Power spectral analysis of the electroencephalogram during increasing end-expiratory concentrations of isoflurane, desflurane and sevoflurane

Dose-related adaptive reconstruction of DMN in isoflurane administration: a study in the rat

BACKGROUND

The anesthetic states are accompanied by functional alterations. However, the dose-related adaptive alterations in the higher-order network under anesthesia, e. g. default mode network (DMN), are poorly revealed.

METHODS

We implanted electrodes in brain regions of the rat DMN to acquire local field potentials to investigate the perturbations produced by anesthesia. Relative power spectral density, static functional connectivity (FC), fuzzy entropy of dynamic FC, and topological features were computed from the data.

RESULTS

The results showed that adaptive reconstruction was induced by isoflurane, exhibiting reduced static and stable long-range FC, and altered topological features. These reconstruction patterns were in a dose-related fashion.

CONCLUSION

These results might impart insights into the neural network mechanisms underlying anesthesia and suggest the potential of monitoring the depth of anesthesia based on the parameters of DMN.

An Entropy-Based Prospective Randomized Controlled Trial to Evaluate the Analgesic and Hypnotic Effects of Equipotent Doses of Sevoflurane and Isoflurane in Patients Presenting for Spine Surgeries

Objectives  Minimal alveolar concentration (MAC) of anesthetic agents has been considered a suitable measure of the potency of inhalational anesthetics. Furthermore, it is assumed that equi-MAC concentrations of different anesthetic agents have a similar potency in suppressing responses to painful stimuli. Isoflurane and sevoflurane are two commonly used volatile anesthetic agents in spine surgeries. Therefore, these agents' hypnotic and analgesic potencies should be distinguished and comprehended for the optimal administration of anesthesia. Consequently, we undertook this study to compare the analgesic and hypnotic potencies between these agents at equi-MAC concentrations, using the entropy monitor.

Materials and Methods  Forty patients undergoing lumbar spine surgery were randomly assigned to two groups receiving either isoflurane ( n  = 20) or sevoflurane ( n  = 20). After induction, maintenance of anesthesia was done with age-corrected 1.0 MAC of either isoflurane or sevoflurane. A standardized noxious stimulus was provided to all the patients after achieving a steady state of 1.0 MAC. The state entropy (SE), response entropy (RE), and RE–SE were recorded at baseline, prestimulus, and poststimulus time points in both groups.

Statistical Analyses  Data are presented as frequency and percentages for categorical variables and mean ± standard deviation for continuous variables. The comparison of categorical variables between the two groups was made using the Fisher's exact test, and the Student's t -test was used for continuous variables. A p -value of < 0.05 was considered to be statistically significant.

Results  At age-corrected 1.0 MAC, there was no significant difference in the SE, RE, and RE–SE in both the groups at any time point.

Conclusion  Our study shows that during a steady state of age-corrected 1.0 MAC single-agent anesthesia, sevoflurane and isoflurane have comparable analgesic and hypnotic potencies as measured by entropy indices when a standardized nociceptive stimulus is provided.

Evaluation of Anesthetic Specific EEG Dynamics during State Transitions between Loss and Return of Responsiveness

Purpose: electroencephalographic (EEG) information is used to monitor the level of cortical depression of a patient undergoing surgical intervention under general anesthesia. The dynamic state transitions into and out of anesthetic-induced loss and return of responsiveness (LOR, ROR) present a possibility to evaluate the dynamics of the EEG induced by different substances. We evaluated changes in the EEG power spectrum during anesthesia emergence for three different anesthetic regimens. We also assessed the possible impact of these changes on processed EEG parameters such as the permutation entropy (PeEn) and the cerebral state index (CSI). Methods: we analyzed the EEG from 45 patients, equally assigned to three groups. All patients were induced with propofol and the groups differed by the maintenance anesthetic regimen, i.e., sevoflurane, isoflurane, or propofol. We evaluated the EEG and parameter dynamics during LOR and ROR. For the emergence period, we focused on possible differences in the EEG dynamics in the different groups. Results: depending on the substance, the EEG emergence patterns showed significant differences that led to a substance-specific early activation of higher frequencies as indicated by the “wake” CSI values that occurred minutes before ROR in the inhalational anesthetic groups. Conclusion: our results highlight substance-specific differences in the emergence from anesthesia that can influence the EEG-based monitoring that probably have to be considered in order to improve neuromonitoring during general anesthesia.

Desflurane and sevoflurane differentially affect activity of the subthalamic nucleus in Parkinson's disease

BACKGROUND

Desflurane and sevoflurane are commonly used during inhalational anaesthesia, but few studies have investigated their effects on deep cerebral neuronal activity. In addition, the association between subthalamic nucleus (STN) neurophysiology and general anaesthesia induced by volatile anaesthetics are not yet identified. This study aimed to identify differences in neurophysiological characteristics of the STN during comparable minimal alveolar concentration (MAC) desflurane and sevoflurane anaesthesia for deep brain stimulation (DBS) in patients with Parkinson's disease.

METHODS

Twelve patients with similar Parkinson's disease severity received desflurane (n=6) or sevoflurane (n=6) during DBS surgery. We obtained STN spike firing using microelectrode recording at 0.5-0.6 MAC and compared firing rate, power spectral density, and coherence.

RESULTS

Neuronal firing rate was lower with desflurane (47.4 [26.7] Hz) than with sevoflurane (63.9 [36.5] Hz) anaesthesia (P<0.001). Sevoflurane entrained greater gamma oscillation power than desflurane (62.9% [0.9%] vs 57.0% [1.5%], respectively; P=0.002). There was greater coherence in the theta band of the desflurane group compared with the sevoflurane group (13% vs 6%, respectively). Anaesthetic choice did not differentially influence STN mapping accuracy or the clinical outcome of DBS electrode implantation.

CONCLUSIONS

Desflurane and sevoflurane produced distinct neurophysiological profiles in humans that may be associated with their analgesic and hypnotic actions.

Use of Multiple EEG Features and Artificial Neural Network to Monitor the Depth of Anesthesia

The electroencephalogram (EEG) can reflect brain activity and contains abundant information of different anesthetic states of the brain. It has been widely used for monitoring depth of anesthesia (DoA). In this study, we propose a method that combines multiple EEG-based features with artificial neural network (ANN) to assess the DoA. Multiple EEG-based features can express the states of the brain more comprehensively during anesthesia. First, four parameters including permutation entropy, 95% spectral edge frequency, BetaRatio and SynchFastSlow were extracted from the EEG signal. Then, the four parameters were set as the inputs to an ANN which used bispectral index (BIS) as the reference output. 16 patient datasets during propofol anesthesia were used to evaluate this method. The results indicated that the accuracies of detecting each state were 86.4% (awake), 73.6% (light anesthesia), 84.4% (general anesthesia), and 14% (deep anesthesia). The correlation coefficient between BIS and the index of this method was 0.892 (p<0.001). The results showed that the proposed method could well distinguish between awake and other anesthesia states. This method is promising and feasible for a monitoring system to assess the DoA.

