ANESTHETIC DEPTH DEFINED USING MULTIPLE NOXIOUS STIMULI DURING ISOFLURANE OXYGEN ANESTHESIA .1. MOTOR REACTIONS/

Background: Potency of Inhaled anesthetics usually is defined by deter mining the minimal alveolar concentration (MAC) that prevents movement in 50% of patients in response to skin incision. Skin incision, howev er, is usually only a single event and, thus, determination of potency cannot be repeated in one patient. Traditional MAC(skin incision) can not be used to predict response to other noxious stimuli. The aim of t his study was to investigate the effects of other noxious stimulation patterns and then compare these to MAC(skin incision) measuring the en d-tidal isoflurane concentrations with the corresponding arterial conc entrations. Methods: In 26 patients, the end-tidal and corresponding a rterial isoflurane concentrations needed to suppress eye opening to ve rbal command and motor response after trapezius squeeze, 50 Hz electri c tetanic stimulation laryngoscopy, skin incision, and tracheal intuba tion in 50% of all patients were determined. Results: The end-tidal (e quivalent arterial) isoflurane concentrations (mean +/- SE, adjusted t o 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. Tetan ic stimulation and, to some extent, trapezius squeeze are reproducible and noninvasive stimulation patterns that can be used as an alternati ve to skin incision when evaluating potency of an anesthetic agent. In contrast to skin incision, they can be repeated.