Phenylephrine

Uses

Hypotension: Treatment of hypotension during anesthesia and other clinical settings.
Nasal Decongestion: Relief of nasal congestion associated with colds, sinusitis, and allergies.
Ophthalmic: Used to dilate pupils for eye examinations.
Vasopressor Support: Management of septic shock, neurogenic shock, and hypotension induced by spinal anesthesia to increase systemic vascular resistance.

Dosage and Administration

Intravenous (IV) Bolus: 1-2 mcg/kg. Repeat as needed. Typical bolus range: 0.5-2 mcg/kg.
IV Infusion: Start at 0.1-0.5 mcg/kg/min, titrate to achieve desired effect. In severe cases, can increase up to 5 mcg/kg/min.
Nasal Administration: 2-3 drops or sprays (0.25%, 0.5%, or 1% solution) per nostril every 4 hours as needed.
Ophthalmic Administration: 2.5% or 10% solution; instill 1 drop in the eye for pupil dilation.

Dose Adjustments in Special Conditions

Renal Impairment: Use with caution; reduced clearance may prolong drug effects.
Hepatic Impairment: No specific dose adjustment; monitor for heightened sensitivity.
Geriatric Population: Start at the lower end of dosing range due to increased risk of side effects.
Pediatric Population: Limited data; use with caution and under specialist supervision.

Presentation or Form

IV Solution: 10 mg/mL (must be diluted prior to administration).
Nasal Spray/Drops: Available as 0.25%, 0.5%, and 1% solutions.
Ophthalmic Solution: 2.5% and 10%.

Pharmacokinetics

Absorption: Rapid absorption after IV administration.
Distribution: Volume of distribution (Vd) ~ 340 L.
Metabolism: Primarily metabolized in the liver by monoamine oxidase.
Excretion: Mainly excreted in urine; approximately 80% of a dose is excreted within 48 hours.
Half-life: Approximately 2-3 hours.

Pharmacodynamics

Phenylephrine is a selective alpha-1 adrenergic receptor agonist that induces vasoconstriction, leading to increased systemic vascular resistance and elevated blood pressure. Unlike non-selective adrenergic agonists, phenylephrine has minimal effects on beta-adrenergic receptors, causing limited changes in heart rate and myocardial contractility.

Onset of Action: 30 seconds to 1 minute (IV administration).
Duration of Action: 15-20 minutes after a single IV bolus dose; continuous effects with infusion.

Drug Interactions

Beta Blockers: Enhanced vasoconstrictive effects and potential for severe hypertension.
Monoamine Oxidase Inhibitors (MAOIs): Prolonged and intensified hypertensive response.
Tricyclic Antidepressants: Increased risk of hypertensive crisis.
Digitalis Glycosides: Risk of cardiac arrhythmias.
General Anesthetics: Exaggerated pressor effects when used concomitantly.

Precautions and Special Considerations

Cardiac Conditions: Use with caution in patients with coronary artery disease or heart failure.
Hypertension: Contraindicated in patients with severe or uncontrolled hypertension.
Peripheral Vascular Disease: Risk of ischemia due to vasoconstriction.
Pregnancy and Lactation: Use only if the potential benefit justifies the potential risk to the fetus.
Rebound Congestion: Overuse of nasal formulations can lead to rebound nasal congestion.

Side Effects

Common: Reflex bradycardia, hypertension, headache, dizziness, anxiety, and nausea.
Serious: Tissue ischemia or necrosis (especially at the infusion site), arrhythmias, and severe hypertension.
Ophthalmic Form: Transient stinging, blurred vision, and photophobia.
Nasal Form: Burning sensation, dryness, and rebound congestion with prolonged use.

Recent Updates and Guidelines

Sepsis Guidelines: Phenylephrine is considered a second-line agent in septic shock when tachyarrhythmias limit the use of norepinephrine.
Vasopressor Selection: Recent critical care guidelines emphasize norepinephrine as the first-line agent for shock; phenylephrine is reserved for cases where norepinephrine is not tolerated or effective.
Updated Dilution Protocols: Revised recommendations suggest standardized dilution protocols to reduce dosing errors and improve patient safety.

Comparison with Other Vasopressors

Phenylephrine vs. Norepinephrine: Phenylephrine is a pure alpha-1 agonist with minimal effect on heart rate, making it preferred in patients with tachycardia. Norepinephrine affects both alpha-1 and beta-1 receptors, increasing both blood pressure and cardiac output.
Phenylephrine vs. Epinephrine: Epinephrine activates alpha-1, beta-1, and beta-2 receptors, leading to increases in heart rate, cardiac output, and vasoconstriction. Phenylephrine provides selective vasoconstriction without significant heart rate changes.
Phenylephrine vs. Dopamine: Dopamine, at higher doses, activates beta-1 and alpha-1 receptors, increasing heart rate and vasoconstriction. Phenylephrine, being a selective alpha-1 agonist, primarily focuses on vasoconstriction without significant tachycardia.