Nitrous Oxide Analgesia — FRCA Primary MCQ
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Correct answer: A — It produces clinically useful analgesia involving endogenous opioid pathways
Explanation lettering: E = shown as A · D = shown as B · A = shown as C · B = shown as D · C = shown as E
E is correct. Nitrous oxide provides clinically useful analgesia involving release of endogenous opioid peptides in the periaqueductal grey and activation of descending inhibitory pathways, particularly noradrenergic spinal pathways. Its blood:gas partition coefficient is approximately 0.46–0.47, so it has low rather than high blood solubility and equilibrates rapidly. At normal cylinder temperatures it is below its critical temperature and is stored in liquid–vapour equilibrium, not solely as compressed gas. It is eliminated predominantly unchanged through the lungs, with negligible metabolism. Nitrous oxide also enters gas-filled spaces faster than nitrogen can leave; it therefore expands compliant spaces or raises pressure in non-compliant spaces, including the middle ear.
Reference: Fujinaga M, Maze M. Neurobiology of nitrous oxide-induced antinociceptive effects. Molecular Neurobiology. 2002;25:167–189. https://pubmed.ncbi.nlm.nih.gov/17683915/