Gentle abdominal stroking ('belly rubbing') of pigs by a human reduces EEG total power and increases EEG frequencies

The neurobiological response to gentle touch remains poorly understood, especially in the context of human-animal interaction. A novel approach allowed recording the pig electroencephalogram (EEG) cranially epidurally and wirelessly during positive interactions with a human. Stroking of the pig's abdomen ('belly rubbing'), applied opportunistically, elicited a distinct behavioral response characterized by lateral recumbency, limb stretching, frequent short-lasting grunts and eye closure. Pigs varied in their responsiveness to belly rubbing but all pigs showed it. Their EEG was compared to EEG during human presence and other positive interactions except belly rubbing; isolation; and in the home pen as a baseline. Total EEG power ('Ptot') was lower during belly rubbing, whereas the median frequency ('F50', 5.3 ± 0.9 Hz vs. 3.8 ± 0.9 Hz for other contexts) and the 95% spectral edge frequency ('F95', 45.2 ± 3.2 Hz vs. 40.0 ± 3.2 Hz for other contexts) were higher during belly rubbing compared to other contexts. Lower EEG total power combined with a shift in spectral power distribution toward higher frequencies were linked to behavioral changes indicative of a positive welfare state during belly rubbing. The effects of belly rubbing on animal psychobiology and well-being warrant further research as a model of positive welfare state induced by touch.

The effect of sevoflurane and isoflurane anesthesia on single unit and local field potentials

Volatile general anesthetics are used commonly in adults and children, yet their mechanisms of action are complex and the changes in single unit firing and synaptic activity that underlie the broad decreases in neuronal activity induced by these drugs have not been well characterized. Capturing such changes throughout the anesthesia process is important for comparing the effects of different anesthetics and gaining a better understanding of their mechanisms of action and their impact on different brain regions. Using chronically implanted electrodes in the rabbit somatosensory cortex, we compared the effects of two common general anesthetics, isoflurane, and sevoflurane, on cortical neurons. Single unit activity and local field potentials (LFP) were recorded continuously before and during anesthetic delivery at 1 MAC, as well as during recovery. Our findings show that although isoflurane and sevoflurane belong to the same class of volatile general anesthetics, their effects upon cortical single units and LFP were quite different. Overall, the suppression of neuronal firing was greater and more uniform under sevoflurane. Moreover, the changes in LFP frequency bands suggest that effect of anesthesia upon beta oscillations does not necessarily depend on the level of single unit activity, but rather on the changes in GABA/glutamate neurotransmission induced by each drug.

The Efficacy of Intraoperative EEG to Predict the Occurrence of Emergence Agitation in the Postanesthetic Room After Sevoflurane Anesthesia in Children

PURPOSE

Emergence agitation (EA) is common after sevoflurane anesthesia, but there are no definite predictors. This study investigated whether intraoperative electroencephalography (EEG) can indicate the occurrence of EA in children.

DESIGN

A prospective predictive study design was used.

METHODS

EEG-derived parameters (spectral edge frequency 95, beta, alpha, theta, and delta power) were measured at 1.0 minimum alveolar concentration (MAC) and 0.3 MAC of end-tidal sevoflurane (EtSEVO) in 29 patients. EA was evaluated using an EA score (EAS) in the postanesthetic care unit on arrival (EAS 0) and at 15 and 30 minutes after arrival (EAS 15 and EAS 30). The correlation between EEG-derived parameters and EAS was analyzed using Spearman correlation, and receiver-operating characteristic curve analysis was used to measure the predictability.

FINDINGS

EA occurred in 11 patients. The alpha power at 1.0 MAC of EtSEVO was correlated with EAS 15 and EAS 30. The theta/alpha ratio at 0.3 MAC of EtSEVO was correlated with EAS 30. The area under the receiver-operating characteristic curve of percentage of alpha bands at 0.3 MAC of EtSEVO and the occurrence of EA was 0.672.

CONCLUSIONS

Children showing high-alpha powers and low theta powers (= low theta/alpha ratio) during emergence from sevoflurane anesthesia are at high risk of EA in the postanesthetic care unit.

Effects of halothane on the electroencephalogram of the chicken

Little is known about the effects of inhalant anaesthetics on the avian electroencephalogram (EEG). The effects of halothane on the avian EEG are of interest, as this agent has been widely used to study nociception and analgesia in mammals. The objective of this study was to characterize the effects of halothane anaesthesia on the EEG of the chicken. Twelve female Hyline Brown chickens aged 8–10 weeks were anaesthetized with halothane in oxygen. For each bird, anaesthesia was progressively increased from 1–1.5 to 2 times the Minimum Anesthetic Concentration (MAC), then progressively decreased again. At each concentration, a sample of EEG was recorded after a 10‐min stabilization period. The mean Total Power (P_TOT), Median Frequency (F50) and 95% Spectral Edge Frequency (F95) were calculated at each halothane MAC, along with the Burst Suppression Ratio (BSR). Burst suppression was rare and BSR did not differ between halothane concentrations. Increasing halothane concentration from 1 to 2 MAC resulted in a decrease in F50 and increase in P_TOT, while F95 increased when MAC was reduced from 1.5 to 1. The results indicate dose‐dependent spectral EEG changes consistent with deepening anaesthesia in response to increasing halothane MAC. As burst suppression was rare, even at 1.5 or 2 times MAC, halothane may be a suitable anaesthetic agent for use in future studies exploring EEG activity in anaesthetized birds.

Sevoflurane affects evoked electromyography monitoring in cerebral palsy

Background

To explore the effect of sevoflurane inhalation anesthesia on evoked electromyography monitoring of spinal nerve root in children associated with cerebral palsy.

Methodology

Children with cerebral palsy (n=40) were selected and further divided into 1MAC (minimum alveolar concentration) sevoflurane group and 2MAC sevoflurane group. Following the induction of anesthesia, Nicolet Endeavor-CR16 channel electrophysiological monitor was used to implement three times of successive electrical stimulation with interval of 5 sec at 3.50 mA.

