How adrenaline works in anaphylaxis
Adrenaline treats anaphylaxis by constricting dilated, leaky vessels and relaxing narrowed airways, but its effects are short-lived and it does not remove the trigger.
Remember
Adrenaline reverses the vasodilation, swelling and narrowed airways of anaphylaxis by stimulating alpha receptors (which squeeze the vessels) and beta-2 receptors (which open the airways), but its effects are short-lived.
Memory aid
alpha squeezes the vessels, beta-2 opens the airways
Link each receptor type to its job: alpha receptors narrow the vessels and beta-2 receptors widen the airways.
In anaphylaxis, mast cells (the cells that release histamine and other mediators) release mediators that narrow the airways and dilate the blood vessels. The mediators make the vessels leaky, so blood pressure falls and fluid escapes into the tissues as swelling. They also tighten the muscle in the airway walls, and the airways can narrow dangerously quickly.
Adrenaline reverses these problems through receptors, the sites on the cell surface that it stimulates. Alpha receptors in the walls of blood vessels cause vasoconstriction, which reverses the vasodilation, raises blood pressure and reduces the leakage of fluid. That reduces tissue oedema (swelling), as well as swelling of the airway lining.
Beta-2 receptors relax bronchial smooth muscle (the muscular walls of the airways). This reverses bronchoconstriction (narrowing of the airways) and improves airflow. Adrenaline also acts on beta-2 receptors on mast cells, which inhibits their activation and limits further release of mediators.
Adrenaline is short-acting (its effects are short-lived), so a dose is repeated after 5 minutes if problems persist. It does not remove the allergen (the trigger), so removing the trigger, where possible, is a separate step. It is given by intramuscular injection into the outer thigh as early as possible.
Key facts
- Alpha effect
- vasoconstriction reverses peripheral vasodilation and reduces tissue oedema.
- Airway effect
- beta-2 receptors dilate the bronchial airways by relaxing bronchial smooth muscle.
- Heart effect
- beta-1 receptors increase the force of the heart's contraction.
- Effect on mast cells
- adrenaline acts on beta-2 receptors on mast cells to inhibit their activation.
- Limits
- it does not remove the allergen and is short-acting.
- Repeat dose
- a dose is repeated after 5 minutes if problems persist.
Exam traps
- Adrenaline does not increase vascular permeability, the leakiness of the vessels. It reduces it through alpha-receptor vasoconstriction, whereas increased permeability is what histamine does, worsening swelling and hypotension.
- Adrenaline does not remove the allergen, so the trigger remains. Removing the trigger, where possible, is a separate step.
- Alpha-1 stimulation constricts the vessels of the airway lining, which can reduce swelling, but it does not relax bronchial smooth muscle. Opening the airways is a beta-2 effect.
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