Phenylephrine

Phenylephrine is a predominantly alpha1-adrenergic agonist used for vascular and other smooth-muscle effects. Veterinary applications include selected vasodilatory hypotension, local vasoconstriction and ophthalmic procedures. A distinct equine use involves splenic contraction during conservative treatment of left dorsal displacement of the large colon, also called nephrosplenic entrapment.

Group

Alpha1-adrenergic agonist vasopressor

Mechanism of Action

Phenylephrine activates alpha1 receptors and increases intracellular calcium signaling in responsive smooth muscle. Vascular contraction raises systemic resistance and can increase arterial pressure. Local vasoconstriction reduces superficial blood flow, while contraction of the iris dilator produces mydriasis. In horses, adrenergic contraction of splenic tissue reduces splenic size and can facilitate correction of nephrosplenic entrapment.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Hypotension	10 mcg/kg	IV	Every 0.25 h	Repeat as required.
Hypotension	0.1 mg/kg	IM, SC	Every 0.25 h	Repeat as required.
Vasopressor infusion — Loading	10 mcg/kg	IV	Once
Vasopressor infusion — Maintenance	3 mcg/kg/min	IV	Continuous
Vasopressor infusion	1–3 mcg/kg/min	IV	Continuous	Titrate to perfusion and BP; monitor reflex bradycardia and organ vasoconstriction.
Septic vasodilation	2–10 mcg/kg/min	IV	Continuous	Titrate to perfusion and BP; monitor reflex bradycardia and organ vasoconstriction.
Hypovolemic shock vasopressor	1–3 mcg/kg/min	IV	Continuous	Titrate to perfusion and BP; monitor reflex bradycardia and organ vasoconstriction.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Hypotension	10 mcg/kg	IV	Every 0.25 h	Repeat as required.
Hypotension	0.1 mg/kg	IM, SC	Every 0.25 h	Repeat as required.
Vasopressor infusion — Loading	10 mcg/kg	IV	Once
Vasopressor infusion — Maintenance	3 mcg/kg/min	IV	Continuous
Vasopressor infusion	1–3 mcg/kg/min	IV	Continuous	Titrate to perfusion and BP; monitor reflex bradycardia and organ vasoconstriction.
Septic vasodilation	2–10 mcg/kg/min	IV	Continuous	Titrate to perfusion and BP; monitor reflex bradycardia and organ vasoconstriction.
Hypovolemic shock vasopressor	1–3 mcg/kg/min	IV	Continuous	Titrate to perfusion and BP; monitor reflex bradycardia and organ vasoconstriction.
Vasodilatory hypotension	1–2 mcg/kg/min	IV	Continuous

Horses

Use / stage	Dose	Route	Schedule	Instructions
Nephrosplenic entrapment	Reference only	IV		Infuse 10-20 mg in 500 mL saline over 10-15 minutes, or 20 mg in 1000 mL over 10 minutes. Admixture volume and duration must remain linked.
Reported vasoconstrictor treatment	5 mg/animal	IV		Specific indication and repeat frequency are not given in this dose line.

Indications

Vasopressor support in selected veterinary patients with vasodilatory or drug-associated hypotension.
Local vasoconstriction for selected mucosal procedures or control of superficial bleeding.
Pharmacologic mydriasis or selected diagnostic ophthalmic applications with an appropriate eye preparation.
Adjunctive conservative management of equine nephrosplenic entrapment within an appropriate colic treatment plan.

Contraindications

Compromised cardiovascular status

Interactions

Other alpha agonists, including alpha2 sedatives, can intensify cardiovascular effects.

Adverse effects

Reflex bradycardia, excessive vasoconstriction and local irritation; equine hemorrhagic shock has occurred.

Toxicity

Marked exposure can cause hypertension, tachyarrhythmias, agitation, tremor, vomiting or seizures. Monitor ECG and BP; treatment depends on the cardiovascular state rather than the oral amount alone.

Preparations

Preparation	Form	Strength	Route
Phenylephrine injection 10 mg/mL	Injection	10 mg / mL	IV

Monitoring and Administration

During systemic treatment monitor blood pressure, pulse and rhythm. Equine nephrosplenic-entrapment use has potentially severe complications.

Protect from light and discard darkened solution; alkaline conditions accelerate oxidation.