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EC50 for LMA insertion using an effect site model during rapid wash-in of sevoflurane

Ross Kennedy1,2, Margaretta McKellow1, Christian Brett1, Richard French1

1Christchurch Hospital, Christchurch, New Zealand.
2University of Otago, Christchurch, Christchurch, New Zealand.

Background. The primary aim of this study was to determine the effect site EC50 for LMA insertion during rapid wash of sevoflurane using real time estimates of effect site levels. Previous slow wash-in studies have established an EC50 ("MAC-LMA") of 2.2 - 2.5vol% [1].

Methods. ASA 1 or 2 subjects. Mask induction with sevoflurane, 6 l/min fgf, 6% dial into an unprimed circuit. Real time estimation of effect site concentration using end-tidal as a measure of central compartment and an effect site equilibrium half time (t1/2ke0) of 3.2min based on studies using BIS as a measure of effect [2]. No other agents used.

LMA inserted at predetermined effect site, which for the first subject was 2.5vol%. Using the Dixon up-and-down method, if the subject responded the target level was increased by 0.2 vol% for the next subject. If no response the target level was decreased. The study continued until seven consecutive response/no response pairs occurred.

Results. 30 female subjects. Median age 39yr (range 22-66). The mean crossing point (calculated EC50) for the seven pairs was 1.6vol%. sd. 0.2vol%. Reprocessing the raw data with t1/2=2.0min gave a mean crossing point of 2.2 (sd 0.3) vol%, significantly different from the original analysis (p=0.001, CI of difference -0.9 to -0.3).

Conclusions. The appropriate effect site level for LMA insertion was reached more rapidly than suggested by BIS based studies. This may be due to differences inherent in having rapidly changing end-tidal and effect site values or reflect real differences in effect site equilibration times for the hypnotic and airway reflex suppressant effects of sevoflurane. These data also allow development of sevoflurane delivery and time based guidelines for LMA insertion.

References 1. Kodaka BJA 92:242; 2. Kennedy BJA 100:72; 3 Mourisse BJA 98:746