What is the management of an allergic reaction in an emergency room (ER) setting?

Management of Allergic Reactions in the Emergency Room

Immediate First-Line Treatment: Epinephrine

Intramuscular epinephrine is the only first-line treatment for anaphylaxis and must be administered immediately—never delay or substitute with antihistamines or corticosteroids, as this is the most common error that places patients at significantly increased risk for life-threatening progression. 1, 2

Epinephrine Dosing Protocol

For adults and children ≥30 kg:

• Administer 0.3-0.5 mg (0.3-0.5 mL of 1:1,000 solution) intramuscularly into the anterolateral thigh (vastus lateralis muscle) 1, 3
• Repeat every 5-10 minutes as necessary if symptoms persist or progress 3

For children <30 kg:

• Administer 0.01 mg/kg (maximum 0.3 mg) intramuscularly into the anterolateral thigh 1, 3
• Repeat every 5-10 minutes as necessary 3

Critical technical points:

• The anterolateral thigh is the only appropriate site—never inject into buttocks, digits, hands, or feet 3
• Use a needle of adequate length to reach muscle beneath subcutaneous fat, particularly in obese patients 1
• IM administration provides more rapid plasma concentrations than subcutaneous injection 1

Graded Response Based on Severity

For Grade II reactions (moderate hypotension or bronchospasm):

• Initial dose: 20 mcg IV epinephrine 1
• If inadequate response at 2 minutes: escalate to 50 mcg 1

For Grade III reactions (life-threatening hypotension or severe bronchospasm):

• Initial dose: 50 mcg IV epinephrine (or 100 mcg if no prior treatment given) 1
• If inadequate response at 2 minutes: escalate to 200 mcg 1

For Grade IV reactions (cardiac arrest):

• Initial dose: 1 mg IV epinephrine 1
• Follow advanced cardiac life support protocols with repeat dosing every 3-5 minutes 1

Supportive Measures (Concurrent with Epinephrine)

Airway Management

• Assess for laryngeal edema, stridor, or respiratory distress 1
• Prepare for early intubation if airway obstruction is present or progressing 4
• Position patient supine with legs elevated to improve venous return 1

Fluid Resuscitation

For adults:

• Administer 500 mL-1 L crystalloid (normal saline or balanced salt solution) as rapid bolus 1
• Repeat as needed based on response 1

For children:

• Administer up to 30 mL/kg crystalloid as rapid bolus 1

Oxygen

• Provide high-flow oxygen to maintain saturation >90% 1

Adjunctive Medications (ONLY After Epinephrine)

H1 Antihistamines

• Diphenhydramine 25-50 mg IV or oral for adults 1, 2, 5
• Diphenhydramine 1-2 mg/kg (maximum 50 mg) for children 2
• Alternative: Cetirizine 10 mg oral (less sedating, rapid onset) 1
• Important caveat: H1 antihistamines only relieve itching and urticaria—they do NOT treat stridor, bronchospasm, GI symptoms, or shock 1, 2

H2 Antihistamines

• Ranitidine 1-2 mg/kg (maximum 75-150 mg) IV or oral 2
• Famotidine is an alternative H2 blocker 2
• Combination of H1 + H2 antihistamines works better than either alone 2

Bronchodilators (for persistent bronchospasm)

• Albuterol via nebulizer (preferred over MDI in severe respiratory distress) 1
• Albuterol does NOT relieve laryngeal edema and should never substitute for epinephrine 1

Corticosteroids

• Prednisone 1 mg/kg (maximum 60-80 mg) oral or methylprednisolone 1-2 mg/kg IV 1, 2
• Theoretical benefit for preventing biphasic reactions, though evidence is limited 1
• Slow onset of action (4-6 hours) makes them ineffective for acute symptoms 1

Management of Refractory Anaphylaxis

If inadequate response >10 minutes after symptom onset despite repeated epinephrine:

Additional Vasopressors

• Epinephrine infusion 0.05-0.1 mcg/kg/min 1
• Vasopressin 1-2 IU bolus with or without infusion at 2 IU/hour 1
• Norepinephrine infusion 0.05-0.5 mcg/kg/min 1

Special Consideration: Beta-Blocker Patients

• Glucagon 1-5 mg IV for adults (or 20-30 mcg/kg for children) 2
• Glucagon bypasses beta-receptor blockade and can reverse refractory hypotension 2

Consider Extracorporeal Life Support (ECLS)

• If systolic BP <50 mmHg or end-tidal CO₂ <20 mmHg despite maximal therapy 1

Observation Period

Observe for 4-6 hours minimum after symptom resolution 1

Extend observation if:

• Severe initial presentation requiring multiple epinephrine doses 1
• History of biphasic reactions 1
• Delayed presentation (>1 hour from exposure) 1
• Inadequate access to emergency care from home 1

Key point: Biphasic reactions (recurrence without re-exposure) occur in up to 20% of cases and can happen outside typical observation windows, so clinical judgment is essential 1, 4

Discharge Planning

Prescriptions and Education

• Two epinephrine auto-injectors (0.15 mg for <25 kg; 0.3 mg for ≥25 kg) with hands-on training 1, 2
• Diphenhydramine every 6 hours for 2-3 days 1, 2
• Ranitidine twice daily for 2-3 days 1, 2
• Prednisone daily for 2-3 days 1, 2

Patient Instructions

• Written anaphylaxis emergency action plan 1
• Medical identification jewelry or wallet card 1
• Strict allergen avoidance counseling 1

Follow-Up

• Mandatory referral to allergist/immunologist for identification of trigger and long-term management 1
• Follow-up with primary care within 1-2 weeks 2

Common Pitfalls to Avoid

Never use antihistamines as primary treatment—this is the most frequently reported reason for delayed epinephrine administration and significantly increases mortality risk 1, 2

Never delay epinephrine for "mild" symptoms—progression from mild to severe can occur within minutes, and early epinephrine prevents fatal outcomes 6

Never rely on inhaled epinephrine—plasma concentrations achieved are insufficient for hemodynamic stabilization 7

Never assume observation period prevents all biphasic reactions—they can occur up to 72 hours later, though most occur within 8 hours 4