23.3 Anaphylaxis Diagnosis, Epinephrine Dosing & Airway Protection

Anaphylaxis Pathophysiology & Classification

Anaphylaxis is an acute, life-threatening, systemic hypersensitivity reaction characterized by rapid onset of airway, breathing, or circulatory compromise, usually accompanied by cutaneous manifestations. Without prompt recognition and epinephrine administration, progression to asphyxiation or cardiovascular collapse can occur within minutes.

Immunologic IgE-Mediated Anaphylaxis (Type I Hypersensitivity)

• Sensitization Phase: Initial exposure to an allergen (e.g., peanut protein, beta-lactam antibiotic, hymenoptera venom) stimulates allergen-specific T helper 2 (Th2) cells, which induce B-cell class switching to produce allergen-specific Immunoglobulin E (IgE). Secreted IgE binds with high affinity to Fc-epsilon-RI receptors on the surface of tissue mast cells and circulating basophils.
• Effector Phase (Re-Exposure):
  • Re-exposure to the allergen leads to antigen cross-linking of adjacent IgE-Fc-epsilon-RI receptor complexes, triggering cellular activation and explosive exocytosis of storage granules.
  • Preformed Mediators (Released Immediately):
    • Histamine: Stimulates H1 receptors (causing peripheral vasodilation, increased capillary permeability, pruritus, and bronchoconstriction) and H2 receptors (causing gastric acid secretion, coronary vasospasm, and chronotropic/inotropic cardiac stimulation).
    • Tryptase: A serine protease released exclusively by mast cells. Serum tryptase levels peak at 1 to 2 hours post-reaction and remain elevated for up to 4 to 6 hours. While not useful in acute emergency triage, an elevated serum tryptase helps retrospectively confirm anaphylaxis.
    • Carboxypeptidase A3 and Chymase.
  • Newly Synthesized Lipid Mediators (Eicosanoids):
    • Leukotrienes (LTC4, LTD4, LTE4): Formerly known as the slow-reacting substance of anaphylaxis. Exert potent bronchoconstrictor effects 1,000 times more potent than histamine and dramatically increase microvascular permeability.
    • Prostaglandin D2 (PGD2): Promotes bronchospasm, peripheral vasodilation, and facial flushing.
    • Platelet-Activating Factor (PAF): Causes profound peripheral vasodilation, vascular leakage, and myocardial depression; circulating PAF levels correlate directly with the clinical severity of anaphylaxis.

Non-IgE-Mediated (Non-Immunologic) Anaphylaxis

• Historically referred to as anaphylactoid reactions, these episodes occur without IgE antibody cross-linking.
• Mechanism: Agents directly stimulate mast cell and basophil degranulation (e.g., via the MRGPRX2 receptor) or activate the complement cascade (yielding anaphylatoxins C3a and C5a).
• Common Triggers: Intravenous iodinated radiocontrast media, vancomycin ("red man syndrome"), intravenous opioids (morphine, codeine), dextrans, and extreme cold or physical exertion.
• Clinical Management Rule: Non-IgE reactions are clinically indistinguishable from IgE-mediated anaphylaxis and are treated identically. Epinephrine remains the immediate first-line drug of choice.

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NIAID/FAAN Clinical Diagnostic Criteria

Anaphylaxis is an urgent clinical diagnosis. Laboratory testing must never delay management. The National Institute of Allergy and Infectious Disease and Food Allergy and Anaphylaxis Network (NIAID/FAAN) consensus criteria outline three distinct clinical scenarios:

                  NIAID/FAAN CLINICAL DIAGNOSTIC CRITERIA

  ┌─────────────────────────────────────────────────────────────────┐
  │ CRITERION 1: ACUTE ONSET + SKIN/MUCOSA + RESPIRATORY or BP      │
  │ • Acute onset of illness (minutes to hours) with skin/mucosal   │
  │   involvement (generalized hives, pruritus, flushing, angioedema)│
  │   PLUS AT LEAST ONE OF:                                         │
  │   a) Respiratory Compromise (dyspnea, wheeze, stridor, hypoxemia)│
  │   b) Reduced BP or End-Organ Dysfunction (syncope, collapse)    │
  ├─────────────────────────────────────────────────────────────────┤
  │ CRITERION 2: EXPOSURE TO LIKELY ALLERGEN + >=2 SYSTEMS          │
  │ • Two or more occurring rapidly after exposure to LIKELY allergen:│
  │   a) Skin/Mucosal Tissue (hives, pruritus, lip/tongue swelling) │
  │   b) Respiratory Compromise (dyspnea, bronchospasm, stridor)    │
  │   c) Reduced BP or End-Organ Dysfunction (hypotonia, syncope)   │
  │   d) Persistent GI Symptoms (cramping abdominal pain, vomiting) │
  ├─────────────────────────────────────────────────────────────────┤
  │ CRITERION 3: EXPOSURE TO KNOWN ALLERGEN + REDUCED BP            │
  │ • Reduced BP alone after exposure to a KNOWN allergen:          │
  │   - Adults: SBP <90 mmHg or >30% decrease from baseline         │
  │   - Children: Age-specific low SBP or >30% decrease             │
  └─────────────────────────────────────────────────────────────────┘

The Critical Diagnostic Pitfall: Up to 10% to 20% of patients with severe anaphylaxis have NO cutaneous signs (no hives, erythema, or angioedema). In rapidly evolving, severe reactions—particularly from intravenous drugs or insect venom—cardiovascular collapse (Criterion 3) or isolated asphyxia can occur before skin manifestations have time to develop. Clinicians must not rule out anaphylaxis simply because the skin is clear!

