A healthy volunteer receives an intravenous dose of phentolamine followed by an intravenous bolus of epinephrine. Which haemodynamic response is expected?
- A Exaggerated pressor response with reflex bradycardia
- B No change in blood pressure because beta receptors dominate
- C Fall in mean arterial pressure with tachycardia ✓
- D Rise in diastolic pressure with no change in heart rate
Correct answer: C. Fall in mean arterial pressure with tachycardia
Explanation
Phentolamine competitively blocks alpha-1 mediated vasoconstriction. With alpha receptors blocked, epinephrine acts unopposed on vascular beta-2 receptors, causing vasodilatation and a fall in blood pressure, the classical 'epinephrine reversal'. The baroreceptor response to the pressure fall produces tachycardia, augmented by cardiac beta-1 stimulation. An exaggerated pressor response cannot occur because the alpha-1 vasoconstrictor component has been antagonised.
Reference: Katzung Basic and Clinical Pharmacology, 15th ed.
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