Chapter 5  ·  Module 3  ·  Visual Summary

Synthetic Adrenergic Agonists

Phenylephrine · Alpha-2 Agonists · Beta-2 Bronchodilators · Tocolytics

Drug Profiles at a Glance

Phenylephrine

Pure Alpha-1 Agonist

ReceptorAlpha-1 only (no beta activity)
EffectVasoconstriction; reflex bradycardia (no beta-1 to offset baroreceptor response)
UsesSpinal anesthesia hypotension; hypertrophic obstructive cardiomyopathy; ophthalmic mydriasis; topical decongestant
NoteOral form not effective as decongestant (extensive first-pass metabolism)

Clonidine / Dexmedetomidine

Central Alpha-2 Agonists

MechanismActivate alpha-2 autoreceptors in locus coeruleus → reduced sympathetic outflow
EffectsAntihypertensive + sedative (same mechanism)
ClonidineOral/patch; hypertension, attention deficit hyperactivity disorder, opioid withdrawal
Dexmed.Intravenous; intensive care unit sedation — cooperative, arousable, no respiratory depression
WarningNever stop abruptly — rebound hypertensive crisis

Midodrine / Fludrocortisone

Orthostatic Hypotension

MidodrineProdrug → desglymidodrine; alpha-1 agonist; raises standing blood pressure
RuleNo dose within 4 hours of bedtime — supine hypertension risk
Fludro.Mineralocorticoid → volume expansion; hypokalemia adverse effect
TogetherComplementary: vasoconstriction (midodrine) + volume (fludrocortisone)

Terbutaline

Beta-2 Tocolytic

MechanismBeta-2 → cyclic AMP ↑ → protein kinase A → myosin light-chain kinase inhibited → uterine relaxation
UseAcute short-term tocolysis (24–34 weeks) to allow fetal lung maturation
WarningFood and Drug Administration black box: no prolonged injectable use (>48–72 hrs); oral form not approved for tocolysis

Beta-2 Bronchodilators — Short-Acting vs. Long-Acting

Drug Class Onset Duration / Role Key Rule
Albuterol Short-acting 5–15 min 4–6 hrs — rescue only Use >2×/week = inadequate control; add controller therapy. Monitor potassium with high-dose nebulized use.
Salmeterol Long-acting 15–20 min 12 hrs — maintenance only, NOT rescue Never without inhaled corticosteroid in asthma (SMART trial: increased asthma-related death).
Formoterol Long-acting 1–3 min 12 hrs — maintenance + rescue protocols Never without inhaled corticosteroid in asthma. Rapid onset allows dual rescue/maintenance use.