What is the immediate management of anaphylaxis to antivenom in a patient being treated for a snake bite?

Management of Anaphylaxis to Anti-Snake Venom

Immediately stop the antivenom infusion, administer intramuscular epinephrine 0.3-0.5 mg (1:1000 solution) into the anterolateral thigh, and prepare for aggressive resuscitation—epinephrine is the only proven life-saving intervention and must never be delayed. 1

Immediate First-Line Management

Stop the antivenom immediately upon recognizing any signs of anaphylaxis (dyspnea, wheezing, hypotension, urticaria, angioedema, or cardiovascular collapse). 2

Administer intramuscular epinephrine as the primary intervention:

• Adults/adolescents: 0.3-0.5 mg (0.3-0.5 mL of 1:1000 solution) IM into the anterolateral thigh 1
• Children: 0.01 mg/kg (maximum 0.3 mg) of 1:1000 epinephrine IM 1
• Repeat every 5 minutes as clinically needed if symptoms persist or worsen 1
• The anterolateral thigh is superior to deltoid injection, achieving higher plasma epinephrine levels more rapidly 2

Call for help immediately and use the ABC approach (Airway, Breathing, Circulation) with team-based simultaneous interventions. 2

Secure the airway and provide 100% oxygen:

• Intubate the trachea if necessary and ventilate with oxygen 2
• Maintain airway patency, as laryngeal edema can develop 3

Elevate the patient's legs if hypotension is present to improve venous return. 2

Administer aggressive IV fluid resuscitation:

• Give 1-2 L normal saline or lactated Ringer's solution rapidly via large-bore IV cannula 1
• Large volumes may be required (up to 3 L in some cases) for persistent hypotension 2

Secondary Adjunctive Therapies

Administer corticosteroids (though they do not treat acute anaphylaxis, they may prevent biphasic reactions):

• Hydrocortisone 200 mg IV or methylprednisolone 125 mg IV every 6 hours 1
• Evidence for preventing protracted/biphasic reactions is limited but extrapolated from asthma management 2

Give antihistamines as adjuncts only (never as monotherapy):

• Chlorphenamine 10 mg IV (adult dose) 2
• Antihistamines alone have shown no benefit in preventing acute antivenom reactions 4

For persistent bronchospasm despite epinephrine:

• Administer inhaled albuterol 2.5 mg nebulized 1
• Consider IV aminophylline or magnesium sulfate for refractory cases 2

For persistent hypotension despite epinephrine and fluids:

• Consider starting an epinephrine infusion (adrenaline has a short half-life) 2
• Alternative vasopressors (e.g., metaraminol) may be considered by experienced clinicians 2

Intravenous Epinephrine Infusion Protocol

If multiple IM epinephrine doses are required, transition to IV epinephrine infusion with continuous hemodynamic monitoring:

• Prepare 1 mg (1 mL) of 1:1000 epinephrine in 250 mL D5W (yields 4 mcg/mL concentration) 2
• Infuse at 1-4 mcg/min initially, increasing to maximum 10 mcg/min for adults 2
• Pediatric dosing: 0.01 mg/kg (0.1 mL/kg of 1:10,000 solution; maximum 0.3 mg) 2
• Alternative adult protocol: 1 mg in 100 mL saline at 30-100 mL/h (5-15 mcg/min) titrated to response 2

Critical Pitfalls to Avoid

Never delay epinephrine while administering antihistamines or corticosteroids first—this is the most common fatal error, as epinephrine is the only intervention proven to save lives in anaphylaxis. 1

Do not rely on skin testing to predict antivenom reactions—studies demonstrate zero positive predictive value for equine-derived antivenoms. 1

Do not withhold necessary antivenom due to fear of reactions—the risk of untreated envenomation (coagulopathy, neurotoxicity, tissue necrosis, death) far exceeds the risk of treatable allergic reactions when epinephrine is immediately available. 1

Recognize overlapping signs between severe anaphylaxis and severe envenomation (hypotension, respiratory distress, altered consciousness)—both may coexist, requiring simultaneous management. 3

Special Populations Requiring Modified Approach

Patients on beta-blockers:

• Are at greater risk for severe anaphylaxis with blunted response to epinephrine 1
• May require higher or more frequent epinephrine doses 1
• Need more aggressive fluid resuscitation 1

Patients on ACE inhibitors:

• Have increased risk of more severe anaphylaxis (OR 2.27,95% CI 1.13-4.56) 1
• Require heightened vigilance with epinephrine immediately available 1

Pregnant patients:

• Epinephrine should be used despite pregnancy category C—maternal survival takes priority 5
• May result in uterine vasoconstriction and decreased uterine blood flow, but maternal hypotension from untreated anaphylaxis poses greater fetal risk 5

Resuming Antivenom After Anaphylaxis

Once anaphylaxis is controlled and the patient is stabilized, antivenom administration may need to be resumed if envenomation is severe and life-threatening:

• Pretreat with IV hydrocortisone and diphenhydramine 6
• Dilute antivenom and administer by slow IV infusion over 60 minutes (rather than IV push) 6
• Have epinephrine drawn up and immediately available 1
• Consider prophylactic epinephrine infusion for patients requiring additional antivenom after prior severe anaphylaxis 3
• One study showed marked reduction in reaction rates (from 49% to 2%) using premedication plus slow infusion, though this requires confirmation in randomized trials 6

Monitoring and Disposition

Observe all patients for minimum 48 hours after antivenom administration, as delayed serum sickness-type reactions can occur 5-14 days post-administration. 1, 7

Use pulse oximetry or arterial blood gas monitoring where hypoxemia is a concern, particularly with prolonged reactions. 1

Transfer to appropriate critical care area for continued monitoring and management. 2

Collect mast cell tryptase levels at three time points to confirm anaphylaxis diagnosis:

• Initial sample as soon as feasible after resuscitation starts (do not delay resuscitation) 2
• Second sample at 1-2 hours after symptom onset 2
• Third sample at 24 hours or in convalescence (baseline measurement) 2