Results

Our results suggested a statistical significance of amplitude retention ratio and latency in the sevoflurane inhalation time (P<0.01), with an interaction effect between the sevoflurane inhalation time and concentration for amplitude retention ratio (P<0.01), while there is no interaction effect between the sevoflurane inhalation time and concentration for latency (P>0.05). Compared to 1MAC sevoflurane group, the amplitude retention ratio of 2MAC sevoflurane group decreased remarkably (P<0.01) and the latency of 2MAC sevoflurane group extended at T3 and T4 (P<0.05 or P<0.01).

Conclusions

In evoked electromyography monitoring of spinal nerve root in children with cerebral palsy, with the increasing of concentration and duration of sevoflurane inhalation, evoked electromyogram retention ratio reduces gradually, latency extends and the retention ratio has more changes than the latency.

Improved spectrum analysis in EEG for measure of depth of anesthesia based on phase-rectified signal averaging

The definition of the depth of anesthesia (DOA) is still controversial and its measurement is not completely standardized in modern anesthesia. Power spectral analysis is an important method for feature detection in electroencephalogram (EEG) signals. Several spectral parameters derived from EEG have been proposed for measuring DOA in clinical applications. In the present paper, an improved method based on phase-rectified signal averaging (PRSA) is designed to improve the predictive accuracy of relative alpha and beta power, a frequency band power ratio, total power, median frequency (MF), spectral edge frequency 95 (SEF95), and spectral entropy for assessing anesthetic drug effects. Fifty-six patients undergoing general anesthesia in an operating theatre are studied. All EEG signals are continuously recorded from the awake state to the end of the recovery state and then filtered using multivariate empirical mode decomposition (MEMD). All parameters are evaluated using the commercial bispectral index (BIS) and expert assessment of conscious level (EACL), respectively. The ability to predict DOA is estimated using the area under the receiver-operator characteristics curve (AUC). All indicators based on the improved method can clearly discriminate the conscious state from the anesthetized state after filtration (p  <  0.05). A significantly larger mean AUC (p  <  0.05) shows that the improved method performs better than the conventional method to measure the DOA in most circumstances. Especially for raw EEG contaminated by artifacts, when the BIS index is used to indicate the consciousness level, the improvement is 7.37% (p  <  0.05), 9.04% (p  <  0.05), 18.46% (p  <  0.05), 27.73% (p  <  0.05), 14.65% (p  <  0.05), 2.52%, 5.38% and 6.24% (p  <  0.05) for relative alpha and beta power, power ratio, total power, MF, SEF, RE and SE, respectively. However, when the EACL is used to indicate the consciousness level, the improvement is 3.30% (p  <  0.05), 16.69% (p  <  0.05), 15.08% (p  <  0.05), 34.83% (p  <  0.05), 27.78% (p  <  0.05), 5.89% (p  <  0.05), 26.05% (p  <  0.05) and 23.42% (p  <  0.05). Spectral parameters derived from PRSA are more useful to measure the DOA in noisy cases.

Characterizing Awake and Anesthetized States Using a Dimensionality Reduction Method

Distinguishing between awake and anesthetized states is one of the important problems in surgery. Vital signals contain valuable information that can be used in prediction of different levels of anesthesia. Some monitors based on electroencephalogram (EEG) such as the Bispectral (BIS) index have been proposed in recent years. This study proposes a new method for characterizing between awake and anesthetized states. We validated our method by obtaining data from 25 patients during the cardiac surgery that requires cardiopulmonary bypass. At first, some linear and non-linear features are extracted from EEG signals. Then a method called "LLE"(Locally Linear Embedding) is used to map high-dimensional features in a three-dimensional output space. Finally, low dimensional data are used as an input to a quadratic discriminant analyzer (QDA). The experimental results indicate that an overall accuracy of 88.4 % can be obtained using this method for classifying the EEG signal into conscious and unconscious states for all patients. Considering the reliability of this method, we can develop a new EEG monitoring system that could assist the anesthesiologists to estimate the depth of anesthesia accurately.

Classifying multiple types of hand motions using electrocorticography during intraoperative awake craniotomy and seizure monitoring processes-case studies

In this work, some case studies were conducted to classify several kinds of hand motions from electrocorticography (ECoG) signals during intraoperative awake craniotomy & extraoperative seizure monitoring processes. Four subjects (P1, P2 with intractable epilepsy during seizure monitoring and P3, P4 with brain tumor during awake craniotomy) participated in the experiments. Subjects performed three types of hand motions (Grasp, Thumb-finger motion and Index-finger motion) contralateral to the motor cortex covered with ECoG electrodes. Two methods were used for signal processing. Method I: autoregressive (AR) model with burg method was applied to extract features, and additional waveform length (WL) feature has been considered, finally the linear discriminative analysis (LDA) was used as the classifier. Method II: stationary subspace analysis (SSA) was applied for data preprocessing, and the common spatial pattern (CSP) was used for feature extraction before LDA decoding process. Applying method I, the three-class accuracy of P1~P4 were 90.17, 96.00, 91.77, and 92.95% respectively. For method II, the three-class accuracy of P1~P4 were 72.00, 93.17, 95.22, and 90.36% respectively. This study verified the possibility of decoding multiple hand motion types during an awake craniotomy, which is the first step toward dexterous neuroprosthetic control during surgical implantation, in order to verify the optimal placement of electrodes. The accuracy during awake craniotomy was comparable to results during seizure monitoring. This study also indicated that ECoG was a promising approach for precise identification of eloquent cortex during awake craniotomy, and might form a promising BCI system that could benefit both patients and neurosurgeons.

The effects of stellate ganglion block on the electroencephalogram in rats

PURPOSE

It is reported that the sympathetic nervous system may play an important role in the arousal response. The present study evaluated the effect of stellate ganglion block (SGB) on electroencephalogram (EEG) activity in rats.

MATERIALS AND METHODS

Adult male Sprague-Dawley rats were divided into two groups: SGB (n = 10) or intramuscular (IM, n = 10) injection was performed with 0.2 ml 0.25 % bupivacaine. The spectral edge frequency 95 % (SEF 95 %), median frequency (MF), beta to theta ratio (BTR), and beta to delta ratio (BDR) were estimated 30 min before bupivacaine injection and 15, 20, 25, 30, 45, 55, and 100 min after SGB or IM injection.

RESULTS

Ipsilateral ptosis occurred in all the rats that underwent SGB but did not occur in the IM group. Significant decrease of the 95 % SEF value, MF, BTR, and BDR was observed from 15 to 45 min after SGB compared with those of the IM group, respectively (p < 0.05).

CONCLUSIONS

SGB with 0.2 ml 0.25 % bupivacaine significantly decreased EEG activities in rats. These results suggest that SGB can induce a sedative effect in rats. Further studies are required to investigate the behavioral tests for sedative effects of SGB.

