Effect Of Remifentanil In Different Whole Blood Concentrations On The Minimum Alveolar Concentration Of Enflurane

Objective: The interaction between opioids and volatileanesthetics is complex.It is necessary to define this interactionand provide a rational dosing schemes when such combinationsare used for anesthesia.Remifentanil is a new,ultra-short-actingμ-receptor agonist.It is susceptible to rapid metabolism byesterases in the blood and other tissues for it's uniquestructure.Remifentanil has a rapid onset and offset of action andrapid blood-brain equilibration properties that facilitate thecontrol of the depth when anesthetizing.Currently,it'sinvestigated more and more.In this study,we aimed to determinethe effect of remifentanil on the minimum alveolarconcentration(MAC) of enflurane under general anesthesia;monitor the depth of anesthesia and predict the movement atskin incision by BIS values;and observe the accuracy andstability of the system of TCI-â… when used for infusingremifentanil.Methods: we studied 70 ASAâ… -â…¡patients undergoingelective abdominal surgery,aged 45-65yr and body massindex(BMI)<30% of both sexes.They were randomly assignedto one of seven groups(each group has ten patients).Thefollowing patients were excluded:(1)those in whom aninhalational induction or opioid drugs were contraindicated;(2)those having any significant cardiovascular,respiratory,renal,orhepatic disease;(3)those receiving medications known to affectMAC within a week,or having a history of alcohol or drugsabuse.On arrival of the patient at the operating room,Atropine0.5mg was administered intravenously.Patient monitoringconsisted of electrocardiogram,noninvasive blood pressure,pulseoximetry,capnography,bispectral index and end-tidal enfluraneconcentration. Remifentanil was administered via TCI device toachieve a predetermined blood concentration of : 0.0,1.0,1.5,2.0 ,4.0 ,8.0 ,16.0ng.ml-1,and the corresponding expiredconcentration of enflurane in each group was 1.6%,1.0%,0.8%,0.6%,0.5%,0.3%,0.1%.Enflurane in increasing inspiredconcentration was then administered via a face mask.Fiveminutes after infusion,2.0mg.kg-1 propofol and 1.5 mg.kg-1succinylcholine was administered.Tracheal intubation wasperformed.Oxygen flow rate was adjusted to 1.0L/min and theinspired enflurane concentration was adjusted to maintain thetargeted end-tidal enflurane concentration for at least 15 minutesbefore skin incision.End-tidal carbon dioxide was maintainedbetween 30-35 mmHg.The first patient in each treatment groupreceived a targeted end-tidal concentration of enflurane thoughtto reflect MAC.If the first patient moved at skin incision,thenext patient in that treatment group received a 0.1% increase inthe targeted end-tidal enflurane concentration.If the firstpatient didn't moved at skin incision,the next patient in thattreatment group received a 0.1% decrease in the targetedend-tidal enflurane concentration.Before skin incision,the returnof neuromuscular function was checked using a peripheral nervestimulator(T₄/T₁>0.75).A radial artery cannula was insertedand the 5ml arterial blood samples for remifentanil whole bloodconcentrations were taken 10 minutes after infusion,at skinincision and two minutes after skin incision respectively. Foreach blood sample,5ml arterial blood samples were collectedinto a vacutainer with sodium heparin and then frozened at-76 ℃. The remifentanil whole blood concentrations wereanalyzed by High liquid chromato-graphy. After observation, theanesthesia was performed normally.Results:patient characteristics was similar in each group,there were no significant differences in AGE,MAP,HR,BMI(p>0.05).We failed in determining the remifentanil concentrationof samples in the 1.0ng.ml-1 and 1.5ng.ml-1 groups. The MDPE,MDAPE of TCI-â… when infusing remifentanil was -19.4%and 20.3% respectively. The measured remifentanil bloodconcentration was lower than the predetermined bloodconcentration. TCI-â… is able to maintain the measuredremifentanil whole blood concentration stable during theadministration of remifentanil. The MAC of enflurane, whencalculated from the enflurane-only group, was 1.52%,remifentanil concentration of 4-8ng.ml-1 can result in thereduction of enflurane MAC remarkably.A ceiling effect on...