Chapter 5  ·  Module 4  ·  Visual Summary

Alpha-Adrenergic Antagonists

Non-selective blockers · Selective alpha-1 blockers · Uroselective agents · Pheochromocytoma

Drug Profiles at a Glance

Phenoxybenzamine

Irreversible Alpha-1/2 Blocker

BindingCovalent, irreversible — cannot be overcome by agonist
RouteOral; onset 1–2 hours; duration 24–48 hours
UsePreoperative pheochromocytoma preparation (1–2 weeks titration)
SEReflex tachycardia (alpha-2 also blocked), orthostatic hypotension, nasal congestion, miosis

Phentolamine

Reversible Alpha-1/2 Blocker

BindingCompetitive, reversible — can be overcome by high agonist doses
RouteIntravenous or intramuscular only; onset 1–2 min; duration 15–30 min
UsesAcute hypertensive emergency (catecholamine excess); norepinephrine extravasation

Prazosin / Terazosin / Doxazosin

Selective Alpha-1 Blockers

SelectivityAlpha-1 only; preserves alpha-2 feedback → less reflex tachycardia
UsesHypertension; benign prostatic hyperplasia; prazosin for post-traumatic stress disorder nightmares
WarningFirst-dose phenomenon — take first dose at bedtime; start at 1 mg; no phosphodiesterase type 5 inhibitors within 4–6 hours
ALLHATDoxazosin → more heart failure than chlorthalidone; not first-line antihypertensive

Tamsulosin / Silodosin

Uroselective Alpha-1A Blockers

SelectivityAlpha-1A dominant (prostate, urethra) → less vascular hypotension
UseBenign prostatic hyperplasia first-line; no bedtime dosing rule needed
SERetrograde ejaculation (silodosin 28%, tamsulosin 4–11%)
AlertIntraoperative floppy iris syndrome during cataract surgery — risk persists indefinitely after stopping drug. Inform ophthalmologist.

Pheochromocytoma Preoperative Protocol — Alpha Before Beta

Correct Sequence — Never Reverse

Step 1

Start phenoxybenzamine 10 mg twice daily. Titrate over 1–2 weeks. Confirm alpha blockade.

Step 2

High-sodium diet + liberal fluids to correct chronic volume depletion throughout titration.

Step 3

ONLY after alpha blockade confirmed: add beta-blocker (propranolol, atenolol) for tachycardia.

Goals

Blood pressure <130/80; no orthostatic drop >10 mmHg; heart rate 60–80; reflex tachycardia and nasal congestion confirm blockade.

Beta-blocker before alpha blockade → unopposed alpha-1 vasoconstriction → hypertensive crisis