Surgical stimulation shifts EEG concentration-response relationship of desflurane

Background: Anesthesiologists routinely increase the delivered anesthetic c oncentration before surgical stimulation in anticipation of increased anest hetic requirement to achieve certain goals (e.g., amnesia, unconsciousness, and immobility). Electroencephalographic monitoring is one method of deter mining indirectly anesthetic effect on the brain. The present study investi gated the effect of surgical stimuli on the concentration-response relation of desflurane-induced electroencephalographic changes. Methods: The electroencephalographic activity was recorded from 24 female p atients who received only desflurane after a single induction dose of propo fol. Twelve patients served as a control group before surgical stimulation. The other 12 patients, all undergoing lower abdominal surgery, were invest igated between opening and closure of the peritoneum. Desflurane vaporizer settings were randomly increased and decreased between 0.5 and 1.6 minimum alveolar concentration as long as anesthesia was considered adequate. Spect ral edge frequency 95, median power frequency, and Bispectral Index were ca lculated Desflurane effect-site concentrations and the concentration-effect curves for spectral edge frequency 95, median power frequency, and Bispect ral Index were determined by simultaneous pharmacokinetic and pharmacodynam ic modeling. Results: Surgical stimulation shifted the desflurane concentration-electroe ncephalographic effect curves for spectral edge frequency 95, median power frequency, and Bispectral Index toward higher desflurane concentrations. In the unstimulated group, 2.2 +/- 0.74 vol% desflurane were necessary to ach ieve a Bispectral Index of 50, whereas during surgery, 6.8 +/- 0.98 vol% (m ean +/- SE) were required. Conclusions: During surgery, higher concentrations of the volatile anesthet ic are required to achieve a desired level of cortical electrical activity and, presumably, anesthesia.