dehydration (≥10% fluid deficit)
• Circulatory shock
• Altered consciousness
• Intestinal ileus
• Complete inability to tolerate oral or nasogastric intake