BJA Advance Access originally published online on October 22, 2006
British Journal of Anaesthesia 2006 97(6):817-821; doi:10.1093/bja/ael278
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Skin conductance monitoring compared with Bispectral Index® to assess emergence from total i.v. anaesthesia using propofol and remifentanil
1 Department of Anaesthesia and Pain Medicine, Royal Perth Hospital Wellington Street Campus, Perth WA 6000, Australia.
2 School of Medicine and Pharmacology, The University of Western Australia Perth, Australia.
3 The Skills Training Centre, University of Oslo Norway
*Corresponding author. E-mail: thomas.ledowski{at}health.wa.gov.au
Accepted for publication July 5, 2006.
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Abstract |
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Background. Arousal after sevoflurane anaesthesia has been detectable by monitoring changes in skin conductance (SC) with similar accuracy as monitoring Bispectral Index (BIS®). As SC monitoring detects changes in sympathetic tone, the measurements might be confounded by the sympatholytic properties of propofol, a component of total i.v. anaesthesia (TIVA). Therefore in this study, monitoring of SC during emergence from TIVA was compared with the monitoring of BIS®.
Methods. Twenty-five patients undergoing plastic surgery were investigated. The number of fluctuations of SC per second (NFSC), BIS® and haemodynamic variables [systolic blood pressure (SBP) and heart rate (HR)] were recorded simultaneously. The performance of the monitoring devices in distinguishing between the clinical states ‘steady-state anaesthesia’, ‘first clinical reaction’ and ‘extubation’ were compared using the method of prediction probability (Pk) calculation.
Results. BIS® showed the best performance in distinguishing between ‘steady-state anaesthesia’ and ‘first reaction’ (Pk BIS® 0.99 vs NFSC 0.80; P<0.01), and ‘steady-state anaesthesia’ and ‘extubation’ (Pk BIS® 1.00 vs NFSC 0.91; P<0.05); the time from first change of BIS® or NFSC to a first clinical reaction was significantly longer for NFSC (median BIS® 135 s vs NFSC 191 s; P<0.05). BIS® and NFSC performed better in distinguishing between the investigated clinical states than SBP and HR.
Conclusions. In this study, BIS® was found to predict arousal with a higher probability but slower response times than NFSC in patients waking after TIVA.
Keywords: anaesthetics i.v., propofol; monitoring, bispectral index; sympathetic nervous system, skin conductance
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Introduction |
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Both arousal and noxious stimuli increase subcortical and cortical cerebral activity resulting in firing of sympathetic, post-ganglionic cholinergic neurons.1 2 Increased sympathetic tone then leads to sweat gland filling and this can be measured in terms of skin conductance (SC).3
Periods of perioperative stress during laparoscopic surgery have recently been shown to correlate well with both catecholamine plasma levels and SC.4 It has also been shown that neurophysiologic arousal produces characteristic SC changes.5
In a previous study we compared monitoring of the SC variable ‘number of fluctuations of SC per second (NFSC)’ with monitoring of Bispectral Index® (BIS®) during emergence after a sevoflurane-based anaesthetic. Both NFSC and BIS® were able to distinguish between different investigated clinical states with a similar accuracy.6
As the monitoring of NFSC is based on tracking changes in sympathetic tone, the sympatholytic properties of some anaesthetic drugs, such as propofol,7 may interfere with the assessment of NFSC. Propofol as a component of a total i.v. anaesthesia (TIVA) could therefore impair the ability of NFSC to detect arousal.
Hence, the aim of this study was to determine the performance of NFSC during emergence from TIVA with propofol and remifentanil, and to compare it with the monitoring of BIS® as a means of detecting arousal.
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Methods |
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After approval by the Ethics Committee of Royal Perth Hospital, 25 patients undergoing minor elective plastic surgery (hand surgical procedures such as k-wires, nerve repairs and tendon repairs) gave consent to participate in the study. None of the participants received premedication on the day of surgery. The BIS® and SC monitors were connected when the patient was on the table in a supine position.
BIS® monitoring was performed using BIS XP A 2000TM monitor (Aspect Medical Systems, USA) with BIS QUATTROTM single use electrodes (Aspect Medical Systems, USA) and a smoothing rate of 15 s. A BIS® value greater than 60 was considered an indicator of light anaesthesia.
The SC monitoring was performed using MEDSTORM AS 2005 monitor (Medstorm Innovations, Norway) with three single use Ag/AgCl paediatric ECG electrodes (NEOTRODE®, ConMed Corp., USA) attached to the palmar surface of the hand. This method has been described previously in detail.6 The software was able to define peaks and troughs to determine the amplitude of fluctuations within the mean SC, and from this count the number of fluctuations per second (NFSC) (Fig. 1). According to the study by Storm and colleagues,5 an increase in the mean SC of >0.1 µS from baseline and/or a NFSC of >0.1 s–1 in a period of 15 s can be counted as a significant change, indicating an increase in the sympathetic outflow that may indicate arousal.
