1 Department of Anaesthesiology, Vejle Hospital, Vejle, Denmark. 2 Department of Anaesthesiology and Intensive Care Medicine, Odense University Hospital, Odense C, Denmark. 3 Department of Statistics, University of Southern Denmark, Odense, Denmark
* Corresponding author. E-mail: alpiger{at}dadlnet.dk
Accepted for publication October 10, 2004.
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Abstract |
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Methods. We studied 108 patients anaesthetized by mask with increasing concentration of sevoflurane in 30% oxygen and 70% nitrous oxide. Fentanyl 1.5 µg kg1 and glycopyrrolate 0.2 mg were administered intravenously immediately before starting induction of anaesthesia. The monitor was programmed to give an alarm at AAI 10, 15, 20, 25 or 30 according to randomization. When the alarm sounded, the end-expiratory sevoflurane concentration was registered and endotracheal intubation was attempted. Intubation conditions were assessed by an observer blinded to the AAI.
Results. At AAI 10 we found acceptable conditions in 91% (confidence interval [CI 7299%]) of patients. The prediction probability value PK of AAI was 0.69 (CI 0.590.79) and the PK of end-expiratory sevoflurane concentration was 0.93 (CI 0.870.99). ED90 (the AAI with a 90% probability of acceptable intubation conditions) was calculated as 8.5 (CI 017.5).
Conclusions. AAI indicates the depth of anaesthesia necessary for acceptable endotracheal intubation conditions. Under the conditions of the present study, end-expiratory sevoflurane concentration was a better predictor and may turn out to be more useful in the clinical setting.
Keywords: anaesthetic techniques, inhalation ; anaesthetics, gases, nitrous oxide ; anaesthetics, volatile, sevoflurane ; evoked potentials, auditory ; intubation, endotracheal
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Introduction |
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Currently, several technologies and devices are available for determining the level of general anaesthesia. While some of them have already been introduced in clinical practice, evaluation of the efficacy of these devices is still ongoing.2
Auditory evoked potential (AEP) measurement may provide a method for estimating depth of anaesthesia.3 TheA-LineTM AEP Monitor, introduced a few years ago, is a new monitor for AEP measurement. It is much faster than earlier models, and furthermore converts a complex curve to a numerical index (A-Line ARX IndexTM [AAI]).4 The AAI range is 0100. Generally speaking, the lower the index, the deeper is the state of unconsciousness.5
It has been reported that AEP is able to predict a patient's reaction to a well-defined stimulus such as the insertion of a laryngeal mask airway6 7 and skin incision.8 Another strong stimulus, endotracheal intubation, has not yet been investigated.
The aim of the present randomized study was to evaluate AAI measured by the A-lineTM AEP Monitor as a predictor for endotracheal intubation conditions.
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Materials and methods |
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Patients were assigned to undergo tracheal intubation at one of six different AAIs by stratified randomization according to sex and age with cut-off points at 30 and 60 yr. They were randomized within each stratum in blocks of five. The first 60 patients were allocated to AAI 10, 15, 20, 25 and 30. As a result of a high incidence of unacceptable insertion conditions at AAI 25 and 30, allocation to these groups was stopped early and the remaining 48 patients were randomized to AAI 10, 15 and 20, which resulted in an over-representation of these levels. An assistant chose a sealed envelope and adjusted the alarm limit according to the group assignment, concealing the display of the AEP monitor. In that way, the investigator performing induction of anaesthesia, intubation and assessment of intubation conditions was blinded with regard to the AAI.
