Drug Information: Phenylephrine
Detailed information regarding Phenylephrine, a synthetic sympathomimetic amine that primarily acts as a selective alpha-1 adrenergic receptor agonist. This results in vasoconstriction, leading to an increase in blood pressure. It is commonly used to treat hypotension, particularly in settings such as anesthesia-induced hypotension and septic shock. Phenylephrine is also available in over-the-counter formulations as a nasal decongestant, although its efficacy for this purpose has been debated. Its ability to increase blood pressure rapidly makes it a valuable tool in emergency medicine and critical care. It’s crucial to monitor patients closely when administering Phenylephrine due to the risk of excessive vasoconstriction and related complications.
Category
Vasopressor
Mechanism of Action
Phenylephrine is a potent alpha-1 adrenergic agonist. Its primary mechanism of action involves binding to and activating alpha-1 adrenergic receptors located on vascular smooth muscle cells. This activation triggers a cascade of intracellular events, including the activation of phospholipase C, which leads to the production of inositol triphosphate (IP3) and diacylglycerol (DAG). IP3 then stimulates the release of calcium ions from intracellular stores, while DAG activates protein kinase C. The increased intracellular calcium concentration, coupled with protein kinase C activation, results in the contraction of vascular smooth muscle. This vasoconstriction increases systemic vascular resistance (SVR), leading to an elevation in blood pressure. Unlike some other vasopressors, phenylephrine has minimal beta-adrenergic activity. This selectivity means it primarily increases blood pressure without significantly affecting heart rate, making it useful when tachycardia is undesirable. The resulting increase in blood pressure also causes a reflex bradycardia through baroreceptor activation.
Clinical Uses
Phenylephrine is primarily used to treat hypotension. This includes hypotension associated with anesthesia, particularly spinal anesthesia, as well as hypotension in critical care settings such as septic shock (though it is often used as a second-line agent in this context after norepinephrine). It’s also used to treat hypotension resulting from certain medications or medical conditions. Another common use is as a nasal decongestant in over-the-counter cold and allergy medications. In ophthalmic preparations, phenylephrine is used as a mydriatic agent to dilate the pupils for eye examinations or surgical procedures. Its vasoconstrictive properties also make it useful in controlling minor bleeding during surgical procedures and treating priapism. Finally, Phenylephrine can be used to manage episodes of paroxysmal supraventricular tachycardia (PSVT) by increasing blood pressure and stimulating a vagal response, although other treatments are typically preferred.
Dosage
**Adult:**
* **IV Bolus:** 50-200 mcg every 10-15 minutes as needed, or a continuous infusion of 10-35 mcg/min, adjusted to maintain blood pressure.
* **Intramuscular/Subcutaneous:** 2-5 mg every 1-2 hours as needed.
* **Nasal Decongestant (Topical):** 1-2 sprays of 0.25% to 1% solution in each nostril every 4 hours as needed.
**Pediatric:**
* **IV Bolus:** 5-20 mcg/kg every 10-15 minutes as needed.
* **Continuous IV Infusion:** 0.15-0.75 mcg/kg/min, adjusted to maintain blood pressure.
* **Nasal Decongestant (Topical):** Consult product labeling for age-appropriate dosing. Use with caution and under medical supervision in infants.
**Important Notes:**
* Dosage may vary based on individual patient factors and clinical situation. Always consult prescribing information and relevant medical guidelines.
* Monitor blood pressure and heart rate closely during administration.
Indications
* Hypotension (e.g., anesthesia-induced, septic shock)
* Nasal congestion
* Mydriasis (pupil dilation) for ophthalmic procedures
* Treatment of priapism
* Management of paroxysmal supraventricular tachycardia (PSVT)
Contraindications
* Severe hypertension
* Ventricular tachycardia
* Narrow-angle glaucoma
* Hypersensitivity to phenylephrine or any component of the formulation
* Use with monoamine oxidase inhibitors (MAOIs) or within 14 days of stopping MAOI treatment (due to risk of hypertensive crisis)
* Severe coronary artery disease or peripheral vascular disease
FAQ
**Q1: What is Phenylephrine used for?**
A1: Phenylephrine is used to treat hypotension (low blood pressure), nasal congestion, and to dilate pupils for eye exams.
**Q2: How does Phenylephrine work?**
A2: It works by constricting blood vessels, which increases blood pressure and reduces swelling in nasal passages. It also stimulates receptors in the eye, causing pupil dilation.
**Q3: What are the common side effects of Phenylephrine?**
A3: Common side effects include headache, nervousness, dizziness, increased blood pressure, and reflex bradycardia (slow heart rate).
**Q4: Can I take Phenylephrine if I have high blood pressure?**
A4: No, Phenylephrine is generally contraindicated in patients with severe hypertension.
**Q5: Can I use Phenylephrine nasal spray if I am pregnant?**
A5: Consult your doctor before using Phenylephrine nasal spray during pregnancy.
**Q6: What should I do if I miss a dose of Phenylephrine?**
A6: If you are taking it regularly, take the missed dose as soon as you remember. If it is close to your next dose, skip the missed dose and continue with your regular schedule. Do not double the dose. For single dose applications, you don’t need to worry about missing a dose.
**Q7: Can Phenylephrine interact with other medications?**
A7: Yes, Phenylephrine can interact with MAOIs, tricyclic antidepressants, and beta-blockers. Always inform your doctor about all medications you are taking.
**Q8: Is Phenylephrine a substitute for Epinephrine?**
A8: While both are vasopressors, they have different mechanisms and effects. Phenylephrine primarily acts on alpha-1 receptors, while Epinephrine acts on both alpha and beta receptors. Epinephrine is generally preferred in anaphylaxis and cardiac arrest, while Phenylephrine might be used in other hypotensive situations where beta-adrenergic effects are undesirable. They are not direct substitutes and should be used according to clinical guidelines.
“`