Phenylephrine administered intravenously to a normotensive subject causes a rise in blood pressure followed by bradycardia. The bradycardia is best explained by:
- A Reflex vagal discharge triggered by baroreceptor activation ✓
- B Direct negative chronotropic action on the SA node
- C Alpha-1 receptor stimulation at the SA node
- D Release of acetylcholine from adrenal medulla
Correct answer: A. Reflex vagal discharge triggered by baroreceptor activation
Explanation
Phenylephrine is a pure alpha-1 agonist with no direct cardiac action. The rise in blood pressure activates aortic and carotid baroreceptors, triggering a reflex increase in vagal (parasympathetic) tone to the SA node, which slows heart rate. This reflex bradycardia can be abolished by atropine, confirming its vagal origin.
Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.
High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP
Written and medically reviewed by the StethoPrep medical team.