Auditory evoked response and awareness: a study in volunteers at sub-MAC concentrations of isoflurane

Establishing a health-based recommended occupational exposure limit for isoflurane using experimental animal data: a systematic review protocol

BACKGROUND

Isoflurane is used as an inhalation anesthetic in medical, paramedical, and veterinary practice. Epidemiological studies suggest an increased risk of miscarriages and malformations at birth related to maternal exposure to isoflurane and other inhalation anesthetics. However, these studies cannot be used to derive an occupational exposure level (OEL), because exposure was not determined quantitatively and other risk factors such as co-exposures to other inhalation anesthetics and other work-related factors may also have contributed to the observed adverse outcomes. The aim of this systematic review project is to assess all available evidence on the effects of isoflurane in studies of controlled exposures in laboratory animals to derive a health-based recommended OEL.

METHODS

A comprehensive search strategy was developed to retrieve all animal studies addressing isoflurane exposure from PubMed, EMBASE, and Web of Science. Title-abstract screening will be performed by machine learning, and full-text screening by one reviewer. Discrepancies will be resolved by discussion. We will include primary research in healthy, sexually mature (non human) vertebrates of single exposure to isoflurane. Studies describing combined exposure and treatments with >  = 1 vol% isoflurane will be excluded. Subsequently, details regarding study identification, study design, animal model, and intervention will be summarized. All relevant exposure characteristics and outcomes will be extracted. The risk of bias will be assessed by two independent reviewers using an adapted version of the SYRCLE's risk of bias tool and an addition of the OHAT tool. For all outcomes for which dose-response curves can be derived, the benchmark dose (BMD) approach will be used to establish a point of departure for deriving a recommended health-based recommended OEL for 8 h (workshift exposure) and for 15 min (short-term exposure).

DISCUSSION

Included studies should be sufficiently sensitive to detect the adverse health outcomes of interest. Uncertainties in the extrapolation from animals to humans will be addressed using assessment factor. These factors are justified in accordance with current practice in chemical risk assessment. A panel of experts will be involved to reach consensus decisions regarding significant steps in this project, such as determination of the critical effects and how to extrapolate from animals to humans.

SYSTEMATIC REVIEW REGISTRATION

PROSPERO CRD42022308978.

Human Auditory Evoked Potentials in the Assessment of Brain Function During Major Cardiovascular Surgery

Focal neurologic and intellectual deficits or memory problems are relatively frequent after cardiac surgery. These complications have been associated with cerebral hypoperfusion, embolization, and inflammation that occur during or after surgery. Auditory evoked potentials, a neurophysiologic technique that evaluates the function of neural structures from the auditory nerve to the cortex, provide useful information about the functional status of the brain during major cardiovascular procedures. Skepticism regarding the presence of artifacts or difficulty in their interpretation has outweighed considerations of its potential utility and noninvasiveness. This paper reviews the evidence of their potential applications in several aspects of the management of cardiac surgery patients. The sensitivity of auditory evoked potentials to the effects of changes in brain temperature makes them useful for monitoring cerebral hypothermia and rewarming during cardiopulmonary bypass. The close relationship between evoked potential waveforms and specific anatomic structures facilitates the assessment of the functional integrity of the central nervous system in cardiac surgery patients. This feature may also be relevant in the management of critical patients under sedation and coma or in the evaluation of their prognosis during critical care. Their objectivity, reproducibility, and relative insensitivity to learning effects make auditory evoked potentials attractive for the cognitive assessment of cardiac surgery patients. From a clinical perspective, auditory evoked potentials represent an additional window for the study of underlying cerebral processes in healthy and diseased patients. From a research standpoint, this technology offers opportunities for a better understanding of the particular cerebral deficits associated with patients who are undergoing major cardiovascular procedures.

