Before surgery for a biochemically confirmed phaeochromocytoma, phenoxybenzamine is always initiated before propranolol. The reason is that beta blockade alone can cause:

• A Severe reflex bradycardia and atrioventricular block
• B Rebound catecholamine excess after withdrawal
• C Precipitation of acute pulmonary oedema from negative inotropy
• D Hypertensive crisis through unopposed alpha-mediated vasoconstriction ✓

Explanation

Catecholamines act on both alpha and beta receptors. If beta receptors are blocked first, vasodilatory beta-2 effects are removed while alpha-mediated vasoconstriction persists unopposed, raising blood pressure catastrophically. Hence irreversible alpha blockade with phenoxybenzamine precedes any beta blocker. Beta blockers are then added mainly to control reflex tachycardia caused by alpha blockade itself. Bradycardia, pulmonary oedema and withdrawal rebound are not the basis of this sequencing rule.

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

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Written and medically reviewed by the StethoPrep medical team.