A 45-year-old man with a catecholamine-secreting adrenal tumour presents with headache, palpitations, and paroxysmal hypertension. Before surgical removal, phenoxybenzamine must be started before propranolol. Giving propranolol first is dangerous primarily because:
- A Blocking vasodilatory beta-2 receptors leaves alpha-mediated vasoconstriction unopposed, causing severe hypertensive crises ✓
- B Beta blockade enhances adrenal medullary catecholamine synthesis
- C Propranolol precipitates acute adrenal insufficiency in these patients
- D Beta blockade abolishes the reflex tachycardia needed to maintain cardiac output in these patients
Correct answer: A. Blocking vasodilatory beta-2 receptors leaves alpha-mediated vasoconstriction unopposed, causing severe hypertensive crises
Explanation
Circulating catecholamines stimulate both alpha-1 receptors, causing vasoconstriction, and beta-2 receptors, causing vasodilation in skeletal muscle beds. Administering a beta blocker first removes the beta-2 vasodilatory counterbalance, leaving alpha-mediated vasoconstriction completely unopposed, which can precipitate extreme hypertension and worsen myocardial ischaemia. Alpha blockade with phenoxybenzamine first restores vascular tone control before any beta blockade is introduced.
Reference: Katzung's Basic and Clinical Pharmacology, 16th ed.
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