Although studies have noted an inverse relationship between age and anesthetic or hypnotic requirement for sevoflurane and isoflurane, the influence of patient age on bispectral index (BIS) or 95% spectral edge frequency (SEF) has not been documented. Accordingly, Katoh et al.  designed a study to determine the influence of age on hypnotic requirement, BIS and 95% SEF associated with sevoflurane-induced sedation.

The 96 patients included in the study were assigned to three separate age groups — 18 to 39, 40 to 64, and 65 to 80 — of 32 each. Scheduled for elective surgery, patients fasted for 8 hours but received no premedication. EEG parameters, including BIS and 95% SEF, were recorded continuously throughout the study, and serial output files were collected on a personal computer.

Patients breathed through a face mask connected to a semiclosed circuit, and gas was drawn continuously from the sampling port located between the face mask and the dead space. Concentrations of carbon dioxide, sevoflurane and oxygen were also measured continuously, using an infrared gas analyzer. Patients were sedated with sevoflurane; after maintaining target CO2 concentrations of between 35 and 45 mmHg for 15 minutes, depth of sedation was assessed using the OAA/S rating scale. Assessments were performed twice per patient at different sevoflurane concentrations, by an observer blinded to the sevoflurane concentration. Multiple regression analysis of the data showed that end-tidal sevoflurane concentration and age significantly affected both BIS and 95% SEF. However, the researchers also found that during sevoflurane, aging reduced MACawake, but did not change BIS and 95% SEF associated with response to a verbal comment. They conclude that in a population with a wide age range, BIS would predict depth of sedation better than end-tidal sevoflurane concentration.