Nitrous Oxide Analgesia — FRCA Primary MCQ
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Correct answer: D — Its analgesic effect is mediated partly by endogenous opioid pathways.
Explanation lettering: E = shown as A · A = shown as B · D = shown as C · C = shown as D · B = shown as E
C is correct. Nitrous oxide is a potent analgesic but a weak anaesthetic. Its antinociceptive action involves endogenous opioid release, including within the periaqueductal grey, with activation of descending inhibitory pathways; NMDA-receptor antagonism also contributes. A is false because its blood:gas partition coefficient is approximately 0.46, not 2.3. B is false: its critical temperature is about 36.4°C, so cylinders at usual room temperature contain liquid nitrous oxide in equilibrium with vapour. D is false because it undergoes negligible biotransformation and is eliminated predominantly unchanged through the lungs. E is false because nitrous oxide enters enclosed gas spaces rapidly and can increase middle-ear pressure.
Reference: Brunt TM, van den Brink W, van Amsterdam J. Mechanisms Involved in the Neurotoxicity and Abuse Liability of Nitrous Oxide: A Narrative Review, mechanism of action section. International Journal of Molecular Sciences, 2022. https://pubmed.ncbi.nlm.nih.gov/36499072/