Autonomic nervous system: phenylephrine reflex bradycardia: practice question

A Pharmacology question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Pharmacology · Autonomic nervous system: phenylephrine reflex bradycardia

Sample item

During spinal anaesthesia an anaesthetist gives a phenylephrine bolus for hypotension. Blood pressure rises, but the heart rate falls from 80 to 58 beats per minute. What explains the fall in heart rate?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Baroreceptor-mediated reflex vagal activity

Phenylephrine is a pure alpha-1 agonist with no direct cardiac stimulation. The rise in blood pressure activates the baroreceptor reflex, increasing vagal tone to the sinoatrial node and slowing the heart.

Option by option

  • A. Baroreceptor-mediated reflex vagal activity (correct)

    Correct. The pressor effect triggers a vagal reflex that slows the heart.

  • B. Beta-1 receptor blockade by phenylephrine

    It is not a beta blocker, and it has no action on beta-1 receptors at these doses.

  • C. Direct stimulation of cardiac muscarinic receptors

    Phenylephrine does not stimulate cardiac muscarinic receptors directly.

  • D. Central alpha-2 mediated sympathetic inhibition

    Phenylephrine acts on peripheral alpha-1 receptors and does not act as a central alpha-2 agonist.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

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Questions about this Pharmacology question

What is the answer to this Pharmacology question?

The best answer is A, Baroreceptor-mediated reflex vagal activity. Phenylephrine is a pure alpha-1 agonist with no direct cardiac stimulation.

Why is "Beta-1 receptor blockade by phenylephrine" not the answer?

It is not a beta blocker, and it has no action on beta-1 receptors at these doses.

Why is "Direct stimulation of cardiac muscarinic receptors" not the answer?

Phenylephrine does not stimulate cardiac muscarinic receptors directly.

Why is "Central alpha-2 mediated sympathetic inhibition" not the answer?

Phenylephrine acts on peripheral alpha-1 receptors and does not act as a central alpha-2 agonist.

What concept does this question test?

It tests Autonomic nervous system: phenylephrine reflex bradycardia in Pharmacology. The rise in blood pressure activates the baroreceptor reflex, increasing vagal tone to the sinoatrial node and slowing the heart.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Autonomic nervous system: phenylephrine reflex bradycardia?

Learn drugs as classes, then the exception in each class that an examiner would pick. Then use the flashcards and the other Pharmacology questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.