Department of Anesthesiology, Saitama Medical Center, Saitama Medical School, 1981, Tsujido-cho, Kamoda, Kawagoe, Saitama 3508550, Japan
*Corresponding author. E-mail:kmkodaka@cb3.so-net.ne.jp
Accepted for publication: September 6, 2003
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Abstract |
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Methods. Seventy-six elective female patients (aged 2060 yr and ASA III) were randomly assigned to one of four groups. Either a PLMA or a CLMA was inserted using either propofol target controlled infusion or sevoflurane. Both propofol and sevoflurane targets were determined with a modified Dixons up-and-down method. After equilibration between the predetermined blood and effect site concentrations, which had been held steady for more than 10 min, LMA insertion was attempted without neuromuscular block.
Results. The predicted EC50CLMA and EC50PLMA for propofol were 3.14 (0.33) and 4.32 (0.67) µg ml1. E'CLMA and E'PLMA of sevoflurane (mean (SD)) were 2.36 (0.22) and 2.82 (0.45)% (P<0.01 and 0.05, respectively).
Conclusions. The estimated concentration of propofol and the sevoflurane concentration needed to allow insertion of the ProSealTM are respectively 38 and 20% greater than those needed for insertion of the Classic LMA.
Br J Anaesth 2004; 92: 2425
Keywords: anaesthetics i.v., propofol; anaesthetics volatile, sevoflurane; equipment, ProSealTM
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Introduction |
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Patients and methods |
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The depth of anaesthesia was monitored with the bispectral index (BIS; A-1050, Version 3.4; Aspect Medical Systems, Newton, MA, USA) every 15 s. Blood pressure and heart rate were measured non-invasively every minute. Inhaled and exhaled concentrations of sevoflurane, SpO2, FIO2 and end-tidal carbon dioxide (FE'CO2) were also monitored breath by breath using a multiple monitoring system (BP-508; Nippon Colin Co, Aichi, Japan). Predetermined propofol blood and effect site concentrations were held constant for at least 10 min. For sevoflurane, predetermined end-tidal concentrations were maintained for more than 10 min before insertion of the device.
Insertion of either device (size 3), in the deflated state, was attempted by an anaesthetist who had no knowledge of agent concentration. Neuromuscular blocking drugs were not given. The CLMA was inserted without an introducer and the PLMA was inserted using an introducer stylet. Responses by the patient to LMA insertion were classified as either movement or no movement by the surgeons, who were also unaware of the anaesthetic concentration. Movement was defined as bucking or gross purposeful muscular movement within 1 min of LMA insertion. Propofol concentrations (starting with 4 µg ml1 for predicted EC50) were chosen by a modification of Dixons up-and-down method6 7 (with 0.5 µg ml1 as the step size). A single measurement was obtained from each patient. If the patient in the propofol group reacted with movement, the propofol concentration for the next patient was increased by 0.5 µg ml1; if there was no movement, it was decreased by 0.5 µg ml1. In the sevoflurane group, the starting end-tidal concentration was 2.5% and the step size of increase/ decrease was also 0.5%.
One trial for each patient was performed and a decision was made to increase or decrease the dose based on the patients response, i.e. difficulty of LMA insertion because of difficulty in opening the mouth, or if the patient gagged or coughed. If adequate ventilation was not obtained because of a leak, the mask was changed for a larger size.
Values of EC50 and E' were determined by calculating the midpoint concentration after at least seven crossover points were obtained in each subgroup. As a result, 20 and 18 patients for each anaesthetic were used to determine propofol EC50 and E' sevoflurane. These values were obtained by calculating the mean values for crossover midpoint in each group. The standard deviation (SD) of EC50 or E' was the SD of the crossover midpoint of each group.68 We compared BIS values, pulse and blood pressure 5 min before induction and 1 min before and after attempted insertion, whether the insertion was successful or not. Patients were asked after surgery if they had any recall of event, and about sore throat.
