Why the thigh and not the vein
In anaphylaxis, the first-line drug is adrenaline (epinephrine) given intramuscularly into the mid-outer thigh. This route is absorbed quickly and is safe at the anaphylaxis dose. Pushing the same undiluted 1 mg/mL solution into a vein produces a sudden, very high blood level. An undiluted IV push of 1 mg/mL adrenaline can cause fatal ventricular dysrhythmias, severe hypertension, and myocardial ischemia, which is why the thigh muscle was used.
Why intramuscular adrenaline works well
The thigh muscle has a rich blood supply, so adrenaline injected there reaches the circulation within minutes. Its alpha-adrenergic effects constrict blood vessels and raise blood pressure, while its beta-adrenergic effects relax the airways and reduce swelling and the release of inflammatory mediators. This man has hives, hoarseness, wheezing, and a blood pressure of 80/50 mmHg, which together show severe anaphylaxis. Intramuscular adrenaline should be given without delay, and repeated if there is no improvement.
When IV adrenaline is used
IV adrenaline in anaphylaxis is reserved for shock that does not respond to intramuscular doses and fluid resuscitation. It is given only as a diluted, monitored infusion by experienced staff, with continuous cardiac monitoring. It is not a quick push of the 1 mg/mL solution, and it is not reserved for complete airway closure.
Why the other explanations are wrong
| Explanation | Error |
|---|
| The thigh absorbs it slowly, so the effect lasts longer | Absorption from the thigh is fast, not slow |
| A vein route has a slower onset | IV acts faster; the danger is the sudden high level |
| IV is saved for complete airway closure | IV is a diluted infusion for refractory shock |
| IV push at that strength can cause fatal heart rhythms | Accurate |
Watch out! He takes metoprolol, a beta blocker. Beta blockers can make anaphylaxis more severe and reduce the response to adrenaline. The answer is still intramuscular adrenaline; if he does not respond, the team may consider glucagon, but that does not change why the IV push is dangerous. Positioning him lying flat with legs raised and giving IV fluids also support his blood pressure.
Exam takeaway
Give anaphylaxis adrenaline intramuscularly into the mid-outer thigh; never push the 1 mg/mL solution intravenously. Key point! The concentration on the label matters: the 1 mg/mL strength is for intramuscular use, while IV adrenaline for refractory shock is diluted and infused under monitoring.