Intraoperative auditory evoked potential recordings are more reliable at signal detection from different sensor sites on the forehead compared to bispectral index

Bispectral index (BIS) and auditory evoked potential (AEP) monitoring require the attachment of forehead sensors, posing difficulties when the surgical field involves the forehead. This study analyzed the relationship between BIS values and AEP indices from different sites on the head to establish alternative sensor locations for AEP recording. Thirty patients scheduled for elective surgery under sevoflurane anesthesia were randomly assigned to the forehead, nose or mandible groups (n = 10 patients per group). AEP sensors were placed at the assigned position for each group and BIS sensors were placed on the forehead. BIS value and AEP index were simultaneously recorded from induction until emergence from general anesthesia. Relationships between BIS values and AEP indices were analyzed using a regression method and compared between groups using Pearson's correlation coefficients. Square regression models better expressed the relationships than linear models in all groups. The z-transformed coefficient in the forehead group was the same as the nose group (p = 0.24) and significantly different in the mandible group (p = 0.0046). These findings suggest that AEPs can be accurately recorded from sensors placed on the nose. Nasal AEP might be useful for monitoring electrical activity in the brain during surgeries involving the forehead.

aepEX monitor for the measurement of hypnotic depth in patients undergoing balanced xenon anaesthesia

BACKGROUND

Previously, we showed a significant difference in the measurements of hypnotic depth by the bispectral index (BIS) and auditory-evoked potentials (AEPs) using the A-line autoregressive index during xenon anaesthesia. In the present study, we evaluate the alternative AEP-based auditory-evoked potential index (aepEX) for the measurement of hypnotic depth in patients undergoing general anaesthesia with xenon.

METHODS

Forty-two patients undergoing elective abdominal surgery were enrolled in this controlled, double-blinded, randomized, clinical study. Patients were randomized to receive either xenon (n=21) or sevoflurane anaesthesia (n=21). During anaesthesia, BIS values were recorded simultaneously with the aepEX monitoring. The anaesthetist performing the anaesthesia was blinded to the hypnotic depth monitors. After surgery, the incidence of recalls and awareness was evaluated.

RESULTS

Patients' characteristics such as gender, age, and weight did not differ between the groups. The aepEX and BIS values behaved similarly during anaesthesia. The comparison of aepEX values during xenon and sevoflurane anaesthesia revealed significantly lower aepEX values in the xenon group after 25 min [xenon: 32.9 (4.8) vs sevoflurane: 39.3 (9.0); P=0.008] and after 35 min [xenon: 31.4 (6.6) vs sevoflurane: 37.0 (6.8); P=0.012]. During anaesthesia, aepEX values correlated with the clinical evaluation of depth of anaesthesia (e.g. >20% changes of the baseline arterial pressure or heart rate, spontaneous breathing and/or intolerance of mechanical ventilation, coughing, abdominal pressing, sweating, eye tearing).

CONCLUSIONS

We found the aepEX monitor to provide index in the range of adequate depth of xenon anaesthesia, when combined with remifentanil infusion in intubated patients undergoing elective abdominal surgery.

Influences of hypoxia on auditory evoked potentials of conscious humans

Early cortical auditory evoked potentials (AEP) from eleven volunteers undergoing an arterial oxygen desaturation procedure have been recorded and analysed. During the procedure the arterial oxygen saturation was decreased from 100% gradually down to 70%. Changes of the latency and amplitude of the AEP could be detected on the conscious subjects thus excluding any influences of anaesthesia. It was found that as oxygen desaturation went down the latency increased significantly and the amplitude also slightly increased but with no statistic significance. The latency parameters, especially Pb latency, are more sensitive to hypoxia than the amplitude parameters. The changes are similar to the effect of induction of anaesthesia.

