Anaphylaxis

1. THE 2-MINUTE PHYSIOLOGY (Rapid Pathophysiology)

• The Immunologic Trigger: Anaphylaxis is a severe, life-threatening systemic reaction triggered by the sudden, massive degranulation of mast cells and basophils. This can be mediated by IgE (classic allergic reaction to foods or venoms) or non-IgE mechanisms (anaphylactoid reactions to radiocontrast media or certain medications).
• The Mediator Release: The activation of these cells releases a storm of preformed mediators—primarily histamine, tryptase, prostaglandins, and leukotrienes.
• The Hemodynamic and Structural Collapse: These mediators directly cause profound systemic vasodilation (distributive shock), dramatically increased capillary permeability (third-spacing and angioedema), and smooth muscle contraction (bronchospasm and severe gastrointestinal cramping). This combination of vascular leak and vasodilation rapidly compromises venous return, plunging the patient into circulatory collapse, while laryngeal edema simultaneously cuts off the airway.

2. THE BEDSIDE ACTION PLAN (Rapid ER Management)

• Immediate Stabilization: Remove any triggering agent (e.g., insect stinger, infusing medication) and place the patient in the supine position to maximize venous return. Establish continuous cardiac monitoring, pulse oximetry, blood pressure monitoring, and large-bore IV access (16 or 18 gauge). Provide supplemental oxygen if indicated.
• First-Line Pharmacotherapy (Epinephrine): Intramuscular (IM) Epinephrine is the absolute first-line treatment. Administer immediately into the anterolateral thigh.
• Dosing: Use the 1:1,000 (1 mg/mL) concentration. The standard dose is 0.3 to 0.5 mg IM in adults, and 0.01 mg/kg IM in pediatrics. Repeat every 5–15 minutes as necessary for refractory symptoms.
• Fluid Resuscitation: For patients presenting with hypotension and cardiovascular collapse, aggressively expand intravascular volume with isotonic IV fluids.
• Second-Line Adjuncts: Antihistamines (H1 and H2 blockers) and corticosteroids are commonly administered; however, there is no objective evidence that they improve overall acute outcomes, and they must never delay the administration of Epinephrine.
• Airway Management: Be prepared for early, definitive airway management (Endotracheal Intubation or Awake Fiberoptic Intubation) if there are signs of impending obstruction.

3. THE DIAGNOSTIC GRID (Differential Diagnosis & Workup)

• Top "Can't-Miss" Mimics:
• Hereditary Angioedema (HAE) / ACE-Inhibitor Angioedema: Presents with massive localized edema (often face/airway) without hives. It is kinin-mediated, meaning it will not respond to epinephrine, steroids, or antihistamines.
• Severe Asthma Exacerbation: Presents with profound bronchospasm and wheezing, but lacks the sudden urticarial rash, hypotension, or facial angioedema classic to anaphylaxis.
• Acute Coronary Syndrome (ACS): Can present with profound hypotension and dyspnea; notably, anaphylaxis itself can trigger coronary vasospasm (Kounis syndrome).
• Prioritized Diagnostic Workup:
• Tier 1 (Clinical): Anaphylaxis is a purely clinical diagnosis. No laboratory testing should ever delay the administration of epinephrine.
• Tier 2 (Resuscitation Labs): In hypotensive patients, check a point-of-care glucose, basic metabolic panel, and ECG to rule out mimics and assess end-organ perfusion.

4. THE VISUAL BOARD (ECG / POCUS / Imaging)

• The Dermatologic Exam: Visually inspect the patient completely. Look for acute urticaria (hives), flushing, and profound angioedema of the lips, tongue, and uvula.
• The Airway Visuals: Explicitly look for pooling of oral secretions, rapidly progressive facial swelling, and listen for hoarseness or stridor, which visually and audibly signal an impending complete airway obstruction.
• The 12-Lead ECG: Given the profound hemodynamic stress and the administration of exogenous epinephrine, older adults must be monitored for concurrent myocardial ischemia or dangerous arrhythmias.

