AssessmentsAntihistamines for allergies
Antihistamines for allergies
USMLE® Step 1 style questions USMLE
USMLE® Step 2 style questions USMLE
A 35-year-old man is undergoing an emergent appendectomy. During the anesthetic assessment, he states that he has been suffering from an upper respiratory infection for the past 4 days which has caused severe cough and nasal congestion, both of which would impair his ability to lie flat on the operating table and pre-oxygenate well. Which of the following adjuvant medications could be given to help alleviate the patient's symptoms?
Content Reviewers:Yifan Xiao, MD
These conditions are related to an increased release of histamine.
Now, H1 blockers work by blocking the effects of histamine in tissues that have H1 receptors, thereby alleviating symptoms of allergic reactions.
In order to understand how antihistamines work, first we need to talk briefly about histamine and allergic reactions.
Once released, they cause local inflammation and vasodilation. However, they are also present in the brain as neurotransmitters, and they are produced by enterochromaffin cells in the stomach to increase gastric acid secretion.
The mast cells are now “primed,” meaning that if pollen enters the body again in the future, the mast cells degranulate and release their histamine into the local tissue.
Now, there are 4 types of histamine receptors: H1, H2, H3, and H4 receptors.
Since we’re going to talk about antihistamines for allergies, we’re going to focus only on histamine H1 receptors.
These receptors are predominantly found on endothelial cells, smooth muscle cells, sensory nerve endings, and in the brain.
On the flip side, on smooth muscle cells in the lungs, stimulation of H1 receptors results in bronchoconstriction, while on sensory nerve endings leads to pain and itching.
Finally, in the brain, H1 receptors promote wakefulness and appetite suppression.
Now, switching gears and moving on to pharmacology. Antihistamines are medications that work by blocking histamine receptors; and depending on the type of the receptor they affect, antihistamines are subdivided into two main groups: H1 blockers, which are used to treat allergies, and H2 blockers, which are primarily used to treat gastric problems like heartburn. Now, let’s focus on H1 blockers.
These medications work by reversibly inhibiting H1 receptors and they are subdivided into 2 main groups: first-generation H1 blockers and second-generation H1 blockers.
These medications are primarily used to treat allergic reactions, such as urticaria, angioedema, and allergic rhinitis.
But, since they have high lipid solubility, these medications can easily cross the blood-brain barrier and block H1 receptors in the brain.
This way, first-generation H1 antihistamines can cause cognitive side effects like sedation, therefore they can be also used as a short-term treatment for sleeping problems.
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