Depth of Anesthesia Monitoring

Depth-of-anesthesia monitoring with EEG or EEG combined with mLAER is becoming widely used in anesthesia practice. Evidence shows that this monitoring improves outcome by reducing the incidence of intra-operative awareness while reducing the average amount of anesthesia that is administered, resulting in faster wake-up and recovery, and perhaps reduced nausea and vomiting. As with any monitoring device, there are limitations in the use of the monitors and the anesthesiologist must be able to interpret the data accordingly. The limitations include the following. The currently available monitoring algorithms do not account for all anesthetic drugs, including ketamine, nitrous oxide and halothane. EMG and other high-frequency electrical artifacts are common and interfere with EEG interpretation. Data processing time produces a lag in the computation of the depth-of-anesthesia monitoring index. Frequently the EEG effects of anesthetic drugs are not good predictors of movement in response to a surgical stimulus because the main site of action for anesthetic drugs to prevent movement is the spinal cord. The use of depth-of-anesthesia monitoring in children is not as well understood as in adults. Several monitoring devices are commercially available. The BIS monitor is the most thoroughly studied and most widely used, but the amount of information about other monitors is growing. In the future, depth-of-anesthesia monitoring will probably help in further refining and better understanding the process of administering anesthesia.

Correlation of the A-Line™ ARX index with acoustically evoked potential amplitude †

BACKGROUND

Automated indices derived from mid-latency auditory evoked potentials (MLAEP) have been proposed for monitoring the state of anaesthesia. The A-Line ARX index (AAI) has been implemented in the A-Line monitor (Danmeter, V1.4). Several studies have reported variable and, in awake patients, sometimes surprisingly low AAI values. The purpose of this study was to reproduce these findings under steady-state conditions and to investigate their causes.

METHODS

Ten awake unmedicated volunteers were studied under steady-state conditions. For each subject, the raw EEG and the AAI were recorded with an A-Line monitor (V1.4) during three separate sessions of 45.0 (1.6) min duration each. MATLAB (Mathworks) routines were used to derive MLAEP responses from EEG data and to calculate maximal MLAEP amplitudes.

RESULTS

The AAI values ranged from 15 to 99, while 11.4% fell below levels which, according to the manufacturer, indicate an anaesthetic depth suitable for surgery. Inter-individual and intra-individual variation was observed despite stable recording conditions. The amplitudes of the MLAEP varied from 0.8 to 42.0 microV. The MLAEP amplitude exceeded 2 microV in 75.3% of readings. The Spearman's rank correlation coefficient between the MLAEP amplitude and the AAI value was r=0.89 (P<0.0001).

CONCLUSIONS

The version of the A-Line monitor used in this study does not exclude contaminated MLAEP signals. Previous publications involving this version of the A-Line monitor (as opposed to the newer A-Line/2 monitor series) should be reassessed in the light of these findings. Before exclusively MLAEP-based monitors can be evaluated as suitable monitors of depth of anaesthesia, it is essential to ensure that inbuilt validity tests eliminate contaminated MLAEP signals.

Application of Generalized Dynamic Neural Networks to Biomedical Data

This paper reviews the application of continuous recurrent neural networks with time-varying weights to pattern recognition tasks in medicine. A general learning algorithm based on Pontryagin's maximum principle is recapitulated, and possibilities of improving the generalization capabilities of these networks are given. The effectiveness of the methods is demonstrated by three different real-world examples taken from the fields of anesthesiology, orthopedics, and radiology.

Detektion intraoperativer Wachheit bei einem Kleinkind mit akustisch evozierten Potenzialen

Intraoperative wakefulness is not only limited to adults and can also be found at a similar percentage (0.8%) in paediatric anaesthesia. For prevention of awareness neurophysiologic methods like auditory evoked potentials might be helpful. We report a case of a 2-year-old boy receiving balanced anaesthesia with sevoflurane and alfentanil. Midlatency auditory evoked potentials (MLAEPs) were recorded continuously before, during and after the surgical procedure. During the surgical procedure sevoflurane was withdrawn unintentionally. After a short period of time the boy started coughing and moved his legs, which was interpreted as insufficient analgesia. Several boli of alfentanil did not lead to the expected clinical effect on the depth of anaesthesia. After a recheck of the anaesthetic ventilator the error was determined and delivery of the volatile anaesthetic restored. The postoperative evaluation of the MLAEPs revealed the inadequate suppression of auditory processing during this incident with latencies comparable to the awake state. After reapplication of sevoflurane the MLAEPs were almost completely suppressed demonstrating adequate anesthetic depth. Exemplarily this case suggests that MLAEPs could be used to detect intraoperative awareness also in paediatric anaesthesia. Investigations to prove the validity and reproducibility of MLAEPs in children will be necessary.

