A patient with pheochromocytoma crisis is given phentolamine. During infusion, the heart rate increases markedly. This tachycardia is primarily due to:
- A Direct cardiac beta-1 stimulation by phentolamine
- B Blockade of cardiac alpha-1 receptors increasing conduction velocity
- C Reflex tachycardia from hypotension plus blockade of presynaptic alpha-2 receptors increasing norepinephrine release onto beta-1 receptors ✓
- D Phentolamine-mediated inhibition of vagal tone
Correct answer: C. Reflex tachycardia from hypotension plus blockade of presynaptic alpha-2 receptors increasing norepinephrine release onto beta-1 receptors
Explanation
Phentolamine is a non-selective competitive alpha-blocker (alpha-1 and alpha-2). The tachycardia is dual in origin: (1) reflex baroreceptor activation from vasodilation-induced hypotension, and (2) blockade of presynaptic alpha-2 receptors (which normally inhibit norepinephrine release), leading to unrestrained norepinephrine release onto unblocked cardiac beta-1 receptors. This makes phentolamine less suitable for long-term therapy and explains the need for concurrent beta-blockade — but only AFTER alpha-blockade.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.