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.

Now, one important feature that is unique to first generation antihistamines is that they can more easily cross the blood brain barrier, meaning that they can also block H1 receptors in the brain, which can cause sedation.

For this reason, second generation antihistamines are better suited for use during the day.First generation antihistamines are also more commonly associated with side effects, since they can block other types of receptors.

Blockade of muscarinic receptors results in anticholinergic side effects, such as dry mouth, blurred vision, urinary retention and constipation, while blockade of alpha-1 adrenergic receptors may cause orthostatic hypotension.

Some antihistamines can also cause QT interval prolongation, which may predispose some clients to arrhythmias. Other side effects include irritability, confusion, hearing problems like tinnitus and wheezing.As for contraindications, some first generation antihistamines are contraindicated during pregnancy, breastfeeding and in newborns; if needed, second generation antihistamines are preferred.

In addition, first generation antihistamines should generally be avoided in clients with narrow-angle glaucoma, arrhythmias, and in those who take other medications that prolong the QT interval, such as erythromycin and amitriptyline.

Also, due to their sedative effect, first generation antihistamines should be avoided in older clients because they increase their risk of falls, as well as in clients who take other medications or substances that impair their mental status, such as alcohol, barbiturates, opioids and hypnotics.

In addition, first generation antihistamines are contraindicated in clients with benign prostatic hyperplasia, as they decrease urinary bladder contraction, which might cause urinary retention.

Promethazine, in particular, has a boxed warning for use in clients under 2 years of age, as it may lead to respiratory depression; as well as for severe tissue injury when used intravenously, since it is a vesicant.

Finally, second generation antihistamines should be used cautiously in clients with renal or hepatic disease.Okay, if your client is prescribed an antihistamine, first ask them about the symptoms they are experiencing, such as sneezing, nasal congestion and runny nose, and itchy or watery eyes.

Then, determine if their symptoms happen occasionally, if they tend to be seasonal or if they occur year around, and ask them about any environmental exposure to an allergen, such as pollen, mold, pets, or allergens in their work environment.

Assist your client to identify and reduce exposure to allergens. For example, since most pollen counts are highest in the early morning hours, advise your client to avoid outdoor activities during these hours.

If dust mite allergies are a problem, suggest measures such as washing bedding weekly, as well as dusting and vacuuming often.

Next, review your client’s recent laboratory results, including liver and renal function tests. In addition, be sure to check their medical history for the presence of QT interval prolongation or any current medications that could affect the QT interval, such as antidepressants or antiarrhythmics, as well as any contraindications like narrow-angle glaucoma, prostatic enlargement, or advanced age.

Also be sure to determine if your client is pregnant, plans to become pregnant, or is breastfeeding. Lastly, conduct a focused physical assessment, including vital signs and lung sounds.Then, explain how the medication works to relieve their symptoms, and to take it as directed.

Teach your client that most antihistamines can be taken with or without food. However, if your client is prescribed a fexofenadine oral disintegrating tablet, instruct them to take it on an empty stomach without water or juice; and teach them to place the tablet directly on their tongue until it dissolves; importantly, be sure to let them know that juices like apple or orange juice can decrease the amount of medication they absorb, so they should avoid the consumption of juice for at least 4 hours before and after taking their medication.Also be sure to inform your client about any side effects they may experience.

Let them know that antihistamines will cause some degree of sedation, so advise them to avoid activities that require alertness or concentration, like driving until the degree of sedation is known.

Also instruct your client to avoid alcohol and other sedating medications when taking an antihistamine. If your client is experiencing discomfort from a dry mouth, advise them to take sips of water or use sugarless gum or hard candy.

In addition, remind them that increasing fluid, fiber, and physical activity can help prevent constipation. Lastly, some antihistamines can cause orthostatic hypotension, so caution them to make position changes slowly until the effect of the antihistamine is known.Now, while your client is taking an antihistamine, continue to monitor their respiratory status, and evaluate for the therapeutic effects, including a reduction of symptoms and an improved quality of life.

Finally, prompt them to report any persistent or bothersome side effects to their healthcare provider.Alright, as a quick recap ….

Antihistamines are primarily used to treat symptoms of allergic conditions. They work by blocking H1 receptors and preventing the effects of histamine on target organs.

First generation antihistamines tend to have more noticeable side effects than second generation antihistamines; they more easily cross the blood brain barrier so are more likely to cause sedation, and they can block other types of receptors, causing anticholinergic side effects and orthostatic hypotension.

Important nursing considerations include performing a thorough assessment and providing instruction on correct self-administration.

Nurses are also responsible for assisting clients to find ways to reduce their exposure to allergens; to help them manage side effects; and to monitor for