Prehospital Academy Lesson 25.2
EMT Treatment for Anaphylaxis
Anaphylaxis is a life-threatening, multi-system Type I Hypersensitivity Reaction that can progress to distributive shock and airway obstruction within seconds. For the EMT, success is determined by the speed of recognition and the aggressive administration of Epinephrine—the only definitive treatment that addresses the physiological "triple threat" of vasodilation, bronchoconstriction, and edema.
Pathophysiology: The Mast Cell "Explosion"
Anaphylaxis is an IgE-mediated event. Upon first exposure to an allergen (e.g., bee venom, peanuts), the body produces IgE antibodies that "prime" Mast Cells and Basophils.
Upon re-exposure, the allergen binds to these IgE molecules, triggering Degranulation—the violent release of chemical mediators, primarily Histamine and Leukotrienes.
Vascular Collapse: Histamine increases capillary permeability (leakiness). Up to 50% of the patient’s plasma can "leak" into the interstitial space within 10 minutes, leading to rapid Distributive Shock.
Respiratory Failure: Leukotrienes cause intense bronchoconstriction (lower airway) while histamines cause laryngeal edema (upper airway).
Epinephrine: The Receptor-Level Antidote
Epinephrine is a natural catecholamine that works as a physiological antagonist to histamine. It acts on three specific receptors:
Alpha-1 (Vasoconstriction): Shrinks dilated blood vessels to raise blood pressure and reduce angioedema (swelling).
Beta-1 (Inotropy/Chronotropy): Increases the heart rate and force of contraction to support the failing circulation.
Beta-2 (Bronchodilation): Relaxes the smooth muscles of the bronchioles to restore air movement.
The Beta-Blocker Challenge: Patients on chronic Beta-Blockers (e.g., Atenolol) may be resistant to Epinephrine. If the patient does not respond to standard Epi doses, ALS may need to administer **Glucagon**, which bypasses the blocked beta receptors to increase cAMP levels in the heart.
Identifying the "Biphasic Threat"
Approximately 20% of anaphylactic patients experience a Biphasic Reaction. This is a "second wave" of the reaction that occurs 4 to 12 hours after the initial event, even without further exposure to the allergen.
This second wave is often more severe and refractory to treatment. This is why mandatory transport is required for every patient who receives an EpiPen, regardless of how "well" they feel in the moment.
Diagnostic Spotlight: Kounis Syndrome
In rare cases, the massive histamine release during anaphylaxis can cause the coronary arteries to spasm, leading to an acute Myocardial Infarction. This is known as Kounis Syndrome (Allergic Angina). The EMT should be aware that chest pain during an allergic reaction may not just be "anxiety"—it could be a real cardiac event.
Assessment: Critical Thinking Questions
Explain the mechanism of "Third Spacing" in anaphylaxis. How can a patient be "hypovolemic" if they haven't lost a single drop of blood?
Contrast the roles of **Epinephrine** (the first-line drug) and **Diphenhydramine** (Benadryl, the supportive drug). Why can't Benadryl "save" a patient in respiratory arrest?
Why is a history of "Beta-Blocker" use a significant risk factor for a poor outcome in anaphylaxis?
Describe the difference between **Urticaria** (hives) and **Angioedema**. Which one is a more reliable indicator of potential airway obstruction?
You treat a patient for a bee sting. They improve instantly after an EpiPen and refuse transport. How do you explain the concept of a **Biphasic Reaction** to them to encourage transport?