Brain monitoring revisited: what is it all about?

To easily measure the depth of anaesthesia during routine surgical procedures has always been a goal in anaesthesiology. For decades, scientists have been developing indices to describe and evaluate the depth of anaesthesia. Historically, mean alveolar gas concentration (MAC) values for volatile anaesthetics have been used to target a predefined level of anaesthesia. MAC values were however not established to differentiate between the hypnotic and analgesic components of anaesthesia. Indices were therefore developed that measure the effect of hypnotics predominantly on the brain (in contrast to an effect on the spinal cord) with the vision to be able to measure the transition from consciousness to unconsciousness. Although monitors measuring the depth of anaesthesia are still not capable of measuring the transition from consciousness to unconsciousness, brain monitoring has proved to help clinicians control the depth of anaesthesia. Clinical trials have shown that the use of brain-monitoring devices can lead to a reduction of intraoperative drug consumption, reduced incidence of postoperative nausea and vomiting, facilitate recovery from anaesthesia compared to routine care and can also lead to a reduction of intraoperative awareness. However a study demonstrating both a reduced intraoperative drug consumption and at the same time a reduction of intraoperative awareness due to the use of brain-monitoring devices has not been published yet.