Age-related changes in EEG power spectra in infants during sevoflurane wash-out

BACKGROUND

Few electroencephalography (EEG) data are available in anaesthetized infants. This study aimed to identify EEG characteristics that might warn of awakening (AW) from sevoflurane anaesthesia in infants.

METHODS

Twenty intubated infants [aged 39-77 weeks post-menstrual age (PMA)] were studied after surgery during sevoflurane wash-out. EEG was recorded at the end of surgery and throughout emergence. Changes in EEG time and frequency domains were described.

RESULTS

At the end of surgery, mean end-tidal sevoflurane concentration was 2.3% (range 1.5-3.5) before wash-out and reduced to 0.3% (0.1-0.6) when AW began. On AW, movement artifacts made signals difficult to interpret. Before awakening, most power was within frequencies ≤4 Hz, but trends over time were variable. Summated power in frequencies between 20 and 70 Hz was almost always <5 µV(2). During anaesthesia, there were two common power spectra: infants >52 weeks PMA had obvious summated power in the frequency range 5-20 Hz (P5-20 Hz) (mean 308, median 320, range 110-542 µV(2)), which decreased before awakening began [mean decrease 252 µV(2) (95% CI 153-351)], whereas younger infants had low P5-20 Hz throughout. P5-20 Hz during anaesthesia increased with age; power in this frequency band of ~100 µV(2) separated infants younger and older than 52 weeks PMA.

CONCLUSIONS

During sevoflurane wash-out, decreasing P5-20 Hz might warn of impending AW in infants >3 months old, but not in younger infants.

Brain monitoring in children

Applying scalp sensors in the operating theater, intensive care, or resuscitation scenarios to detect and monitor brain function is achievable, practical, and affordable. The modalities are complex and the output of the monitor needs careful interpretation. The monitor may have technical problems, and a single reading must be considered with caution. These monitors may have a use for monitoring trends in specific situations, but evidence does not support their widespread use. Nevertheless, research should continue to investigate their role. Future techniques and treatments may show that these monitors can monitor brain function and prevent harm.

Slow EEG pattern predicts reduced intrinsic functional connectivity in the default mode network: an inter-subject analysis

The last two decades have witnessed great progress in mapping neural networks associated with task-induced brain activation. More recently, identification of resting state networks (RSN) paved the way to investigate spontaneous task-unrelated brain activity. The cardinal features characterising RSN are low-frequency fluctuations of blood oxygenation level dependent (BOLD) signals synchronised between spatially distinct, but functionally connected brain areas. Simultaneous EEG/fMRI has been previously deployed to study the neurophysiological signature of RSN by comparing EEG power with BOLD amplitudes. We hypothesised that band-limited EEG power may be directly related to network-specific functional connectivity (FC) of BOLD signal time courses. Hence, we studied the association between individual EEG signature and FC in a core RSN, the so-called default mode network (DMN). Combined EEG/fMRI data of 20 healthy volunteers collected during a 15-minute rest period were analysed. Using an inter-subject analysis design, we demonstrated a network and frequency specific relation between RSN FC and EEG. In a multiple regression model, EEG band-powers explained 70% of DMN FC variance, with significant partial correlations of DMN FC to delta (r=-0.73) and beta (r=0.53) power. The identified EEG pattern has been previously associated with increased alertness. Conversely, an established EEG-derived sedation index (spectral edge frequency SEF95) closely correlated with DMN FC. The study presents an approach that opens a new perspective to EEG/fMRI correlation. Direct evidence was provided for a distinct neurophysiological correlate of DMN FC. This finding further validates the biological relevance of network-specific intrinsic FC and provides an initial neurophysiological basis for interpreting studies of DMN FC alterations.

Anesthetic-specific electroencephalographic patterns during emergence from sevoflurane and isoflurane in infants and children

BACKGROUND

Devices that monitor the depth of anesthesia are increasingly used to titrate sedation and avoid awareness during anesthesia. Many of these monitors are based upon electroencephalography (EEG) collected from large adult reference populations and not pediatric populations (Anesthesiology, 86, 1997, 836; Journal of Anaesthesia, 92, 2004, 393; Anesthesiology, 99, 2003, 34). We hypothesized that EEG patterns in children would be different from those previously reported in adults and that they would show anesthetic-specific characteristics.

METHODS

This prospective observational study was approved by the Institutional Review Board, and informed written consent was obtained. Patients were randomized to receive maintenance anesthesia with isoflurane or sevoflurane. EEG data collection included at least 10 min at steady-state maintenance anesthesia. The EEG was recorded continuously through emergence until after extubation. A mixed model procedure was performed on global and regional power by pooled data analysis and by analyzing each anesthetic group separately. Statistical significance was defined as P < 0.05.

RESULTS

Thirty-seven children completed the study (ages 22 days-3.6 years). Isoflurane and sevoflurane had different effects on global and regional EEG power during emergence from anesthesia, and frontal predominance patterns were significantly different between these two anesthetic agents.

CONCLUSIONS

The principal finding of the present study was that there are anesthetic-specific and concentration-dependent EEG effects in children. Depth-of-anesthesia monitors that utilize algorithms based on the EEGs of adult reference populations therefore may not be appropriate for use in children.

EEG power spectrum analysis for monitoring depth of anaesthesia during experimental surgery

The first attempts to introduce computerized power spectrum analysis of the electroencephalogram (EEG) as an intraoperative anaesthesia monitoring device started approximately 30 years ago. Since that time, the effects of various anaesthetic agents, sedative and analgesic drugs on the EEG pattern have been addressed in numerous studies in human patients and different animal species. These studies revealed dose-dependent changes in the EEG power spectrum for many intravenous and volatile anaesthetics. Moreover, EEG responses evoked by surgical stimuli during relative light levels of surgical anaesthesia have been classified as 'arousal' and 'paradoxical arousal' reaction, previously referred to as 'desynchronization' and 'synchronization', respectively. Contrasting reports on the correlation between quantitative EEG (QEEG) variables derived from power spectrum analysis (i.e. spectral edge frequency, median frequency) and simultaneously recorded clinical signs such as movement and haemodynamic responses, however, limited the routine use of intraoperative EEG monitoring. In addition, the appearance of EEG burst suppression pattern and isoelectricity at clinically relevant concentrations/doses of newer general anaesthetics (i.e. isoflurane, sevoflurane, propofol) may have weakened the dose-related EEG changes previously reported. Despite these findings, the EEG power spectrum analysis may still provide valuable information during intraoperative monitoring in the individual subject. The information obtained from EEG power spectrum analysis may be further supplemented by newer EEG indices such as bispectral index and approximate entropy or other neurophysiological monitors including auditory evoked potentials or somatosensory evoked potentials.

