Anaphylaxis in a 10 kg Toddler With No IV
A 1-year-old (10 kg) with a known peanut allergy develops facial hives, lip swelling, and vomiting 10 minutes after eating a cookie at a birthday party. On arrival he has inspiratory stridor, a hoarse cry, respiratory rate 50, SpO2 91% on room air, heart rate 170, and blood pressure 60/35 mmHg. He is irritable and inconsolable, and there is no IV access. Two nurses ask for orders: one requests IV diphenhydramine 1 mg/kg, the other requests IM epinephrine. The nearest epinephrine is a 1:1000 vial (1 mg/mL) with 1 mL syringes; a 1:10,000 syringe (0.1 mg/mL) is also available in the code cart.
1. Which drug is first, what exact dose and volume do you give, and by what route? Show the calculation and explain why the antihistamine request is the wrong first move.
Read the worked answer
IM epinephrine is first, immediately, without waiting for IV access.
- Dose: 0.01 mg/kg of 1:1000 = 0.01 × 10 = 0.1 mg
- Volume from the 1 mg/mL vial: 0.1 mL
- Route: anterolateral thigh (vastus lateralis), IM, using the 1 mL syringe
- Maximum single dose 0.3 mg in a child (0.5 mg in an adolescent); repeat every 5–15 minutes if needed
Why not diphenhydramine first: antihistamines only block H1 receptors - they do nothing for laryngeal edema or vasodilatory shock. This child has airway obstruction (stridor, hoarse cry, SpO2 91%) plus hypotension, which is anaphylaxis with both respiratory and cardiovascular compromise. Only epinephrine's alpha-1 effect reduces mucosal edema and restores vascular tone, and its beta-2 effect bronchodilates and stabilizes mast cells. Delaying epinephrine to give an antihistamine is associated with worse outcomes.
Never give the 1:1000 concentration IV.
2. Five minutes after the IM dose, stridor persists and the blood pressure is 55/30 mmHg. What are your next steps, including fluid volume and how epinephrine may be given intravenously?
Read the worked answer
1. Repeat IM epinephrine 0.1 mg (0.1 mL of 1:1000) in the other thigh now.
2. Give a rapid isotonic crystalloid bolus: 20 mL/kg = 200 mL over 5–10 minutes, and reassess; a second bolus may be needed, since anaphylaxis causes profound capillary leak and vasodilation.
3. Establish IV/IO access and start an epinephrine infusion if hypotension persists after 1–2 boluses and repeated IM dosing: typically 0.05–0.1 mcg/kg/min, titrated to blood pressure, with continuous monitoring.
4. If no infusion is available and the child is peri-arrest, a dilute IV push of 0.01 mg/kg of 1:10,000 (0.1 mg = 1 mL of the 10 mL 1:10,000 syringe) may be given slowly with extreme caution and continuous cardiac monitoring - this is a different drug concentration and route from the IM dose.
5. Prepare for a difficult airway - the stridor means supraglottic edema is progressing; call anesthesia and have suction, a smaller ETT, and surgical backup ready. Consider a second vasopressor (norepinephrine) for refractory shock.
The key safety judgment: the same milligram dose given by the wrong concentration (1:1000 IV) can be fatal, so the concentration–route pairing must be stated explicitly out loud.
3. What is the appropriate role of nebulized epinephrine, corticosteroids, and antihistamines in this child, and what mistake might a team make with them?
Read the worked answer
All three are adjuncts, never substitutes for IM epinephrine and fluids:
- Nebulized epinephrine (0.5 mL/kg of 2.25% solution, max 5 mL) can reduce stridor by local mucosal vasoconstriction, but it does not treat shock and its effect wears off in about 2 hours. The risk is false reassurance: a child whose stridor improves may still be profoundly hypotensive.
- Corticosteroids (e.g., methylprednisolone 1–2 mg/kg IV) may reduce the risk of a biphasic or protracted reaction, but onset is hours - they cannot rescue an airway now.
- H1/H2 antihistamines (e.g., diphenhydramine 1 mg/kg) relieve urticaria and itching only; they do not treat airway edema or shock.
The team-level error to avoid is sequencing: giving the antihistamine and steroid first because they feel safer and are easier to order, then giving epinephrine 'if he doesn't improve.' Epinephrine goes first, and the adjuncts are added in parallel, not instead.