A 28-year-old woman presents with severe headache, palpitations and a heart rate of 42 per minute after using an over-the-counter nasal decongestant spray containing phenylephrine excessively. The mechanism of her bradycardia is:

• A Direct stimulation of cardiac muscarinic receptors
• B Blockade of cardiac beta-1 receptors by phenylephrine metabolites
• C Baroreceptor-mediated vagal reflex triggered by raised systolic pressure ✓
• D Central activation of the vasomotor center causing vagal outflow

Explanation

Phenylephrine is a selective alpha-1 agonist that raises peripheral vascular resistance and arterial pressure without any direct cardiac action. The rise in pressure stimulates carotid sinus and aortic arch baroreceptors, increasing vagal outflow and slowing the sinoatrial node. This reflex bradycardia is characteristic of pure alpha agonists. Phenylephrine has no affinity for muscarinic receptors, no beta-blocking activity, and its cardiovascular actions are peripheral rather than mediated centrally through the vasomotor center.

Reference: Katzung's Basic and Clinical Pharmacology, 15th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.