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Anaesthesia was induced with propofol 2 mg kg–1 and remifentanil 0.5 µg kg–1 min–1. If required, muscle relaxation was achieved with rocuronium 0.6 mg kg–1. After placement of either a tracheal tube or a laryngeal mask, anaesthesia was maintained using propofol and remifentanil, targeting a BIS® of 45–55. If the BIS® was >55, the target-controlled infusion (TCI) of propofol was adjusted in steps of 0.5 µg ml–1. A deviation of blood pressure of >15% from baseline resulted in a decreased or increased remifentanil infusion (in steps of 0.05 µg kg–1 min–1). Ten minutes before the anticipated end of surgery, the remifentanil infusion was stopped and fentanyl was given in a dose considered appropriate for postoperative analgesia (dose range 25–150 µg). If a local block was administered by the surgeon, no fentanyl was given before extubation. Propofol was ceased at the end of surgery. Neuromuscular block was monitored using a nerve stimulator. No reversal agents were given. At the time of stopping the remifentanil infusion, a stopwatch was started and systolic blood pressure (SBP), heart rate (HR), BIS®, NFSC and a clinical score of depth of sedation, the Observer Alertness Assessment Scale (OAAS),8 were recorded every 2 min and at defined time points (‘first clinical reaction’, ‘extubation’). The times from cessation of remifentanil and propofol to the first BIS® value >60 and NFSC >0.1 s–1, and from a BIS® >60 and NFSC >0.1 s–1 to the time of first clinical reaction (defined as a first reaction of the patient indicating a light anaesthesia, such as coughing, movement, eye opening) and extubation were recorded. The patients were extubated as soon as they were considered clinically suitable and showed a minimum OAAS of 3 points.
OAAS assessment was based on the scale 5=alert; 4=lethargic response to name spoken in normal tone; 3=response only after name is called loudly or repeatedly; 2=response only after mild prodding or shaking; 1=no response to mild prodding or shaking; 0=no response to noxious stimulus (sternal rub).
The accuracy in distinguishing between the anaesthetic states of ‘steady state’ vs ‘first clinical reaction’ and ‘steady state’ vs ‘extubation’ was assessed by means of prediction probability (Pk). This method has originally been described by Smith and colleagues9 and recently published to compare the performance of BIS® and NARCOTREND® monitoring by Schmidt and colleagues.10 For the calculation of the Pk values a custom spreadsheet macro (PkMACRO) as described by Smith and colleagues9 was used. A second spreadsheet, PkDMACRO, was used to compute the t-value for a comparison between Pk values of the different monitors. The SE of the estimate was computed by the jackknife method.9 The jackknife method is a non-parametric method for estimating the distribution of a statistic within a sample population. Given a sample data set (n) and a desired statistic (e.g. the mean), the jackknife works by repeatedly computing the desired statistic with 1 data point deleted at each time (n–1). This is done for each point of the data set. It is expected that this method returns a bias-reduced estimation, especially if the distribution is wide or extreme scores are present in the data set.
A Pk value of 1 means a 100% correct prediction of a certain clinical state by a specific monitor, whereas a value of 0.5 represents only a 50:50 chance.
Correlation of NFSC and BIS® was estimated using the Spearman rank correlation coefficient (
). The probability for first reaction (OAAS>0) compared with steady-state anaesthesia was calculated for certain values of BIS® and NFSC.
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Results |
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The data collected from 25 ASA I or II patients [6 female, 19 male; mean (SD) age 38 (12) yr; BMI 28.3 (4.6)] were included in the analysis.
A laryngeal mask was used as an airway device in 23 patients and a tracheal tube in 2 patients.
Fentanyl was given in a mean dose of 37 µg (0–150 µg) at the time the remifentanil infusion [0.09 (0.05) µg kg–1 min–1] was stopped. No correlation was found between the dose of fentanyl and the investigated response times, or the NFSC at steady state, first clinical reaction and extubation.
The investigated response times ‘Cessation of remifentanil to first BIS®>60/NFSC>0.1 s–1’, ‘Cessation of remifentanil to first BIS®>60/NFSC>0.1 s–1’, ‘First BIS®>60/NFSC>0.1 s–1 to first clinical reaction’ and ‘First BIS®>60/NFSC>0.1 s–1 to extubation’ were significantly different (Table 1) with NFSC reacting faster after cessation of remifentanil and giving a longer pre-warning time from its change to a clinical first reaction.
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First changes of BIS® vs NFSC to first clinical reaction, P<0.05; #First changes of BIS® vs NFSC to extubation, P=0.030; n=25There were no significant differences between patients with a laryngeal mask or a tracheal tube. BIS® performed significantly better than NFSC regarding the differentiation between ‘steady-state anaesthesia’ and ‘first clinical reaction’ and ‘steady-state anaesthesia’ and ‘extubation’ (Table 2). Both BIS® and NFSC were able to distinguish the investigated clinical states with a higher Pk than the HR or blood pressure (Table 2). HR appeared to be the worst predictor of a first clinical reaction with a Pk of only 0.54, meaning it was no better than tossing a coin.
