Antihistamines: Nursing pharmacology

Antihistamines are medications that can be used for a variety of conditions, such as alleviating symptoms of a common cold, to decrease motion sickness, and they can occasionally be used to help induce sleep.

But the primary use of antihistamines is to alleviate symptoms of allergic conditions, such as allergic rhinitis, allergic conjunctivitis, and urticaria, as well as as an adjunctive therapy for anaphylaxis, which is a severe systemic allergic reaction.

Now, these allergic conditions are usually triggered by an environmental factor, like pollen, which causes mast cells to release inflammatory mediators like histamine, triggering a widespread response.

When acting on the brain, histamine promotes wakefulness. In the bronchi, histamine causes smooth muscle contraction, leading to bronchoconstriction, while in blood vessels, it causes smooth muscle relaxation, resulting in vasodilation.

In addition, histamine increases vascular permeability, allowing fluid to accumulate in the airways, which increases nasal and bronchial secretions.

Now, antihistamines can be administered orally, topically, and injected intravenously or intramuscularly.

Once administered, they block the histamine H1 receptors, preventing histamine from binding and triggering its effects on target organs.

There are two main groups of antihistamines: first generation antihistamines include chlorpheniramine, diphenhydramine, hydroxyzine and promethazine, while second generation antihistamines include loratadine, fexofenadine, cetirizine, and levocetirizine.

Now, peripherally, both generations of antihistamines can block H1 receptors in the bronchi, resulting in bronchodilation, which improves airflow, while in the blood vessels of the skin, they cause smooth muscle contraction, which leads to vasoconstriction, in turn decreasing localized flushing; in the capillaries, they decrease vascular permeability, reducing edema; in mucous membranes, they reduce nasal and bronchial secretions; and in peripheral nerves, they reduce the sensation of itching.