A 58-year-old man with hypertension and benign prostatic hyperplasia is started on an alpha-1 blocker. He develops orthostatic hypotension and syncope on the first dose. Which feature best distinguishes the drug most likely responsible from other alpha-1 blockers?

Explanation

## First-Dose Syncope in Alpha-1 Blockers: Prazosin's Unique Risk **Key Point:** **Prazosin** is notorious for causing **first-dose syncope** — an acute, severe orthostatic hypotension and syncope occurring within 30–90 minutes of the first dose. This is the hallmark distinguishing feature of prazosin among α₁-blockers. ### Mechanism of First-Dose Syncope 1. Prazosin has **rapid absorption** (peak plasma levels in 1–2 hours) 2. Rapid onset of potent **peripheral α₁-blockade** → acute vasodilation 3. **Poor CNS penetration** (hydrophilic, does not cross blood–brain barrier significantly) 4. Lack of central sympathetic compensation → unopposed peripheral vasodilation 5. Result: **acute, severe orthostatic hypotension** and syncope **Clinical Pearl:** First-dose syncope is **preventable** by: - Starting with a low dose (0.5–1 mg) - Giving the **first dose at bedtime** - Advising the patient to lie down immediately if dizziness occurs - Gradual dose titration over days ### Why Other Alpha-1 Blockers Do NOT Cause First-Dose Syncope | Drug | Why No First-Dose Syncope | |------|---------------------------| | **Doxazosin** | Slower absorption (peak 2–3 hours); gradual onset allows compensatory mechanisms | | **Terazosin** | Intermediate absorption; slower rise in plasma levels | | **Alfuzosin** | Extended-release formulation; slower, more gradual absorption | | **Tamsulosin** | α₁A-selective (urethral > vascular selectivity); minimal vascular effects | **High-Yield:** Prazosin's **rapid absorption + poor CNS penetration + lack of reflex tachycardia compensation** = first-dose syncope. This is a **classic NEET PG trap** — students often confuse prazosin's lipophilicity (it is actually hydrophilic) or assume all α₁-blockers carry equal syncope risk. ### Mnemonic **"PRAZOSIN = PRONE to first-dose SYNCOPE"** - **P**razosin - **R**apid absorption - **A**cute vasodilation - **Z**ero CNS compensation - **O**rthostatic hypotension - **S**yncope on first dose [cite:KD Tripathi 8e Ch 12]