Anesth 2001; 94:390-9

Reviewed by: R. Prasad, MD

Summary:

• Surgical stimulation shifted des concentration-EEG effect curves for SEF95, MPF, BIS toward higher des concentations.
• To achieve BIS 50, needed desflurane concentation of
• 2.2 +/- 0.74% in unstimulated pts
• 6.8 +/- 0.98% in pts during surgery

• Of course! Everyone knows to increase anesthetic levels in anticipation of stimulation!
• Nevertheless, interesting study because
• no study (apparently) has shown this in any systematic way
• nice confirmation that in unstimulated pts (e.g., those with blocks), you really DON'T need all that much anesthetic
• You do, of course, have to believe that a BIS of 50 is OK. There is plenty of data saying that it is, and no one here (small group) had recall.

• 24 ASA I-II females for gyn surgery.
• Premed 7.5 midaz po. GA induced with propofol/vecuronium, then on desflurane alone (no nitrous or opioids)
• 45min waiting period to allow propofol effects to go away before beginning study
• 12 pts had data collected before start of surgery, 12 after peritoneum opened.
• Data collected by increasing or decreasing des levels, btwn 0.5 MAC (=1.3 MACawake) and 1.6 MAC. MD free to inc levels if suspect inadequate anesthetic level, or specific criteria met (inc SBP, inc HR, autonomic signs, movement).
• EEG determinations: spectral edge frequency 95 (SEF 95; the frequency below which 95% of the power of EEG lies), median power frequency (MPF; frequency below which 50% of the power lies), and bispectral index (BIS).
• Due to hysteresis between end-tidal concentrations and EEG values (because, depending on whether des concentrations are being increased or decreased, effect site (brain) concentrations will be lower or higher, respectively), they used a lot of statistical/mathematical manipulation I don't begin to understand to model effect site concentration, and plotted that against the EEG parameters.

• Periods of burst-suppression noted in all pts without stimulation when end-tidal des >1.5 MAC; in none during surgery.
• No recall
• Developed models for each parameter, relating effect-site concentration to parameter

• For up to 8% des, SEF 95, MPF, and BIS were about 2-3 times higher during surgical stimulation ... a right-shift of the concentration-response curves
• Painful stimuli may increase level of consciousness