Why must alpha-adrenergic blockade be established before starting a beta blocker in a patient with phaeochromocytoma scheduled for surgery?
- A Beta blockade without alpha blockade leaves vasoconstriction unopposed and precipitates hypertensive crisis ✓
- B Beta blockers reduce tumour catecholamine synthesis
- C Alpha blockers prevent intraoperative hypoglycaemia caused by beta blockers
- D Beta blockers antagonise the effect of phenoxybenzamine at vascular receptors
Correct answer: A. Beta blockade without alpha blockade leaves vasoconstriction unopposed and precipitates hypertensive crisis
Explanation
Circulating catecholamines stimulate both alpha-mediated vasoconstriction and beta-mediated vasodilation in skeletal muscle vessels. Blocking beta-mediated vasodilation first leaves alpha vasoconstriction completely unopposed, causing severe hypertension and reduced perfusion. Hence phenoxybenzamine or another alpha blocker is started first, and beta blockade is added only afterwards, mainly for reflex tachycardia. Beta blockers do not affect catecholamine synthesis within the tumour.
Reference: Williams Textbook of Endocrinology, 14th ed.
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