Pathophysiology & Risk Factors

A severe, rapid-onset systemic allergic reaction from massive IgE-triggered histamine and mediator release by mast cells and basophils. It is distributive shock from widespread vasodilation and capillary leak — not volume loss or pump failure. The defining criterion is involvement of two or more body systems within minutes to hours of exposure.

Signs & Symptoms

EarlyProgresses →

urticaria

present in up to 90%, but can be absent

flushing

angioedema of lips and tongue Hallmark

signals airway threat before wheezing

wheezing

throat tightness

tachycardia

distinguishes from vasovagal bradycardia

GI cramping

vomiting/diarrhea counts as a system

Late / Severe

stridor Hallmark

laryngeal edema — closing window

hypotension

weak pulse

Interventions & Priorities

Anaphylaxis emergency sequence

Stop the triggerhalt infusion / remove stinger
IM epinephrine0.3-0.5 mg 1:1,000, lateral thigh
Airway + O2call for help, ready intubation
IV fluid bolus1-2 L 0.9% NS, large-bore
Adjunct medsantihistamine + corticosteroid

Treatments & Medications

epinephrine 0.3-0.5 mg IMPrototype

1:1,000 (1 mg/mL), repeat q5-15 min

epinephrine IV 1:10,000

cardiac arrest / refractory shock only

diphenhydramine 25-50 mg IV

H1 blockade — treats hives, not airway

methylprednisolone 125 mg IV

prevents biphasic reaction, not acute fix

glucagon

for beta-blocked patients — non-adrenergic

Patient Teaching

carry epinephrine auto-injector

prescribed at discharge

inject into outer thigh

call 911 after every use

stay for 4-6 hour observation

biphasic risk window

avoid the known trigger

wear medical alert identification

Complications

biphasic reaction

recurs 1-72 hr without re-exposure

complete airway obstruction

cardiovascular collapse

Report Nowescalate immediately

stridor Hallmark

impending airway obstruction

angioedema of tongue and throat

airway threat — act before wheezing

hypotension with tachycardia

cardiovascular collapse

oxygen saturation below 90%

recurrence of symptoms

biphasic reaction 1-72 hr later

Clinical Pearl

Two systems, think anaphylaxis — epi first, epi fast, epi in the thigh. Benadryl treats hives; epinephrine saves lives.