The influence of wavelets on multiscale analysis and parametrization of midlatency auditory evoked potentials

This work shows methodological aspects of heuristic pattern recognition in auditory evoked potentials. A linear and a nonlinear transformation based on wavelet transform are presented. They result in a statistical error model and an entropy function related to the Gibbs function and describe changes in midlatency auditory evoked potentials induced by general anaesthesia. The same transformations were calculated using 12 common wavelets. We present a method to compare the two defined parametrizations with respect to their ability to discriminate two defined states which is responsive and unresponsive depending on the wavelet used for the analysis. Auditory evoked potentials of 60 patients undergoing general anaesthesia were analysed. We propose the defined statistical error model and the entropy function as a very robust measure of changes in auditory evoked potentials. The influence of the wavelets suggest that for each parametrization the goodness of the wavelet should be validated.

Auditory evoked potentials

This chapter will focus on the two auditory evoked potentials (AEP) most commonly used to assess the effects of general anesthetics on the brain, the auditory middle latency response (AMLR) and the 40 Hz auditory steady-state response (40 Hz-ASSR). We will review their physiological basis, the recording methodology, the effects of general anesthetics, their ability to track changes in level of consciousness and their clinical applications. Because of space constraints, this review will be limited to human studies.

The effects of propofol, small-dose isoflurane, and nitrous oxide on cortical somatosensory evoked potential and bispectral index monitoring in adolescents undergoing spinal fusion

In this study we compared the effects of propofol, small-dose isoflurane, and nitrous oxide (N(2)O) on cortical somatosensory evoked potentials (SSEP) and bispectral index (BIS) monitoring in adolescents undergoing spinal fusion. Twelve patients received the following anesthetic maintenance combinations in a randomly determined order: treatment #1: isoflurane 0.4% + N(2)O 70% + O(2) 30%; treatment #2: isoflurane 0.6% + N(2)O 70% + O(2) 30%; treatment #3: isoflurane 0.6% + air + O(2) 30%; treatment #4: propofol 120 microg . kg(-1) . min(-1) + air + O(2) 30%. Cortical SSEP amplitudes measured during anesthesia maintenance with treatment #3 (isoflurane 0.6%/air) were more than those measured during maintenance with treatment #1 (isoflurane 0.4%/N(2)O 70%) (P < 0.0001) and treatment #2 (isoflurane 0.6%/N(2)O 70%) (P < 0.0052). Cortical SSEP amplitudes measured during treatment #4 (propofol 120 microg . kg(-1) . min(-1)/air) were more than treatment #1 (isoflurane 0.4%/N(2)O 70%) (P < 0.0001), treatment #2 (Iso 0.6%/N(2)O 70%) (P < 0.0007), and treatment #3 (isoflurane 0.6%/air) (P < 0.0191). In addition, average BIS values measured during treatments 1, 2, 3 and 4 were 62, 62, 61, and 44 respectively. Only treatment #4 (propofol 120 microg . kg(-1) . min(-1)/air) uniformly maintained BIS values less than 60. Our study demonstrates that propofol better preserves cortical SSEP amplitude measurement and provides a deeper level of hypnosis as measured by BIS values than combinations of small-dose isoflurane/N(2)O or small-dose isoflurane alone.

