Comparative effects of ketamine on Bispectral Index and spectral entropy of the electroencephalogram under sevoflurane anaesthesia

Background. The Bispectral Index (BIS) and spectral entropyof the electroencephalogram can be used to assess the depthof hypnosis. Ketamine is known to increase BIS in anaesthetizedpatients and may confound that index as a guide to steer administrationof hypnotics. We compared the effects of ketamine on BIS, responseentropy (RE) and state entropy (SE) during surgery under sevofluraneanaesthesia. Methods. Twenty-two women undergoing gynaecological surgerywere enrolled in this double-blind, randomized study. Anaesthesiawas induced i.v. and maintained with sevoflurane. Under stablesurgical and anaesthetic conditions, patients were assignedto receive either a bolus of ketamine 0.5 mg kg^–1 or thesame volume of saline. Blood pressure, heart rate, BIS, RE andSE were measured every 2.5 min from 10 min before (baseline)until 15 min after ketamine or saline administration. The maximumrelative increase in BIS, RE and SE compared with baseline wascalculated for each patient. Values are mean (SD). Results. Baseline values were BIS 33 (4), RE 31 (5), SE 30 (5)for the ketamine patients and BIS 35 (3), RE 33 (5) and SE 32(6) for the patients receiving saline. BIS, RE and SE increasedsignificantly from 5 min (BIS) and 2.5 min (RE and SE) afterketamine administration, peaking at 46 (8) (BIS), 52 (12) (RE)and 50 (12) (SE) respectively. The maximum relative increasein RE 42.2 (10.4%)] and SE 41.6 (10.9)%] was higher than thatof BIS 29.4 (10.4%)]. Blood pressure, heart rate and RE–SEgradient did not change in either group. Conclusions. Ketamine administered under sevoflurane anaesthesiacauses a significant increase in BIS, RE and SE without modificationof the RE–SE gradient. This increase is paradoxical inthat it is associated with a deepening level of hypnosis.