از ویکیپدیا، دانشنامهٔ آزاد
فنیل افرین (به انگلیسی: Phenylephrine)
رده درمانی: داروهای سمپاتومیمتیک
موارد مصرف [ویرایش]
فنیل افرین یکی از رایج ترین ضد احتقانهای بینی است و در بسیاری از فرآوردههای موضعی برای برطرف کردن نشانههای آلرژی و سرماخوردگی وجود دارد و جزء برخی از قرصهای ضد سرماخوردگی است. این دارو با منقبض کردن رگهای خونی داخل چشم میتواند التهاب ملتحمه را کاهش دهد. فنیل افرین برای گشادکردن مردمک در حین معاینات و جراحی چشم و نیز درمان افت شدید فشارخون بکار میرود.
مکانیسم اثر [ویرایش]
فنیل افرین حاصیت سمپاتومیمتیک دارد لذا موجب انقباض عروق و کاهش احتقان میشود.
عوارض جانبی [ویرایش]
سردرد، تاری دید، افزایش فشارخون، تاکیکاردی، اختلال ریتم قلب و...
جستارهای وابسته [ویرایش]
Phenylephrine is a selective α1-adrenergic receptor agonist used primarily as a decongestant, as an agent to dilate the pupil, and to increase blood pressure. Phenylephrine is marketed as a substitute for the decongestant pseudoephedrine, though clinical studies differ regarding phenylephrine's effectiveness in this role.
Phenylephrine is used as a decongestant sold as an oral medicine, as a nasal spray, or as eye drops. Phenylephrine is now the most common over-the-counter decongestant in the United States; oxymetazoline is a more common nasal spray.
Oral phenylephrine is extensively metabolised by monoamine oxidase, an enzyme that is present in the intestinal wall and in the liver. Compared to intravenous pseudoephedrine, it has a reduced and variable bioavailability; only up to 38%. Because phenylephrine is a direct selective α-adrenergic receptor agonist, it does not cause the release of endogenous noradrenaline, as pseudoephedrine does. Both Pseudoephdrine, and Phenylephrine are likely to cause side effects such as central nervous system stimulation, insomnia, anxiety, irritability, and restlessness. and unlike Pseudoephdrine, Phenylephrine's effectiveness as a decongestant stems from its vasoconstriction of nasal blood vessels, thereby decreasing blood flow to the sinusoidal vessels, leading to decreased mucosal edema.
Phenylephrine is used as an eye drop to dilate the pupil in order to facilitate visualization of the retina. It is often used in combination with tropicamide. Narrow-angle glaucoma is a contraindication to phenylephrine use.
Phenylephrine is commonly used as a vasopressor to increase the blood pressure in unstable patients with hypotension, especially resulting from septic shock. Such use is common in anesthesia or critical-care practices; phenylephrine is especially useful in counteracting the hypotensive effect of epidural and subarachnoid anesthetics, as well as the vasodilating effect of bacterial toxins and the inflammatory response in sepsis and systemic inflammatory response syndrome. It has the advantage of not being inotropic or chronotropic, and so it strictly elevates the blood pressure without increasing the heart rate or contractility (reflex bradycardia may result from the blood pressure increase, however). This is especially useful if the heart is already tachycardic and/or has a cardiomyopathy. The elimination half life of phenylephrine is about 2.5 to 3 hours.
Because of its vasoconstrictive effect, phenylephrine (Neo-Synephrine) can cause severe necrosis if it infiltrates the surrounding tissues. Because of this, it should be given through a central line if at all possible. Damage may be prevented or mitigated by infiltrating the tissue with the alpha blocker phentolamine by subcutaneous injection.
Phenylephrine is used by urologists to abort priapism. It is diluted significantly and injected directly into the corpora cavernosa. The mechanism of action is to cause constriction of the blood vessels entering into the penis thus breaking the pathophysiologic cycle that continues the priapism.
Side effects 
The primary side effect of phenylephrine is hypertension. Patients with hypertension are typically advised to avoid products containing phenylephrine. Prostatic hyperplasia can also be symptomatically worsened by use, and chronic use can lead to rebound hyperemia. Patients with a history of epilepsy and on anticonvulsant medication should not take this substance. The drug interaction might produce seizures. Some patients have been shown to have an upset stomach, severe abdominal cramping, and vomiting issues connected to taking this drug.
Substitute for pseudoephedrine 
Pseudoephedrine and phenylephrine are both used as decongestants; and, until recently, pseudoephedrine was much more commonly available in the United States. This has changed because provisions of the Combat Methamphetamine Epidemic Act of 2005 placed restrictions on the sale of pseudoephedrine products in order to prevent the clandestine manufacture of methamphetamine. Since 2004, phenylephrine has been increasingly marketed as a substitute for pseudoephedrine; some manufacturers have changed the active ingredients of products to avoid the restrictions on sales. Phenylephrine has been off patent for some time, and there are many generic brands available.
Questions about effectiveness 
Pharmacists Leslie Hendeles and Randy Hatton of the University of Florida suggested in 2006 that oral phenylephrine is ineffective as a decongestant at the 10-mg dose used, arguing that the studies used for the regulatory approval of the drug in the United States in 1976 were inadequate to prove effectiveness at the 10-mg dose and safety at higher doses. Other pharmacists have expressed concerns over phenylephrine's effectiveness as a nasal decongestant, and other clinicians have indicated concern for regulatory actions that reduced the availability of pseudoephedrine. A subsequent meta-analysis by the same researchers concluded that there is insufficient evidence for its effectiveness, though another meta-analysis published shortly thereafter by researchers from GlaxoSmithKline found the standard 10 mg dose to be significantly more effective than a placebo. Additionally, two studies published in 2009 examined the effects of phenylephrine on symptoms of allergic rhinitis by exposing sufferers to pollen in a controlled, indoor environment. Neither study was able to distinguish between the effects of phenylephrine or a placebo. Pseudoephedrine and loratadine-montelukast therapy were found to be significantly more effective than both phenylephrine and placebo.