The Effect of Music on the Neurohormonal Stress Response to Surgery Under General Anesthesia

Several pharmacological interventions reduce perioperative stress hormone release during surgery under general anesthesia. Listening to music and therapeutic suggestions were also studied, but mostly in awake patients, and these have a positive effect on postoperative recovery and the need for analgesia. In this study, we evaluated the effect of listening to music under general anesthesia on the neurohormonal response to surgical stress as measured by epinephrine, norepinephrine, cortisol, and adrenocorticotropic hormone (ACTH) blood levels. Thirty female patients scheduled for abdominal gynecological procedures were enrolled and randomly divided into two groups: group NM (no music) and group M (music). In group M, music was played from after the induction of anesthesia until the end of surgery. In the NM group, the patients wore the headphones but no music was played. We established three sample times for hormonal dosage during the procedure and one in the recovery room. Hemodynamic data were recorded at all times, and postoperative consumption of morphine in the first 24 h was noted. There was no group difference at any sample time or in the postoperative period in terms of mean arterial blood pressure, heart rate, isoflurane end-tidal concentration, time of the day at which the surgery was performed, bispectral index (BIS) value, doses of fentanyl, or consumption of postoperative morphine. There was no difference between the two groups with regard to plasmatic levels of norepinephrine, epinephrine, cortisol, or ACTH at any sample time, although the blood level of these hormones significantly increased in each group with surgical stimulation. In conclusion, we could not demonstrate a significant effect of intraoperative music on surgical stress when used under general anesthesia.

IMPLICATIONS

Listening to music under general anesthesia did not reduce perioperative stress hormone release or opioid consumption in patients undergoing gynecological surgery.

Entropy during propofol hypnosis, including an episode of wakefulness

Depth of anaesthesia has proved to be a complex process to quantify. Monitors based on bispectral analysis of the electroencephalogram and auditory evoked potential have been available, but only recently has a monitor based on entropy become available. This study determined state entropy and response entropy in nine healthy volunteers during propofol hypnosis with a brief intervening period of wakefulness. Both the calculated entropy indices decreased with increasing levels of sedation (r2 = 0.58 and 0.61, respectively) and they showed a high correlation with each other (r2 = 0.94). However, an overlap was observed in real time indices between different stages of the Observer's Assessment of Alertness/Sedation Scale. Only three of the nine volunteers had explicit memories from the episode of wakefulness. Electroencephalographic entropy monitors seem to have potential for staging clinical hypnotic effects.

Xenon: elemental anaesthesia in clinical practice

The 'noble' gases have been known to have anaesthetic properties for 50 years yet only recently has their application become a clinical reality. In this review we describe the preclinical and clinical studies that have led to a resurgence of interest in the use of the element xenon as an anaesthetic. Furthermore, we highlight specific areas where xenon demonstrates advantages over other anaesthetics, including safety, beneficial pharmacokinetics, cardiovascular stability, analgesia and neuroprotection.

Mid-latency auditory evoked response during propofol and alfentanil anaesthesia

BACKGROUND

Propofol has been shown to affect the mid-latency auditory evoked response (MLAER) in a dose-dependant manner. Few studies have investigated the addition of alfentanil. Myogenic responses, such as the post-auricular responses (PAR), can confound the MLAER but there has been little investigation as to which electrode site reduces this interference.

METHODS

We studied the MLAER in 27 women. They received an infusion of alfentanil 15 micro g kg(-1) h(-1), followed by either a high or low infusion regimen of propofol (final infusion rates 6 and 3 mg kg(-1) h(-1)). We compared the results with those of our study using propofol alone. We collected the data from two electrode sites: vertex-inion and vertex-mastoid. We evaluated the occurrence of the PAR and the shape of the MLAER at each electrode site.

RESULTS

The infusion rate of propofol associated with loss of the eyelash response in 50% of subjects was 3.3 mg kg(-1) h(-1). This was significantly lower than using propofol alone (5.8 mg kg(-1) h(-1)). Nb latency was the best MLAER discriminator of unconsciousness (sensitivity 94%, specificity 88%), with a threshold of 46 ms (propofol alone was 53 ms). The addition of alfentanil did not alter the relationship between propofol infusion rate and MLAER. The vertex-inion electrode site gave the best protection against PAR in awake subjects (P=0.0003), and after 30 min of propofol infusion (P=0.06). The magnitude of the MLAER obtained from the vertex-mastoid electrodes was larger than from the other site, although the increase was not consistent throughout the waveform (brain stem 100%, Nb 14%).

