Alpha Blocker Drugs
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Direct answer
Alpha blockers inhibit adrenergic alpha receptors: non-selective agents (phenoxybenzamine, phentolamine) block alpha-1 and alpha-2 and are reserved for pheochromocytoma and catecholamine-excess crises, while selective alpha-1 blockers (prazosin, doxazosin, tamsulosin) lower blood pressure by relaxing arterioles and venules and relieve bladder-outlet obstruction in benign prostatic hyperplasia. Their signature adverse effects are first-dose hypotension and syncope with prazosin, and ejaculatory dysfunction with tamsulosin.
What you must remember
- Phenoxybenzamine is an irreversible, non-competitive alpha blocker; phentolamine is reversible — both are non-selective.
- In pheochromocytoma, alpha blockade (phenoxybenzamine 10 mg twice daily, titrated 1–2 weeks before surgery) must precede beta blockade; giving a beta-blocker first causes unopposed alpha-mediated hypertensive crisis.
- Phentolamine IV rescues the tyramine–cheese reaction in patients on monoamine oxidase inhibitors and reverses noradrenaline extravasation necrosis (5–10 mg local infiltration).
- Prazosin causes first-dose syncope — start 0.5 mg at bedtime; it also remains a specific treatment for post-traumatic stress disorder nightmares.
- Tamsulosin is alpha-1A selective: it relaxes prostate and bladder-neck smooth muscle with little effect on blood pressure, ideal for BPH patients on other antihypertensives.
- Intraoperative floppy iris syndrome during cataract surgery is a recognised complication of tamsulosin — warn the ophthalmologist.
- Doxazosin is once-daily and the most lipid-soluble alpha-1 blocker; terazosin and doxazosin improve BPH symptoms and pressure together.
- Yohimbine (alpha-2 blocker) has no routine clinical role but is exam-famous as the pharmacological opposite of clonidine.
How to work through a pheochromocytoma preparation
A 34-year-old with paroxysmal headaches, palpitations and sweating has a 24-hour urine metanephrine level five times normal and an adrenal mass on CT. Preoperative pharmacological preparation proceeds in a fixed sequence, and the reasoning matters more than the doses. Begin with phenoxybenzamine 10 mg twice daily, increasing every few days until blood pressure is controlled and nasal congestion appears — nasal stuffiness is the traditional clinical sign that alpha blockade is adequate, because it is the one alpha-mediated function you can see from the foot of the bed. Only after at least three to four days of alpha blockade add a beta-blocker (propranolol or metoprolol) for reflex tachycardia; if beta-blockade is given first, the unopposed alpha stimulation can precipitate a hypertensive emergency or pulmonary oedema. Restore volume — chronic vasoconstriction masks a contracted plasma volume that would otherwise crash the pressure the moment the tumour is clamped. Continue phenoxybenzamine until the day of surgery; intraoperative surges are handled by phentolamine boluses or nitroprusside, and hypotension after tumour removal by volume and noradrenaline. For inoperable tumours, phenoxybenzamine plus metyrosine is long-term therapy. This pathway explains why the non-selective, irreversible drug survives in modern practice at all.
Where students slip
Candidates routinely write "alpha blockers are first-line antihypertensives" — they are not; thiazides, calcium channel blockers, ACE inhibitors and ARBs hold that position, with prazosin reserved for add-on use or specific situations because the first-dose phenomenon and reflex tachycardia limit it. The second recurring mistake is misassigning selectivity: tamsulosin is alpha-1A (prostate-selective, pressure-sparing), while prazosin, terazosin and doxazosin block alpha-1 broadly and lower pressure. A viva favourite is the postural hypotension shared by prazosin and the phenothiazines through different mechanisms — receptor blockade versus alpha-adrenergic suppression — and the examiner wants you to name first-dose timing as the practical discriminator. Finally, do not forget that clonidine withdrawal can mimic pheochromocytoma; the drug history distinguishes rebound sympathetic overactivity from a catecholamine-secreting tumour faster than any assay.
Frequently asked questions
Why must beta-blockade never precede alpha-blockade in pheochromocytoma?
Beta-blockade leaves alpha-mediated vasoconstriction unopposed, so blood pressure rises precipitously and can precipitate hypertensive encephalopathy or pulmonary oedema; always block alpha first.
Which alpha blocker suits a BPH patient who is already on amlodipine?
Tamsulosin 0.4 mg at bedtime, because alpha-1A selectivity relaxes prostatic smooth muscle without adding meaningful antihypertensive effect or first-dose syncope.
What is first-dose phenomenon and how is it prevented?
Prazosin's initial dose can cause sudden syncope within 30–90 minutes due to venodilatation with absent baroreflex compensation; start with 0.5 mg at bedtime and warn against standing.
How is noradrenaline extravasation managed?
Stop the infusion and infiltrate the area with phentolamine 5–10 mg in saline to reverse local alpha-1 vasoconstriction, preventing tissue necrosis; topical nitroglycerine is an alternative.
Which alpha blocker has a psychiatric indication?
Prazosin reduces trauma nightmares and improves sleep in post-traumatic stress disorder by damping central adrenergic signalling during rapid eye movement sleep.