SummaryThis study investigated the effect of intrathecal fentanyl on the dose of propofol during sedation guided by Cerebral State Index monitoring. Seventy patients were randomly assigned to receive either fentanyl 25 lg (n = 35) or normal saline (n = 35) with hyperbaric bupivacaine 12.5 mg for spinal anaesthesia. Propofol was infused to maintain a Cerebral State Index value of 65-75 for 30 min. The propofol infusion time and dose required to reach a Cerebral State Index value of 75 were recorded together with the time required to reach a Cerebral State Index value higher than 90 after cessation of sedation. The onset time for sedation was faster and the recovery time was slower in the fentanyl group compared to those in the saline group (p = 0.018 and 0.027, respectively). The propofol doses required for onset and maintenance of sedation were significantly lower in the fentanyl group compared to those in the control group (p = 0.018 and < 0.001, respectively). In conclusion, adding intrathecal fentanyl 25 lg during spinal anaesthesia significantly reduced the dose of propofol required for sedation and prolonged the subsequent recovery time. During spinal anaesthesia, conscious sedation can be provided for patient comfort. A minimal dose of intravenous sedative agents such as midazolam and propofol is often used to reduce patients' anxiety with minimal influence on haemodynamic variables. Neuraxial anaesthesia with local anaesthetics alone has been reported to reduce sedative requirements but with significant sedation achieved only with a high sensory block level, which may result in haemodynamic instability [1][2][3][4][5][6]. Intrathecal fentanyl is often combined with a local anaesthetic agent to enhance and prolong the sensory block produced by spinal anaesthesia [7,8], and a recent study reported the sedative effect of intrathecal fentanyl as monitored using Bispectral Index (BIS) [9]. The level of sedation can be monitored using the Observer's Assessment of Alertness ⁄ Sedation scale (OAA ⁄ S) and other commercially available monitors such as BIS, entropy and the Cerebral State Index (CSI), which uses the spontaneous EEG to calculate a numerical index between 100 and 0. The CSI has been reported to distinguish between graduated levels of propofol anaesthesia [1,10].The purpose of this study was to investigate the effect of intrathecal fentanyl on the dose of propofol required for sedation guided by CSI monitoring and the OAA ⁄ S scale in patients undergoing lower extremity surgery. We hypothesised that the addition of intrathecal fentanyl would decrease the propofol dose for an adequate level of sedation during spinal anaesthesia. MethodsThis study was approved by our Institutional Review Board and informed consent was obtained from all patients. Seventy patients of ASA physical status 1 or 2, undergoing lower extremity surgery, were assigned by random number in sealed envelopes to either a fentanyl group (n = 35) or the saline group (n = 35). Exclusion criteria included age kg.m )2 over 65 years,