Phenylephrine

Usage

• Phenylephrine is primarily used to treat hypotension (low blood pressure), particularly during anesthesia or surgery, and shock (inadequate blood flow to tissues). It is also used as a nasal decongestant to relieve nasal congestion caused by colds, allergies, or sinusitis. In ophthalmic solutions, it is used to dilate the pupil and reduce redness of the eye.
• Pharmacological classification: Phenylephrine is a sympathomimetic agent, specifically an alpha-1 adrenergic receptor agonist and a vasopressor.
• Mechanism of action: Phenylephrine acts by stimulating alpha-1 adrenergic receptors, leading to vasoconstriction (narrowing of blood vessels). This increases peripheral vascular resistance and blood pressure.

Alternate Names

• It is sometimes referred to as “phenyl spritzer” (when prepared as a diluted injection for mini boluses) or “Neosynephrine” (a common brand name, though there are many other brand names).

How It Works

• Pharmacodynamics: Phenylephrine exerts its primary effect by stimulating alpha-1 adrenergic receptors on vascular smooth muscle, causing vasoconstriction and increasing blood pressure. It has minimal beta-adrenergic activity, resulting in less impact on heart rate and contractility compared to other sympathomimetics.
• Pharmacokinetics: Phenylephrine is poorly absorbed orally, therefore the oral formulation is only used for nasal decongestion. It is rapidly absorbed after intravenous administration and is metabolized by monoamine oxidase in the liver and intestine. The elimination half-life is about 2-3 hours, primarily through renal excretion.
• Mode of action: Phenylephrine selectively binds to alpha-1 adrenergic receptors located on the cell membranes of vascular smooth muscle. This binding activates intracellular signaling pathways, leading to an increase in intracellular calcium levels, ultimately causing muscle contraction and vasoconstriction. It can also bind to some alpha-2 receptors at high doses, reducing sympathetic outflow from the central nervous system, but this effect is much weaker compared to other alpha-2 agonists.
• Elimination pathways: Metabolism primarily involves monoamine oxidase and elimination occurs via the kidneys.

Dosage

Standard Dosage

Adults:

• Hypotension/Shock (IV):

  • Bolus: 50-250 mcg IV (common initial doses: 50-100 mcg), repeated as needed every 10-15 minutes. Maximum total dose: 500 mcg. Consider consultation if more is needed.
  • Infusion: Initial dose: 25-50 mcg/min IV; titrate to maintain systolic blood pressure close to normal (25-100 mcg/min has been shown to be effective). Some guidelines suggest an initial maximum rate of 180 mcg/min, then adjusted to 30-60 mcg/min according to response. For vasodilatory shock, 0.5 to 6 mcg/kg/min IV continuous infusion, titrated to blood pressure. No bolus dose.

• Nasal Congestion (Oral): 10 mg every 4 hours as needed. Maximum dose: 60 mg in 24 hours.

Children:

• Hypotension/Shock (IV): 0.1 mg/kg IM/SC every 1-2 hours as needed (maximum single dose 5mg). 5-20 mcg/kg IV bolus every 10-15 minutes PRN. 0.1-0.5 mcg/kg/min IV infusion, titrate as needed.

• Nasal Congestion (Oral): For children under 12 years, consult a doctor before administrating. 12 years and older: 10 mg every 4 hours, not exceeding 60 mg per 24 hrs.

Special Cases:

• Elderly Patients: Initiate therapy at the lower end of the dosing range and adjust as needed. Exercise caution, considering age-related decline in organ function and potential for drug interactions.
• Patients with Renal Impairment: Start at the lower end of the dose range due to potential for increased responsiveness and adjust based on blood pressure goals.
• Patients with Hepatic Dysfunction: May require higher doses due to decreased responsiveness, but start with the recommended dose range.
• Patients with Comorbid Conditions: Use with caution in patients with hypertension, cardiovascular disease, hyperthyroidism, or diabetes.

Clinical Use Cases

• Intubation/Surgical Procedures/Mechanical Ventilation/ICU Use: Similar to hypotension/shock dosing, titrating to maintain blood pressure targets and hemodynamic stability. Perioperative: Initial bolus dose of 50-250 mcg (most common initial doses: 50 or 100 mcg) followed by 0.5-1.4 mcg/kg/min continuous IV infusion titrated to maintain hemodynamic stability.
• Emergency Situations (e.g., cardiac arrest): Not commonly used as the first-line agent. Adrenaline is preferred. Some studies suggest 1 mg phenylephrine may be used as an alternative.

