A patient receives intravenous phenylephrine for hypotension during surgery. Shortly after, the heart rate drops significantly. The mechanism of this bradycardia is:

• A Baroreceptor-mediated reflex increase in vagal tone ✓
• B Direct suppression of sinoatrial node automaticity by alpha-1 stimulation
• C Direct stimulation of cardiac muscarinic receptors
• D Decreased norepinephrine release from presynaptic alpha-2 autoreceptors

Explanation

Phenylephrine is a pure alpha-1 agonist that raises blood pressure by vasoconstriction. The increased blood pressure activates the baroreceptor reflex, increasing vagal tone to the sinoatrial node, causing reflex bradycardia. This is a reflex phenomenon, not a direct cardiac effect. Phenylephrine has no direct muscarinic action or direct SA node suppression. Alpha-2 autoreceptor activation would reduce sympathetic outflow but is not the primary mechanism here.

Reference: Goodman and Gilman's The Pharmacological Basis of Therapeutics, 14th ed.

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