End-tidal concentrations (CET) have been used to guide delivery of inhaled anesthetic drugs for many years. Effect-site concentrations (Ceff) are a frequently used guide to therapy with IV drugs and should also be of benefit with inhaled drugs, especially during periods of rapid change. For Ceff to be useful, the appropriate levels required for any given end point, and the delay between central compartment and effect, need to be defined. In this study, we explored these relationships for the effect of response to insertion of the classic laryngeal mask airway (cLMA) and compared the utility of CET and Ceff-guided cLMA insertion.METHOD:
We studied 30 ASA physical status I or II patients in whom induction with sevoflurane alone and use of the cLMA were appropriate. After oxygen administration from a circle system with a total gas flow of 6 L/min, the sevoflurane vaporizer dial was set to 6%. cLMA insertion was attempted at a predetermined Ceff calculated in real time based on measured CET. Target levels were chosen using up-and-down methodology. The initial value was 2.5 vol% with a step size of 0.2 vol%. Subjects showing a gross motor response were responders, and the target was increased for the next subject. Those without such a response were nonresponders, and the target was decreased for the next subject. Data collection continued until after 7 transitions from nonresponder to responder. For each subject, after the first transition, we calculated a Ceff time series from the measured CET time series for 11 t1/2ke0 values between 0.5 and 5.0 minutes. We combined data from 2 studies of equilibrium 50% effective concentration (EC50) for LMA insertion to derive a pooled EC50 of 2.17%. We determined graphically the t1/2ke0 that gave a mean EC50 of 2.17% in our subjects. We constructed receiver operator characteristic curves to compare the utility of CET and Ceff-guided cLMA insertion.RESULTS:
The 30 patients studied were all women, ASA physical status I or II, aged between 22 and 66 years (mean 38). Consciousness was lost after 99.2 (SD 11.1) seconds, and the target for cLMA insertion reached after 256 (57) seconds. The optimum t1/2ke0 was 2.25 minutes (95% confidence interval, 2.0–2.5 minutes). The area under the receiver operator characteristic curves was significantly different at 0.87 (SE 0.06) for Ceff and 0.63 (0.11) for CET.CONCLUSIONS:
This study confirmed that real-time calculation and display of Ceff based on measured CET values are feasible. We determined the optimum t1/2ke0 for sevoflurane for the effect of cLMA insertion as 2.25 minutes, similar to that determined for loss of consciousness using the raw electroencephalogram. We also showed that Ceff is a more reliable (P < 0.05) guide to successful cLMA insertion than CET.