Pitfalls and challenges when assessing the depth of hypnosis during general anaesthesia by clinical signs and electronic indices

The objective of this article was to review the present methods used for validating the depth of hypnosis. We introduce three concepts, the real depth of hypnosis (DH real ), the observed depth of hypnosis (DH obs ), and the electronic indices of depth of hypnosis (DH el‐ind ). The DH real is the real state of hypnosis that the patient has in a given moment during the general anaesthesia. The DH obs is the subjective assessment of the anaesthesiologist based on clinical signs. The DH el‐ind is any estimation of the depth of hypnosis given by an electronic device. The three entities DH real , DH obs and DH el‐ind should in the ideal situation be identical. However, this is rarely the case. The correlation between the DH obs and the DH el‐ind can be affected by a number of factors such as the stimuli used for the assessment of the level of consciousness or the administration of analgesic agents or neuro muscular blocking agents. Opioids, for example, can block the response to tactile and noxious stimuli, and even the response to verbal command could vanish, hence deeming the patient in a lower depth of hypnosis than the real patient state. The DH el‐ind can be disturbed by the presence of facial muscular activity. In conclusion, although several monitors and clinical scoring scales are available to assess the depth of hypnosis during general anaesthesia, care should be taken when interpreting their results.