Alpha Adrenergic Agonists Pharmacology

Vocabulary flashcards covering alpha-1 and alpha-2 adrenergic receptor agonists, including their mechanisms of action, clinical indications, major adverse reactions, contraindications, drug interactions, and high-yield clinical pearls based on lecture notes.

Phenylephrine (Neo-Synephrine®, Vazculep®) (IV/Oral)

A selective $\alpha_1$ agonist that stimulates peripheral $\alpha_1$ receptors to cause arterial and venous vasoconstriction, increasing SVR and BP without direct $\beta$ stimulation. Used as an IV treatment for hypotension (e.g., septic shock, anesthesia-induced hypotension) and as an oral decongestant. Preferred vasopressor when tachyarrhythmias are present.

Phenylephrine Nasal Spray (Neo-Synephrine®)

A selective $\alpha_1$ agonist that causes vasoconstriction of nasal mucosal blood vessels for temporary relief of nasal congestion. Usage must be limited to $\le 3$ days to prevent rebound congestion (rhinitis medicamentosa).

Phenylephrine Ophthalmic (Mydfrin®, AK-Dilate®)

A selective $\alpha_1$ agonist that contracts the iris radial muscle to produce diagnostic mydriasis without cycloplegia (preserving accommodation). Used for retinal examination and ophthalmic surgery; contraindicated in narrow-angle glaucoma.

Midodrine (ProAmatine®)

A selective $\alpha_1$ agonist prodrug converted to desglymidodrine, indicated as a first-line oral agent for symptomatic neurogenic orthostatic hypotension and autonomic dysfunction. Major ADRs include supine hypertension, piloerection, urinary retention, scalp pruritus, and paresthesias. Do not administer within $3\text{--}4$ hours of bedtime.

Oxymetazoline (Afrin®)

A predominantly $\alpha_1$ agonist (with some $\alpha_2$ activity) that causes potent vasoconstriction of nasal mucosal vessels for nasal congestion and off-label control of epistaxis. More potent and longer acting than phenylephrine; it is the most common cause of rhinitis medicamentosa.

Clonidine (Catapres®, Catapres-TTS®, Duraclon®)

A central $\alpha_2$ agonist available in oral, transdermal, and epidural formulations that decreases sympathetic outflow from the CNS, reducing HR, BP, and peripheral vascular resistance. Used for HTN, ADHD, opioid/nicotine withdrawal, menopausal hot flashes, and severe cancer pain. Must never be stopped abruptly due to severe rebound hypertension caused by a catecholamine surge.

Clonidine ER (Kapvay®)

An extended-release central $\alpha_2$ agonist formulation indicated for ADHD in children and adults. Often combined with stimulants to reduce hyperactivity and insomnia.

Guanfacine (Tenex®, Intuniv®)

A selective central $\alpha_{2A}$ agonist that stimulates receptors in the prefrontal cortex and brainstem, reducing sympathetic tone and improving executive function. Indicated for hypertension (Tenex) and ADHD (Intuniv). More selective than clonidine, producing less sedation and less rebound hypertension.

Methyldopa (Aldomet®)

A central $\alpha_2$ agonist prodrug converted to $\alpha$-methylnorepinephrine, which stimulates central $\alpha_2$ receptors to reduce sympathetic outflow. Primary clinical use is hypertension during pregnancy. Major ADRs include positive Coombs test, hemolytic anemia, and hepatotoxicity.

Dexmedetomidine (Precedex®)

A highly selective $\alpha_2$ agonist that stimulates central $\alpha_2$ receptors in the locus coeruleus to produce sedation, anxiolysis, and analgesia with minimal respiratory depression. Indicated for ICU sedation, procedural sedation, and awake fiberoptic intubation, producing "cooperative sedation."

Tizanidine (Zanaflex®)

A central $\alpha_2$ agonist that inhibits presynaptic motor neurons in the spinal cord to reduce muscle spasticity due to multiple sclerosis, spinal cord injury, or stroke. Concomitant use with ciprofloxacin or fluvoxamine is strictly contraindicated.

Brimonidine Ophthalmic (Alphagan P®)

A selective $\alpha_2$ agonist that decreases aqueous humor production and increases uveoscleral outflow. Used for open-angle glaucoma and ocular hypertension. Preferred over apraclonidine for chronic glaucoma because of better tolerability. Contraindicated in neonates and infants ($<2$ years).

Brimonidine Topical (Mirvaso®)

A selective $\alpha_2$ agonist causing vasoconstriction of superficial cutaneous blood vessels, indicated for persistent facial erythema of rosacea. Can cause rebound erythema if overused.

Apraclonidine Ophthalmic (Iopidine®)

An $\alpha_2$ agonist (less selective than brimonidine) that decreases aqueous humor production. Used for prevention and treatment of transient increases in intraocular pressure after laser eye surgery. Not used chronically because tachyphylaxis and allergic reactions are common.

Rhinitis Medicamentosa

Rebound nasal congestion caused by the prolonged use ($>3$ days) of topical nasal decongestants such as Phenylephrine nasal spray or Oxymetazoline (Afrin®).

Cooperative Sedation

A clinical feature of Dexmedetomidine (Precedex®) characterized by patients being easily arousable with minimal respiratory depression during sedation.