Phenylephrine

Phenylephrine In Dogs & Cats: Uses, Dosage and Side Effects

Overview

• Alpha-1 selective adrenergic agonist that causes peripheral vasoconstriction when given i.v., resulting in increased diastolic and systolic blood pressure, a small decrease in cardiac output and an increased circulation time.
• Directly stimulates the alpha-adrenergic receptors in the iris dilator musculature.

Uses of Phenylephrine

• Phenylephrine is utilized alongside fluid therapy to address hypotension resulting from drug effects or vascular failure.
• When applied topically to the eye, it induces vasoconstriction and pupil dilation.
• Ophthalmic applications involve facilitating mydriasis before intraocular surgery, distinguishing between superficial and deep vasculature, and minimizing haemorrhage during eye surgery.
• Additionally, it aids in diagnosing Horner’s syndrome by assessing the time to pupillary dilation after topical administration, where shorter times suggest lesions closer to the iris.
• The use of 10% phenylephrine can expedite mydriasis in post-ganglionic lesions.

Dose of Phenylephrine in Dogs and Cats

Dogs:

Hypotension:

• Correct blood volume then infuse 0.01 mg/kg very slowly i.v. q15min.
• Continuously monitor blood pressure if possible.

Ophthalmic use:

• 1 drop approximately 2 hours before intraocular surgery (for mydriasis).
• 1 drop as a single dose for vasoconstriction.
• 1 drop of 1% solution to both eyes for diagnosis of Horner’s syndrome.

Cats:

Ophthalmic use: as for dogs (NB: ineffective for mydriasis as sole agent).

Side Effects of Phenylephrine in Dogs and Cats

• These include hypertension, tachycardia, and reflex bradycardia.
• Extravasation injuries can be serious (necrosis and sloughing).

Contraindications of Phenylephrine in Dogs and Cats

• No information is available.
• Care in cats and small dogs; use lower concentrate solutions.
• Do not apply once ophthalmic surgery has started (to avoid direct arterial absorption).

Some Notes:

• There is a risk of arrhythmias if phenylephrine is used in patients receiving digoxin or with volatile anaesthetic agents.
• When used concurrently with oxytocic agents the pressor effects may be enhanced, leading to severe hypertension.