A patient with pheochromocytoma is being prepared for surgery. Phenoxybenzamine is preferred over phentolamine for preoperative alpha-blockade. What is the key pharmacological reason?
- A Phenoxybenzamine is an irreversible (covalent) alpha blocker providing stable, non-surmountable blockade ✓
- B Phenoxybenzamine is a competitive antagonist with longer duration of action
- C Phenoxybenzamine selectively blocks alpha-2 receptors, preventing inhibition of norepinephrine release
- D Phenoxybenzamine has an additional beta-blocking effect, preventing tachycardia
Correct answer: A. Phenoxybenzamine is an irreversible (covalent) alpha blocker providing stable, non-surmountable blockade
Explanation
Phenoxybenzamine is a haloalkylamine that alkylates and irreversibly blocks alpha-1 and alpha-2 adrenoceptors. This non-competitive (non-surmountable) blockade cannot be overcome by massive catecholamine surges during tumor manipulation, making it superior for preoperative blood pressure control in pheochromocytoma. Phentolamine is a competitive (reversible) antagonist that can be overcome by high catecholamine levels. Beta blockers are added only after alpha blockade to prevent reflex tachycardia, not as a property of phenoxybenzamine.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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