Anesthetic depth defined using multiple noxious stimuli during isoflurane/oxygen anesthesia. I. Motor reactions.

Background: Potency of inhaled anesthetics usually is defined by determining the minimal alveolar concentration (MAC) that prevents movement in 50% of patients in response to skin incision. Skin incision, however, is usually only a single event and, thus, determination of potency cannot be repeated in one patient. Traditional MACskin incision cannot be used to predict response to other noxious stimuli. The aim of this study was to investigate the effects of other noxious stimulation patterns and then compare these to MACskin incision measuring the end-tidal isoflurane concentrations with the corresponding arterial concentrations.
Methods: In 26 patients, the end-tidal and corresponding arterial isoflurane concentrations needed to suppress eye opening to verbal command and motor response after trapezius squeeze, 50 Hz electric tetanic stimulation, laryngoscopy, skin incision, and tracheal intubation in 50% of all patients were determined.
Results: The end-tidal (equivalent arterial) isoflurane concentrations (mean +/- SE, adjusted to sea level) expressed in vol% (to allow comparison) increased in the following order (mean +/- SE): vocal command 0.37 +/- 0.09 (0.36 +/- 0.09); trapezius squeeze 0.84 +/- 0.07 (0.65 +/- 0.07); laryngoscopy 1.00 +/- 0.12 (0.78 +/- 0.09); tetanic stimulation 1.03 +/- 0.09 (0.80 +/- 0.06); skin incision 1.16 +/- 0.10 (0.97 +/- 0.17); and intubation 1.76 +/- 0.13 (1.32 +/- 0.11).
Conclusions: Different stimuli require different isoflurane concentrations to suppress motor responses. Tetanic stimulation and, to some extent, trapezius squeeze are reproducible and noninvasive stimulation patterns that can be used as an alternative to skin incision when evaluating potency of an anesthetic agent. In contrast to skin incision, they can be repeated.