Department of Anaesthesiology, Shanghai First Peoples Hospital, Shanghai 200080, China*Corresponding author
Accepted for publication: February 22, 2002
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Abstract |
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Methods. Thirty-two ASA I and II adult patients undergoing elective gynaecological surgery under low-thoracolumbar epidural block were studied. Eighteen patients received propofol (Group P: 20 mg bolus every 3 min) and 14 received midazolam (Group M: 0.5 mg bolus every 5 min) until an observers assessment of alertness/sedation (OAA/S) scale score of 1 was achieved. AAI and BIS were monitored for different OAA/S scores.
Results. AAI and BIS decreased and increased following the changes on the patients OAA/S scores and correlated with sedation significantly. During the onset phase, the coefficients of Spearmans rank correlation for AAI and BIS were respectively 0.958 and 0.898 (P<0.001) for Group P, and 0.973 and 0.945 (P<0.001) for Group M. During the recovery phase in Group P, the coefficients were respectively 0.946 and 0.702 (P<0.001). Linear regression analysis showed that both AAI and BIS were linearly related to the OAA/S scores. The coefficients of Spearmans rank correlation and linear regression for AAI were all greater than those for BIS (P<0.05).
Conclusions. Both AAI and BIS correlated well with the depth of sedation induced by propofol or midazolam under epidural block. AAI may be more valuable when monitoring depth of sedation.
Br J Anaesth 2002; 89: 2604
Keywords: anaesthesia, depth; anaesthetics i.v., propofol; brain, evoked potentials; hypnotics benzodiazepine, midazolam; monitoring, bispectral index
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Introduction |
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In this study, we analysed the changes in AAI and BIS during sedation induced by propofol or midazolam under low-thoracolumbar epidural block.
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Methods |
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The patients were divided into two groups, 18 in the propofol group (Group P) and 14 in the midazolam group (Group M). In Group P, they ranged in age from 38 to 64 yr with a mean of 47.9 (SD 7.7) yr, and in weight from 48 to 72.5 kg with a mean of 59.3 (5.3) kg, while in Group M they ranged in age from 25 to 60 yr with a mean of 43.2 (8.8) yr and in weight from 42.5 to 72 yr with a mean of 58.8 (8.9) kg.
Patients were not premedicated and were fasted for at least 8 h. After they had rested for 10 min in the operating room, AAI, BIS, arterial pressure, heart rate (HR) and SpO2 monitoring was commenced. In order to minimize artefacts, patients were asked to close their eyes.
Epidural puncture was performed through vertebral interspace L12 for surgery on the uterine appendages or hysteromyomectomy, or T12L1 and L34 for hysterectomy. A mixture of 1.6% lidocaine, 0.2% tetracaine and 1:200 000 epinephrine, was administered in increments of 35 ml every 5 min until satisfactory blockade was achieved. All patients received supplemental oxygen (34 litres min1) via nasal cannula and an experienced anaesthetist monitored respiratory and cardiovascular function and determined the need for interventions such as jaw support to maintain an adequate airway or a tight-fitting face mask to supply oxygen.
When surgery commenced on the uterus or uterine appendages, an i.v. bolus dose of propofol 20 mg every 3 min was given to patients in Group P or midazolam 0.5 mg every 5 min in Group M until an observers assessment of alertness/sedation (OAA/S) scale7 score of 1 was achieved (Table 1). The OAA/S scores were assessed by one blinded investigator. AAI and BIS values were recorded at different OAA/S scores during the onset and recovery phases.
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BIS, arterial pressure, HR and SpO2 were monitored using the HXD-1 series block-type multifunctional monitor (Huaxiang, Heilongjiang, China). BIS was continuously calculated from two bipolar electroencephalographic channels using five silversilver chloride electrodes (Meditrace, Ludlow com., Canada) applied to the scalp (Fp1F7, Fp2F8, international 1020 system, with one placed at the midline near mid-forehead as the reference). The update time was 2.5 s.
