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Adrenaline in anaphylaxis
Minutes after an intramuscular injection a 30-year-old woman develops widespread urticaria, stridor, wheeze and a blood pressure of 74/40 mmHg. Which drug, dose, concentration and route should be used first?
- correctA. Adrenaline 0.5 mg of 1 in 1000, intramuscularly into the anterolateral thigh
- B. Adrenaline 0.5 mg of 1 in 10 000, intravenously as a bolus
- C. Chlorphenamine 10 mg intravenously
- D. Hydrocortisone 200 mg intravenously
Explanation
Key fact
Anaphylaxis is treated with intramuscular adrenaline into the anterolateral thigh, and nothing else comes first. Steroids and antihistamines are adjuncts.
0.5 mg of 1 in 1000 intramuscularly into the anterolateral thigh. That is 0.5 mL of the 1 mg/mL ampoule.
Why
Alpha-1 action reverses vasodilation and the loss of capillary integrity.
Beta-1 action supports the heart; beta-2 action relieves bronchospasm.
The thigh gives faster and more reliable absorption than the deltoid.
The intramuscular route needs no line and has a wide safety margin.
The dose may be repeated every 5 minutes if there is no response.
Adrenaline in anaphylaxis: site, strength and volume
Why not the others
- B. Adrenaline 0.5 mg of 1 in 10 000, intravenously as a bolus — is the cardiac arrest preparation and dose. Given as a bolus to a perfusing patient it risks hypertensive crisis and arrhythmia.
- C. Chlorphenamine 10 mg intravenously — helps the urticaria and itch only. Antihistamines do not reverse airway obstruction or shock.
- D. Hydrocortisone 200 mg intravenously — takes hours to act. It may reduce a biphasic reaction but does nothing for the airway or the blood pressure now.
Remember
Thigh, 1 in 1000, half a millilitre. Everything else can wait.
Go deeper
Patients on beta blockers may respond poorly, and glucagon is the recognised rescue because it raises cardiac cyclic AMP independently of beta receptors.
Lie the patient flat with the legs raised - sitting up abruptly has been associated with sudden cardiac arrest in anaphylaxis.
Serum mast cell tryptase taken within a few hours, with a later baseline sample, confirms the diagnosis retrospectively.
Tripathi KD. Essentials of Medical Pharmacology, 9th ed. (Jaypee, 2024); Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th ed. (McGraw Hill, 2023).