A 58-year-old man from Delhi presents with a 6-month history of lower urinary tract symptoms (LUTS) including nocturia (3–4 times per night), hesitancy, and weak urinary stream. Digital rectal examination reveals a smooth, enlarged prostate (approximately 40 g). PSA is 2.8 ng/mL. He is started on an alpha blocker. Within 2 hours of the first dose, he develops severe dizziness, syncope, and a blood pressure drop from 145/90 mmHg to 95/55 mmHg. Which of the following alpha blockers is MOST likely responsible for this adverse effect, and what is the underlying mechanism?

Explanation

## First-Dose Syncope in Alpha Blocker Therapy ### Mechanism of First-Dose Syncope **Key Point:** First-dose syncope is a well-recognized adverse effect of non-selective α1-adrenergic antagonists, particularly terazosin and doxazosin. It occurs within 30 minutes to 3 hours of the first dose due to acute, non-selective blockade of α1A and α1B receptors on vascular smooth muscle. ### Pathophysiology 1. **Non-selective α1 blockade** → vasodilation in arterioles and veins 2. **Loss of peripheral vascular resistance** → sudden drop in blood pressure 3. **Inadequate compensatory mechanisms** → syncope before reflex tachycardia can stabilize perfusion 4. **Supine position during sleep** → exacerbates orthostatic component on arising ### Comparison of Alpha Blockers by Selectivity and First-Dose Risk | Alpha Blocker | α1A Selectivity | α1B Selectivity | Vascular Effects | First-Dose Syncope Risk | Titration Needed | | --- | --- | --- | --- | --- | --- | | **Terazosin** | Non-selective | Non-selective | Marked vasodilation | **HIGH** | Yes (start 0.5–1 mg) | | **Doxazosin** | Non-selective | Non-selective | Marked vasodilation | **HIGH** | Yes (start 0.5–1 mg) | | **Tamsulosin** | Selective (α1A) | Minimal | Minimal at therapeutic doses | Low | Minimal | | **Alfuzosin** | Selective (α1A) | Minimal | Minimal at therapeutic doses | Low | Minimal | **High-Yield:** Terazosin and doxazosin are **non-selective α1 antagonists** and carry the highest risk of first-dose syncope. They require slow titration starting at 0.5–1 mg at bedtime. ### Clinical Pearl **Clinical Pearl:** First-dose syncope is preventable by: - Starting at low dose (0.5–1 mg) - Administering at bedtime - Advising patient to remain supine for 2–3 hours after first dose - Gradual dose escalation over 2–4 weeks ### Why Tamsulosin and Alfuzosin Are Safer **Key Point:** Tamsulosin and alfuzosin are **uroselective α1A antagonists**. They preferentially block α1A receptors in the prostate and bladder neck while sparing α1B receptors on blood vessels, resulting in: - Minimal vasodilation - No first-dose syncope - No need for bedtime dosing or slow titration ### Mnemonic **Mnemonic:** **"TOAD" = Terazosin, Doxazosin = Orthostatic hypotension, Adjust dose** - **T**erazosin → **T**itrate slowly - **D**oxazosin → **D**rop in BP (first-dose syncope) - **T**amsulosin → **T**olerant (uroselective, safe) - **A**lfuzosin → **A**void syncope (uroselective, safe)