Predicted EC50 and sevoflurane E' were compared by Students t-test. Heart rate, arterial pressure and BIS data changes were compared by repeated measures ANOVA, within either the propofol or the sevoflurane group. P<0.05 was considered significant.
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Results |
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Discussion |
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We previously found that the predicted concentration of propofol for classic LMA insertion was 3.24 µg ml1 when premedication was not used, which is slightly greater than in this study9 (3.14 µg ml1). This is probably because midazolam 2 mg was given in the present study. Previous studies1012 have reported the success rate for LMA insertion with TCI target values between 6 and 8 µg ml1, with or without premedication. The greater concentrations provided good conditions for insertion of the LMA. However, in all these studies the device was inserted within a few minutes of starting the propofol infusion, so there would be limited equilibration between target and effect site. Their study designs are quite different from ours. Two studies13 14 reported the EC50 of propofol for LMA insertion using TCI infusions. Casati and colleagues13 reported the EC50 of propofol for LMA placement to be 4.3 µg ml1, but did not state the time given for equilibration, and allowed more than one attempt if necessary in each patient, by increasing the target until satisfactory insertion was achieved.
Higuchi and colleagues14 found the effective median measured propofol concentration for LMA insertion was 8.7 µg ml1. This value is 2.8 times higher than that of our study. An explanation for this large difference could be the gender, because they studied men. A second possibility is giving lidocaine, since this can reduce the reflex response to LMA insertion during propofol induction15. Thirdly, we used premedication, with midazolam 2 mg. In addition, we used a size 3 LMA for all patients, although a size 4 would have been better in several cases. Finally, actual measured values after induction of anaesthesia are often under-estimated by prediction software.16 Tanaka and colleagues17 found the end-tidal sevoflurane for insertion of the Classic LMA was 2.0% for both young and elderly patients. The value we found was 2.38%. The difference could have been caused by age or by the time from equilibration to insertion (20 min) and the number of attempts (one to three) for each patient in each study. Because brain equilibration with arterial sevoflurane is >90% complete in 48 min,18 a different effect site partial pressure is unlikely. Our findings support studies2 3 which showed that the PLMA needs more attempts and a longer time for insertion than the CLMA. The semi-rigid distal end of the drain tube at the leading edge of the ProSealTM and the use of an introducer makes the PLMA more rigid than the CLMA. The PLMA can be inserted without the stylet or with a gum elastic bougie,19 and the difference in insertion technique might explain the difference in the success rate of insertion and the estimate of EC50/E' sevoflurane that we found. In our study, anaesthetists who had used the CLMA for more than 2 yr and the PLMA in more than 50 cases took part.
The patients in our study were smaller than those reported in the study by Brimacombe and colleagues20 and half of our patients were suitable for size 3 LMA. We inserted just one size 3 for all of the patients, which could have made the EC50 lower. In six patients with a PLMA we needed to exchange the LMA for a bigger one. We did not have to change any Classic LMA devices because most of the patients in this group were breathing spontaneously.
Table 2 shows that BIS values with PLMA insertion during propofol anaesthesia were less than those for CLMA insertion. This difference was not seen with sevoflurane. Propofol decreases BIS values21 in a dose-related way, but we did not see an effect for sevoflurane despite the fact that sevoflurane concentration for PLMA insertion was also greater than for CLMA insertion. Sebel and colleagues21 studied the relative effects of propofol, isoflurane and opioids on BIS in relation to the probability of movement at induction. The BIS value of isoflurane was greater than that of propofol in relation to the probability of movement, and it is likely that a similar effect exists for sevoflurane.
We found no adverse effects during the induction of anaesthesia in the middle-aged female patients we studied. However, in older or less fit patients a greater dose of anaesthetic agent could have adverse effects, and in such patients a classical LMA would allow less anaesthetic and could avoid circulatory effects, if the other features of the PLMA were not required.
In conclusion, we found that more anaesthesia is needed for insertion of the ProSealTM laryngeal mask airway, particularly when propofol is used.
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References |
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