None of the patients received sedative premedication. Monitoring during anaesthesia included non-invasive measurement of arterial pressure every 5 min, three-lead ECG, pulse oximetry, end-expiratory sevoflurane concentration (FE'sevoflurane) and end-expiratory carbon dioxide pressure (JulianTM, Dräger Medical, Germany). Three AgAgCl disk electrodes (A-lineTM AEP Monitor Electrodes, Danmeter A/S, Odense, Denmark) were placed at mid-forehead (positive), left forehead (indifferent) and left mastoid (negative) after cleaning the skin with abrasion paste (NuprepTM, Weaver & Co., USA). The AEP signals were monitored using an A-lineTM AEP monitor (AAI version 4.1, software version 1.5, Danmeter A/S, Odense, Denmark). Binaural 1000 Hz click stimuli of duration 2 ms and intensity 65 dB (sound pressure level) were applied through headphones at a rate of 9 Hz. AEP sweeps in the window from 20 to 80 ms following the stimulus (mid-latency AEP [MLAEP]) were recorded and preprocessed by artifact rejection and 2565 Hz finite impulse response digital bandpass filtering. A system identification model (an autoregressive model with exogenous input [ARX model]),4 was used to facilitate faster extraction of the AEP than obtained with the classical moving time averaging (MTA) method. The ARX model, which has previously been applied to the extraction of both visual and auditory evoked potentials,4 911 made it possible to extract the MLAEP within 15 sweeps. Post-smoothing of the index results in a total update delay of 6 s. In comparison, 256 sweeps corresponding to a delay of approximately 45 s are required with the classical MTA method. The AEPs were converted into an index by using the ARX model and defined as the AAI.5 The AAIs were stored on a computer hard disk for further off-line analysis. The A-lineTM AEP monitor was equipped with a built-in electrode impedance meter. If the electrode impedance was >5 k, electrodes were changed and the skin was prepared again. The software was modified for the purpose of the present study by reversing the alarm function. Thus the alarm sounded when the AAI remained below the assigned value for a time period
3 s.
Before induction, fentanyl 1.5 µg kg1 and glycopyrrolate 0.2 mg were given intravenously. Anaesthesia was induced with sevoflurane in 30% oxygen and 70% nitrous oxide via a facemask, using a circle system with a carbon dioxide absorber and a total fresh gas flow of 4 litre min1. The vaporizer was initially set at 1% and successively increased by 1% every minute until either a maximum of 8% or the appointed depth of anaesthesia was reached. Ventilation was assisted if respiration became shallow, as estimated clinically and by observing the pulse oximeter and capnograph, targeting and an end-expiratory carbon dioxide pressure of 3545 mmHg. Once the alarm sounded, indicating that the required level of anaesthesia had been reached, intubation of the trachea was attempted and conditions graded according to the criteria given in Table 1. If the patient opened their eyes or responded verbally, intubation was postponed and the conditions were judged to be unacceptable. Endotracheal tube sizes 78 and a direct laryngoscopy technique with a Macintosh blade were used. After observation for 1 min, anaesthesia continued according to the routine of the department. In all eight categories in Table 1, excellent or good scores were required for grading the intubation conditions as acceptable. Ephedrine 510 mg i.v. was given if hypotension occurred. Hypotension was defined as systolic blood pressure <65 mm Hg in patients <60 yr and <75 mm Hg in patients
60 yr. Patients receiving ephedrine were withdrawn from the study.
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As suggested by Smith and colleagues,12 prediction probability (PK) values were used to compare the predictive values of the AAI and FE'sevoflurane. PK values were computed using a custom spreadsheet macro (PKMACRO).12 PK values can be used to compare the performance of indicators with different units of measurements. A PK value of 1 means that if we compare two randomly selected subjects with different measurements, an increase in the measurement always implies an increase in the outcome. A PK value of 0.5 would imply no better than chance.
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Results |
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Discussion |
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The ability of an AEP monitor to predict acceptable conditions for the insertion of laryngeal mask airway has been investigated.6 7 In the present study, investigating the intubation conditions, we expected that the addition of nitrous oxide13 and opioids14 would facilitate intubation by more intense reflex suppression.
The effect of sevoflurane on AAI has been described as non-linear.15 Schwender and colleagues16 have demonstrated that fentanyl has no effect on AEP. The effect of nitrous oxide on AEP has been studied by several authors and the general result is that there is an effect on the potential amplitude but only a small effect on latency.1719 The effect of nitrous oxide combined with sevoflurane on AAI has recently been described by Barr and colleagues.20 The addition or withdrawal of nitrous oxide during constant sevoflurane anaesthesia was found to cause only minor changes in AAI. During sevoflurane anaesthesia with AAI <30, a nitrous oxide end-expiratory concentration of 66% had no additional effect. Therefore the small but statistically significant difference in end-expiratory nitrous oxide concentrations between the acceptable and unacceptable groups (Table 4) is considered to be clinically negligible.
Five patients were awake or aroused when the alarm sounded at AAI between 20 and 40. Despite the higher stability compared with earlier versions, software version 1.5 still produced AAI with high short-term fluctuation (data not shown). Others have demonstrated a similar variability with earlier software versions.5 7 21 This suggests that software version 1.5 may also face some problems with electrical noise or myogenic potentials from the temporalis or postauricular muscles.