Comparative effects of halothane, isoflurane, sevoflurane and desflurane on the electroencephalogram of the rat

Inhalant anaesthetic agents are commonly used in studies investigating the electroencephalographic (EEG) effects of noxious stimuli in animals. Halothane causes less EEG depression than isoflurane, however, the EEG effects of halothane, isoflurane, sevoflurane and desflurane have not been compared in the same model. This study aimed to compare the EEG effects of these inhalational agents in the rat. Forty male Sprague-Dawley rats were assigned to four groups and anaesthetized with halothane, isoflurane, sevoflurane or desflurane. EEG was recorded from the left and right somatosensory cortices for 5 min at three different multiples of minimal alveolar concentration (MAC) (1.25, 1.5 and 1.75). Median, 95% spectral edge frequency and total power were derived and a single mean value for each was calculated for the first 60 s of each recording period. When the raw EEG contained burst suppression (BS), the BS ratio (BSR) over 60 s was calculated. No BS was found in EEG recorded from the halothane group at any concentration. BS was present at all concentrations with the other anaesthetic agents. BS was almost complete at all concentrations of isoflurane, whereas BSR increased significantly with increasing concentrations of sevoflurane and desflurane. No significant differences were found between the BSR due to the 1.75 MAC multiple of isoflurane, sevoflurane or desflurane. Halothane causes significantly less depression of cortical activity than the newer inhalant agents at equivalent multiples of MAC. These data support the hypothesis that halothane has a fundamentally different mechanism of action than the other inhalant agents.

Effects of averaging data series on the electroencephalographic response to noxious visceral stimulation in isoflurane-anaesthetized dogs

The effects of averaging epochs on electroencephalographic (EEG) responses to visceral stimulation has been determined in seven isoflurane-anaesthetized dogs. Quantitative EEG variables including 80% spectral edge frequency (SEF80), median frequency (MF), relative power in the delta, theta, alpha, beta band and power band ratios (theta/delta, alpha/delta, beta/delta) were recorded over 1min before stimulation and during a 1-minute stimulation period. During off-line analysis EEG variables were derived from either single 2-second EEG epochs or as an average from 5, 10, 15 and 30 consecutive 2-second epochs. Noxious stimulation resulted in significant increases in SEF80, MF, alpha power, beta power, alpha/delta ratio and beta/delta ratio. The number of variables that were significantly affected as well as the strength of changes as indicated by p-values, however, varied with the number of epochs subjected to averaging. The data suggest that stimulation-induced EEG changes may be more pronounced at lower rather than at higher averaging rates.

Classifying depth of anesthesia using EEG features, a comparison

Various EEG features have been used in depth of anesthesia (DOA) studies. The objective of this study was to find the excellent features or combination of them than can discriminate between different anesthesia states. Conducting a clinical study on 22 patients we could define 4 distinct anesthetic states: awake, moderate, general anesthesia, and isoelectric. We examined features that have been used in earlier studies using single-channel EEG signal processing method. The maximum accuracy (99.02%) achieved using approximate entropy as the feature. Some other features could well discriminate a particular state of anesthesia. We could completely classify the patterns by means of 3 features and Bayesian classifier.

Differences in need for hemodynamic support in horses anesthetized with sevoflurane as compared to isoflurane

OBJECTIVE

To study whether hemodynamic function in horses, particularly mean arterial blood pressure (MAP), is better maintained with sevoflurane than isoflurane, thus requiring less pharmacological support.

STUDY DESIGN

Prospective randomized clinical investigation. Animals Thirty-nine racehorses undergoing arthroscopy in lateral recumbency.

METHODS

Horses were assigned to receive either isoflurane (n = 20) or sevoflurane (n = 19) at 0.9-1.0 minimum alveolar concentration (MAC) for maintenance of anesthesia. Besides routine clinical monitoring, cardiac output (CO) was measured by lithium dilution. Hemodynamic support was prescribed as follows: when MAP decreased to <70 mmHg, patients were to receive infusion of 0.1% dobutamine, which was to be discontinued at MAP >85 mmHg or heart rate >60 beats minute(-1). Statistical analysis of results, given as mean +/- SD, included a clustered regression approach.

RESULTS

Average inhalant anesthetic time [91 +/- 35 (isoflurane group) versus 97 +/- 26 minutes (sevoflurane group)] and dose (in MAC multiples), volume of crystalloid solution infused, and cardiopulmonary parameters including CO were similar in the two groups, except heart rate was 8% higher in isoflurane than sevoflurane horses (p < 0.05). To maintain MAP >70 mmHg, isoflurane horses received dobutamine over a significantly longer period (55 +/- 26 versus 28 +/- 21% of total anesthetic time, p < 0.01) and at a 51% higher dose than sevoflurane horses (41 +/- 19 versus 27 +/- 23 microg kg(-1) MAC hour(-1); p = 0.058), with 14/20 isoflurane animals and only 9/19 sevoflurane horses being infused with dobutamine at >30 microg kg(-1) MAC hour(-1) (p < 0.05). Dobutamine infusion rates were consistently lower in the sevoflurane as compared to the isoflurane group, with differences reaching significance level during the 0-30 minutes (p < 0.01) and 61-90 minutes periods (p < 0.05).

CONCLUSIONS AND CLINICAL RELEVANCE

Horses under sevoflurane anesthesia may require less pharmacological support in the form of dobutamine than isoflurane-anesthetized horses. This could be due to less suppression of vasomotor tone.

Bispectral index, spectral edge frequency 95% and median frequency recorded at varying desflurane concentrations in pigs

The objective of this study is to evaluate the usefulness of bispectral index (BIS), spectral edge frequency 95% (SEF) and median frequency (MED) in relation to a simple descriptive scale (SDS) as indicators of anaesthetic depth at different desflurane concentrations in swine. Sixteen pigs were randomly allocated to four groups. Electroencephalograms (EEG) were recorded during desflurane anaesthesia, and BIS, SEF and MED were calculated from the EEG. The agent was administered in pure oxygen at 1, 1.25, 1.5 and 1.7 MAC in randomized order. Anaesthetic depth was evaluated on a SDS. BIS decreased significantly (P<0.001) at the different anaesthetic dosages used. SEF decreased significantly (P<0.001) from basal to 1 MAC of desflurane. MED decreased significantly (P<0.001) from basal to 1 MAC and from 1 to 1.75 MAC. Good correlation was seen between SDS scores and BIS values and between SDS scores and MED values. BIS appeared to be useful to predict changes in anaesthetic depth at clinically used dosages of inhalant anaesthesia.

