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USMLE Step 1Pharmacology: principles and autonomic drugsEP 12 of 16

Vasopressors compared: receptors, pressure and heart rate

Four adrenergic drugs, four different effects on mean arterial pressure and heart rate. Work them out from the receptors and the baroreflex instead of memorising them.

  • 5:46 video
  • 3 min read
  • 11 sources
  • Published 15 Nov 2026
Video lesson · 5:46 · Watch on YouTubeSeries: Pharmacology: Pharmacokinetics & Autonomic Drugs · @medservatory
Chapters (13)
By the end of this lesson

Predict the changes in mean arterial pressure, systemic vascular resistance and heart rate after norepinephrine, phenylephrine, isoproterenol and epinephrine (including epinephrine reversal after alpha blockade) from each drug's receptor selectivity.

The short version

  1. Mean arterial pressure is approximately cardiac output × systemic vascular resistance, and the baroreflex pushes heart rate against any change in pressure.
  2. Phenylephrine is alpha-1 only: resistance and pressure rise, and the heart rate falls reflexly.
  3. Norepinephrine is alpha-1 plus modest beta-1: pressure rises, heart rate changes little, and it is first-line in septic shock.
  4. Isoproterenol is beta-1 plus beta-2: heart rate rises while diastolic and mean pressure fall.
  5. Epinephrine is dose-dependent (beta effects at low doses, alpha-1 at high doses), and after an alpha blocker it reverses into an isoproterenol-like drug.

Vasopressors compared: why a pressor can slow the heart

A man under spinal anesthesia has a blood pressure of 82/48 mm Hg and a heart rate of 88/min. He is given phenylephrine. His pressure climbs to 118/70 mm Hg, but his heart rate falls to 62/min. A drug that raises pressure and slows the heart: why?

Vasopressors compared by receptor give the answer. Each drug has a receptor profile, and each profile predicts what happens to mean arterial pressure and to heart rate.

Two ideas first: MAP = CO × SVR, and the baroreflex

Mean arterial pressure (MAP) is, to a good approximation, cardiac output times systemic vascular resistance. A drug therefore raises pressure by squeezing vessels, by driving the heart, or by both.[1][3][4]

Estimating MAP
MAP ≈ diastolic pressure + pulse pressure / 3

pulse pressure = systolic − diastolic pressure[1]

The second idea is the baroreflex. When arterial pressure rises, baroreceptors signal the brain, vagal input to the heart goes up, sympathetic drive goes down, and the heart rate falls. A fall in pressure does the opposite.[2][5][11]

Phenylephrine: alpha-1 only

Phenylephrine is an almost pure alpha-1 agonist with almost no beta activity. Alpha-1 constricts the arterioles, so resistance and pressure rise.[3][6][11]

The baroreflex then slows the heart. There is no beta-1 drive to fight it, so the pattern is pressure up, heart rate down: the patient from the opening.[3][11]

Norepinephrine: alpha-1 plus a little beta-1

Norepinephrine acts mainly on alpha-1, with modest beta-1 activity. It raises vascular resistance and both systolic and diastolic pressure.[3][11]

The beta-1 drive on the heart is partly cancelled by the reflex, so the heart rate changes little and may even fall. Norepinephrine is the first-line vasopressor in septic shock.[3][10][11]

Isoproterenol: beta-1 and beta-2, with a worked MAP example

Isoproterenol is a nonselective beta agonist with little alpha activity. Beta-1 speeds the heart and strengthens the contraction. Beta-2 relaxes arteriolar smooth muscle, so resistance falls and the diastolic pressure drops.[3][7]

MAP before and after isoproterenol (illustrative values)
  1. Baseline 120/80 mm Hg: pulse pressure 40, MAP = 80 + 40/3 ≈ 93 mm Hg
  2. Under isoproterenol, systolic 126 and diastolic 60: pulse pressure 66
  3. MAP = 60 + 66/3 = 60 + 22 = 82 mm Hg
  4. A faster heart, a lower mean pressure