5. THE SCORING MATRIX (Risk Stratification & Guidelines)

• Diagnostic Criteria: Anaphylaxis is highly likely when any of the following occur suddenly:

1. Acute onset of skin/mucosal tissue involvement (hives, swollen lips) AND either respiratory compromise or hypotension.
2. Two or more of the following occurring rapidly after exposure to a likely allergen: skin/mucosal involvement, respiratory compromise, hypotension, or persistent GI symptoms (e.g., severe abdominal pain).
3. Hypotension alone rapidly after exposure to a known allergen for that patient.

• Observation & Disposition Rules:
• Patients whose symptoms resolve completely must be observed for 4 to 6 hours before safe discharge due to the risk of biphasic reactions. Up to 15% of pediatric patients may experience biphasic anaphylaxis.
• Exclusion from Clinical Decision Unit (CDU): Patients must be admitted to the hospital (not observed and released) if they have ongoing hypotension (SBP <100), tachycardia >110, an SpO2 <94% on room air, stridor/hoarseness, or require IV vasopressors.

6. THE DANGER ZONE (Pitfalls & Critical Actions)

• Pitfall - Delayed Epinephrine: Hesitating to give epinephrine, or giving antihistamines and steroids first. Critical Action: Epinephrine must be given fast and early; delays directly correlate with hypoxia, circulatory collapse, and death.
• Pitfall - The Dosing Error: Administering the IV cardiac arrest concentration of Epinephrine (1:10,000) instead of the IM concentration (1:1,000) for intramuscular injection. Critical Action: Always verify the 1:1,000 concentration for IM use to avoid lethal arrhythmias or dosing failures.
• Pitfall - Missing the Airway: Dismissing a patient's hoarse voice as a simple allergic symptom. Critical Action: Hoarseness is a dire sign of impending laryngeal edema and airway obstruction; prepare for an immediate surgical or fiberoptic airway.
• Pitfall - Mismanaging HAE: Using epinephrine to treat Hereditary Angioedema. Critical Action: HAE requires C1 inhibitor concentrate, ecallantide, icatibant, or Fresh Frozen Plasma (FFP), not epinephrine.

7. MCQ MASTERCLASS (Written Exam Tips)

• Buzzwords: "Sudden urticarial rash," "hoarseness/stridor," "abdominal pain after eating," and "biphasic reaction".
• Common Distractor: A patient presents with generalized hives, wheezing, and a blood pressure of 85/50. An option suggests "Administer IV Diphenhydramine and IV Methylprednisolone." Differentiate: This is a lethal trap. H1/H2 blockers and steroids are second-line and do not treat cardiovascular collapse. The correct answer is always IM Epinephrine.
• Common Distractor: A patient is brought in with severe lip and tongue swelling but no hives, actively taking Lisinopril. An option suggests "Administer IM Epinephrine." Differentiate: This is bradykinin-mediated ACE-inhibitor angioedema, not IgE-mediated anaphylaxis. Epinephrine will not work. Secure the airway and consider FFP or icatibant.

8. THE BOARDROOM SCRIPT (OSCE & Oral Board Tips)

• The Opening Salvo: "I recognize this patient is experiencing anaphylaxis with cardiovascular collapse. My immediate priorities are to place the patient supine, establish large-bore IV access, place them on a cardiac monitor, and administer IM Epinephrine 1:1,000 into the anterolateral thigh without delay."
• Articulating the Airway Assessment: "I will continuously evaluate the patient's airway for hoarseness, stridor, or pooling secretions. Because laryngeal edema can progress rapidly, I will have the difficult airway cart, fiberoptic scope, and a cricothyrotomy kit immediately at the bedside."
• Addressing the Shock: "The patient remains hypotensive. I am ordering aggressive isotonic IV fluid boluses and will repeat the IM Epinephrine dose every 5 to 15 minutes as needed. I will administer H1/H2 blockers and steroids solely as adjuncts."
• The Disposition: "The patient's symptoms have resolved. Because up to 15% of patients can experience a life-threatening biphasic reaction, I will observe the patient in the ED for a minimum of 4 to 6 hours. Upon discharge, I will provide strict anticipatory guidance, an epinephrine auto-injector prescription, and education on its proper use."