Monitoring level of sedation with bispectral EEG analysis: comparison between hypothermic and normothermic cardiopulmonary bypass

The level of sedation of 28 patients undergoing elective coronary artery by pass grafting with fentanyl-propofol anaesthesia was monitored with bispect ral analysis (BIS), spectral edge frequency, and band power of the electroe ncephalogram. Fourteen patients underwent hypothermic cardiopulmonary bypas s (CPB) (32 degreesC, group H), and 14 normothermic CPB (group N). The leve l of sedation was measured with Observer's Assessment of Alertness/Sedation Score and with Ramsay Sedation Score. BIS was the only EEG measurement tha t paralleled the clinical course of the patients' sedation level. Values (m edian, 95% confidence intervals (CI)) changed significantly over time in bo th groups (P <0.0001). In group H, BIS decreased from 97 (95, 99) the day b efore surgery to 48 (44, 52) after tracheal intubation, to 46 (41, 52) befo re going off CPB, to 91 (85, 97) immediately before extubation. In group N, values were 93 (91, 97) the day before surgery, 53 (47, 59) after tracheal intubation, 48 (43, 53) before going off CPB, and 90 (84, 96) before extub ation. During CPB, BIS values were significantly different between the two groups. Group H had a median of 41 (95% CI, 39, 42), and group N had a medi an of 49 (95% CI, 48, 51, P <0.0001). Peak values of all other processed EE G parameters during anaesthesia and surgery overlapped with values from the day before, when patients had no sedating medication, and these values did not correlate to the patients' course of sedation during the study. There was no explicit recall of the surgery in either group. During the phases of anaesthesia and surgery without CPB, the progression of BIS levels was com parable with previously published data for non-cardiac surgery. During norm othermic CPB, the highest BIS values were close to values representing insu fficient depth of sedation. It remains to be elucidated whether the much lo wer BIS values in the hypothermic group were solely a result of brain cooli ng or if increased serum propofol concentrations, because of slowed pharmac odynamics during hypothermia, also contributed.