Which of the following adverse effects is MOST characteristic of non-selective alpha blockers like prazosin but NOT expected with tamsulosin?

Explanation

## Hemodynamic Side Effects: Selectivity Matters **Key Point:** Non-selective α1 antagonists (prazosin, doxazosin, terazosin) block α1B receptors on vascular smooth muscle, causing vasodilation, hypotension, and orthostatic symptoms. Selective α1A blockers (tamsulosin) spare vascular α1B receptors and thus avoid these hemodynamic complications. ### Mechanism of Orthostatic Hypotension **Non-selective blockers:** 1. Block α1A receptors in prostate → smooth muscle relaxation (desired effect) 2. Block α1B receptors in blood vessels → vasodilation (unwanted effect) 3. Result: Reduced peripheral vascular resistance → orthostatic hypotension **Selective α1A blockers (tamsulosin):** 1. Block α1A receptors in prostate → smooth muscle relaxation (desired effect) 2. Spare α1B receptors in blood vessels → minimal vasodilation (avoided) 3. Result: Minimal hemodynamic changes ### First-Dose Syncope ```mermaid flowchart TD A[Non-selective α blocker given]:::action --> B[Rapid α1B blockade on blood vessels]:::outcome B --> C[Acute vasodilation]:::outcome C --> D[Sudden drop in peripheral vascular resistance]:::urgent D --> E[Orthostatic hypotension + syncope]:::urgent F[Tamsulosin given]:::action --> G[Selective α1A blockade in prostate]:::outcome G --> H[Minimal vascular α1B blockade]:::outcome H --> I[Preserved vascular tone]:::outcome I --> J[No first-dose syncope]:::outcome ``` **High-Yield:** First-dose syncope is a classic teaching point for prazosin. It occurs within 30–90 minutes of the first dose and is due to sudden vasodilation. Tamsulosin does NOT cause first-dose syncope because it does not block vascular α1B receptors. **Mnemonic:** **VAST** = Vasodilation, Alpha non-selective, Syncope, Tachycardia (in non-selective blockers) ### Comparison of Hemodynamic Effects | Adverse Effect | Prazosin | Doxazosin | Terazosin | Tamsulosin | |---|---|---|---|---| | Orthostatic hypotension | +++++ | ++++ | ++++ | + | | First-dose syncope | Yes | Yes | Yes | No | | Tachycardia (reflex) | Yes | Yes | Yes | No | | Fluid retention | Yes | Yes | Yes | No | | Blood pressure reduction | Marked | Marked | Marked | Minimal | **Clinical Pearl:** Prazosin requires slow dose titration starting at 0.5 mg at bedtime to minimize first-dose syncope. Tamsulosin can be started at the full therapeutic dose (0.4 mg daily) without titration because syncope risk is negligible.