As FE'sevoflurane appeared to correlate more closely with LMA insertion conditions,7 we also performed a post hoc analysis of FE'sevoflurane as a predictor for intubation conditions, although this was not part of the original protocol. Great care should be taken when evaluating these data as the investigator was not blinded with regard to the FE'sevoflurane. Under the conditions of the present study, FE'sevoflurane turned out to be a better predictor than AAI. FE'sevoflurane measurements also estimate sevoflurane's effect on the spinal cord. The success of intubation is dependent on sufficient spinal-cord-related reflex suppression. However, the presence of a time lag between end-expiratory and effect-site concentrations may make it difficult to generalize from these results. In theory, if anaesthesia is induced more slowly the difference between the end-expiratory and effect-site concentrations will be less, and if it is induced more quickly the difference will be greater, shifting the doseresponse curve in Figure 2 to the left and to the right, respectively. In the case when the anaesthetic technique is not standardized with regard to how quickly anaesthesia is induced, the relationship between FE'sevoflurane and response (intubation conditions) would be expected to weaken and AAI, mirroring the effect-site concentration, might be a better predictor, but this remains to be investigated. Furthermore, exact monitoring of the end-expiratory concentrations requires a tight-fitting mask to overcome the problem of contamination of the gas sample with room air.
Several investigators have described the required end-expiratory sevoflurane concentration for intubation.2226 Some of them combined sevoflurane with nitrous oxide and opioids. Iamaroon and colleagues22 used a vital capacity induction technique (VCIT), 66% nitrous oxide and fentanyl 1.5 µg kg1 i.v. They found acceptable intubation conditions in 93.3% of the cases with FE'sevoflurane=6%, which is comparable to our results. Sivalingam and colleagues24 used VCIT, 60% nitrous oxide and alfentanyl 30 µg kg1. The success rate was 92% with FE'sevoflurane=3.2% (CI 2.93.4). The difference from our results can be explained by the use of alfentanil. Katoh and colleagues23 used an equilibration time of 10 min and fentanyl 1 or 2 µg kg1 i.v. ED50 values were 2.07% and 1.45%, respectively; ED95 values were 2.7% and 2.2%, respectively. Aantaa and colleagues25 used sevoflurane alone with an equilibration time of 10 min and found ED50=2.2% (SD 0.31) and ED95=2.62. Kimura and colleagues26 also used sevoflurane alone with an equilibration time of 20 min and found ED50=4.52% and ED95=8.07%. The overall tendency is that endotracheal intubation can be facilitated by a long equilibration time and combination with nitrous oxide and opioids.
In summary, the A-LineTM AEP Monitor (AAI version 4.1, software version 1.5) can indicate the depth of anaesthesia necessary for 90% acceptable intubation conditions during sevoflurane inhalation combined with nitrous oxide and fentanyl. Under the conditions of the present study, the prediction probability of FE'sevoflurane was higher and hence might be found to be more useful in clinical practice.
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Acknowledgments |
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References |
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2 Kalkman CJ, Drummond JC. Monitors of depth of anesthesia: quo vadis? Anesthesiology 2002; 96: 7847[CrossRef][ISI][Medline]
3 Thornton C, Sharpe RM. Evoked responses in anaesthesia. Br J Anaesth 1998; 81: 77181
4 Jensen EW, Lindholm P, Henneberg SW. Autoregressive modeling with exogenous input of middle-latency auditory-evoked potentials to measure rapid changes in depth of anesthesia. Methods Inf Med 1996; 35: 25660[ISI][Medline]
5 Struys MM, Jensen EW, Smith W et al. Performance of the ARX-derived auditory evoked potential index as an indicator of anesthetic depth: a comparison with bispectral index and hemodynamic measures during propofol administration. Anesthesiology 2002; 96: 80316[CrossRef][ISI][Medline]
