Management of anaphylactic shock
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PMID: 38108678
Excellent - I now have the full treatment algorithm from Rosen's Emergency Medicine and the 2023 practice guideline update. I have enough to write a comprehensive, well-structured response.
Management of Anaphylactic Shock
Anaphylaxis is a severe, life-threatening systemic hypersensitivity reaction caused by rapid mast cell and basophil degranulation - releasing histamine, tryptase, prostaglandins, leukotrienes, and platelet-activating factor - that produces simultaneous cardiovascular collapse, bronchospasm, and cutaneous/mucosal changes.
Diagnostic Criteria
Anaphylaxis is highly likely when any one of the following three criteria is met:
- Acute onset involving skin/mucosa + respiratory compromise OR hypotension after exposure to a likely allergen
- Two or more of the following after exposure to a known allergen: skin/mucosal signs, respiratory symptoms, hypotension, or persistent GI symptoms
- Reduced blood pressure alone after exposure to a known allergen for that patient
Common Triggers
| Category | Examples |
|---|---|
| Medications | Antibiotics (esp. penicillins), NSAIDs, radiocontrast media, chemotherapy, monoclonal antibodies |
| Foods | Peanuts, tree nuts, shellfish, fish, milk, eggs |
| Insect stings | Hymenoptera (bees, wasps, hornets) |
| Latex | Medical gloves, catheters, balloons |
| Idiopathic | ~20% of cases |
Step-by-Step Management
Immediate Emergency Measures (all done simultaneously)
- Remove the triggering agent (stop infusion, remove stinger)
- Position the patient supine; elevate legs if hypotensive (do NOT allow upright posture - upright position at onset is a risk factor for fatal outcome)
- Cardiac monitoring, pulse oximetry, and continuous BP monitoring
- Secure airway - be prepared for endotracheal intubation (with RSI or surgical airway if angioedema threatens the airway)
- Large-bore IV access (16-18 gauge preferred)
- Supplemental oxygen if respiratory compromise present
- IV fluids - start rapid infusion of isotonic crystalloid (normal saline):
- Adults: 1000 mL IV in the first 5 minutes, several liters may be required
- Pediatrics: 20-30 mL/kg IV boluses
First-Line Drug: Epinephrine (Adrenaline) - GIVE IMMEDIATELY
Epinephrine is the only first-line agent. It must not be delayed. All other drugs are second-line adjuncts.
Route of choice: Intramuscular (IM) into the anterolateral thigh (fastest absorption; superior to deltoid or subcutaneous routes)
| Patient | Dose | Route |
|---|---|---|
| Adult | 0.3-0.5 mg (of 1 mg/mL = 1:1000 solution) | IM, anterolateral thigh |
| Pediatric | 0.01 mg/kg (1:1000 solution) | IM, anterolateral thigh |
| Autoinjector | EpiPen 0.3 mg (adult) / EpiPen Jr 0.15 mg (child) | IM, anterolateral thigh |
- Repeat every 5-10 minutes as needed
- IV epinephrine (1:10,000 dilution, 0.1 mg slow IV) is reserved for refractory cases with cardiovascular collapse or when IM access is not feasible
- IV infusion: 1-10 mcg/min, titrated to response
Why epinephrine first? It is the only agent that addresses all pathophysiologic mechanisms: alpha-1 agonism reverses vasodilation, beta-1 agonism improves cardiac output, and beta-2 agonism reverses bronchospasm and reduces mediator release from mast cells.
Second-Line Agents (adjuncts - should NOT precede or replace epinephrine)
1. Antihistamines
H1 blocker - Diphenhydramine:
- Adults: 50 mg IV or oral
- Pediatric: 1 mg/kg IV or oral
H2 blocker - Famotidine:
- Adults: 40 mg IV or oral
- Pediatric: 0.25 mg/kg IV or oral
Note: Antihistamines relieve urticaria and pruritus but do NOT treat bronchospasm or hypotension. They are purely symptomatic adjuncts.
