Efficacy of A-lineTM AEP Monitor as a tool for predicting acceptable tracheal intubation conditions during sevoflurane anaesthesia

S. Alpiger1,*, H. S. Helbo-Hansen2, W. Vach3 and H. Ording1

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.


    Abstract
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Background. It is essential for the clinical anaesthetist to know whether patients are sufficiently anaesthetized to tolerate direct laryngoscopy and endotracheal intubation. Because of the lack of an accurate objective method to determine the level of general anaesthesia, under- or overdosing of anaesthetics may occur. Auditory evoked potential (AEP) is one of several physiological parameters under investigation. We aimed to determine the clinically required depth of anaesthesia, measured by the A-lineTM AEP Monitor and expressed as A-Line ARX IndexTM (AAI) for 90% probability of acceptable conditions for endotracheal intubation.

Methods. We studied 108 patients anaesthetized by mask with increasing concentration of sevoflurane in 30% oxygen and 70% nitrous oxide. Fentanyl 1.5 µg kg–1 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 72–99%]) of patients. The prediction probability value PK of AAI was 0.69 (CI 0.59–0.79) and the PK of end-expiratory sevoflurane concentration was 0.93 (CI 0.87–0.99). ED90 (the AAI with a 90% probability of acceptable intubation conditions) was calculated as 8.5 (CI 0–17.5).

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


    Introduction
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
Clinical signs to assess depth of anaesthesia are unreliable during periods with little noxious stimulation. A patient may appear adequately anaesthetized at a given level of stimulation, but later, when facing other more intense stimuli e.g. at endotracheal intubation, may show signs of insufficient anaesthesia. Current anaesthesia depth monitors are accurate with regard to assessment of the hypnotic component of general anaesthesia, but are less reliable when it comes to assessment of the analgesia and reflex suppression components of anaesthesia.1

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 0–100. 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.


    Materials and methods
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
We studied 108 ASA I or II patients, aged 19–86 yr, undergoing routine minor elective surgery at Vejle Hospital. Patients with gastro-oesophageal reflux or with neurological, mental or severe otological disorders were not included. All participants gave written informed consent to this double-blind study, which was approved by the regional ethics committee.

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 Ag–AgCl 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 25–65 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{Omega}, 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 kg–1 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 min–1. 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 35–45 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 7–8 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 5–10 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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Table 1 Intubation score.

 
Statistical analysis
Continuous variables are represented by mean (SD) or by median (range). We computed the rate of patients with acceptable insertion conditions within each AAI and sevoflurane class together with a 95% confidence interval (CI). We further fitted a dose–response curve using logistic regression. The effects of age and sex on the acceptability of intubation conditions were assessed by a logistic regression model with age, sex and the measured AAI value as covariates. The joint effect of FE'sevoflurane and AAI as predictors for the acceptability of insertion conditions was also investigated in a logistic regression model with these two covariates using Stata 8.0 (Stata Corporation, College Station, Texas, USA) for logistic regression analysis.

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’.


    Results
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
We recruited 108 patients to the study. Three patients were withdrawn because of failed intubation. Despite total neuromuscular relaxation with suxamethonium 100 mg i.v., their tracheas could not be intubated without using extra equipment. One patient received ephedrine because of low blood pressure, and two patients were withdrawn because the assigned AAI was not reached within 20 min. In the first patient assigned to AAI 10, FE'sevoflurane was 6.1% and AAI was 17 at 20 min, while in the second patient assigned to AAI 15, FE'sevoflurane was 6.1% and AAI was 20 at 20 min. The AAI variability in these two patients was pronounced, and at the same time we noted that the EMG values were high. One patient was excluded because the AEP monitor displayed artifacts for several minutes. A total of 101 patients completed the study as allocated. Most of the patients allocated to AAI 10, 15 and 20, needed assisted ventilation before intubation. There were no significant differences in demographic characteristics among the groups (Table 2).


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Table 2 Patient characteristics for the different groups of AAI.

