title: Increasing isoflurane concentration may cause paradoxical increases in the EEG bispectral index in surgical patients.
abstract: We 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.
authors.affiliation: Klinik für Anaesthesiologie, Klinikum rechts der Isar, Technische Universität München, Germany.
authors.forename: O
authors.initials: O
authors.lastname: Detsch
authors.forename: G
authors.initials: G
authors.lastname: Schneider
authors.forename: E
authors.initials: E
authors.lastname: Kochs
authors.forename: G
authors.initials: G
authors.lastname: Hapfelmeier
authors.forename: C
authors.initials: C
authors.lastname: Werner
mesh_terms: D000005:Abdomen / Q000601:surgery; D000293:Adolescent; D000328:Adult; D000368:Aged; D018685:Anesthetics, Inhalation / Q000008:administration & dosage / Q000494:pharmacology*; D004305:Dose-Response Relationship, Drug; D004569:Electroencephalography / Q000187:drug effects*; D005260:Female; D006439:Hemodynamics / Q000187:drug effects; D006801:Humans; D007530:Isoflurane / Q000008:administration & dosage / Q000494:pharmacology*; D008297:Male; D008875:Middle Aged; D016343:Monitoring, Intraoperative / Q000379:methods*; D011446:Prospective Studies
chemical_list: D018685:Anesthetics, Inhalation; D007530:Isoflurane