Why must an alpha-blocker be started before a beta-blocker in a patient with phaeochromocytoma who requires perioperative blood pressure control?
- A Beta-blockers accelerate catecholamine synthesis in the tumour
- B Beta-blockade without prior alpha-blockade causes unopposed alpha-mediated vasoconstriction and hypertensive crisis ✓
- C Alpha-blockers are required to prevent rebound tachycardia after surgery
- D Beta-blockers interfere with the metabolism of metanephrines used for monitoring
Correct answer: B. Beta-blockade without prior alpha-blockade causes unopposed alpha-mediated vasoconstriction and hypertensive crisis
Explanation
Blocking vasodilatory beta-2 receptors while alpha-1 receptors remain open removes peripheral vasodilation and leaves circulating catecholamines acting purely on alpha receptors, producing severe vasoconstriction and hypertensive crises. Phenoxybenzamine or doxazosin achieves alpha-blockade first; beta-blockade is added afterwards to control reflex tachycardia. Beta-blockers do not alter catecholamine synthesis or metanephrine assays.
Reference: Harrison's Principles of Internal Medicine, 21st ed.
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