Nitrous Oxide and Xenon

Mechanism

Mechanism of action

Unlike halogenated hydrocarbons that predominantly modulate GABA_A receptors, nitrous oxide (N₂O) and xenon (Xe) exert their primary anesthetic effect by blocking the NMDA glutamate receptor. This inhibition prevents Ca²⁺ flux induced by neuronal depolarization, limiting synaptic plasticity and selectively suppressing spinal and supraspinal nociceptive transmission. Likewise, they activate two-pore potassium channels coupled to neuronal metabotropic receptors.

Second Gas Effect and Diffusion of Nitrous Oxide

Nitrous oxide is administered in high volumetric concentrations (50% - 70%). Due to its high diffusibility, it is rapidly absorbed from the alveolus into the pulmonary capillary blood. This massive movement of gas generates a reactive concentration of the remaining gases in the alveolus (such as oxygen or a co-administered halogenate), accelerating the alveolar uptake of the latter. This biophysical phenomenon is known as the second gas effect.

Security

Contraindications of the Class

• Nitrous Oxide: Suspected or confirmed pneumothorax, mechanical intestinal obstruction, active air embolism, severe pulmonary hypertension (causes reactive pulmonary arterial vasoconstriction), first trimester of pregnancy (potential teratogenesis due to inhibition of folate synthesis).
• Xenon: No specific absolute contraindications have been described apart from those derived from the controlled mechanical ventilation technique.

Clinical

Clinical Differentiation: Nitrous Oxide vs. Xenon

🎈 Nitrous Oxide (N₂O)

• MAC: 104% (cannot be used as a sole anesthetic agent at normal atmospheric pressure).
• Solubility: Blood/gas coefficient of 0.47.
• Inactivates methionine synthase by irreversible oxidation of the cobalt of vitamin B₁₂, altering DNA synthesis.
• It diffuses into closed cavities containing air at a speed greater than that of nitrogen, expanding its volume (pneumothorax, intestinal obstruction, intraocular gas bubbles after vitrectomy).

💎 Xenon (Xe)

• MAC: 63% to 71% (sufficient power for general surgical anesthesia).
• Solubility: Extremely low blood/gas ratio of 0.115 (ultra-rapid induction and awakening).
• Hemodynamic Profile: Exceptional hemodynamic stability without cardiac depression or peripheral vasodilation.
• It has marked cellular neuroprotection properties by attenuating glutamate excitotoxicity. High cost of extraction and distribution.