Changes in the bispectral index during intraabdominal irrigation in patients anesthetized with nitrous oxide and sevoflurane

Abstract

Surgical stimulation typically results in an activation of electroencephalographic activity. In some instances, painful stimulation in the presence of inadequate anesthesia results in a suppression of the electroencephalogram. This phenomenon has been referred to as a "paradoxical arousal." In our daily practice, we have noted a marked decrease in the bispectral index (BIS) with large delta waves during abdominal surgery when the abdominal cavity was irrigated with normal saline. In the present study, we sought to evaluate changes in BIS during intraabdominal irrigation. Eighteen ASA physical status I-II patients scheduled for elective abdominal surgery were enrolled in the study and allocated randomly to the control group (group C) or the fentanyl group (group F). Anesthesia was induced with 3 mg/kg of thiopental and was maintained with sevoflurane and 50% nitrous oxide. BIS, 95% spectral edge frequency (SEF95), and burst-suppression ratio were recorded using a BIS monitor. Near the end of the procedure, but before irrigation of the abdominal cavity, 1.5 microg/kg fentanyl was given IV to group F. There was no significant change in BIS or SEF95 in group F patients during subsequent irrigation of the abdominal cavity. In contrast, BIS and SEF95 decreased significantly after start of irrigation in group C patients. These data show that the stimulation occurring during intraabdominal irrigation might cause a paradoxical arousal response, as evidenced by a decrease in processed electroencephalographic parameters. Pretreatment with fentanyl suppressed these changes. Anesthesiologists should be aware of this paradoxical arousal response to avoid an inappropriate decrease in the anesthetic concentration in such situations.