Side Effects

Common Side Effects:

• Headache
• Nausea
• Vomiting
• Restlessness
• Anxiety
• Reflex bradycardia (slow heart rate)

Rare but Serious Side Effects:

• Severe hypertension (high blood pressure)
• Cardiac arrhythmias (irregular heartbeat)
• Myocardial ischemia (reduced blood flow to the heart)
• Pulmonary edema (fluid in the lungs)
• Tissue necrosis (tissue death) with extravasation

Long-Term Effects:

Limited data available. Chronic use as a nasal decongestant can lead to rebound congestion.

Adverse Drug Reactions (ADR):

• Severe hypertension
• Bradycardia
• Ischemic events

Contraindications

• Hypersensitivity to phenylephrine
• Severe hypertension
• Severe coronary artery disease
• Monoamine oxidase inhibitor (MAOI) use within 14 days
• Closed-angle glaucoma
• Severe bradycardia

Drug Interactions

• MAOIs can potentiate the pressor effects of phenylephrine.
• Tricyclic antidepressants and other sympathomimetics can have additive effects.
• Beta-blockers may antagonize the cardiovascular effects of phenylephrine.
• Oxytocic drugs may potentiate the pressor response.

Pregnancy and Breastfeeding

• Pregnancy Safety Category: C (FDA classification). Data in pregnant women are limited. Use only if the potential benefit justifies the potential risk to the fetus.
• Breastfeeding: Minimal excretion in breast milk. Use cautiously.

Drug Profile Summary

• Mechanism of Action: Alpha-1 adrenergic receptor agonist causing vasoconstriction.
• Side Effects: Headache, nausea, reflex bradycardia, hypertension, arrhythmias.
• Contraindications: Hypersensitivity, severe hypertension, MAOI use, closed-angle glaucoma.
• Drug Interactions: MAOIs, tricyclic antidepressants, beta-blockers, oxytocic drugs.
• Pregnancy & Breastfeeding: Use with caution. FDA category C.
• Dosage: See detailed section above.
• Monitoring Parameters: Blood pressure, heart rate, cardiac rhythm, peripheral perfusion.

Popular Combinations

Phenylephrine is often used alone but may be combined with other vasopressors or inotropes for synergistic effects in treating shock. It is sometimes given with pyrilamine as part of an antihistamine/decongestant combination.

Precautions

• Monitor blood pressure, heart rate, and ECG continuously during IV administration.
• Avoid extravasation; use a large vein and monitor the IV site closely.
• Use with caution in patients with pre-existing cardiovascular disease, hyperthyroidism, or diabetes.
• Inform patients about potential side effects.
• Advise patients to report any adverse effects to their physician.

FAQs (Frequently Asked Questions)

Q1: What is the recommended dosage for Phenylephrine?

A: See the detailed dosage section above.

Q2: How should Phenylephrine be administered for hypotension during surgery?

A: Administer as an IV bolus (50-250 mcg) or continuous infusion (0.5-1.4 mcg/kg/min or 25-100 mcg/min), titrating to the desired blood pressure.

Q3: What are the common side effects of Phenylephrine?

A: Common side effects include headache, nausea, vomiting, restlessness, anxiety, and reflex bradycardia.

Q4: What are the contraindications for using Phenylephrine?

A: Contraindications include hypersensitivity to phenylephrine, severe hypertension, severe coronary artery disease, MAOI use within 14 days, and closed-angle glaucoma.

Q5: How does Phenylephrine interact with other medications?

A: Phenylephrine can interact with MAOIs, tricyclic antidepressants, beta-blockers, and oxytocic drugs. Refer to the drug interactions section above for more information.

Q6: Can Phenylephrine be used during pregnancy and breastfeeding?

A: Use cautiously during pregnancy (FDA Category C). Minimal data are available. Excretion into breast milk appears minimal.

Q7: What precautions should be taken when administering Phenylephrine intravenously?

A: Monitor blood pressure, heart rate, and ECG continuously. Avoid extravasation, use a large vein, and monitor IV site for signs of infiltration or inflammation. Re-site canula immediately if signs of inflammation are detected.

Q8: How is Phenylephrine metabolized and eliminated?

A: Phenylephrine is metabolized by monoamine oxidase in the liver and intestine and eliminated primarily through renal excretion.

Q9: Is Phenylephrine effective for treating septic shock?

A: It is not a first-line agent and is usually only used in septic or other vasodilatory shock when other agents (e.g., norepinephrine, epinephrine, vasopressin) are ineffective or cause serious adverse effects.

Q10: What is the role of Phenylephrine in ophthalmic procedures?

A: Phenylephrine eye drops are used to dilate pupils (mydriasis) and reduce redness of the eye. It is commonly used in ophthalmic examinations and some surgical procedures.

Please note that this information is current as of February 17, 2025, and is intended for educational purposes only. It should not be considered medical advice. Consult with a qualified healthcare professional for any health concerns or before making any decisions related to your health or treatment.