Management of Anaphylaxis
A high-yield, management-focused lesson on anaphylaxis emphasizing prompt IM epinephrine as the only first-line, mortality-reducing therapy, with an ordered treatment algorithm, first-line-vs-adjunct comparison, trustworthy dosing, escalation (IV epinephrine infusion; glucagon for beta-blocked patients), and the classic exam pitfalls. Built around "next best step in management" for STEP 1 and STEP 2 CK.
The one thing that saves lives: epinephrine, now
Anaphylaxis is a life-threatening, IgE-mediated (or non-IgE) systemic hypersensitivity reaction that must be recognized and treated clinically — never wait for labs. Diagnose it when any one of the following is met:
- Acute onset of illness with skin/mucosal involvement (hives, itch, lip/tongue swelling) PLUS respiratory compromise (dyspnea, wheeze, stridor) or hypotension.
- Two or more organ systems involved rapidly after a likely allergen (skin, respiratory, GI cramping/vomiting, cardiovascular).
- Hypotension alone after exposure to a known allergen for that patient.
The single most important management principle for the exam and the bedside: prompt intramuscular (IM) epinephrine is first-line and the only therapy proven to reduce mortality. Antihistamines and glucocorticoids are adjuncts — they do nothing for airway edema or shock, and giving them instead of or before epinephrine is the leading modifiable cause of anaphylaxis death. If a vignette meets criteria, the "next best step" is almost always IM epinephrine — before IV access, before antihistamines, before imaging or tryptase. Serum tryptase (peaks ~1–2 h) can support the diagnosis retrospectively but must never delay treatment.
- IM epinephrine into the anterolateral thigh (vastus lateralis) is FIRST-LINE and the only life-saving drug — give immediately, alongside (not after) everything else.
- Dose: epinephrine 1 mg/mL (the "1:1000" concentration), 0.01 mg/kg IM; adult 0.3–0.5 mg, child up to 0.3 mg. Autoinjector: 0.3 mg for ≥30 kg, 0.15 mg for ~15–30 kg.
- Repeat every 5–15 min if response is inadequate; most patients respond to ≤2 doses.
- IM > SC (faster, more reliable absorption). Reserve IV epinephrine (infusion, monitored) for refractory shock — an IV push of the 1:1000 concentration is a dangerous error.
- Position supine with legs elevated; do NOT let the patient sit or stand up abruptly (risk of fatal "empty ventricle / empty vena cava" syndrome). Left lateral decubitus if vomiting or pregnant.
- Adjuncts never replace epinephrine: high-flow O₂, rapid IV crystalloid for hypotension, inhaled β₂-agonist for bronchospasm, H1 ± H2 antihistamines for cutaneous symptoms, glucocorticoids.
- On a beta-blocker and refractory to epinephrine → give IV glucagon.
- At discharge: observe for a biphasic reaction, prescribe two epinephrine autoinjectors, give an anaphylaxis action plan + avoidance education, and refer to allergy/immunology.
Step-by-step management algorithm
| Order | Intervention | Key detail |
|---|---|---|
| 1 | Remove the trigger + call for help | Stop the IV drug/blood product/contrast; scrape (don't squeeze) a retained stinger |
| 2 | IM epinephrine — anterolateral thigh | 0.01 mg/kg of 1 mg/mL; adult 0.3–0.5 mg; repeat q5–15 min as needed |
| 3 | Position supine, legs elevated | Do not sit/stand the patient up; left lateral if vomiting or pregnant |
| 4 | High-flow O₂ + airway assessment | Prepare for early intubation if stridor/angioedema — airway can close fast |
| 5 | IV access + rapid crystalloid bolus | 1–2 L isotonic crystalloid (normal saline or lactated Ringer's) wide open for hypotension (peds 20 mL/kg rapid bolus) |
| 6 | Adjuncts | Inhaled β₂-agonist (albuterol) for bronchospasm; H1 (± H2) antihistamine and glucocorticoid for symptom control |
| 7 | Refractory → escalate | IV epinephrine infusion in a monitored setting; glucagon if on a β-blocker |
| 8 | Observe + discharge planning | Monitor for biphasic reaction; discharge with 2 autoinjectors, action plan, allergy referral |
A 19-year-old with known peanut allergy develops diffuse urticaria, lip swelling, wheezing, and light-headedness minutes after biting a cookie. BP 82/48, HR 124, diffuse expiratory wheeze, no stridor yet.
