Nitrous Oxide

Nitrous oxide (N2O; dinitrogen monoxide; CAS 10024-97-2; molecular weight 44.01) is the oldest inhalational anesthetic agent in continuous clinical use, introduced by Horace Wells in 1844 for dental analgesia and by William Morton in operative anesthesia (in combination with ether) in the 1840s. The minimum alveolar concentration (MAC) is 104 percent at one atmosphere, indicating that nitrous oxide cannot produce surgical anesthesia at atmospheric pressure when administered alone (because hypoxia precedes anesthetic depth at concentrations approaching 100 percent inspired) and is used clinically as a co-administered agent with halogenated volatiles or intravenous induction agents to achieve MAC additivity at safe inspired oxygen fractions. Mechanism differs from the halogenated ethers: the principal target is NMDA glutamate receptor antagonism (similar to ketamine and xenon), with secondary effects on opioid receptor function (modest mu-receptor agonism contributing to analgesia), GABA-A modulation, and two-pore domain potassium channel activation. Pharmacokinetics: the blood-gas partition coefficient is 0.47 (rapid uptake and elimination); recovery is fast. The principal clinical risks are pneumothorax expansion (nitrous diffuses into closed gas spaces faster than nitrogen escapes), middle ear pressure increase, hyperhomocysteinemia from oxidation of cobalamin (B12) cofactor in methionine synthase (clinically apparent with prolonged or repeated exposure), and postoperative nausea and vomiting at higher rates than with halogenated agents alone. Recreational and substance-use disorder concerns have grown with availability of food-grade nitrous oxide cartridges; chronic non-medical use produces subacute combined degeneration of the spinal cord through methionine synthase inhibition. Dental and obstetric analgesic use (Entonox, 50:50 N2O/O2 premix) remains widespread.