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First-Line Pharmacotherapy: Intramuscular Epinephrine

Intramuscular epinephrine is the primary, first-line, lifesaving medication of choice. There are absolute zero contraindications to epinephrine in a patient experiencing life-threatening anaphylaxis.

Pharmacodynamic Actions Across Adrenergic Receptors

1. Alpha-1 Adrenergic Agonism:
   • Induces potent vasoconstriction in peripheral vascular beds;
   • Reverses profound peripheral vasodilation and distributive shock;
   • Decreases post-capillary venular leakage, directly reducing laryngeal edema and macroglossia;
   • Elevates systemic vascular resistance and raises blood pressure.
2. Beta-1 Adrenergic Agonism:
   • Increases myocardial contractility (positive inotropy) and heart rate (positive chronotropy), augmenting cardiac output to reverse circulatory collapse.
3. Beta-2 Adrenergic Agonism:
   • Promotes rapid bronchial smooth muscle relaxation, reversing bronchospasm;
   • Crucial Cellular Downregulation: Beta-2 receptor activation on mast cells and basophils stimulates adenylate cyclase, elevating intracellular cyclic adenosine monophosphate (cAMP). High intracellular cAMP directly inhibits further exocytosis of mast cell and basophil storage granules, shutting down the inflammatory mediator cascade at its cellular source.

Dosing and Administration Protocol

• Adult Dosing: 0.3 mg to 0.5 mg IM (0.3 to 0.5 mL of 1:1,000 [1 mg/mL] concentration).
• Pediatric Dosing: 0.01 mg/kg IM of 1:1,000 concentration (maximum single dose: 0.3 mg).
  • Autoinjector Weight Tiers: 0.15 mg autoinjector for children weighing 10 kg to 25 kg; 0.3 mg autoinjector for individuals weighing >25 kg.
• Anatomical Route of Administration: Anterolateral middle third of the thigh (Vastus Lateralis muscle).
  • Pharmacokinetic Superiority: Landmark pharmacokinetic studies demonstrated that intramuscular injection into the vastus lateralis achieves a peak plasma concentration (Cmax) significantly faster (mean time to peak ~8 minutes) and up to 10-fold higher compared to subcutaneous administration or intramuscular injection into the deltoid muscle. Subcutaneous administration causes local alpha-1 vasoconstriction, trapping the drug locally and severely delaying systemic absorption.
• Repeat Dosing: Can be repeated every 5 to 15 minutes as clinically indicated for persistent, refractory, or progressive symptoms. Approximately 10% to 20% of patients require a second dose.

               EPINEPHRINE ROUTE PHARMACOKINETICS

  Vastus Lateralis IM:  Peak Cmax achieved in ~8 minutes (HIGHEST & FASTEST)
  Deltoid IM:           Peak Cmax achieved in ~34 minutes (Delayed)
  Subcutaneous (SQ):    Peak Cmax achieved in ~45 minutes (Slowest; local vasoconstriction)

Intravenous Epinephrine Warning

• Intravenous Epinephrine Boluses (1:1,000) are LETHAL: Rapid IV push of concentrated 1:1,000 epinephrine causes severe hypertensive crises, intracranial hemorrhage, ventricular fibrillation, and myocardial infarction.
• IV Epinephrine is Reserved Strictly for Refractory Shock or Cardiac Arrest: Administer only under continuous telemetry monitoring via a dedicated infusion pump, diluted as an infusion (0.1 mcg/kg/min or 1 to 10 mcg/min) or using dilute 1:10,000 (0.1 mg/mL) or 1:100,000 push-dose formulations.

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Biphasic Anaphylaxis & Post-Stabilization Monitoring

• Definition: The recurrence of systemic anaphylactic symptoms after complete resolution of the initial acute episode, occurring without any re-exposure to the inciting allergen.
• Incidence & Timing: Biphasic reactions develop in 10% to 20% of anaphylaxis cases. While they characteristically emerge within 4 to 12 hours after the initial reaction, onset has been documented up to 72 hours later.
• Risk Factors for Biphasic Reactions:
  1. Severe initial clinical presentation (e.g., severe hypotension, respiratory arrest, wide pulse pressure);
  2. Delayed epinephrine administration (>30 to 60 minutes from symptom onset);
  3. Requirement for more than one dose of epinephrine to achieve initial stabilization;
  4. Ingestion of large allergen loads (e.g., oral food allergen).
• Mandatory Clinical Observation Period:
  • All patients with anaphylaxis must be observed in a medical facility for a minimum of 4 to 8 hours following complete resolution of symptoms.
  • Patients with risk factors for biphasic reactions (severe hypotension, requirement for multiple epinephrine doses, or underlying severe asthma) warrant prolonged observation or overnight hospital admission (12 to 24 hours).
• Discharge Care Bundle:
  • Prescribe two epinephrine autoinjectors (never a single pen);
  • Provide hands-on patient education on self-administration into the outer thigh;
  • Provide a written Anaphylaxis Emergency Action Plan;
  • Recommend medical alert identification and arrange formal referral to an allergist/immunologist.

