PROMETHAZINE
Promethazine is a first-generation antihistamine with antiemetic, sedative, and anticholinergic properties. It works by blocking histamine H1 receptors and inhibiting the action of histamine in the body. In the context of anesthesia, promethazine has several important roles:
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Prevention and treatment of postoperative nausea and vomiting (PONV): Promethazine can be administered before or after surgery to help prevent or treat PONV, improving patient comfort and satisfaction.
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Sedation: Promethazine has sedative properties, which can be useful for preoperative sedation and anxiety relief. When administered preoperatively, it can help patients relax and minimize their anxiety about the upcoming procedure.
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Adjunctive analgesic: Promethazine has been shown to potentiate the analgesic effects of opioids, which can help improve pain management in the perioperative period. By reducing the required doses of opioids, promethazine may also contribute to a lower incidence of opioid-related side effects, such as respiratory depression or excessive sedation.
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Antipruritic: Promethazine's antihistaminic properties can help alleviate itching associated with certain medications, such as opioids, or allergic reactions in the perioperative period. This can improve patient comfort during recovery.
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Treatment of mild allergic reactions: In some cases, patients may experience mild allergic reactions during the perioperative period. Promethazine can be administered to help manage symptoms such as itching, rash, or mild swelling.
The mechanism of action of promethazine can be summarized as follows:
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H1 histamine receptor antagonism: Promethazine primarily works by competitively blocking H1 histamine receptors in various tissues, including smooth muscles, endothelial cells, and the central nervous system. By inhibiting the effects of histamine, promethazine reduces symptoms associated with allergic reactions, such as itching, redness, and swelling.
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Central nervous system effects: Promethazine readily crosses the blood-brain barrier and antagonizes H1 receptors in the brain. This action leads to sedation, which is one of the reasons it is used as a sedative or pre-anesthetic medication. Additionally, promethazine's sedative effects contribute to its efficacy as a nighttime cold and allergy relief medication.
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Antiemetic effects: Promethazine is thought to exert its antiemetic effects through several mechanisms. The primary mechanism involves its action on the chemoreceptor trigger zone (CTZ) in the area postrema of the medulla, where it blocks H1 and possibly dopamine receptors. This inhibits the transmission of emetic signals to the vomiting center in the brain, reducing the sensation of nausea and preventing vomiting.
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Anticholinergic effects: Promethazine also has anticholinergic properties, as it blocks muscarinic acetylcholine receptors. This action may contribute to its antiemetic effects and can also cause side effects, such as dry mouth, urinary retention, and constipation.
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α-adrenergic receptor antagonism: Promethazine has weak α-adrenergic blocking effects, which may contribute to its potential hypotensive (blood pressure-lowering) effects.
Key points about Promethazine
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Promethazine (Phenergan) is a phenothiazine derivative with antidopaminergic, antihistamine, and anticholinergic properties used to treat postoperative nausea and vomiting (PONV).
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It has alpha-adrenergic blocking properties.
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Side effects include sedation, delirium, confusion, and vision changes.
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Anticholinergic side effects include blurry vision, xerostomia (dry mouth), dry nasal passages, mydriasis (dilated pupils), constipation, and urinary retention.
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Antidopaminergic properties may result in extrapyramidal symptoms.
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The extrapyramidal symptoms include acute dyskinesias and dystonic reactions, tardive dyskinesia, akinesia, akathisia, and neuroleptic malignant syndrome.
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Promethazine may cause serious tissue injury on injection, including gangrene, regardless of route of administration.