2. Corticosteroids
Methylprednisolone:
- Adults: 125 mg IV
- Pediatric: 2 mg/kg IV
Alternative - Hydrocortisone:
- Adults: 200-300 mg IV
Primary purpose: Prevention of biphasic anaphylaxis (recurrence 4-12 hours after initial reaction, occurring in ~5% of cases). Corticosteroids have NO role in acute reversal of anaphylaxis - their onset is too slow (4-6 hours).
3. Inhaled Beta-2 Agonists (for bronchospasm)
- Albuterol (salbutamol) via nebulizer: 2.5 mg in 3 mL normal saline, can repeat
- Indicated when wheezing/bronchospasm persists after epinephrine
4. Vasopressors (for refractory hypotension)
When hypotension persists despite epinephrine and adequate fluid resuscitation:
- Norepinephrine: 0.1-0.2 mcg/kg/min IV infusion (first-choice vasopressor)
- Vasopressin: 40 units IV bolus or 0.01-0.04 units/min infusion
- Dopamine: 5-20 mcg/kg/min IV infusion
- Glucagon: Especially important in patients on beta-blockers (bypasses beta-receptor blockade; dose 1-2 mg IV over 5 min, then 1-15 mg/min infusion)
Special Situation: Beta-Blocker Patients
Patients on beta-blockers are at risk for:
- Epinephrine resistance (beta-2 blockade prevents bronchodilation)
- Paradoxical bradycardia from unopposed alpha stimulation from epinephrine
Use glucagon as a key adjunct - it acts directly on myocardial glucagon receptors, bypassing adrenergic blockade. Also consider atropine for bradycardia and higher doses of epinephrine.
Biphasic Anaphylaxis
A second-phase reaction occurs in ~5% of cases, typically 4-12 hours (range 1-72 hours) after the initial reaction. All patients should be observed for a minimum of 4-6 hours after resolution of initial symptoms. High-risk patients (severe initial reaction, uncertain trigger, biphasic history) warrant 24-hour observation.
Disposition and Follow-Up
| Criteria | Action |
|---|---|
| Mild reaction, complete resolution, no comorbidities | Observe 4-6 hours, discharge |
| Severe reaction, respiratory compromise, or refractory shock | ICU admission |
| All discharged patients | Prescribe epinephrine autoinjector x2 (EpiPen) |
| All discharged patients | Referral to allergist/immunologist |
| All discharged patients | Medical alert bracelet |
| Consider | Serum tryptase at 1-3 hours post-reaction (elevated >11.4 ng/mL confirms diagnosis; also screens for underlying mastocytosis) |
Summary Algorithm
Suspect anaphylaxis?
|
v
EPINEPHRINE IM (anterolateral thigh) IMMEDIATELY
|
v
Remove trigger + Supine position + O2 + IV access + Monitoring
|
v
IV fluids (NS bolus)
|
Bronchospasm? Refractory hypotension? Airway compromise?
| | |
Albuterol Vasopressors/Glucagon Intubation/Surgical airway
|
v
H1/H2 antihistamines + Corticosteroids (adjuncts)
|
v
Observe ≥4-6 hours for biphasic reaction
|
v
Discharge: EpiPen x2 + Allergen avoidance + Allergy referral
Key Points to Remember
- Epinephrine is the ONLY first-line drug - delay increases mortality
- IM into anterolateral thigh is the preferred route (not deltoid, not subcutaneous)
- Antihistamines and steroids are adjuncts, not substitutes for epinephrine
- Upright posture at onset worsens outcomes - always lay the patient flat
- Beta-blocker use makes anaphylaxis harder to treat - glucagon is essential
- Serum tryptase should be drawn 1-3 hours post-reaction to confirm diagnosis
- Always send patients home with an epinephrine autoinjector and allergy referral
Sources:
- ROSEN's Emergency Medicine: Concepts and Clinical Practice - Anaphylaxis chapter (Box 106.7 Treatment Algorithm)
- Goldman-Cecil Medicine International Edition - Anaphylaxis chapter
- Golden et al., 2024 - Anaphylaxis: A 2023 Practice Parameter Update (PMID: 38108678, Annals of Allergy, Asthma & Immunology)
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