The relationship between end-tidal isoflurane concentration and electroencephalographic bispectral index during isoflurane/epidural anesthesia

We studied the effects of increases in isoflurane concentration on the bispectral index (BIS) in 16 patients undergoing lower abdominal surgery during isoflurane/epidural anesthesia. In 8 patients, the lungs were ventilated with an air/oxygen mixture (inspired oxygen fraction 0.33) [N(-) group], and in another 8 patients, the lungs were ventilated with 66% nitrous oxide in oxygen [N(+) group]. During surgery, patients received 1.0 MAC (1.15%) end-tidal isoflurane and the BIS was recorded after 10 min of unchanged end-tidal concentration. After this, we increased the end-tidal concentration of isoflurane by 0.2 MAC to 1.8 MAC. At each concentration step, the BIS was recorded again after 10 min of unchanged end-tidal concentration. At isoflurane concentration < 1.4 MAC, the BIS did not change with increasing isoflurane concentration in both groups (BIS values = about 40). In N (-) group, the BIS decreased in all patients at isoflurane concentration > 1.6 MAC. The mean BIS values were 22 (SD 18) at 1.6 MAC and 2(4) at 1.8 MAC, respectively. In N (+) group, the BIS decreased in four patients at isoflurane concentration > 1.6 MAC, and the BIS did not decrease at 1.8 MAC in another four patients. The mean BIS values were 27 (17) at 1.6 MAC and 21(21) at 1.8 MAC. The present data suggest that BIS may not correlate with anesthetic effect of isoflurane at isoflurane concentration > 1.0 MAC.