Phenylephrine
**Phenylephrine** is a potent, direct-acting synthetic sympathomimetic amine primarily utilized in veterinary medicine for its profound vasoconstrictive properties. Key clinical applications include: - **Hypotension and Shock**: Used parenterally to elevate blood pressure (after adequate volume replacement) without causing overt cardiostimulation. - **Drug-Induced Hypotension**: Highly effective for hypotensive crises secondary to drug overdoses or idiosyncratic reactions (e.g., phenothiazines, adrenergic blocking agents). - **Ophthalmic Use**: Applied topically for diagnostic eye examinations, reducing posterior synechiae formation, and relieving pain associated with complicated uveitis. - **Nasal Decongestion**: Applied intranasally to reduce mucosal congestion. *Clinical Pearl*: Because phenylephrine lacks significant beta-adrenergic activity at normal doses, it is particularly useful when an increase in blood pressure is needed but an increase in heart rate or myocardial work is contraindicated (e.g., in patients with hypertrophic cardiomyopathy).
Mechanism: Phenylephrine is a highly selective **alpha-1 adrenergic receptor agonist**. - **Vascular Smooth Muscle**: Binds to post-synaptic **alpha-1 receptors** โ activates **Gq-proteins** โ stimulates **Phospholipase C (PLC)** โ cleaves PIP2 into **Inositol Triphosphate (IP3)** and **Diacylglycerol (DAG)** โ IP3 triggers the release of intracellular calcium from the sarcoplasmic reticulum โ profound **vasoconstriction**. - **Cardiovascular Effects**: Intravenous administration causes peripheral vasoconstriction, leading to significant increases in both diastolic and systolic blood pressures. This sudden increase in blood pressure often triggers a **baroreceptor-mediated reflex bradycardia** (which can be blocked by anticholinergics like atropine). - **Organ Perfusion**: Constricts most vascular beds (renal, splanchnic, pulmonary, cutaneous), though coronary blood flow is typically increased due to elevated aortic diastolic pressure.
Dosing by species
- To increase blood pressure by vasoconstriction in pronounced systemic vasodilation or when increasing myocardial contractility is disadvantageous (e.g., hypertrophic cardiomyopathy) ยท 1-2 micrograms/kg/minute ยท IV ยท CRI ยท Infusions of 1 microgram/kg/min significantly increased mean arterial pressure without a change in cardiac output. At 2 micrograms/kg/min, cardiac index also was increased with an increase in stroke volume index.
- As a constant rate infusion for profound vasodilation due to septic shock ยท 2-10 micrograms/kg/minute ยท IV ยท CRI
- As a vasopressor in catastrophic stages of hypovolemic shock ยท 1-3 micrograms/kg/min ยท IV ยท CRI
- As a CRI to increase peripheral vascular resistance and mean arterial blood pressure ยท Low dose is 1 microgram/kg/min; high dose is 3 micrograms/kg/min ยท IV ยท CRI ยท May see reflex bradycardia, and vasoconstriction can lead to excessive decreases in blood flow to liver, GI tract, and kidneys, although coronary blood flow is increased.
- Diagnosis of Horner's syndrome (denervation hypersensitivity) ยท 1% solution topically to both eyes ยท topical ยท once ยท single dose ยท Use lower concentrate solutions in cats to avoid systemic hypertension.
- General use ยท 5 mg ยท IV ยท Single dose ยท ARCI UCGFS Class 3 Drug
- As a CRI to increase peripheral vascular resistance and mean arterial blood pressure ยท Low dose is 1 microgram/kg/min; high dose is 3 micrograms/kg/min ยท IV ยท CRI ยท May see reflex bradycardia, and vasoconstriction can lead to excessive decreases in blood flow to liver, GI tract, and kidneys, although coronary blood flow is increased.
Routes of administration
Contraindications
- Severe hypertension
- Ventricular tachycardia
- Hypersensitivity to phenylephrine
- Do not apply topically once ophthalmic surgery has started (to avoid direct arterial absorption)
Adverse effects
- Reflex bradycardia
- CNS effects (excitement, restlessness, headache)
- Arrhythmias (rare)
- Severe extravasation injuries (tissue necrosis and sloughing)
- Hypertension
- Tachycardia
- Tissue necrosis and sloughing (if IV extravasation occurs)
Drug interactions
- Alpha-Adrenergic Blockers (phentolamine, phenothiazines, phenoxybenzamine) ยท Higher dosages of phenylephrine may be required to attain a pressor effect if these agents have been used prior to therapy.
- Anesthetics, General (halogenated) ยท Potentially may induce cardiac arrhythmias when used with halothane anesthesia.
- Atropine (and other anticholinergics) ยท Block the reflex bradycardia caused by phenylephrine.
- Beta-Adrenergic Blockers ยท The cardiostimulatory effects of phenylephrine (seen at high doses) can be blocked.
- Digoxin ยท Use with phenylephrine may cause increased myocardium sensitization. ยท major
- Monoamine Oxidase (MAO) Inhibitors (e.g., amitraz, selegiline) ยท Should not be used with phenylephrine because of a pronounced pressor effect.
- Oxytocin ยท When used concurrently with oxytocic agents, pressor effects may be enhanced.
- Sympathomimetic Agents (epinephrine) ยท Tachycardia and serious arrhythmias are possible.
- Volatile anaesthetic agents ยท Increased risk of cardiac arrhythmias ยท major
- Oxytocic agents ยท Enhanced pressor effects leading to severe hypertension ยท major
Monitoring
- Cardiac rate and rhythm
- Blood pressure
- Blood gases (if possible)
- IV catheter site (for signs of extravasation)
- Blood pressure (systemic use or high-concentration topical use)
- Heart rate and rhythm (ECG)
- Pupillary response and time to dilation (for Horner's syndrome testing)
- IV catheter site for signs of extravasation
Overdose
The margin of safety with phenylephrine overdose is fairly wide, especially after oral administration. **Clinical Signs**: - Common findings in dogs include vomiting, lethargy, depression, hyperactivity, and tachycardia. - Severe overdosage can cause hypertension, seizures, paresthesias, ventricular extrasystoles, and cerebral hemorrhage. **Treatment**: - Vomiting is commonly seen and may self-decontaminate oral exposures. - Cardiovascular changes often respond well to IV fluids. - Beta-blockers or nitroprusside may be indicated when signs (tachycardia, severe hypertension) are refractory to fluids.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturerโs current label.