Interventions: The patients were allocated into 2 groups, group M (middle aged, <65 years) and group H (elderly, >65 years). Midazolam was administered at a bolus dose of 0.1 mg・kg -1 , followed by a continuous dose of 0.04 mg・kg -1 ・hr -1 , which was adjusted every 2 hrs to achieve a target level of sedation at 3-6 on Ramsay sedation scale (RSS), and buprenorphine was administered at a constant rate (0.625 μg・kg -1 ・hr -1 ).Measurements: BIS value, RSS, the dose of midazolam, body temperature, heart rate, the dose of dopamine, and mean arterial pressure were recorded every 2 hrs by an independent nurse. Data were analyzed using Spearman's rank correlation and Mann-Whitney U test.Main Results: The BIS values decreased depending on the depth of sedation, and there were significant correlation between RSS and BIS in both groups. The BIS values at levels of RSS 5 and 6 were significantly lower in group H than in group M.Conclusion: BIS correlated with sedation depth, and BIS in older patients were lower than that in middle-aged patients at a deep sedation depth.