DIPHENHYDRAMINE

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Diphenhydramine is a first-generation antihistamine that also possesses anticholinergic properties. It plays a useful role in the context of anesthesia for various reasons:

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1. Prevention and treatment of allergic reactions: Diphenhydramine is used to manage allergic reactions, which can occur during the perioperative period due to exposure to certain medications, latex, or other substances. It works by blocking histamine H1 receptors, thereby reducing symptoms such as itching, hives, and redness.

2. Sedation: Diphenhydramine has sedative effects, which can be beneficial for preoperative anxiolysis and sleep induction. It is sometimes administered preoperatively to help patients relax before surgery.

3. Antiemetic properties: Diphenhydramine has been shown to possess some antiemetic properties, which can help reduce nausea and vomiting during the perioperative period. It may be used alone or in combination with other antiemetic medications for the prevention and treatment of postoperative nausea and vomiting (PONV).

4. Treatment of extrapyramidal symptoms: Diphenhydramine can help manage extrapyramidal symptoms, which may arise as a side effect of certain medications (such as metoclopramide) used in the perioperative setting.

5. Anticholinergic effects: The anticholinergic properties of diphenhydramine may help in the management of excessive secretions or bronchospasms in certain situations, although it is not typically the first choice for these purposes.

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The mechanisms of action of diphenhydramine can be summarized as follows:

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1. Histamine H1 receptor antagonism: Diphenhydramine competitively binds to and blocks histamine H1 receptors, which are found in various tissues throughout the body. By inhibiting the action of histamine at these receptors, diphenhydramine reduces symptoms of allergic reactions, such as itching, redness, and swelling.

2. Sedative effects: Diphenhydramine can readily cross the blood-brain barrier and block H1 receptors in the central nervous system (CNS), which contributes to its sedative effects. This property can be useful for promoting sleep and reducing anxiety before surgery.

3. Antiemetic properties: The exact mechanism behind diphenhydramine's antiemetic effects is not entirely understood, but it is believed to involve its action on histamine H1 receptors in the brain's chemoreceptor trigger zone (CTZ) and vestibular system. By blocking these receptors, diphenhydramine helps to reduce nausea and vomiting.

4. Anticholinergic effects: Diphenhydramine also has anticholinergic properties, meaning it can block the action of acetylcholine at muscarinic receptors. This can result in reduced smooth muscle contraction, decreased secretions, and bronchodilation. However, these effects are generally less pronounced compared to its antihistamine properties and are not the primary reason for its use in anesthesia.

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                                                                                         Key points about Diphenhydramine

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• Histamine is a chemical messenger mostly generated in mast cells.

 

• Histamine functions as a neurotransmitter in the brain.

 

• The H1 receptors are important in producing smooth muscle contraction and increasing capillary permeability.

 

• Histamine H1 receptors mediate many pathological processes, including allergic rhinitis, atopic dermatitis, conjunctivitis, urticaria, bronchoconstriction, asthma, and anaphylaxis.

 

• Antagonizes the effects of histamine at H1-receptor sites.

 

• Moderate anticholinergic (antimuscarinic) and weak antidopaminergic activity.

 

• Side effects include drowsiness, dry mouth, blurred vision, urinary retention, and extrapyramidal symptoms.

 

• H1-receptor blockers are useful in treating and preventing allergic reactions caused by antigens acting on immunoglobulin E antibody.

 

• Effective agent for prevention of the symptoms of motion sickness.

 

• Strong sedative properties and used in the treatment of insomnia.

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