CA-1 · intraoperative teaching CA-1 Bootcamp day 5. Authored from cited abstracts; every claim carries a PMID.
THE QUESTIONWhere the evidence disagrees
This topic was selected because an evidence synthesis beats a textbook on it: the trials disagree, or the guidance has moved recently.
THE EVIDENCE
What each source contributes, and how strong it is
Study design first — a cohort and a randomised trial do not carry the same weight.
Design
Year
Journal
What it found
Other
1993
British Journal of Anaesthesia
The minimum alveolar concentration of sevoflurane measured in patients averagi
Other
1996
Acta Anaesthesiologica Scandinavic
In patients with coronary artery disease, isoflurane and desflurane given in r
Other
1996
Drugs
Sevoflurane has lower blood solubility than isoflurane and negligible airway i
Other
2003
Anesthesia and Analgesia
During inhalational induction the alveolar concentration of nitrous oxide rise
Other
2007
Anesthesiology
In ENIGMA, avoiding nitrous oxide during major surgery lowered the rate of maj
Other
2014
Lancet
In ENIGMA-II, 7,112 patients with known or suspected coronary disease had the
7 resolved citations behind this deck; every point above traces to one of them.
IN PRACTICE
What the cohorts and reviews add
6 findings, each on the slide that follows.
British Journal of Anaesthesia 1993
The minimum alveolar concentration of sevoflurane measured in patients averagi
The minimum alveolar concentration of sevoflurane measured in patients averaging 71 years was 1.48%, lower than the values for adults and children
Acta Anaesthesiologica Scandinavica 1996
In patients with coronary artery disease, isoflurane and desflurane given in r
In patients with coronary artery disease, isoflurane and desflurane given in rising concentrations up to 1 MAC each lowered mean arterial pressure by…
Drugs 1996
Sevoflurane has lower blood solubility than isoflurane and negligible airway i
Sevoflurane has lower blood solubility than isoflurane and negligible airway irritant effects, which is exactly why it is the agent you can use for a…
Anesthesia and Analgesia 2003
During inhalational induction the alveolar concentration of nitrous oxide rise
During inhalational induction the alveolar concentration of nitrous oxide rises towards the inspired concentration faster than sevoflurane's does,…
IN PRACTICE
The minimum alveolar concentration of sevoflurane measured in patients averagi
Other · British Journal of Anaesthesia
The minimum alveolar concentration of sevoflurane measured in patients averaging 71 years was 1.48%, lower than the values for adults and children, and the concentration needed to stop 95% of patients moving was 1.98% - so MAC is a half-of-patients dose and it falls with age.
Nakajima et al., British Journal of Anaesthesia 1993 · PMID 8471368
IN PRACTICE
In patients with coronary artery disease, isoflurane and desflurane given in r
Other · Acta Anaesthesiologica Scandinavic
In patients with coronary artery disease, isoflurane and desflurane given in rising concentrations up to 1 MAC each lowered mean arterial pressure by about 16 to 18%, and that fall came primarily from a 21 to 26% drop in systemic vascular resistance rather than from any change in cardiac index.
Grundmann et al., Acta Anaesthesiologica Scandinavica 1996 · PMID 8933850
IN PRACTICE
Sevoflurane has lower blood solubility than isoflurane and negligible airway i
Other · Drugs
Sevoflurane has lower blood solubility than isoflurane and negligible airway irritant effects, which is exactly why it is the agent you can use for a smooth mask induction and isoflurane is not.
During inhalational induction the alveolar concentration of nitrous oxide rise
Other · Anesthesia and Analgesia
During inhalational induction the alveolar concentration of nitrous oxide rises towards the inspired concentration faster than sevoflurane's does, consistent with their blood/gas partition coefficients, and raising the inspired nitrous oxide concentration shifts the whole equilibration curve leftward - the concentration effect, with a brief second gas effect on the sevoflurane.
Goldman, Anesthesia and Analgesia 2003 · PMID 12538185
IN PRACTICE
In ENIGMA, avoiding nitrous oxide during major surgery lowered the rate of maj
Other · Anesthesiology
In ENIGMA, avoiding nitrous oxide during major surgery lowered the rate of major complications (odds ratio 0.71) and severe nausea and vomiting (odds ratio 0.40), though it did not shorten hospital stay.
In ENIGMA-II, 7,112 patients with known or suspected coronary disease had the
Other · Lancet
In ENIGMA-II, 7,112 patients with known or suspected coronary disease had the same rate of death and cardiovascular complications with or without nitrous oxide (8% in both groups) and no excess risk at one year, but severe nausea and vomiting rose from 11% to 15% - so the case against nitrous oxide is emetogenic, not cardiac.
Colour is the strength of the evidence behind each step, not the urgency.
1
1
The minimum alveolar concentration of sevoflurane measured in patients averaging 71 years was 1.48%, lower than the values for adults and children, and the concentration needed to stop 95% of patients moving was 1.98% - so MAC is a half-of-patients dose and it falls with age.
2
2
In patients with coronary artery disease, isoflurane and desflurane given in rising concentrations up to 1 MAC each lowered mean arterial pressure by about 16 to 18%, and that fall came primarily from a 21 to 26% drop in systemic vascular resistance rather than from any change in cardiac index.
3
3
Sevoflurane has lower blood solubility than isoflurane and negligible airway irritant effects, which is exactly why it is the agent you can use for a smooth mask induction and isoflurane is not.
4
4
During inhalational induction the alveolar concentration of nitrous oxide rises towards the inspired concentration faster than sevoflurane's does, consistent with their blood/gas partition coefficients, and raising the inspired nitrous oxide concentration shifts the whole equilibration curve leftward - the concentration effect, with a brief second gas effect on the sevoflurane.
5
5
In ENIGMA, avoiding nitrous oxide during major surgery lowered the rate of major complications (odds ratio 0.71) and severe nausea and vomiting (odds ratio 0.40), though it did not shorten hospital stay.
The oral-boards stem on the next slide puts these into one scenario.
KEY TAKEAWAYS
What to carry into the next case
The minimum alveolar concentration of sevoflurane measured in patients averagi
British Journal of Anaesthesia 1993
In patients with coronary artery disease, isoflurane and desflurane given in r
Acta Anaesthesiologica Scandinavica 1996
Sevoflurane has lower blood solubility than isoflurane and negligible airway i
Drugs 1996
During inhalational induction the alveolar concentration of nitrous oxide rise
Anesthesia and Analgesia 2003
MAC is the currency of every volatile - it is the dose, it falls with age, and every volatile buys its depth partly by dropping the systemic vascular resistance and with it the blood pressure.
Questions I'll ask you in the room
What does one MAC actually mean, and in what fraction of patients?
This patient is 80 rather than 30 - does the number on the dial change?
Where is the blood pressure going as you deepen, and through which mechanism?
What would nitrous oxide add here, and in which patient would you leave it off?
Oral boards stem
You are maintaining a healthy 30-year-old for a knee arthroscopy on sevoflurane in oxygen and air. Your attending reaches over, taps the vaporiser dial, and asks what one MAC actually means, what it means for this particular patient, and what would change if you added 60% nitrous oxide or swapped the sevoflurane for isoflurane.
The bottom lineMAC is the currency of every volatile - it is the dose, it falls with age, and every volatile buys its depth partly by dropping the systemic vascular resistance and with it the blood pressure.
Sources
[1] Nakajima et al., British Journal of Anaesthesia 1993 · PMID 8471368 open
[2] Grundmann et al., Acta Anaesthesiologica Scandinavica 1996 · PMID 8933850 open