Inhaled Anesthetics: Sevoflurane, Isoflurane, Nitrous Oxide
CA-1 · draft
CA-1 Bootcamp day 5. Authored from cited abstracts; every claim carries a PMID.
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Before induction
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.
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.
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 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.
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 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.
Questions 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.
Sources
Nakajima et al., British Journal of Anaesthesia 1993
PMID 8471368
Grundmann et al., Acta Anaesthesiologica Scandinavica 1996
PMID 8933850
Patel et al., Drugs 1996
PMID 8706599
Goldman, Anesthesia and Analgesia 2003
PMID 12538185
Myles et al., Anesthesiology 2007
PMID 17667565
Myles et al., Lancet 2014
PMID 25142708
Leslie et al., Anesthesiology 2015
PMID 26501387
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