Alpha-blockers

Mechanism of Action - The Alpha Blockade

• Primary Action: Competitively block postsynaptic α₁-adrenergic receptors in vasculature.
• Vascular Effects:
  • Relaxation of arterial smooth muscle → ↓ systemic vascular resistance (afterload).
  • Relaxation of venous smooth muscle → ↑ venous capacitance → ↓ venous return (preload).
• Net Result: ↓ Blood pressure.

⭐ The "first-dose effect" is a key adverse event, causing marked postural hypotension and syncope ~30-90 minutes after the initial dose. This is due to abrupt vasodilation.

📌 Remember the "-osin" suffix for α₁ selective blockers (Prazosin, Doxazosin, Terazosin).

Classification & Drugs - The Alpha Team

• Non-selective (α1 & α2): Used for managing pheochromocytoma.
  • Phenoxybenzamine: Irreversible, long-acting.
  • Phentolamine: Reversible, used for diagnosis and acute hypertensive crises.
• α1-selective: Primary use in BPH and hypertension.
  • Prazosin, Terazosin, Doxazosin.
  • 📌 The -osin suffix is your team identifier!
• Uroselective (α1A/D): Tamsulosin, primarily for BPH with minimal effect on blood pressure.

⭐ Be cautious of "first-dose" orthostatic hypotension with α1-blockers like Prazosin. Counsel patients to take the initial dose at bedtime.

Clinical Uses - Mission Possible

• Benign Prostatic Hyperplasia (BPH): Primary use. Relaxes smooth muscle in the bladder neck & prostate, improving urinary flow.
  • Uroselective agents (tamsulosin, silodosin) are preferred to minimize systemic BP effects.
• Hypertension: Not first-line. Used as add-on therapy, especially in men with concurrent BPH.
• Pheochromocytoma: Phenoxybenzamine (irreversible, non-selective) is used pre-operatively to control BP and prevent catecholamine-induced hypertensive crisis.

📌 Mnemonic: Alpha-blockers are used for the 3 P's: Prostatic hyperplasia, Pheochromocytoma, and high Pressure (hypertension).

⭐ First-Dose Orthostatic Hypotension: A classic, testable side effect. Significant drop in blood pressure can occur after the initial dose, potentially causing syncope. Always start with the lowest possible dose and administer at bedtime.

Adverse Effects & Toxicity - Friendly Fire

• Orthostatic Hypotension: The most common effect due to α₁-blockade preventing vasoconstriction upon standing. Leads to dizziness, lightheadedness, and syncope.
  • ⚠️ First-Dose Effect: A severe hypotensive reaction can occur with the initial dose, especially with prazosin. Minimized by starting with a low dose at bedtime.
• Reflex Tachycardia: Baroreceptor reflex responds to the drop in blood pressure by ↑ heart rate.
• Other Common Effects: Headache, nasal congestion (from vasodilation), and sexual dysfunction (impaired ejaculation).

⭐ Intraoperative Floppy Iris Syndrome (IFIS): A major concern during cataract surgery in patients taking α₁-antagonists, particularly tamsulosin. The iris billows and prolapses, complicating the procedure.

High‑Yield Points - ⚡ Biggest Takeaways

• Alpha-blockers, like prazosin and doxazosin, primarily cause arterial and venous dilation by blocking α1 receptors.
• They are used for both hypertension and benign prostatic hyperplasia (BPH), relaxing smooth muscle in the bladder neck and prostate.
• The most significant adverse effect is first-dose orthostatic hypotension, which can cause syncope; advise patients to take it at bedtime.
• Tamsulosin is uroselective for the α1A receptor in the prostate, having less effect on blood pressure.
• Phenoxybenzamine (irreversible) and phentolamine (reversible) are used to manage pheochromocytoma.