What is the appropriate initial management for a suspected allergic reaction, including anaphylaxis?

Initial Management of Allergic Reactions and Anaphylaxis

Immediately administer intramuscular epinephrine 0.3-0.5 mg (1:1,000 solution) into the anterolateral thigh for any suspected anaphylaxis—this is the single most critical intervention and should never be delayed. 1, 2, 3

Immediate Recognition and First-Line Treatment

ABC Approach and Epinephrine Administration

• Stop all potential triggers immediately including IV medications, colloids, latex exposure, and any ongoing infusions 4, 1
• Call for help and note the time of reaction onset 4, 1
• Secure airway with 100% oxygen; intubate if there is evidence of upper airway obstruction, progressive laryngeal edema, or respiratory compromise 4, 1, 5
• Position patient supine with legs elevated to improve venous return and cerebral perfusion 4, 5

Epinephrine Dosing Strategy

The route and dose depend critically on setting and IV access:

For patients WITHOUT established IV access (most common scenario):

• Adults: 0.3-0.5 mg IM (0.3-0.5 mL of 1:1,000 solution) into anterolateral thigh 5, 2, 3
• Children >12 years: 500 mcg IM (0.5 mL of 1:1,000 solution) 4
• Children 6-12 years: 300 mcg IM (0.3 mL of 1:1,000 solution) 4
• Children <6 years: 150 mcg IM (0.15 mL of 1:1,000 solution) 4

For patients WITH established IV access (perioperative/ICU settings):

• Grade II reactions (moderate symptoms): 50 mcg IV (0.5 mL of 1:10,000 solution), escalate to 100 mcg at 2 minutes if inadequate response 1
• Grade III reactions (severe, life-threatening): 100 mcg IV initially, escalate to 200 mcg at 2 minutes if unresponsive 1
• Grade IV reactions (cardiac arrest): 1 mg IV following advanced life support protocols 1

Critical pitfall: IV epinephrine requires extreme caution with dosing—use 1:10,000 dilution (not 1:1,000) and titrate carefully to avoid cardiovascular complications. 4, 1

Aggressive Fluid Resuscitation

• Administer crystalloid (0.9% saline or lactated Ringer's) immediately via large-bore IV 4, 1
• Grade II reactions: 500 mL rapid bolus 1
• Grade III reactions: 1 L rapid bolus, escalate up to 20-30 mL/kg for refractory hypotension 1
• Large volumes (multiple liters) may be required due to massive capillary leak 4

Management of Refractory Anaphylaxis (After 10 Minutes Without Response)

For Persistent Hypotension:

• Double the epinephrine bolus dose 1, 5
• Start peripheral epinephrine infusion at 0.05-0.1 mcg/kg/min 1, 5
• Add norepinephrine infusion (0.05-0.5 mcg/kg/min), phenylephrine, or metaraminol 4, 1, 5

For Persistent Bronchospasm:

• Administer inhaled bronchodilators (salbutamol) or volatile anesthetics 4, 1
• Consider IV bronchodilators such as ketamine, salbutamol, aminophylline, or magnesium sulfate 4

Adjunct Medications (ONLY After Epinephrine and Fluids)

These are secondary interventions and should never delay epinephrine:

Antihistamines:

• Adults: Chlorphenamine 10 mg IV or diphenhydramine 25-50 mg IV 4, 5
• Children 6-12 years: Chlorphenamine 5 mg IV 4
• Children <6 years: Chlorphenamine 2.5 mg IV 4

Corticosteroids:

• Adults: Hydrocortisone 200 mg IV or methylprednisolone 1 mg/kg IV 4, 5
• Children 6-12 years: Hydrocortisone 100 mg IV 4
• Children <6 years: Hydrocortisone 50 mg IV 4

Important caveat: There is no clear evidence that corticosteroids prevent biphasic reactions, but they are traditionally given after adequate resuscitation 5

Diagnostic Testing

• Obtain serum mast cell tryptase at three specific time points: (1) as soon as feasible during resuscitation, (2) at 1-2 hours after symptom onset, and (3) at 24 hours or in convalescence for baseline 4, 1
• An increase >1.2 × baseline + 2 μg/L confirms mast cell degranulation 1
• Do not delay resuscitation to obtain samples 4

Observation Period

• Monitor in a supervised area for minimum 6 hours from reaction onset or until stable with regressing symptoms 4, 1, 5
• Risk of biphasic reactions is likely low, but prolonged observation is prudent for severe cases (Grades III-IV) 4, 5
• Most patients with Grade III-IV reactions require ICU admission, particularly with prolonged resuscitation or ongoing vasopressor requirements 4

Documentation and Follow-Up

• Document exact timing of all drug/substance exposures before reaction, symptom onset and progression, and all treatments with times and doses 1
• All patients with Grade II-IV reactions must be referred to specialized allergy clinics for formal investigation 4-6 weeks post-reaction 1
• Prescribe two epinephrine auto-injectors with training on proper use before discharge 5, 3
• Develop an emergency action plan with the patient 3

Special Populations

Pregnant Patients:

• Follow same treatment principles with no dose adjustments 4
• Position with left uterine displacement to avoid aortocaval compression 4
• Consider emergent Caesarean section if persistent hypotension despite resuscitation 4
• Perimortem Caesarean delivery should be considered if persistent hypotension after 4 minutes of cardiac arrest 4

Patients with Mastocytosis:

• Avoid known histamine-releasing agents (atracurium, mivacurium) unless clinically essential 4
• Preoperative H1/H2 antagonists and corticosteroids are usually recommended, though not evidence-based 4