Vasopressors and Inotropes: Phenylephrine, Ephedrine, Norepinephrine, Epinephrine, Vasopressin
CA-1 · draft
CA-1 Bootcamp day 2. Authored from cited abstracts; every claim carries a PMID.
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Before induction
Phenylephrine raises blood pressure while simultaneously reducing cardiac output, and although it increases cerebral blood flow it lowers cerebral tissue oxygen saturation - so the number on the monitor improves while the flow behind it does not.
Norepinephrine carries weak beta-adrenergic activity alongside its potent alpha effect, and given by infusion at caesarean delivery it held systolic pressure as well as phenylephrine while preserving a greater cardiac output.
In a graded dose-response study the ED50 for restoring blood pressure was 10 micrograms of norepinephrine against 137 micrograms of phenylephrine, so these two drugs can never be exchanged at the same numerical dose.
Ephedrine's pressor effect in vivo comes from releasing stored norepinephrine out of sympathetic nerve terminals rather than from direct receptor activation, since destroying those terminals abolished the response, and in obstetric practice ephedrine has the highest probability of being the worst agent for fetal acid-base status.
Vasopressin raises mean arterial pressure through V1 receptors and lowers cardiac output, has little effect in a healthy subject but potentiates norepinephrine in vasodilatory shock, and above 0.04 units per minute has been reported to cause cardiac arrest.
Adrenaline is the first-line treatment for anaphylaxis and should be given intravenously in the perioperative setting, and anaphylaxis is exactly what you should suspect when hypotension persists despite inotropes and vasopressors even with no rash to look at.
Questions in the room
Which of these raises blood pressure while lowering cardiac output, and when is that trade worth making?
She is bradycardic as well as hypotensive - does that change your first choice, and why?
Ephedrine worked the first two times and did nothing the third time. What happened?
The patient takes an ACE inhibitor and phenylephrine is not holding her. What else is in the drawer, and why might it work when phenylephrine will not?
Oral boards stem
Ten minutes into a laparoscopic hysterectomy in a healthy 45-year-old, the cuff reads 78/44 with a heart rate of 58. The drug drawer beside you holds phenylephrine, ephedrine, norepinephrine, epinephrine and vasopressin. Before you pick one up, your attending asks you to say out loud what each of them does to heart rate, to systemic vascular resistance, and to cardiac output.
Sources
Meng et al., Anaesthesia 2024
PMID 37948131
Ngan Kee et al., Anesthesiology 2015
PMID 25635593
Ngan Kee, Anesthesiology 2017
PMID 28872480
Kobayashi et al., Anesthesia and Analgesia 2003
PMID 14570629
Singh et al., British Journal of Anaesthesia 2020
PMID 31810562
den Ouden et al., Netherlands Journal of Medicine 2005
PMID 15719846
Takazawa et al., Journal of Anesthesia 2021
PMID 34651257
Ma et al., Canadian Journal of Anaesthesia 2025
PMID 40244358
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