Anaphylaxis
USMLE Step 2 CKDifficulty: easyLast updated:
A 26-year-old woman develops diffuse hives, lip swelling, wheezing, and lightheadedness 10 minutes after eating a cookie that contained peanuts. Blood pressure is 78/42 mm Hg, pulse 128/min, and oxygen saturation 92% on room air. Diffuse expiratory wheezes are heard. Large-bore IV access is being obtained.
Which of the following is the most appropriate initial pharmacotherapy?
Answer and explanation
Correct answer: B. Intramuscular epinephrine 0.3–0.5 mg (1 mg/mL solution) into the anterolateral thigh
This patient has anaphylaxis, with skin and mucosal involvement, bronchospasm, and hypotension after allergen exposure. The source describes epinephrine as the cornerstone of treatment: it may help relieve all symptoms and signs and should be given immediately. The usual adult dose is 0.3 to 0.5 mL of a 1:1000 (1 mg/mL) solution, which is 0.3 to 0.5 mg, or 0.01 mL/kg in children. It can be repeated every 5 to 15 minutes, and maximal absorption occurs when it is given IM in the anterolateral (mid-outer) thigh. Antihistamines, inhaled beta-agonists, and IV fluids are adjuncts. Persistent hypotension may require IV fluids and vasopressors. A 1-mg IV push is cardiac-arrest dosing and is inappropriate for a patient with a pulse.
Each option explained
- A. Nebulized albuterolIncorrect. Inhaled beta-agonists can be added for bronchospasm but do not treat hypotension or upper airway edema.
- B. Intramuscular epinephrine 0.3–0.5 mg (1 mg/mL solution) into the anterolateral thighCorrect. Immediate IM epinephrine in the anterolateral thigh is the first-line treatment of anaphylaxis.
- C. Intravenous push of 1 mg epinephrine (0.1 mg/mL solution)Incorrect. A 1-mg IV bolus is cardiac-arrest dosing; in a patient with a pulse it risks arrhythmia and severe hypertension.
- D. Intravenous diphenhydramineIncorrect. H1 antihistamines relieve itching and urticaria but do not reverse airway obstruction or shock; they are adjuncts.
- E. Intravenous methylprednisoloneIncorrect. Glucocorticoids have a delayed onset and are not first-line for acute anaphylaxis.
Key point
Anaphylaxis: give IM epinephrine immediately (adults 0.3–0.5 mg of 1 mg/mL into the anterolateral thigh), repeating every 5–15 minutes as needed.
From the source
“Epinephrine is the cornerstone of treatment for anaphylaxis; it may help relieve all symptoms and signs and should be given immediately”