The effects of nitrous oxide and ketamine on the bispectral index and 95% spectral edge frequency during propofol–fentanyl anaesthesia

Abstract

In this study, we have sought to establish whether N₂O and ketamine alter the bispectral index during propofol–fentanyl anaesthesia. Fourteen surgical patients were randomly assigned to one of two groups: the N₂O group (n=7) and the ketamine group (n=7). In both groups, anaesthesia was induced with propofol 1.5–2 mg kg⁻¹ and fentanyl 2 μg kg⁻¹ and maintained with propofol 5–7 mg kg⁻¹ hr⁻¹ to target the bispectral index between 40 and 50. After the bispectral index value had stabilized the propofol infusion rate was fixed. In the N₂O group, the following concentrations of N₂O were subsequently inhaled at 20-min intervals; 20, 40, 60 and 70%, and then N₂O was terminated. In the ketamine group, ketamine (0.4 mg kg⁻¹+1.0 mg kg⁻¹h⁻¹) was given. The bispectral index and 95% spectral edge frequency were recorded 20 min after each change in concentration of N₂O or ketamine infusion. The bispectral index and 95% spectral edge frequency did not change significantly in the N₂O group, but increased significantly from 44.1±0.7 and 16.0±0.5 to 58.6±1.4 and 19.5±0.3 (P <0.01), respectively, in the ketamine group. Additional N₂O or ketamine did not decrease the bispectral index and 95% spectral edge frequency values. The depth of sedation should be assessed carefully using a bispectral index monitor when these anaesthetic agents are used together.