BIS and AEP machines have sophisticated artefact rejection algorithms, and the auditory clicks generate signals 100 times smaller than the remainder of the EEG, so there is no interference between the two machines that could affect the data when they are used simultaneously.
Statistical analysis of the results was performed by means of Spearmans rank correlation analysis, linear regression analysis and one-way ANOVA using SPSS software 10.0 (SPSS Inc., Chicago, IL, USA). Probability values <0.05 were considered statistically significant.
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Results |
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Group P originally consisted of 22 patients. After several bolus doses of propofol, symptoms including restlessness, excitement and hallucinations occurred in four patients, and both AAI and BIS increased to >90. They were excluded from the study. Of the other 18 patients, sedation deepened and the OAA/S scores decreased following propofol administration. The total dose of propofol administered to obtain an OAA/S score of 1 was 108.9 (45.1) mg. AAI and BIS decreased as sedation deepened, and increased progressively to the baseline as sedation lightened. AAI and BIS showed a significant correlation with the OAA/S scores. During the onset phase, the coefficients of Spearmans rank correlation were respectively 0.958 and 0.898 (P<0.001). During the recovery phase, the coefficients were 0.946 and 0.702 (P<0.001) (Table 2).
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Linear regression analysis showed that both AAI and BIS were linearly related to the OAA/S scores (Fig. 1 and Fig. 2). The coefficients of Spearmans rank correlation and linear regression for AAI all differed significantly from the corresponding ones for BIS (P<0.05).
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As shown in Figure 1 and Figure 2, some AAI and BIS values recorded at one OAA/S score were within the range of values at another score, especially the adjacent score.
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Discussion |
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A great deal of attention has recently been paid to the analysis of AEP for monitoring depth of anaesthesia. In particular, MLAEP are thought to have anatomical significance and to provide a good indication of the responsiveness of the CNS.46 Many previous studies have shown that MLAEP amplitudes and latencies changed with many anaesthetics in a dose-dependent manner, making the analysis of MLAEP a promising way for measuring depth of anaesthesia.1 2 5 6 12 13
Typically, AEP were measured and displayed as amplitudes and latencies off-line. Then a quantitative measure of changes in AEP, the AEP index (AEPindex) was proposed.14 There are two identification models to evaluate the MLAEPindex: the moving time average model (MTA model) and the autoregressive model with exogenous input (ARX model). The MTA model needs an average of 256 sweeps that lasts approximately 45 s, whereas the ARX model enables extraction within 1525 sweeps, needing only 26 s.5 6 15 Many studies have reported that the ARX-model extracted MLAEPindex changed significantly during anaesthesia induced with thiopental, propofol, isoflurane and desflurane.5 6 15 16 This ARX model has been applied to the A-line monitor to facilitate rapid extraction of the MLAEP (2080 ms) and on-line monitoring of the depth of sedation with the AAI.
In this study, AAI and BIS decreased and increased following changes in OAA/S scores and correlated well with the depth of sedation. Linear regression analysis showed that both AAI and BIS were linearly related to the OAA/S scores. However, the coefficients of Spearmans rank correlation and linear regression for AAI were all greater than the corresponding ones for BIS. AAI values differed significantly between any two OAA/S scores during both the onset and recovery phases in the two groups, while BIS values did not differ significantly between the scores 2 and 3 during the onset phase, or between any two of the scores 2, 3 and 4 during the recovery phase in Group P. These results imply that AAI is more valuable.
There was a substantial overlap of the ranges of AAI, and also of BIS, at some levels of sedation. This reveals the same problem of sensitivity and specificity for AAI and BIS that has been apparent with other methods of measuring the depth of sedation. However, since we still lack a satisfactory method of monitoring sedation, AAI and BIS can be used to provide useful trend information in the individual patient.
In summary, both AAI and BIS correlated well with the depth of sedation induced by propofol or midazolam under epidural block. AAI may be more valuable when monitoring the depth of sedation.
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References |
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