6 Doi M, Gajraj RJ, Mantzaridis H, Kenny GN. Prediction of movement at laryngeal mask airway insertion: comparison of auditory evoked potential index, bispectral index, spectral edge frequency and median frequency. Br J Anaesth 1999; 82: 2037
7 Alpiger S, Helbo-Hansen HS, Vach W, Ording H. Efficacy of theA-line trade mark AEP monitor as a tool for predicting successful insertion of a laryngeal mask during sevoflurane anesthesia. Acta Anaesthesiol Scand 2004; 48: 88893[CrossRef][ISI][Medline]
8 Kurita T, Doi M, Katoh T et al. Auditory evoked potential index predicts the depth of sedation and movement in response to skin incision during sevoflurane anesthesia. Anesthesiology 2001; 95: 36470[CrossRef][ISI][Medline]
9 Liberati D, Cerutti S, Di Ponzio E, Ventimiglia V, Zaninelli L. The implementation of an autoregressive model with exogenous input in a single sweep visual evoked potential analysis. J Biomed Eng 1989; 11: 28592[ISI][Medline]
10 Cerutti S, Baselli G, Liberati D, Pavesi G. Single sweep analysis of visual evoked potentials through a model of parametric identification. Biol Cybern 1987; 56: 11120[CrossRef][ISI][Medline]
11 Cerutti S, Chiarenza G, Liberati D, Mascellani P, Pavesi G. A parametric method of identification of single-trial event-related potentials in the brain. IEEE Trans Biomed Eng 1988; 35: 70111[CrossRef][ISI][Medline]
12 Smith WD, Dutton RC, Smith NT. Measuring the performance of anesthetic depth indicators. Anesthesiology 1996; 84: 3851[CrossRef][ISI][Medline]
13 Coste C, Guignard B, Menigaux C, Chauvin M. Nitrous oxide prevents movement during orotracheal intubation without affecting BIS value. Anesth Analg 2000; 91: 1305
14 Helbo-Hansen S, Ravlo O, Trap-Andersen S. The influence of alfentanil on the intubating conditions after priming with vecuronium. Acta Anaesthesiol Scand 1988; 32: 414[ISI][Medline]
15 Alpiger S, Helbo-Hansen HS, Jensen EW. Effect of sevoflurane on the mid-latency auditory evoked potentials measured by a new fast extracting monitor. Acta Anaesthesiol Scand 2002; 46: 2526[CrossRef][ISI][Medline]
16 Schwender D, Rimkus T, Haessler R, Klasing S, Poppel E, Peter K. Effects of increasing doses of alfentanil, fentanyl and morphine on mid- latency auditory evoked potentials. Br J Anaesth 1993; 71: 6228[Abstract]
17 Newton DE, Thornton C, Creagh-Barry P, Dore CJ. Early cortical auditory evoked response in anaesthesia: comparison of the effects of nitrous oxide and isoflurane. Br J Anaesth 1989; 62: 615[Abstract]
18 Porkkala T, Jantti V, Kaukinen S, Hakkinen V. Nitrous oxide has different effects on the EEG and somatosensory evoked potentials during isoflurane anaesthesia in patients. Acta Anaesthesiol Scand 1997; 41: 497501[ISI][Medline]
19 Lam AM, Sharar SR, Mayberg TS, Eng CC. Isoflurane compared with nitrous oxide anaesthesia for intraoperative monitoring of somatosensory-evoked potentials. Can J Anaesth 1994; 41: 295300[Abstract]
20 Barr G, Anderson R, Jakobsson J. The effects of nitrous oxide on the auditory evoked potential index during sevoflurane anaesthesia. Anaesthesia 2002; 57: 7369[CrossRef][ISI][Medline]
21 Anderson RE, Barr G, Assareh H, Jakobsson J. The AAI index, the BIS index and end-tidal concentration during wash in and wash out of sevoflurane. Anaesthesia 2003; 58: 5315[CrossRef][ISI][Medline]
22 Iamaroon A, Pitimana-aree S, Prechawai C, Anusit J, Somcharoen K, Chaiyaroj O. Endotracheal intubation with thiopental/succinylcholine or sevofluranenitrous oxide anesthesia in adults: a comparative study. Anesth Analg 2001; 92: 5238
23 Katoh T, Nakajima Y, Moriwaki G et al. Sevoflurane requirements for tracheal intubation with and without fentanyl. Br J Anaesth 1999; 82: 5615
24 Sivalingam P, Kandasamy R, Dhakshinamoorthi P, Madhavan G. Tracheal intubation without muscle relaxanta technique using sevoflurane vital capacity induction and alfentanil. Anaesth Intensive Care 2001; 29: 3837[ISI][Medline]
25 Aantaa R, Takala R, Muittari P. Sevoflurane EC50 and EC95 values for laryngeal mask insertion and tracheal intubation in children.Br J Anaesth 2001; 86: 21316
26 Kimura T, Watanabe S, Asakura N, Inomata S, Okada M, Taguchi M. Determination of end-tidal sevoflurane concentration for tracheal intubation and minimum alveolar anesthetic concentration in adults. Anesth Analg 1994; 79: 37881[Abstract]
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