10.3 Pediatric Anaphylaxis & Severe Allergic Reactions

10.3 Pediatric Anaphylaxis & Severe Allergic Reactions

Anaphylaxis is a severe, life-threatening systemic hypersensitivity reaction characterized by rapid onset of airway, breathing, or circulatory compromise. It results from the massive, sudden release of inflammatory mediators—including histamine, leukotrienes, tryptase, and platelet-activating factor (PAF)—from tissue mast cells and circulating basophils. In pediatrics, common triggers include foods (peanuts, tree nuts, milk, eggs, shellfish), medications (beta-lactam antibiotics, NSAIDs), insect stings (Hymenoptera), and latex. Delay in administering intramuscular Epinephrine is the single most consistent factor contributing to pediatric anaphylaxis fatalities.

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Diagnostic Criteria for Pediatric Anaphylaxis

According to the National Institute of Allergy and Infectious Disease (NIAID) and Food Allergy and Anaphylaxis Network (FAAN) criteria, anaphylaxis is highly likely when ANY ONE of the following three clinical scenarios is met:

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|                        NIAID/FAAN ANAPHYLAXIS DIAGNOSTIC CRITERIA                       |
+-----------------------------------------------------------------------------------------+
|  SCENARIO 1: ACUTE SKIN/MUCOSA + RESPIRATORY OR CIRCULATORY COMPROMISE                 |
|  - Acute onset (minutes to hours) involving skin/mucosal tissue (urticaria, flushing,   |
|    swollen lips/tongue/uvula) PLUS AT LEAST ONE OF:                                    |
|    a) Respiratory compromise (dyspnea, wheezing, stridor, hypoxemia)                   |
|    b) Reduced BP or end-organ dysfunction (syncope, hypotonia, incontinence)            |
+-----------------------------------------------------------------------------------------+
|  SCENARIO 2: TWO OR MORE SYSTEMS INVOLVED AFTER LIKELY ALLERGEN EXPOSURE               |
|  - Two or more of the following occurring rapidly after exposure to a LIKELY allergen:  |
|    a) Skin/mucosal involvement (hives, itch, flushing, angioedema)                     |
|    b) Respiratory compromise (dyspnea, stridor, wheeze)                                 |
|    c) Reduced BP or associated symptoms (collapse, syncope)                             |
|    d) Persistent gastrointestinal symptoms (cramping abdominal pain, vomiting)           |
+-----------------------------------------------------------------------------------------+
|  SCENARIO 3: HYPOTENSION AFTER KNOWN ALLERGEN EXPOSURE                                  |
|  - Reduced BP after exposure to a KNOWN allergen for that specific patient:             |
|    - Infants/Children: Low SBP for age or > 30% decrease in SBP                        |
|    - Adolescents: SBP < 90 mmHg or > 30% decrease from baseline                        |
+-----------------------------------------------------------------------------------------+

CPEN Exam Tip: Skin symptoms (hives, angioedema) are absent in up to 10–20% of anaphylactic episodes! Never rule out anaphylaxis simply because urticaria is not present, especially if acute respiratory distress or hypotension follows a food or medication exposure.

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First-Line Pharmacotherapy: Intramuscular Epinephrine

Epinephrine is the single primary life-saving medication for anaphylaxis. There are no absolute contraindications to epinephrine in a patient experiencing anaphylaxis.

Pharmacological Actions of Epinephrine

• $\alpha_1$-adrenergic receptor agonist: Induces intense vasoconstriction, reversing peripheral vasodilation, reducing mucosal vascular permeability, and relieving airway edema and cutaneous angioedema.
• $\beta_1$-adrenergic receptor agonist: Increases myocardial contractility (inotropy) and heart rate (chronotropy), boosting cardiac output.
• $\beta_2$-adrenergic receptor agonist: Causes powerful bronchial smooth muscle relaxation (bronchodilation) and inhibits further mast cell and basophil degranulation (mediator suppression).

Administration Protocol & Dosing Guidelines

• Concentration: Epinephrine 1:1,000 (1 mg/mL) formulation.
• Dose: 0.01 mg/kg IM (equivalent to $0.01 \text{ mL/kg}$ of 1:1,000 solution).
  • Maximum single dose (prepubertal children): 0.3 mg ($0.3 \text{ mL}$).
  • Maximum single dose (adolescents/adults): 0.5 mg ($0.5 \text{ mL}$).
• Autoinjector Weight-Based Dosing:
  • Infant Autoinjector ($0.1 \text{ mg}$): Weight 7.5 kg to 15 kg.
  • Junior Autoinjector ($0.15 \text{ mg}$): Weight 15 kg to 30 kg.
  • Adult Autoinjector ($0.3 \text{ mg}$): Weight $\ge 30 \text{ kg}$.
• Route & Site: Intramuscular (IM) injection into the mid-anterolateral thigh (vastus lateralis muscle). Subcutaneous injection must be avoided due to delayed absorption caused by local vasoconstriction.
• Frequency: Repeat every 5 to 15 minutes if clinical symptoms persist or worsen. Up to 20% of pediatric anaphylactic reactions require a second dose of epinephrine.

