What are the appropriate dosing recommendations for intramuscular epinephrine, oral diphenhydramine, and oral famotidine in a one‑year‑old child weighing 8.9 kg presenting with anaphylaxis?

Anaphylaxis Treatment Dosing for a One-Year-Old (8.9 kg)

For this 8.9 kg infant with anaphylaxis, give intramuscular epinephrine 0.09 mg (0.09 mL of 1:1000) in the anterolateral thigh immediately; diphenhydramine 9–18 mg orally is adjunctive only and must never delay epinephrine, and famotidine is not recommended in acute pediatric anaphylaxis.

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Epinephrine: First-Line and Life-Saving

Epinephrine is the only first-line medication for anaphylaxis and must be administered before any other drug. 1, 2

Dosing Calculation

• Weight-based dose: 0.01 mg/kg of 1:1000 (1 mg/mL) epinephrine intramuscularly 1, 2, 3
• For 8.9 kg: 0.01 mg/kg × 8.9 kg = 0.089 mg (round to 0.09 mg or 0.09 mL of 1:1000 solution) 1, 2
• Maximum single dose in children: 0.3 mg 1, 2, 3

Administration Technique

• Inject into the anterolateral mid-thigh (vastus lateralis) at a 90-degree angle to ensure intramuscular delivery 1, 3
• Peak plasma concentration is reached in 8 ± 2 minutes with intramuscular thigh injection, compared to 34 ± 14 minutes with subcutaneous administration 1, 2, 3
• Repeat every 5–15 minutes if symptoms persist or recur; approximately 10–20% of patients require more than one dose 1, 2, 3

Autoinjector Considerations

• For infants 7.5–15 kg, a 0.15 mg autoinjector is appropriate if manual dosing is unavailable 1, 2
• A 0.1 mg autoinjector is preferred for infants where available, but the 0.15 mg dose is acceptable for infants >7.5 kg 1
• At 8.9 kg, a 0.15 mg autoinjector would deliver 0.017 mg/kg—slightly above the 0.01 mg/kg target but well within the safe therapeutic range and preferable to underdosing 3

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Diphenhydramine (Benadryl): Adjunctive Only

Diphenhydramine is second-line adjunctive therapy that treats only cutaneous symptoms (urticaria, itching) and does not relieve airway obstruction, bronchospasm, gastrointestinal symptoms, or shock. 1, 2

Dosing

• 1–2 mg/kg per dose (maximum 50 mg) 1
• For 8.9 kg: 1 mg/kg = 8.9 mg (round to 9 mg); 2 mg/kg = 17.8 mg (round to 18 mg)
• Practical dose range: 9–18 mg orally 1

Critical Caveats

• Never administer diphenhydramine before or in place of epinephrine; delayed epinephrine is directly associated with anaphylaxis fatalities 1, 2
• Give only after epinephrine has been administered and the patient is stabilizing 1, 2
• Diphenhydramine does not prevent or reverse cardiovascular collapse or airway edema 1

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Famotidine (Pepcid): Not Recommended in Acute Pediatric Anaphylaxis

H2 antihistamines such as famotidine have minimal evidence of benefit in acute anaphylaxis and are not part of standard pediatric emergency treatment. 1

Why Famotidine Is Not Recommended

• Evidence of benefit is minimal even when added to H1 antihistamines 1
• Onset of action is too slow (30–60 minutes) to impact the acute phase of anaphylaxis
• Pediatric dosing data for acute anaphylaxis are lacking; famotidine is not mentioned in any pediatric anaphylaxis guideline reviewed 1, 2
• Adult dose (when used): ranitidine 50 mg IV or famotidine 20 mg IV, but this is not extrapolated to infants 1

If Famotidine Is Considered (Off-Label)

• Typical pediatric dose for other indications: 0.5 mg/kg/dose IV (maximum 20 mg)
• For 8.9 kg: 0.5 mg/kg × 8.9 kg = 4.5 mg IV
• This is off-label, not guideline-supported, and should not delay or replace epinephrine 1

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Supportive Measures

Positioning and Oxygen

• Place the infant supine with legs elevated (unless respiratory distress or vomiting dictates otherwise) 1
• Administer supplemental oxygen at 6–8 L/min if respiratory symptoms are present 1

Fluid Resuscitation

• For cardiovascular involvement, give up to 30 mL/kg of isotonic crystalloid (normal saline) within the first hour 1
• For 8.9 kg: up to 267 mL (30 mL/kg × 8.9 kg) 1

Observation

• Minimum 4–6 hours in a facility equipped to manage anaphylaxis 1
• Extended observation (≥6 hours) or admission if:
  • More than one epinephrine dose was required 1
  • Severe initial presentation (hypotension, respiratory compromise) 1
  • Unknown trigger 1

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Common Pitfalls to Avoid

• Never delay epinephrine to give antihistamines or H2 blockers first; this is the leading cause of preventable anaphylaxis deaths 1, 2, 4, 5
• Do not use subcutaneous epinephrine; intramuscular thigh injection is vastly superior 1, 2, 3, 6
• Do not underdose epinephrine in small infants; the 0.01 mg/kg dose is safe and effective 1, 2, 3, 7, 8
• Do not rely on antihistamines or corticosteroids to prevent biphasic reactions; they have no proven efficacy for this purpose 1
• There are no absolute contraindications to epinephrine in anaphylaxis, even in infants with cardiac disease 1, 2, 3

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Discharge Requirements

• Two epinephrine autoinjectors (0.15 mg dose for this weight) with hands-on caregiver training 1
• Written anaphylaxis emergency action plan detailing symptoms, triggers, and epinephrine use 1
• Referral to an allergist within 1–2 weeks 1