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Background: An unintentional large dose of dexmedetomidine during sedation can lead to increased side effects such as bradycardia and hypotension. We investigated whether the frequency of bradycardia in patients undergoing surgery under spinal anesthesia and sedation with dexmedetomidine was effected if bispectral index (BIS) was used to monitor the sedation. Methodology: Fifty patients between 20 and 60 y of age, who underwent spinal anesthesia, were included in this study. The patients were divided into two groups (control and BIS groups). Patients in the control group were administered a loading dose of 1 μg/kg dexmedetomidine for the first 10 min without BIS monitoring, followed by a maintenance dose of 0.2 μg/kg/h. Patients in the BIS group received a loading dose of 1 μg/kg dexmedetomidine for the first 10 min. The maintenance dose in this group was administered at the discretion of the anesthesiologist to maintain the BIS score between 50 and 70. The baseline heart rate (HR), lowest HR, and the difference between baseline and the lowest HR were calculated. Results: The baseline HR was 74.4 ± 11.0 beats per minute (bpm) and 80.9 ± 16.0 bpm, in the control and BIS groups respectively (p = 0.098). The difference between the baseline and lowest HR in the control and BIS groups was 19.4 ± 7.6 bpm and 25.5 ± 8.8 bpm, respectively (p = 0.011). Low baseline HR was positively correlated with a reduced lowest HR. Conclusions: The frequency of bradycardia during sedation with dexmedetomidine decreased with BIS monitoring. Patients with a low baseline HR were more likely to develop bradycardia during sedation with dexmedetomidine. Therefore, BIS monitoring may be helpful in patients with low baseline HR. Key words: Bispectral index monitor; Dexmedetomidine; heart rate; spinal anesthesia Citation: Kim J, Kim HY, Yun M, Lee J, Kim JD, Kang D. Bispectral index monitoring during sedation with dexmedetomidine in spinal anesthesia prevents bradycardia: a randomized clinical trial. Anaesth. pain intensive care 2021;26(1):14-19 ; DOI: 10.35975/apic.v26i1.1760