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Emergency Airway Management & Patient Positioning

Airway Management Escalation

Laryngeal edema and tongue angioedema can cause complete upper airway obstruction within minutes. Emergency nursing interventions include:

1. Administer $100%$ high-flow oxygen via non-rebreather mask.
2. Nebulized Racemic Epinephrine ($0.5 \text{ mL}$ of 2.25% solution in $3 \text{ mL}$ normal saline) or L-epinephrine for stridor and upper airway swelling.
3. Prepare for early endotracheal intubation if hoarseness, stridor, or severe uvular edema progresses. Select an endotracheal tube size 0.5 to 1.0 mm smaller than standard for age to accommodate laryngeal edema.

Patient Positioning: Prevention of "Empty Ventricle Syndrome"

   CORRECT POSITIONING                  FATAL POSITIONING ERROR
+-------------------------+            +-------------------------+
| SUPINE WITH LEGS ELEVATED|            | ABRUPT STANDING / SITTING|
| Maintains venous return  |            | Sudden venous pooling    |
| to cardiac chambers     |            | -> Vena Cava Collapse    |
|                         |            | -> Empty Ventricle      |
|                         |            | -> Sudden Cardiac Arrest|
+-------------------------+            +-------------------------+

CRITICAL CPEN SAFETY MANDATE: Patients in anaphylactic shock MUST be kept supine with their legs elevated. If respiratory distress makes lying flat intolerable, elevate the head slightly, but NEVER allow the child to stand up or sit upright suddenly. Massive venodilation causes blood pooling in the splanchnic bed and lower extremities. Abruptly standing up causes immediate venous collapse, leading to zero preload, empty ventricular contraction, profound bradycardia, and sudden cardiac arrest ("empty ventricle syndrome").

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Refractory Anaphylaxis & Glucagon Therapy

Refractory anaphylaxis is defined as persistent shock or airway obstruction despite repeated IM epinephrine doses and aggressive fluid resuscitation (20 mL/kg isotonic crystalloid boluses).

• Continuous Epinephrine Infusion: Initiate continuous IV/IO Epinephrine infusion at 0.05 to 1.0 mcg/kg/min titrated to blood pressure and perfusion.
• Glucagon Administration for Beta-Blocker Patients: Patients taking chronic beta-blocker therapy (e.g., propranolol for dysrhythmias or hemangiomas) may be resistant to epinephrine because their cardiac beta-receptors are blocked.
  • Mechanism: Glucagon binds to non-beta glucagon receptors on cardiomyocytes, stimulating adenylate cyclase directly to produce cAMP, exerting positive inotropic and chronotropic effects independent of beta-receptors.
  • Pediatric Glucagon Dose: 20–30 mcg/kg IV/IO (maximum single dose 1–5 mg) given over 3–5 minutes, followed by a continuous infusion of 5–15 mcg/min.

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Second-Line Adjunctive Therapies & Biphasic Monitoring

Antihistamines and corticosteroids are second-line adjunctive agents. They MUST NEVER be administered in place of or prior to Epinephrine.

• $\mathbf{H_1}$-Antihistamines: Diphenhydramine ($1 \text{ mg/kg}$ IV/IM, max $50 \text{ mg}$). Relieves pruritus and hives.
• $\mathbf{H_2}$-Antihistamines: Famotidine ($0.5 \text{ mg/kg}$ IV, max $20 \text{ mg}$). Provides dual histamine blockade.
• Systemic Corticosteroids: Methylprednisolone ($1-2 \text{ mg/kg}$ IV) or Dexamethasone ($0.6 \text{ mg/kg}$ IV/IM). Onset of action takes 4 to 6 hours; administered primarily to decrease the risk of biphasic reactions.
• Biphasic Reactions: A recurrence of anaphylactic symptoms hours after complete resolution of the initial event, occurring without re-exposure to the allergen. Biphasic reactions occur in up to 20% of cases, typically within 1 to 72 hours (peak 4–12 hours). Patients requiring multiple epinephrine doses or presenting with severe initial shock must be observed in the ED or pediatric ICU for at least 4 to 8 hours.