A patient with pheochromocytoma is scheduled for surgery. Alpha-blockade is initiated with phenoxybenzamine. Why is a beta-blocker added ONLY AFTER adequate alpha-blockade is established?
- A Unopposed alpha-1 stimulation after beta-blockade causes paradoxical severe hypertension ✓
- B Beta-blockade reduces catecholamine synthesis from the tumor
- C Beta-blockers increase plasma volume needed pre-operatively
- D Alpha-blockade prevents the reflex tachycardia caused by beta-blockers
Correct answer: A. Unopposed alpha-1 stimulation after beta-blockade causes paradoxical severe hypertension
Explanation
In pheochromocytoma, catecholamines cause both alpha-1 (vasoconstriction) and beta-2 (vasodilation) effects. If a beta-blocker is given first, the beta-2 vasodilatory effect is blocked, leaving alpha-1 vasoconstriction unopposed, causing paradoxical severe hypertension. Alpha-blockade is therefore established first to prevent this. Beta-blockers do not reduce catecholamine synthesis; that is the role of alpha-methyl-para-tyrosine. Beta-blockers reduce tachycardia and arrhythmias after alpha-blockade is secured.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
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