Epinephrine: dose-dependent, and epinephrine reversal

Epinephrine acts on alpha-1, beta-1 and beta-2. At low doses beta effects dominate: a faster, stronger heart and beta-2 vasodilation. At high doses alpha-1 vasoconstriction takes over, and resistance rises.[3][11]

Now the classic trap. Block the alpha receptors first, with phentolamine or phenoxybenzamine, then give epinephrine. Its alpha effect is gone, but its beta-2 vasodilation is not. Resistance falls, mean pressure falls, and the heart rate rises, from the drug and from the reflex.[5][8][9]

All four drugs side by side

Receptor action and expected effect
DrugReceptorsResistance and pressureHeart rate
PhenylephrineAlpha-1Resistance and pressure upDown (reflex)
NorepinephrineAlpha-1 plus modest beta-1Resistance, systolic and diastolic pressure upChanges little, may fall
IsoproterenolBeta-1 and beta-2Diastolic and mean pressure downUp
EpinephrineAlpha-1, beta-1, beta-2Low dose: beta-2 vasodilation; high dose: alpha-1 riseUp at low doses (beta-1)
Epinephrine after alpha blockadeBeta-1 and beta-2 onlyMean pressure downUp

Practice question

Original · USMLE style
Your turnQB-MS-S1-PHARM-016-01

A healthy 28-year-old woman in a pharmacology laboratory first receives an alpha-adrenergic antagonist intravenously. She then receives an intravenous infusion of epinephrine at a dose that, before the antagonist, raised her mean arterial pressure.

Which of the following changes in mean arterial pressure and heart rate is most likely after the epinephrine?
Pick one option. Your answers stay on this device.

Common questions

Why does phenylephrine slow the heart rate?

Phenylephrine is an almost pure alpha-1 agonist, so it constricts arterioles and raises pressure. The baroreflex answers by slowing the heart, and there is no beta-1 drive to oppose it.

What is the difference between norepinephrine and phenylephrine?

Norepinephrine acts mainly on alpha-1 with modest beta-1 activity, so the heart rate changes little and may even fall. Phenylephrine is alpha-1 only, so the reflex slowing of the heart is not offset.

What is epinephrine reversal?

After an alpha blocker such as phentolamine or phenoxybenzamine, epinephrine's alpha effect is gone but its beta-2 vasodilation remains. Resistance and mean pressure fall and the heart rate rises.

How do you estimate mean arterial pressure?

MAP is approximately the diastolic pressure plus one third of the pulse pressure. For example, at 120/80 mm Hg the pulse pressure is 40, so MAP ≈ 80 + 40/3 ≈ 93 mm Hg.

Which vasopressor is first-line in septic shock?

Norepinephrine is the first-line vasopressor in septic shock.

Transcript

Read the full narration, by chapter714 words

0:00The case: a pressor that slows the heart

A man under spinal anesthesia has a blood pressure of 82 over 48, and a heart rate of 88. The anesthesiologist gives phenylephrine. His pressure climbs to 118 over 70. But his heart rate falls to 62. A drug that raises pressure and slows the heart. Why?

0:21Today: four drugs, mean pressure and heart rate

Today: compare norepinephrine, phenylephrine, isoproterenol and epinephrine by receptor, and predict what each does to mean arterial pressure and heart rate. First, two ideas.

0:33Two ideas: MAP = CO x SVR, and the baroreflex

Mean arterial pressure is, to a good approximation, cardiac output times systemic vascular resistance. So a drug raises pressure by squeezing vessels, driving the heart, or both. We estimate it as the diastolic pressure plus one third of the pulse pressure. Second, the baroreflex. When arterial pressure rises, baroreceptors signal the brain, vagal input to the heart goes up, sympathetic drive goes down, and the heart rate falls.