 
Intubation conditions improved with decreasing AAI (Fig. 1). They were acceptable in 91% (CI 72–99%) of the patients at AAI 10, which was the only level with an incidence of acceptable intubation conditions >90%. ED50, ED90 and ED95 were calculated as AAI 28.4 (CI 21.8–35), AAI 8.5 (CI 0–17.5) and AAI 1.7 (CI 0–14.9), respectively. It should be noted that the AAI ED95 estimate is based on extrapolation of the data and consequently should be interpreted with caution as the sevoflurane–AAI response curve is not linear. The reasons for assessing insertion conditions as unacceptable are presented in Table 3. Five patients opened their eyes or responded verbally when the alarm sounded. Insertion conditions were judged unacceptable in these cases. None of the patients reported any recall of laryngoscopy or intubation when interviewed after the operation on discharge from the recovery ward.



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Fig 1 Fitted logistic dose–response curve of the probability of acceptable tracheal intubation conditions as a function of AAI. The open symbols show ED50, ED90 and ED95 (horizontal bars, 95% CI), and the closed symbols show the rate (vertical bars, 95% CI) of acceptable intubation conditions.

 

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Table 3 Number of patients with a poor score in each of the eight categories.

 
Intubation conditions also improved with increasing FE'sevoflurane (Fig. 2). ED50, ED90 and ED95 were calculated as FE'sevoflurane 3.75 (CI 3.27–4.22), FE'sevoflurane 5.14 (CI 4.58–5.7) and FE'sevoflurane 5.61 (CI 4.91–6.31), respectively.



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Fig 2 Probability of acceptable tracheal intubation conditions is related to end-expiratory sevoflurane concentration in a dose-dependent manner (success rate [95% CI]). Data according to concentrations between 0.4 and 1.9 are collected and displayed under an end-expiratory sevoflurane concentration of 1%, data according to concentrations between 2 and 2.9 are collected and displayed under an end-expiratory sevoflurane concentration of 2.5%, etc. The open symbols show ED50, ED90 and ED95 (horizontal bars, 95% CI), and the closed symbols show the rate (vertical bars, 95% CI) of acceptable intubation conditions.

 
The PK values were 0.69 (CI 0.59–0.79) for AAI and0.93 (CI 0.87–0.99) for FE'sevoflurane. Both PK values are statistically different from 0.5. Considering the joint effect of the two variables, AAI did not make a significant contribution to the prediction. Also, age and sex had no effect on the intubation conditions. FE'sevoflurane, end-expiratory carbon dioxide pressure, end-expiratory nitrous oxide pressure and induction time among patients with acceptable and unacceptable intubation conditions are given in Table 4.


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Table 4 End-expiratory sevoflurane concentration, end-expiratory carbon dioxide and nitrous oxide pressures, and induction times for acceptable and unacceptable intubation conditions.

 

    Discussion
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
The present study shows that the A-Line ARX IndexTM can indicate the level of depth of anaesthesia necessary for intubation. AAI 10 predicts a 90% probability of acceptable intubation conditions. End-expiratory sevoflurane concentration predicts intubation conditions better than AAI.

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 dose–response 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 kg–1 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 kg–1. The success rate was 92% with FE'sevoflurane=3.2% (CI 2.9–3.4). 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 kg–1 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.


    Acknowledgments
 
This work is a part of a PhD study and was supported by the Foundation for Health Research, Vejle County, the Board of Science and Development, Vejle Hospital, Abbott Laboratories A/S, Dameca A/S, the Holger and Ruth Hesse Foundation, the A.P. Møller and Housewife Chastine McKinney Møller Foundation, the Danish Society of Anaesthesiology and Intensive Care Medicine, the K. Rasmussen Foundation and the Lippmann Foundation, Denmark. We would like to thank them all. Special thanks are due to Mikael Regnér, nurse anaesthetist.


    References
 Top
 Abstract
 Introduction
 Materials and methods
 Results
 Discussion
 References
 
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22 Iamaroon A, Pitimana-aree S, Prechawai C, Anusit J, Somcharoen K, Chaiyaroj O. Endotracheal intubation with thiopental/succinylcholine or sevoflurane–nitrous oxide anesthesia in adults: a comparative study. Anesth Analg 2001; 92: 523–8[Abstract/Free Full Text]

23 Katoh T, Nakajima Y, Moriwaki G et al. Sevoflurane requirements for tracheal intubation with and without fentanyl. Br J Anaesth 1999; 82: 561–5[Abstract/Free Full Text]

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