• O Detsch, G Schneider, E Kochs, G Hapfelmeier, and C Werner.
• Klinik für Anaesthesiologie, Klinikum rechts der Isar, Technische Universität München, Germany.
• Br J Anaesth. 2000 Jan 1; 84 (1): 33-7.

AbstractWe have studied the effects of increases in isoflurane concentration on the EEG bispectral index (BIS) in 70 patients anaesthetized with isoflurane-nitrous oxide-sufentanil for major abdominal surgery. During surgery, baseline BIS was recorded at 0.8% end-tidal isoflurane with nitrous oxide in oxygen (FIO2 0.35). After this, end-tidal isoflurane was increased to 1.6% for 15 min and decreased subsequently to 0.8% for 20 min to assess recovery. In 20 patients, BIS decreased from a mean value of 40 (SD 9) during baseline to 25 (10) at 1.6% isoflurane. In contrast, BIS did not change in 23 patients and increased in 27 patients from 35 (6) to 46 (8) as isoflurane was increased to 1.6%. In all patients, BIS recovered to baseline values at 0.8% isoflurane. The changes in BIS with increasing isoflurane concentration were not related to drugs or differences in physiological variables, which did not differ between groups. Patients with a decrease in BIS were significantly younger (38 (range 18-68) yr) than those with unchanged (55 (26-70) yr) or increased (60 (40-70) yr) BIS values (P < 0.001). It is possible that the paradoxical increase in BIS is related to continuous pre-burst EEG patterns consisting of high-frequency activity. This suggests that the use of BIS as a guide for isoflurane administration may be misleading in some patients undergoing surgical procedures.

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