1:07Phenylephrine: alpha-1 only

Start with phenylephrine. It is an almost pure alpha 1 agonist, with almost no beta activity. Alpha 1 constricts the arterioles, so resistance and pressure rise. The baroreflex answers by slowing the heart. So phenylephrine: pressure up, heart rate down. That is our patient. Norepinephrine acts mainly on alpha 1,

1:29Norepinephrine: alpha-1 plus a little beta-1

with modest beta 1 activity. It raises vascular resistance and both systolic and diastolic pressure. The beta 1 drive on the heart is partly cancelled by the reflex, so the heart rate changes little and may even fall. It is the first line vasopressor in septic shock. Isoproterenol is the opposite:

1:51Isoproterenol: beta-1 and beta-2

a nonselective beta agonist with little alpha activity. Beta 1 speeds the heart and strengthens the contraction. Beta 2 relaxes arteriolar smooth muscle, so resistance falls and the diastolic pressure drops. Take a baseline of 120 over 80.

2:09Worked example: MAP 93 versus 82

The mean pressure is 80 plus 40 over three, about 93. Under isoproterenol, say the systolic rises to 126 and the diastolic falls to 60. Now the pulse pressure is 66, and the mean is 60 plus 22, about 82. A faster heart, a lower mean pressure. These values are illustrative.

2:37Epinephrine: dose-dependent

Epinephrine is the dose-dependent drug. It acts on alpha 1, beta 1 and beta 2. At low doses, beta effects dominate: a faster, stronger heart, and beta 2 vasodilation. At high doses, alpha 1 vasoconstriction takes over, and the resistance rises. Now a famous trap.

2:58Epinephrine reversal after an alpha blocker

Block the alpha receptors first, with phentolamine or phenoxybenzamine. Then give epinephrine. Its alpha effect is gone, but its beta 2 vasodilation is not. Resistance falls, mean pressure falls, and the heart rate rises, from the drug and the reflex. That is epinephrine reversal. An alpha blocker turns epinephrine into an isoproterenol-like drug.

3:27Back to the patient

Back to the patient. Phenylephrine hit alpha 1. Pressure rose, and the baroreflex slowed his heart from 88 to 62. No beta 1 drive was there to fight the reflex.

3:40Practice question

Your turn. A 28-year-old healthy volunteer in a pharmacology laboratory first receives an alpha adrenergic antagonist intravenously. Then she receives an intravenous infusion of epinephrine at a dose that, before the antagonist, raises her mean arterial pressure. Which of the following changes in mean arterial pressure and heart rate is most likely after the epinephrine? A, mean pressure increases and heart rate decreases. B, mean pressure increases and heart rate increases. C, mean pressure is unchanged and heart rate decreases. D, mean pressure decreases and heart rate increases. E, mean pressure decreases and heart rate decreases. Pause here and decide.

4:33Answer and explanation

A is out: that is phenylephrine's pattern, and the alpha receptors are blocked. B is out: a rising mean pressure needs the alpha squeeze, which the antagonist removed. C is out: beta 2 vasodilation is not cancelled by anything, so the mean pressure cannot stay level, and with no pressure rise the reflex would not slow the heart. E is out: a falling pressure triggers the baroreflex, which speeds the heart, not slows it. So the answer is D. Unopposed beta 2 vasodilation lowers the mean pressure, and the heart rate rises.

5:11High-yield recap and next lesson

Three things to keep. One: mean pressure is cardiac output times resistance, and the baroreflex pushes the heart rate against any change in pressure. Two: phenylephrine is alpha 1, norepinephrine is alpha 1 plus a little beta 1, isoproterenol is beta 1 and beta two, and epinephrine is all of them, by dose. Three: block alpha, and epinephrine reverses. Next, cholinergic toxicity and its antidotes.