Changes in Electroencephalographic Bicoherence During Sevoflurane Anesthesia Combined with Intravenous Fentanyl

With the introduction of bispectral index (BIS) as a measure of a patient's sedation during general anesthesia, attention has been directed toward bispectral analysis of electroencephalography (EEG). In the present study we evaluated the relationship between EEG bicoherence and sevoflurane concentration. Sixteen ASA physical status I-II patients scheduled for elective abdominal surgery were enrolled in the study. Anesthesia was induced with 5% sevoflurane and maintained with sevoflurane and oxygen (50%). Just before surgery, IV fentanyl (2 microg/kg) was given and then continuously infused (2 microg x kg(-1) x h(-1)). Using software we developed, EEG bicoherence, BIS, and 95% spectral edge frequency (SEF95) were recorded at end-tidal sevoflurane concentrations of 0.5%, 0.8%, 1.1%, 1.4%, 1.7%, 2.0%, and 2.3%. Under light anesthesia, EEG bicoherence values were low. With increasing sevoflurane concentrations, 2 peaks of bicoherence emerged along the diagonal line (f1 = f2). Both the first (at around 4 Hz) and second (at around 10 Hz) grew higher (37.7% +/- 7.5% and 35.1% +/- 9.0%, respectively) as the sevoflurane concentration increased to 1.4%. However, the first peak leveled off whereas the second tended to decrease slightly with further increases in sevoflurane concentration. The BIS value decreased as the sevoflurane concentration increased and leveled off at 1.4% and higher concentrations of sevoflurane. The SEF 95 also decreased as the sevoflurane concentration increased up to 2.3%. Thus the distribution pattern of the two bicoherence peaks is likely to be better than BIS of the anesthetic effect of sevoflurane during surgery.

Investigation of changes in the middle latency auditory evoked potential during anesthesia with sevoflurane in dogs

OBJECTIVE

To investigate the middle latency auditory evoked potential (MLAEP) in awake dogs and dogs anesthetized with 2 concentrations of sevoflurane.

ANIMALS

10 adult Beagles.

PROCEDURE

The MLAEP was recorded while dogs were awake and anesthetized with sevoflurane (end-tidal concentration, 2.7% or 3.5%). Three needle electrodes were inserted SC, and click stimuli were delivered biaurally. Signal acquisition, averaging, and analysis were performed by use of computer software developed in-house. Signals were recorded for 128 milliseconds, and the responses to 1,024 stimuli were averaged. Waveforms from 10 recordings in each dog were averaged, and latencies of peaks were measured. Data acquired for awake dogs and dogs anesthetized with high and low sevoflurane concentrations were compared statistically.

RESULTS

Sevoflurane anesthesia attenuated the MLAEP so that only peaks P0, Na, and Pa could be identified. The MLAEP changes were maximal at the lower concentration of sevoflurane evaluated. The latencies of these peaks were significantly shorter in awake dogs, compared with values in anesthetized dogs. No difference in the peak latency was detected between the sevoflurane concentrations.

CONCLUSIONS AND CLINICAL RELEVANCE

In terms of CNS responsiveness, the effects of anesthesia with sevoflurane are similar to those of anesthesia with isoflurane. Data suggest that sevoflurane is not the inhalant agent of choice in a research setting where electroencephalographic measurements are to be recorded during anesthesia. The depression of the MLAEP waveform by sevoflurane also suggests that the MLAEP is not a suitable tool with which to monitor anesthetic depth during sevoflurane anesthesia in dogs.

Impact of age on both BIS values and EEG bispectrum during anaesthesia with sevoflurane in children

BACKGROUND

The aim of this study was to evaluate the potential relationship between age, BIS (Aspect), and the EEG bispectrum during anaesthesia with sevoflurane.

METHODS

BIS and raw EEG were recorded at a steady state of 1 MAC in 100 children, and during a decrease from 2 to 0.5 MAC in a sub-group of 29 children. The bispectrum of the EEG was estimated using MATLAB software. For analysis, the bispectrum was divided into 36 frequencies of coupling (P(i))--the MatBis. A multiple correspondence analysis (MCA) was used to establish an underlying structure of the pattern of each individual's MatBis at 1 MAC. Clustering of children into homogeneous groups was conducted by a hierarchical ascending classification (HAC). The level of statistical significance was set at 0.05.

RESULTS

At 1 MAC, the BIS values for all children ranged from 20 to 74 (median 40). Projection of both age and BIS value recorded at 1 MAC onto the structured model of the MCA showed them to be distributed along the same axis, demonstrating that the different values of BIS obtained in younger or older children are mainly dependent on their MatBis. At 1 MAC, six homogeneous groups of children were obtained through the HAC. Groups 5 (30 months; range 23-49) and 6 (18 months; range 6-180) were the younger children and Group 1 (97 months; range 46-162) the older. Groups 5 and 6 had the highest median values of BIS (54; range 50-59) (55; range 26-74) and Group 1 the lowest values (29; range 22-37).

CONCLUSION

The EEG bispectrum, as well as the BIS appeared to be strongly related to the age of children at 1 MAC sevoflurane.

Modeling the effects of anesthesia on the electroencephalogram

Changes to the electroencephalogram (EEG) observed during general anesthesia are modeled with a physiological mean field theory of electrocortical activity. To this end a parametrization of the postsynaptic impulse response is introduced which takes into account pharmacological effects of anesthetic agents on neuronal ligand-gated ionic channels. Parameter sets for this improved theory are then identified which respect known anatomical constraints and predict mean firing rates and power spectra typically encountered in human subjects. Through parallelized simulations of the eight nonlinear, two-dimensional partial differential equations on a grid representing an entire human cortex, it is demonstrated that linear approximations are sufficient for the prediction of a range of quantitative EEG variables. More than 70,000 plausible parameter sets are finally selected and subjected to a simulated induction with the stereotypical inhaled general anesthetic isoflurane. Thereby 86 parameter sets are identified that exhibit a strong "biphasic" rise in total power, a feature often observed in experiments. A sensitivity study suggests that this "biphasic" behavior is distinguishable even at low agent concentrations. Finally, our results are briefly compared with previous work by other groups and an outlook on future fits to experimental data is provided.