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We investigated the correlation between the points on the OAAS scale at the time of ‘first clinical reaction’ and ‘extubation’ and the BIS® and NFSC measurements at these times. The only statistically significant correlation (r=0.407; P=0.044) was found between BIS® and the OAAS scale at the time of extubation.
No significant correlation was found between BIS® and NFSC measurements at any of the investigated time points.
The probability of an OAAS scale value of >0 as a function of BIS® and NFSC for all 295 measurements was calculated and is shown in Figure 2.
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Discussion |
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Methods of clinical observation, such as the OASS,8 are likely to fail in assessing depth of anaesthesia in earlier states of emergence and are unlikely to prevent intraoperative awareness, as it has been shown that this can remain undetected even in unparalysed patients.11 Hence, a device for monitoring depth of anaesthesia is highly desirable.
The concept of assessing sympathetic activity by monitoring SC was introduced many years ago.12 13 More recently, Storm and colleagues5 described NFSC as a parameter of SC. This was found to be more sensitive in detecting clinical stress or arousal during surgical stimulation than the mean level of SC.5
The correlation of arousal, increased subcortical and cortical activity, and an increase of SC has been reported from a functional MRI study.2 This increase in cerebral activity may be the common denominator between SC and BIS®, as otherwise both monitors follow a completely different pathway to produce a clinical parameter.
However, a recent investigation6 comparing the performance of NFSC and BIS® showed both monitors had a similar prediction probability to distinguish between different clinical states during emergence after a sevoflurane-based anaesthetic.
In this study, monitoring of NFSC was able to predict a first clinical reaction in a patient, when emerging from steady-state TIVA, with a still reasonable, but significantly lower accuracy than BIS®.
The performance of BIS® in differentiating the clinical state ‘extubation’ from that of ‘steady-state anaesthesia’ showed a 100% accuracy (Pk 1.0), whereas that of NFSC was 91% (Pk 0.91). Interestingly, NFSC reacted faster and gave a slightly longer pre-warning time before a first clinical reaction than BIS®.
Both NFSC and BIS® were superior predictors of impending arousal to the classic haemodynamic parameters said to be associated with light anaesthesia, namely blood pressure and HR.
The value of the classic haemodynamic parameters is controversial.14–16 In this study, the previously described sympatholytic effects of propofol7 17 18 during TIVA might explain the disappointing performance of SBP and HR, with HR being no better in distinguishing a first clinical reaction from steady-state anaesthesia than tossing a coin.
As NFSC measures sympathetic outflow, the method may be impaired by using a TIVA with propofol, which has previously been described to generate a less pronounced perioperative sympathetic reaction when compared with an anaesthetic with sevoflurane.19 In addition, the drug propofol that is commonly used for TIVA has been reported to have substance-specific sympatholytic properties.7 17 18 These effects may explain the results of the current investigation. However, this remains an assumption, as we did not measure NFSC under different plasma levels of propofol.
As acetylcholine is the transmitter for sympathetic stimulation of sweat gland filling, patients in our trial received no reversal of neuromuscular block. A potential interaction of anticholinergic drugs with SC may limit the clinical use of this method whenever these substances are used. Although there are currently no data to support this, central modulators of the sympathetic nervous system, such as alpha-2-agonists, could also interfere with SC.
In our study we used only the parameter NFSC to represent SC and to compare it with BIS®. Hence, it needs to be said, that the SC parameter NFSC and its definition (>1/15 s) to indicate arousal, were used in this trial according to the experience from a few former investigations,4–6 but do not necessarily reflect the ideal SC setting for this purpose. In fact, data from our own pilot trial suggests that the area under the curve of the fluctuations over a certain period of time might be more specific for awakening stimuli.
In this study, NFSC appeared to react faster than BIS®, which would allow the anaesthetist more time to deepen an unwanted light anaesthetic. This might at least partially be explained by the fast refreshing rate of NFSC (1 s). BIS® readings in contrast, are known to represent the clinical state with a delay of approximately 30 s.20 Hence, a BIS® with a faster sampling rate may well perform better, especially with regard to the investigated response times. However, we found that BIS® performed very well in distinguishing ‘steady-state anaesthesia’ from a ‘first clinical reaction’ (Pk 0.99).
In conclusion, we found BIS® to have a higher prediction probability, but slower response times when compared with NFSC. The performance of NFSC may have been impaired by the sympatholytic properties of propofol. However, both monitors were superior to haemodynamic parameters in distinguishing clinical states during emergence from TIVA with propofol and remifentanil. Being well aware that NFSC may be confounded by factors other than arousal, its fast response time and reasonable prediction probability still suggest that it may have a clinical use. However, the advantages of combining both NFSC and BIS® monitoring may produce a better tool for depth of anaesthesia assessment.
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Acknowledgments |
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This study was funded by the Department of Anaesthesia and Pain Medicine, Royal Perth Hospital. Each monitor was a research bound loan device from the manufacturers: Device for Skin Conductance by Medstorm Innovation, Oslo, Norway; Bispectral Index® Monitor by Aspect Medical Systems Inc., Natick, USA.
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