Emergency Anaphylaxis Medicines

Emergency anaphylaxis medicines reverse life-threatening allergic reactions that cause airway swelling, bronchospasm and shock. Intramuscular adrenaline is the only first-line drug and must be given at once. Other drugs are adjuncts and never replace adrenaline.

Classification. Adrenaline; oxygen and intravenous fluids; adjuncts (antihistamines, corticosteroids, bronchodilators); glucagon for patients on beta-blockers.

Mechanism. Adrenaline reverses vasodilation and oedema through alpha-1 effects, relaxes bronchi through beta-2 effects, supports the heart through beta-1 effects and reduces mediator release from mast cells.

Pharmacokinetics. Intramuscular adrenaline into the outer thigh acts within 5 to 10 minutes and lasts 10 to 20 minutes, so doses may need repeating. Subcutaneous or deltoid injection is slower.

Positioning. Lie the patient flat with legs raised (sit up if breathless; left side if pregnant). Do not stand them up suddenly.

After the event. Observe for at least 6 hours, and 12 hours or longer in severe cases or if adrenaline was repeated. Measure mast cell tryptase within 1 to 2 hours of onset, then again at 24 hours. Refer to allergy services, and prescribe two adrenaline auto-injectors with training.

• First-line. Adrenaline intramuscularly; oxygen; fluids; positioning.
• Alternative. Adrenaline infusion if no response after two to three intramuscular doses; glucagon for beta-blocker users.
• Specialist. Intensive care, desensitisation, allergist assessment, mast cell disorders.
• Emergency. Any suspected anaphylaxis: airway, breathing or circulation compromise after an allergen exposure.

Selection. Give adrenaline first; do not wait for blood pressure or tests. Never give intravenous adrenaline bolus doses outside arrest.

Contraindications. None absolute in anaphylaxis.

Serious adverse effects.

• Tremor, anxiety, palpitations, headache and pallor.
• Hypertension, arrhythmia, myocardial ischaemia (particularly with intravenous dosing errors or heart disease).
• Biphasic reaction in up to 5% of cases.
• Chlorphenamine: sedation and hypotension if given fast.

Major interactions. Beta-blockers cause resistant anaphylaxis and unopposed alpha effects; tricyclic antidepressants and monoamine oxidase inhibitors raise adrenaline effects; ACE inhibitors may worsen reactions.

Common prescribing errors.

• Giving antihistamine or steroid instead of adrenaline.
• Using the wrong adrenaline concentration (1 mg/mL versus 0.1 mg/mL).
• Giving intravenous adrenaline as a bolus.
• Discharging too soon.
• Not providing auto-injectors and allergy follow-up.

Special groups.

• Pregnancy: give adrenaline at the usual dose; left tilt; fetal monitoring after.
• Breastfeeding: compatible.
• Children: weight or age-based doses; auto-injector choice by weight.
• Older adults: give adrenaline without delay; heart disease is not a reason to withhold it.
• Kidney: no adjustment.
• Liver: no adjustment.
• East Africa: adrenaline 1 mg/mL ampoules are widely available and cheap, but auto-injectors are rarely available, so train patients and health workers to draw up and inject the right dose; common triggers include antibiotics (penicillins), foods, insect stings and snake antivenom, which also needs adrenaline on hand.

Monitoring. Airway, respiratory rate, oxygen saturation, blood pressure, heart rate and rhythm, urine output, and repeat observation for 6 hours or longer.

Referral and escalation. No response to two doses of adrenaline, airway oedema, refractory shock, biphasic reaction or recurrent episodes.

Comparison with alternative classes. Adrenaline is the only drug that reverses all features of anaphylaxis. Antihistamines and steroids help skin and later symptoms, and fluids support circulation.