Clinical Pharmacodynamics Of Isoflurane Based On Auditory Evoked Potential Index During Low-flow Inhalation Anesthesia

Objective To investigate the pharmacodynamics of isofluraneduring low-flow inhalation anesthesia and analysethe relevance between theauditory evoked potential index and end-tidal isoflurane concentration, also to evaluate the feasibility of AAI in monitoring the depth of low-flow isoflurane anesthesia. Methods Thirty ASA Ⅰ～Ⅱ patients aged18～50yr scheduled for operation performed under general anesthesia were studied. The patients were not premedicated.General anesthesia was induced with midazolam0.5mg.kg-1, fentanyl3ug.kg-1, vecuronium0.1mg.kg-1and propofol1mg.kg-1and matained with isofluraneinhalation and intermittent iv boluses of vecuronium. Isoflurane anesthesia was started with high-flow for12min followed by low-flow. During high-flow wash-in,FGF was3L/min,VT was set at8-10ml/kg, PETCO2was maintained at4.66-5.33KPa(35-40mmHg) and isofluranevaporizer was set at3%.During low-flowinhaltion FGFwas set at0.5L/min.MAP,HR,End-tital isoflurane concentration(ETI) and AAI were recorded at1，3,5，7,9and12min during high-flow wash-in and at0.8,1.0and1.3MAC during low-flow period and30minutes withdraw period. Results AAIdescended gradually with the increase of end-tidal isoflurane concentrationduring high-flow wash-in period.The differences were statistically different with each other (P<0.05).The correlation coefficients between AAI and ETI,MAP and HR were0.862、0.345and0.398respectively.During low-flow inhalation, AAI at different ETI were significantly different (P<0.05),and the correlation coefficients between AAI and ETI,MAP and HR were0.854、0.454、0.395respectively，Emax model accurately describes the relationship between Isofluraneand AAI. The ke0was0.276±0.116min-1，EC50was1.183±0.216%v/v，andγwas4.268±1.675。Conclusions AAIwas significantly correlated with the end-tidal isoflurane concentration during low-flow isoflurane anesthesia,and it was the feasible parameters to monitor the depth of anesthesia with low-flow isoflurane.But there was no correlation between AAI and MAP,HR, so it can not represent the stress level.