What is the next best step in management?
→ Immediately administer IM epinephrine 0.3–0.5 mg (1 mg/mL) into the anterolateral thigh. Then place the patient supine with legs elevated, apply high-flow O₂, obtain IV access, and give a rapid crystalloid bolus.
Why: The patient meets diagnostic criteria (acute onset with skin plus respiratory plus cardiovascular involvement). Epinephrine is the only agent that simultaneously reverses airway edema (α₁), bronchospasm (β₂), and hypotension (α₁/β₁) and reduces mortality. Do not give diphenhydramine or steroids first — they relieve itch at best and waste critical minutes.
First-line vs. adjunct: what each agent actually does
| Agent | Role | Mechanism / caveat |
|---|---|---|
| Epinephrine (IM) | First-line, life-saving | α₁ vasoconstriction (↓ mucosal edema, ↑ BP), β₁ inotropy/chronotropy, β₂ bronchodilation + mast-cell stabilization. Only drug proven to reduce death; no absolute contraindication in anaphylaxis. |
| IV crystalloid | First-line adjunct for shock | Treats distributive + hypovolemic component; large volumes may be needed |
| Inhaled β₂-agonist (albuterol) | Adjunct | For bronchospasm not relieved by epinephrine; no effect on airway edema or BP |
| H1 antihistamine (diphenhydramine) | Adjunct | Relieves itch/hives only; slow onset; no effect on airway or hemodynamics — never first-line |
| H2 antihistamine (famotidine) | Adjunct | Added to H1 for cutaneous symptoms |
| Glucocorticoid (methylprednisolone) | Adjunct | Slow onset; does not treat the acute event; evidence for preventing biphasic reactions is weak — do not rely on it |
| Glucagon | Rescue | For patients on β-blockers who are refractory to epinephrine; raises cAMP independent of β-receptors |
A 62-year-old on metoprolol for hypertension develops anaphylaxis after a bee sting. Despite two doses of IM epinephrine and a 2 L saline bolus, he remains hypotensive (BP 80/50) and bradycardic.
What is the next best step in management?
→ Give IV glucagon (1–5 mg IV over ~5 min, then a 5–15 mcg/min infusion titrated to response).
Why: Chronic β-blockade blunts epinephrine's β-adrenergic effects, producing epinephrine-resistant hypotension and bronchospasm. Glucagon increases intracellular cAMP by a mechanism that bypasses the β-receptor, restoring inotropy and chronotropy. Simultaneously escalate to an IV epinephrine infusion in a monitored setting, and continue aggressive fluids. (Anticipate vomiting from glucagon — protect the airway.)
ABCDE is the standard Resuscitation Council approach — a real, widely taught mnemonic — applied to anaphylaxis:
- A — Airway: look for swelling, hoarseness, stridor; secure early before edema worsens.
- B — Breathing: high-flow O₂; treat wheeze with epinephrine ± inhaled β₂-agonist.
- C — Circulation: lay the patient supine with legs raised; IV crystalloid bolus; epinephrine for hypotension.
- D — Disability: assess consciousness (a marker of cerebral hypoperfusion).
- E — Exposure: find and remove the trigger; examine the skin for hives/angioedema.
Key caveat: epinephrine is given the moment anaphylaxis is recognized — do not "complete" the ABCDE survey before treating.
- Giving antihistamines or steroids instead of / before epinephrine — the #1 lethal error.
- Wrong route: using SC or the deltoid — use IM into the vastus lateralis (anterolateral thigh).
- Concentration mix-up: IM uses 1 mg/mL (1:1000); IV infusion uses 0.1 mg/mL (1:10,000). An IV push of the 1:1000 concentration can cause fatal arrhythmia or hypertensive crisis.
- Letting the patient stand or sit up — sudden death from an empty ventricle.
- Failing to repeat epinephrine when the response is inadequate.
- Under-recognizing the biphasic reaction — symptoms can recur hours after apparent resolution without re-exposure; observe accordingly and warn the patient.
- Discharging without two autoinjectors, an action plan, avoidance counseling, and allergy/immunology referral.
Practice Clinical Management now
Board-style questions, spaced-repetition flashcards, and a Socratic AI tutor — free to start.