Isofrequency Band-like Zones of Activation Revealed by Optical Imaging of Intrinsic Signals in the Cat Primary Auditory Cortex

Neurons of similar frequency preference are arranged in isofrequency bands (IFBs) across the primary auditory cortex (AI) of many mammals. Across the AI of the cat, one of the most frequently studied species for auditory anatomy and function, we demonstrate IFB-like responses using optical imaging of intrinsic signals (OIS). Optically defined activations were extensively elongated along the dorsoventral axis of AI (the ratio of the major and minor axes was approximately 2:1), and systematically shifted as a function of stimulus frequency. The elongation of this IFB-like zone was more conspicuous at higher frequencies. In the ventral sector of the imaged field, the IFB-like zones of activation evoked at different pure tone frequencies tended to overlap extensively. Electrophysiological recording from loci within the optically defined zones of activation revealed matched responses to the frequencies used for optical imaging at 65% of these loci. The dorsoventral orientation of these zones of activation was also closely matched with the orientation of tangentially spreading intrinsic axon terminals, as revealed anatomically. The visualization of IFB-like architecture and tonotopic organization by OIS provides a basic framework for investigating the relationships of different spectral channels and between multiple acoustic parameters at a neuronal population level.

Comparison of electroencephalogram activity and auditory evoked responses during isoflurane and halothane anaesthesia in the rat

OBJECTIVES

To compare the second differential index (SDI) calculated from the auditory evoked potential (AEP) and electroencephalogram (EEG) parameters: median frequency (MF), spectral edge frequency (SEF) and burst suppression rate (BSR) determined at four equivalent minimum alveolar concentrations (MAC) of isoflurane or halothane.

ANIMALS

Twelve male Wistar rats weighing 418 g (SD +/- 18.4 g).

METHODS

Auditory evoked potentials and EEG responses were recorded in animals implanted with electrodes at established anaesthetic concentrations. Depth of anaesthesia was assessed using the strength of the pedal withdrawal reflex (PWR), and data were analysed using repeated measures anova and paired t-tests.

RESULTS

The SEF tended to decrease with increasing depth of halothane anaesthesia (F = 4.198, p = 0.05), but not with isoflurane. The MF and SDI were significantly higher during halothane than with isoflurane (F = 5.82, p = 0.036 and F = 5.263, p = 0.045, respectively) at equivalent depths of anaesthesia, and EEG burst suppression occurred at deeper planes of isoflurane but not halothane anaesthesia.

CONCLUSIONS

The study demonstrated that EEG and AEP characteristics recorded at MAC equivalent concentrations were suppressed to a greater degree by isoflurane than by halothane. These findings have strong implications for research projects where EEG recordings are collected, and also cast more general doubts upon the value of such parameters for evaluating depth of isoflurane anaesthesia in rats.

Bispectral index, spectral edge frequency 95%, and median frequency recorded for various concentrations of isoflurane and sevoflurane in pigs

OBJECTIVE

To evaluate bispectral index (BIS), spectral edge frequency 95% (SEF), and median frequency (MED) in relation to a visual analogue scale (VAS) as indicators of anesthetic depth for various concentrations of sevoflurane and isoflurane in pigs.

ANIMALS

32 pigs.

PROCEDURE

Pigs were randomly allocated to 8 groups (4 pigs/group). An electroencephalogram (EEG) was recorded in each conscious pig. Pigs were then anesthetized by use of sevoflurane (n = 16) or isoflurane (16). Agents were administered in oxygen at minimum alveolar concentrations (MACs) of 1, 1.25, 1.5, and 1.75 MAC in a randomized order. End-tidal sevoflurane and isoflurane concentrations were maintained for 30 minutes, after which an EEG was recorded for 5 minutes; BIS, SEF, and MED were then calculated. Anesthetic depth was evaluated by use of the VAS. Cardiovascular and EEG responses to nociceptive stimuli were evaluated for each anesthetic agent.

RESULTS

BIS decreased significantly for the various concentrations of each anesthetic. At equivalent MACs, BIS values were significantly higher during sevoflurane-induced anesthesia than during isoflurane-induced anesthesia. Values of MED and SEF decreased significantly from basal values to 1 MAC of sevoflurane and isoflurane. For both agents, there was good correlation between VAS scores and BIS values and between VAS scores and SEF values.

CONCLUSIONS AND CLINICAL RELEVANCE

BIS was useful for predicting changes in anesthetic depth at clinical dosages of inhalant anesthetics. Values of BIS, SEF, and MED were significantly higher during anesthesia induced by administration of sevoflurane than during anesthesia induced by administration of isoflurance at equivalent MACs.

Sevoflurane requirements during coloproctologic surgery: difference between two different epidural regimens

STUDY OBJECTIVE

To examine the influence of epidural morphine on the end-tidal sevoflurane concentration titrated to maintain bispectral index (BIS) values between 40 and 50.

DESIGN

Prospective, double-blinded clinical trial.

SETTINGS

Anesthesia department of a university hospital.

PATIENTS

40 ASA physical status I, II, and III patients scheduled for elective coloproctological surgery.

INTERVENTIONS

Patients were randomized to receive via a thoracic epidural catheter either a) bupivacaine 0.25% (10 mL) and saline 0.9% (2 mL) as a bolus followed by an infusion of bupivacaine 0.25% (5 mL/hr) or b) bupivacaine 0.25% (10 mL) and morphine 0.1% (2 mL) as a bolus followed by an infusion of bupivacaine 0.25% plus morphine 0.025% (5 mL/hr). Anesthesia was induced with propofol, fentanyl 2 microg kg(-1) and atracurium and maintained with sevoflurane and nitrous oxide in oxygen. Sevoflurane level was titrated to maintain a BIS value between 40 and 50. After extubation, patients were asked about the presence of pain.

MAIN RESULTS

There was no significant difference between groups of end-tidal sevoflurane concentrations at identical BIS values and hemodynamic values at any time in the study. However, the morphine group had a lower pain score level at extubation than did the plain bupivacaine group (no pain on movement, 79% vs. 31.5%, p < 0.01).

CONCLUSIONS

Adding morphine to the bupivacaine epidural solution did not reduce sevoflurane requirements but did provide high-quality postoperative analgesia, mainly just after tracheal extubation.

Effect of volatile anesthetics on interhemispheric EEG cross-approximate entropy in the rat

Dissimilarity of the electroencephalogram (EEG) between the two hemispheres was characterized by cross-approximate entropy (C-ApEn), an information statistical parameter applicable to nonlinear, aperiodic signals. EEGs were recorded bipolarly with pairs of epidural electrodes in the left and right frontal cortices. The signals were filtered for 1-100 Hz and digitized at 200 Hz. Inhaled anesthetic concentration was varied between 0.3 and 2.1% with 45-min equilibration periods while the rats were breathing spontaneously. Anesthetics produced concentration-dependent changes in C-ApEn. A greater dynamic range of C-ApEn was obtained by reducing the epoch length from 2 s to 100 ms. At a 0.4% inspired agent concentration halothane caused an increase of C-ApEn, whereas isoflurane did not. When the inspired concentrations of both agents were greater than 0.4%, C-ApEn dose-dependently decreased as agent concentrations increased. Isoflurane depressed C-ApEn more than did halothane at all equivalent minimum alveolar concentration levels, but the two agents became equipotent at 1.5% inspired concentration. C-ApEn fell below the awake baseline at 0.8% anesthetic concentration that also abolished the righting reflex. C-ApEn increased after high-pass filtering (>20 Hz) and decreased after-low pass filtering (<20 Hz) of the digitized EEG; the anesthetic dependence of C-ApEn was diminished by both filters. The results suggest that C-ApEn of bihemispheric EEG is a sensitive, agent-specific correlate of anesthetics' central effect.

