How low can we go?

Intraoperative awareness is a terrifying experience, which patients fear and anaesthetists are anxious to avoid; any clinician who doubts this should read a first hand account.1 Since the early 1990s, the Bispectral Index (BIS) has been developed as a measure of hypnosis during anaesthesia and sedation. As BIS is a linearized and monotonic measure, clinicians have become comfortable with the idea that an increase in BIS may warn of actual or impending awareness. Although there are currently no substantial data proving that generalized use of the monitor would reduce the rate of awareness, the device has become popular, especially in the USA.
In this issue of the British Journal of Anaesthesia, Hoymork and colleagues2 report using BIS to guide the administration of i.v. anaesthesia. They found that measured plasma concentrations of propofol were poorly correlated with BIS and also observed wide discrepancies between predicted and measured concentrations of both propofol and remifentanil in some patients on some occasions.