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Secondary Adjunctive Therapies: What the Board Tests

[!CRITICAL] CRITICAL BOARD PEARL: Antihistamines and corticosteroids are strictly SECOND-LINE adjunctive agents. They do NOT treat or prevent laryngeal edema, bronchospasm, or distributive shock. They have a slow onset of action and must NEVER be used as monotherapy or substitute for epinephrine!

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Refractory Anaphylaxis in Patients Taking Beta-Blockers

Patients maintained on chronic beta-adrenergic antagonist therapy (e.g., metoprolol, propranolol, atenolol, carvedilol) represent a high-risk population prone to catastrophic, treatment-refractory anaphylaxis.

The Pathophysiological Dilemma

• Beta-blockers saturate and antagonize beta-1 and beta-2 adrenergic receptors.
• When standard doses of epinephrine are administered, the drug cannot activate blocked beta-2 receptors (failing to induce bronchodilation or halt mast cell mediator release) or beta-1 receptors (failing to restore cardiac contractility).
• Simultaneously, exogenous epinephrine stimulates unblocked alpha-1 receptors, which can trigger severe unopposed peripheral vasoconstriction, leading to severe paradoxical hypertension, profound reflex bradycardia, coronary spasm, or refractory distributive collapse.

The Specific Treatment of Choice: Intravenous Glucagon

• Pharmacologic Mechanism: Glucagon binds to specific, non-adrenergic glucagon G-protein coupled receptors on cardiac myocytes and vascular smooth muscle cells. This directly stimulates adenylate cyclase, converting ATP to intracellular cAMP, completely bypassing blocked beta-adrenergic receptors. Elevated cAMP restores positive inotropy and chronotropy and promotes smooth muscle relaxation.
• Dosing Protocol:
  • Initial Bolus: 1 mg to 5 mg IV administered slowly over 5 minutes;
  • Maintenance Infusion: Followed by a continuous infusion of 5 to 15 mcg/min, titrated to clinical hemodynamics and resolution of bronchospasm.
• Side Effect: Glucagon causes a high incidence of severe nausea and vomiting. Because the patient may have an unprotected airway, position the patient in the lateral recumbent position to prevent aspiration.

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Emergency Airway Protection & Patient Positioning

                      PATIENT POSITIONING WARNING

     STRICTLY RECUMBENT / SUPINE WITH LEGS ELEVATED
                    ┌─────────────────────┐
         Head       │     Torso           │    Legs Elevated (30-45°)
      O─────────────┴─────────────────────┴────────────/ 

  NEVER ALLOW AN ANAPHYLACTIC PATIENT TO STAND OR SIT UP ABRUPTLY!
  Sudden standing -> Venous pooling in dilated splanchnic & lower limbs
  -> Zero venous return -> "EMPTY VENTRICLE SYNDROME"
  -> Sudden Pulseless Electrical Activity (PEA) & Death within seconds!

1. Patient Positioning and "Empty Ventricle Syndrome"

• Patients experiencing anaphylaxis must be maintained strictly in a recumbent or supine position with lower extremities elevated, unless severe dyspnea dictates reclining in a semi-Fowler position (and even then, never allow them to stand).
• The Fatal Mechanism: Massive histamine- and PAF-mediated vasodilation creates an enormous, low-resistance vascular capacitance bed. If the patient abruptly sits up or stands, gravity immediately causes venous blood to pool in the lower extremities and splanchnic circulation.
• Venous return to the right heart abruptly drops to zero, depriving the left ventricle of preload ("empty ventricle syndrome"). The heart pumps against empty chambers, precipitating sudden pulseless electrical activity (PEA) and cardiac arrest within seconds.

2. Early Airway Protection & Intubation

• Progressive laryngeal edema, epiglottic swelling, and tongue enlargement can completely obliterate the glottic aperture in minutes.
• Indications for Immediate Intubation: Stridor, hoarseness, dysphonia ("hot potato voice"), tongue/pharyngeal swelling, or persistent severe hypoxemia despite epinephrine.
• Intubation Strategy:
  • Intubate early: Attempting intubation late through distorted, edematous pharyngeal tissues is technically hazardous.
  • Have video laryngoscopy ready, select an endotracheal tube 0.5 to 1.0 mm smaller than normal (e.g., 6.0 or 6.5 mm cuffed ETT) to negotiate the narrowed glottis, utilize an endotracheal tube introducer (bougie), and have a surgical cricothyrotomy kit open and immediately available at the bedside.