A 55-year-old man takes his first dose of prazosin at night and collapses while getting up to urinate, with transient loss of consciousness. Blood pressure recorded immediately is 80/50 mmHg. The mechanism of this event is:

• A Reflex bradycardia from baroreceptor activation
• B Central suppression of vasomotor outflow identical to clonidine overdose
• C Direct myocardial depression reducing cardiac output
• D Loss of alpha-1 mediated venous and arteriolar tone without compensatory tachycardia on standing ✓

Explanation

First-dose hypotension is characteristic of alpha-1 antagonists such as prazosin. Blockade of vascular alpha-1 receptors abolishes the standing vasoconstrictor response, causing orthostatic pooling and syncope, often within 30 to 90 minutes of the first dose. Unlike ganglionic or central agents (option B), the effect is peripheral. Prevention is by giving the first dose at bedtime and titrating slowly. Reflex tachycardia may occur but is blunted in older patients, worsening postural falls.

Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.