Effect of Surgical Stimuli on Desflurane-induced Electroencephalographic Changes.Röpcke et al.(page 390)

Röpcke et al.  recruited 24 female patients scheduled to undergo abdominal surgery to investigate the effect of surgical stimuli on the concentration–response relation of desflurane-induced electroencephalographic changes. Oral midazolam, 7.5 mg, was administered to all patients 2 h before surgery, and anesthesia was induced with 2 mg/kg propofol. A 45-min waiting period was allowed for dissipation of the effects of the induction dose of propofol; during this time, 1.0 MAC desflurane was administered to the patients.

The team used 12 patients as controls to determine the desflurane–electroencephalographic effect relation without noxious stimulation. With the desflurane concentration administered during the waiting period serving as a starting point, half the patients were assigned randomly to administration of a decreased desflurane concentration initially, whereas the other half were assigned to administration of an increased amount. Desflurane was increased or decreased in incremental steps, and concentrations did not exceed 1.6 MAC to avoid a high percentage of burst suppressions. Data were collected for at least 30 min.

In the other 12 patients, desflurane vaporizer settings were increased or decreased after opening of the peritoneum. Decreasing the desflurane vaporizer settings was terminated if an end-tidal desflurane concentration of 0.5 MAC was achieved or if the attending anesthesiologist deemed the depth of anesthesia inadequate. Raw signals from electroencephalographic monitors were filtered and digitized. The median power frequency (the frequency below which 50% of the power lies) and spectral edge frequency 95 (the frequency below which 95% of the power lies) were calculated for each epoch. The Bispectral Index was computed from the bilateral frontal channels on a second electroencephalographic monitor. Desflurane effect-site concentrations and the concentration–effect curves for spectral edge frequency 95, median power frequency, and Bispectral Index were determined by simultaneous pharmacokinetic and pharmacodynamic modeling.

The researchers found that surgical stimulation caused a shift in desflurane concentration–electroencephalographic effect curves for spectral edge frequency 95, median power frequency, and Bispectral Index toward higher desflurane concentrations. In the unstimulated group of patients, 2.2 ± 0.74 vol% desflurane was necessary to achieve a Bispectral Index of 50, whereas in the group monitored during surgery, 6.8 ± 0.98 vol% was required. Although all study participants underwent the same type of procedure (gynecologic laparotomy), the researchers concede that the level of noxious stimulation might not have remained constant throughout the data collection period. However, their results showed that surgical stimulation affects the cortical electrical activity measured by univariate electroencephalographic parameters.