Anesthesia adequacy was assessed with mass-spectrometric method by monitoring the ratio of mass concentrations of end-tidal CO₂ and inhaled O₂ in every respiratory cycle during surgery. For real-time monitoring, we used a mass spectrometer with electron ionization connected to the respiratory contour of inhalation anesthesia machine. The study has demonstrated advantages of the novel method in real-time assessment of adequacy of the total intravenous anesthesia.