Phenylephrine, a pure alpha-1 adrenergic agonist, is given IV to a hypotensive patient. Despite raising MAP, the heart rate falls (reflex bradycardia). This bradycardia is mediated by:

• A Direct negative chronotropic effect of phenylephrine on beta-1 receptors
• B Activation of chemoreceptors by reduced tissue oxygen delivery
• C Suppression of sympathetic tone by alpha-1 receptors in the hypothalamus
• D Baroreflex arc: aortic/carotid baroreceptors detect increased arterial pressure and increase vagal tone to the SA node ✓

Explanation

Phenylephrine increases peripheral vascular resistance (via vascular alpha-1 receptors) and thus raises arterial pressure. Elevated pressure is detected by carotid sinus and aortic arch baroreceptors (mechanoreceptors), which increase afferent firing to the nucleus tractus solitarius (NTS) in the medulla. This reflexly increases vagal efferent outflow to the SA node, slowing heart rate (reflex bradycardia). Phenylephrine itself has no direct cardiac receptor activity — the bradycardia is entirely reflex-mediated.

Reference: Guyton & Hall, Textbook of Medical Physiology, 14th ed.

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