Pharmacodynamic interaction of nitrous oxide with sevoflurane, desflurane, isoflurane and enflurane in surgical patients: measurements by effects on EEG median power frequency

BACKGROUND AND OBJECTIVE

This study investigates the interaction of sevoflurane and nitrous oxide on EEG median power frequency of 2.5 Hz during surgery.

METHODS

Sevoflurane concentrations required for electroencephalographic median power frequency between 2 and 3 Hz were measured in 25 patients during gynaecological laparotomies. Nitrous oxide was randomly administered at 0, 20, 40, 60 and 75 vol%, subsequently two different concentrations in each patient. The data were analysed using isobolographic analysis together with previously published data on nitrous oxide-isoflurane, -enflurane, or -desflurane interaction.

RESULTS

The interaction is described by the equation: C volatile anaesthetic/C0 volatile anaesthetic + C N2O/C0 N2O=1 (C is the concentrations for a drug combination to achieve the desired effect; C0 is the concentration for single drug use). The parameters are C0 isoflurane=1.11 vol% (95% CI 1.03-1.19), C0 enflurane=1.64 (1.52-1.77), C0 desflurane=5.31 (4.92-5.73), C0 sevoflurane=2.12 (1.96-2.29), C0 N2O=174 (153-202). These parameters decrease by 6% (2.5-10) for every 10 years of patients' age > 40 years.

CONCLUSIONS

The interaction is compatible with additivity. The potency of nitrous oxide to substitute the volatile anaesthetics is less than anticipated from previously reported MAC values.

Biphasic EEG changes in relation to loss of consciousness during induction with thiopental, propofol, etomidate, midazolam or sevoflurane

The time course of four EEG effect variables, amplitude in the 2-5 Hz and in the 11-15 Hz band, spectral edge frequency 95% (SEF95), and bispectral index (BIS), in response to increasing concentrations of thiopental, propofol, etomidate, midazolam, or sevoflurane during a 10 min induction of anaesthesia was studied in 25 patients to determine the existence of a biphasic effect and to study the relationship of the EEG effect to the moment of loss of consciousness. A biphasic effect, that is, an initial increase of the effect variable followed by a decrease at higher concentrations, during the transition from consciousness to unconsciousness was found in EEG amplitude (both frequency bands) and in SEF95 for all anaesthetics except midazolam. There was a concentration-related decrease in BIS for all anaesthetics. There was no consistent relationship between the time of occurrence of the peak EEG effect, or the value of the EEG variable and the moment of loss of consciousness. With rapidly changing drug concentrations during the induction of anaesthesia, none of these EEG effect variables could be correlated to the moment of loss of consciousness.

The epileptogenic properties of the volatile anesthetics sevoflurane and isoflurane in patients with epilepsy

No study comparing epileptogenicity of sevoflurane to other volatile anesthetics has been performed. We compared the epileptogenic properties of sevoflurane to isoflurane in patients with epilepsy. In 24 mentally and/or physically disabled patients, 12 with epilepsy and 12 without epilepsy, electroencephalograms were recorded under anesthesia with 1.0 minimum alveolar anesthetic concentration (MAC), 1.5 MAC, and then 2.0 MAC sevoflurane or isoflurane under three ventilatory conditions: (A) 100% oxygen, and end-tidal CO(2) partial pressure (ETCO(2)) = 40 mm Hg, (B) 50% oxygen, 50% nitrous oxide, ETCO(2) = 40 mm Hg, and (C) 100% oxygen, ETCO(2) = 20 mm Hg. Spike activity was evaluated as a spike-and-wave index (% durations of spike and wave). The spike-and-wave index increased (P<0.05) from 1.99%+/-0.96% during 1.0 MAC sevoflurane to 6.14% +/- 4.45% during 2.0 MAC sevoflurane in (A) in the epilepsy group, while no spike activity was observed in the nonepilepsy group. Only a few spikes were observed under isoflurane anesthesia, 0.04% +/- 0.04% in (A), with no spikes in (B) and (C). Supplementation with 50% nitrous oxide or hyperventilation (P<0.05) suppressed the occurrence of spikes. Sevoflurane has a stronger epileptogenic property than isoflurane, but nitrous oxide or hyperventilation counteracts this specific epileptogenic property.

IMPLICATIONS

The stronger epileptogenicity of sevoflurane than isoflurane was confirmed in a controlled study in patients with epilepsy. Hyperventilation and supplementation of nitrous oxide under sevoflurane anesthesia suppressed epileptogenicity. A combination of sevoflurane and nitrous oxide may be a safer method for seizure-prone patients than the use of sevoflurane alone.

Effect of epidural bupivacaine on the relationship between the bispectral index and end-expiratory concentrations of desflurane

We compared the relationship between the bispectral index and end-tidal desflurane concentrations in 20 patients undergoing elective surgery. Patients received epidurally either 10 ml saline (group S) or 10 ml bupivacaine 0.125% with epinephrine 1/800 000 (group B) before induction of anaesthesia with sufentanil (0.15 microgram.kg(-1)) and propofol (2 mg.kg(-1)); muscle relaxation was obtained with cisatracurium (0.2 mg.kg(-1)). Patients lungs were ventilated to maintain end-tidal desflurane at 3% in O2/N2O (50/50) until 5 min after skin incision, followed by two consecutive 10 min periods at end-tidal desflurane 6% and 9%. bispectral index values were recorded before induction, at 3% desflurane before and 5 min after skin incision, and at 6% and 9% end-tidal desflurane. Bispectral index decreased with increasing end-tidal desflurane concentration (ANOVA: p < 0.05). The decrease in bispectral index was significant between pre-induction, 3% and 6% desflurane. No significant difference was observed at 3% desflurane before and after skin incision, or between 6 and 9% desflurane. The relationship between bispectral index and end-tidal desflurane concentration was fitted by a linear regression in each group. No significant difference in bispectral index was observed between the groups at any time. We conclude that bispectral index decreases with increasing desflurane concentration and that this relationship is not affected by epidural 0.125% bupivacaine.