Sources

Numbers in the text, e.g. [1], point here
  1. OpenStax Anatomy and Physiology 2e, 20.2 Blood Flow, Blood Pressure, and Resistance (MAP = diastolic pressure + pulse pressure/3). openstax.org/books/anatomy-and-physiology-2e/pages/20-2-blood-flow-blood-pressure-and-resistance
  2. OpenStax Anatomy and Physiology 2e, 20.4 Homeostatic Regulation of the Vascular System (baroreceptor reflex). openstax.org/books/anatomy-and-physiology-2e/pages/20-4-homeostatic-regulation-of-the-vascular-system
  3. Shock: classification and pathophysiological principles of therapeutics. Curr Cardiol Rev 2019, PMC6520577 (phenylephrine, norepinephrine, isoproterenol, epinephrine). ebi.ac.uk/europepmc/webservices/rest/PMC6520577/fullTextXML
  4. What matters in shock? Flow or pressure? Qatar Med J 2019, PMC6851930. ebi.ac.uk/europepmc/webservices/rest/PMC6851930/fullTextXML
  5. Basic Sciences in Anesthesia: Cardiovascular Anatomy and Pharmacology, PMC7121118 (alpha blockade and baroreceptor reflex tachycardia). ebi.ac.uk/europepmc/webservices/rest/PMC7121118/fullTextXML
  6. Phenylephrine, Crit Care 2011, PMC3219266. ebi.ac.uk/europepmc/webservices/rest/PMC3219266/fullTextXML
  7. Recent advances in the management of chronic stable angina II. Vasc Health Risk Manag 2010, PMC2941787 (beta-1 and beta-2 effects). ebi.ac.uk/europepmc/webservices/rest/PMC2941787/fullTextXML
  8. Adrenaline-induced hypotension in the context of significant quetiapine overdose. Clin Case Rep 2023, PMC10017399 (adrenaline reversal). ebi.ac.uk/europepmc/webservices/rest/PMC10017399/fullTextXML
  9. Polypharmacy-related shock with phenothiazine. Intern Med 2024, PMC11239264 (adrenergic reversal). ebi.ac.uk/europepmc/webservices/rest/PMC11239264/fullTextXML
  10. Septic shock in the era of precision medicine. Front Med 2026, PMC13643401 (Surviving Sepsis Campaign 2026: norepinephrine first-line). ebi.ac.uk/europepmc/webservices/rest/PMC13643401/fullTextXML
  11. LITFL: Phenylephrine, Noradrenaline (norepinephrine), Adrenaline (epinephrine). litfl.com/pharmacology/phenylephrine/ · litfl.com/pharmacology/noradrenaline-norepinephrine/ · litfl.com/pharmacology/adrenaline-epinephrine/

The patient values, the isoproterenol blood pressures 120/80 and 126/60, and the question are teaching values written for this lesson, not data from a study.

The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.

Image credits. Illustrations: Servier Medical Art (smart.servier.com), CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/), adapted (recoloured, outlined): "Blood pressure measurement", "Heart", "Arteriole and smooth muscle cells", "Syringe". Infusion bag: "infusion-bag-blue" by Servier via Bioicons (bioicons.com), CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/), adapted (recoloured, outlined). Original diagrams: the baroreflex loop, the pressure bars, the arrows and the pocket card, all drawn in code. No AI-generated teaching images.

Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-PHARM-016.

For exam study only, not medical advice. Read the medical disclaimer.

In this series

Playlist on YouTube
← Previous · EP 11Beta blocker selectivity: beta-1, ISA and alpha blockade
  1. EP 01Potency vs efficacy: EC50, Emax and agonist types5:17
  2. EP 02Competitive vs noncompetitive antagonists: effect on EC50 and Emax5:01
  3. EP 03Therapeutic index explained: TD50 / ED505:01
  4. EP 04First-pass metabolism and bioavailability: calculating oral F from AUC5:18
  5. EP 05Volume of distribution: why Vd can be bigger than the body5:17
  6. EP 06Clearance and half-life: t½ = 0.693 × Vd / CL5:01
  7. EP 07Zero-order vs first-order elimination explained5:12
  8. EP 08Loading dose and maintenance dose, explained5:02
  9. EP 09CYP450 inducers and inhibitors: how they change drug levels5:16
  10. EP 10Autonomic receptors: alpha, beta and muscarinic5:33
  11. EP 11Beta blocker selectivity: beta-1, ISA and alpha blockade5:24
  12. EP 12Vasopressors compared: receptors, pressure and heart rate5:46
  13. EP 13Organophosphate poisoning: muscarinic vs nicotinic signs5:36
  14. EP 14Receptor desensitization, downregulation and upregulation5:01
  15. EP 15Beta-lactams and vancomycin: enzyme versus substrate5:04
  16. EP 16Protein synthesis inhibitors: 30S vs 50S antibiotics5:07