4.3 Allergic Reactions & Anaphylaxis

Allergic reactions occur when the immune system overreacts to a foreign substance (allergen). While most allergic reactions are mild and self-limiting, anaphylaxis is a rapidly progressive, life-threatening emergency that demands immediate intervention.

Mild vs. Severe Allergic Reactions

Anaphylaxis

Anaphylaxis involves two or more body systems and can progress to death within minutes if untreated. It is mediated by massive histamine release from mast cells and basophils.

Pathophysiology

1. Vasodilation -- blood vessels dilate, causing hypotension
2. Increased capillary permeability -- fluid leaks into tissues (edema, angioedema)
3. Bronchospasm -- smooth muscle constriction in airways
4. Increased mucus production -- further airway compromise

Signs and Symptoms of Anaphylaxis

• Skin: Urticaria (hives), flushing, angioedema (swelling of face, lips, tongue, throat)
• Respiratory: Wheezing, stridor, hoarseness, dyspnea, throat tightness
• Cardiovascular: Tachycardia, hypotension, weak/thready pulse, dizziness
• GI: Nausea, vomiting, abdominal pain, diarrhea
• Neurological: Anxiety, altered mental status, sense of impending doom

Common Allergens

• Foods: Peanuts, tree nuts, shellfish, fish, milk, eggs, soy, wheat
• Insect stings: Bees, wasps, hornets, fire ants
• Medications: Antibiotics (penicillin), NSAIDs, aspirin
• Other: Latex, contrast dye, exercise-induced

Epinephrine Auto-Injector

Epinephrine is the first-line and ONLY definitive treatment for anaphylaxis. It works by:

• Bronchodilation -- relaxes smooth muscle in airways
• Vasoconstriction -- increases blood pressure
• Reduces edema -- decreases capillary permeability
• Increases heart rate and contractility -- improves cardiac output

Dosing

Administration Steps

1. Confirm signs/symptoms of anaphylaxis
2. Obtain order from medical control (or follow standing orders/protocol)
3. Remove safety cap from auto-injector
4. Place tip firmly against lateral (outer) mid-thigh
5. Can be administered through clothing -- do not delay to remove pants
6. Press firmly until click is heard -- hold for 10 seconds
7. Remove and massage injection site for 10 seconds
8. Record the time of administration
9. Reassess patient -- a second dose may be needed in 5-15 minutes if no improvement

Side Effects of Epinephrine

• Tachycardia, palpitations
• Anxiety, tremors
• Headache
• Nausea
• Pale skin (from vasoconstriction)
• These are expected and generally well-tolerated -- they are NOT reasons to withhold epinephrine in anaphylaxis

Biphasic Reactions

• Anaphylaxis symptoms can return 4-12 hours after initial resolution
• Occurs in approximately 20% of anaphylaxis cases
• This is why all anaphylaxis patients must be transported to the hospital for monitoring
• Second reaction may be more severe than the first

EMT Treatment for Anaphylaxis

1. Remove the patient from the allergen source if safe to do so
2. Maintain airway -- prepare for potential need for BVM ventilation
3. Administer epinephrine auto-injector (assist with patient's own or per protocol)
4. Administer high-flow oxygen (15 L/min via NRB)
5. If signs of shock: lay patient supine with legs elevated (unless dyspnea prevents this)
6. Monitor vitals continuously
7. Rapid transport
8. Be prepared to administer a second dose of epinephrine

Recognizing Anaphylaxis Quickly

Anaphylaxis can kill within minutes, so the EMT must recognize it fast and treat without hesitation. The practical field rule is to suspect anaphylaxis - and give epinephrine - whenever a patient has a sudden illness after an exposure with either (1) involvement of two or more body systems (for example, hives plus wheezing, or swelling plus vomiting) or (2) airway, breathing, or circulatory compromise alone (stridor, severe wheeze, or hypotension), even with skin findings absent. Waiting for "classic" hives can be a fatal delay, because up to 20% of anaphylaxis cases have no skin findings at all.

Airway Is the Priority

The most rapidly lethal feature of anaphylaxis is upper-airway swelling (angioedema) of the lips, tongue, and larynx. Listen for stridor, hoarseness, or a "lump in the throat" - these signal a closing airway and demand immediate epinephrine, which is the only treatment that reverses the swelling. Give high-flow oxygen, prepare suction and a BVM, and transport without delay, because the airway can deteriorate faster than antihistamines could ever work.

Why Epinephrine, and Why First

A core NREMT trap is choosing an antihistamine or "giving Benadryl and watching" for a patient in true anaphylaxis. Epinephrine comes first; everything else is secondary.

Special Populations and Cautions

Patients on beta-blockers may respond poorly to epinephrine and need ALS support. In pregnancy, anaphylaxis still warrants epinephrine - the risk of untreated shock to mother and fetus outweighs the drug's effects. For an infant or small child under 30 kg, use the 0.15 mg auto-injector. Always reassess after the dose: if the airway, breathing, or blood pressure has not improved (or symptoms return), a second dose is appropriate in 5-15 minutes per protocol.

Because of the risk of a biphasic reaction 4-12 hours later, every patient who receives epinephrine must be transported for monitoring even if they feel completely well.