CONCLUSIONS

Addition of alfentanil lowers the propofol infusion rate required to produce unconsciousness and the Nb latency that predicts it. The better of the two sites to reduce the incidence of PAR is the vertex-inion electrode site.

Effects of isoflurane on auditory middle latency (MLRs) and steady-state (SSRs) responses recorded from the temporal cortex of the rat

Auditory steady-state responses (SSRs) are believed to result from superimposition of middle latency responses (MLRs) evoked by individual stimuli during repetitive stimulation. Our previous studies showed that besides linear addition of MLRs, other phenomena, mainly related to the adaptive properties of neural sources, interact in a complex way to generate the SSRs recorded from the temporal cortex of awake rats. The aim of this study was to evaluate the effects of the inhalational general anesthetic, isoflurane, on MLRs and SSRs at several repetition rates (30-60 Hz) recorded from the temporal cortex of rats. Auditory evoked potentials were obtained by means of epidural electrodes in the awake condition and during anesthesia at three isoflurane concentrations (0.38, 0.76 and 1.13 vol.% in oxygen). MLR latency significantly increased during anesthesia in a concentration-dependent manner, while MLR amplitude, even when significantly attenuated with respect to the mean awake baseline value, failed to correlate with isoflurane concentration. SSRs decreased in amplitude and increased in phase during anesthesia in a concentration-dependent manner and the anesthetic-induced decrease of SSR amplitude appeared to be higher than the corresponding MLR attenuation. SSR prediction curves synthesized by linear addition of MLRs failed to predict SSRs in both amplitude and phase. Moreover, phase discrepancies proved to be higher during anesthesia. Our results suggest that MLRs and SSRs recorded from the temporal cortex of the rat exhibit differential sensitivity to isoflurane and that isoflurane could enhance the role of rate-dependent effects in SSR generation.

Effects of isoflurane on the auditory brainstem responses and middle latency responses of rats

OBJECTIVE

To evaluate the effects of a volatile anesthetic, isoflurane, on auditory brainstem responses (ABRs) and middle latency responses (MLRs) recorded in rats.

MATERIAL AND METHODS

ABRs and MLRs evoked by click stimuli were simultaneously recorded in eight rats in the awake condition and during anesthesia with isoflurane at clinically relevant concentrations.

RESULTS

Vertex-recorded ABRs showed a significant increase in the latency of waves I-IV during anesthesia and the latency changes appeared to be significantly related to the isoflurane concentration. The I-IV interval also appeared to be significantly increased in comparison to the awake condition, while minor changes in ABR amplitudes were induced by isoflurane. MLRs, which were recorded by means of epidural electrodes implanted over the auditory cortex, appeared to be attenuated in amplitude and increased in latency during anesthesia. Only latency changes appeared to be significantly related to the isoflurane concentration. Moreover, "bursts" of high amplitude MLRs were observed during anesthesia at each concentration.

CONCLUSION

The present findings indicate that both ABR and MLR latencies are increased by isoflurane in a concentration-dependent manner, whilst the anesthetic-induced attenuation in amplitude does not appear to be related to the isoflurane concentration.

Midlatency auditory evoked potentials do not allow the prediction of recovery from general anesthesia with isoflurane

PURPOSE

To investigate midlatency auditory evoked potentials (MLAEP) waveforms during recovery from anesthesia. The hypothesis was that MLAEP are sensitive variables to discriminate between states of consciousness and unconsciousness during emergence from anesthesia.

METHODS

MLAEP were recorded in the awake state and during the wake-up phase from isoflurane anesthesia in 22 female patients undergoing ophthalmologic surgery. During emergence from anesthesia the changes in latency and amplitude of MLAEP components Na, Pa and Nb were compared with the awake level. The next day the patients were asked for explicit memory for the recovery period.

RESULTS

In 72% of the patients the MLAEP waveforms were completely suppressed during isoflurane anesthesia. When the patients responded and opened their eyes spontaneously 38 +/- 12 min after anesthesia, the latencies of Na (18.3 +/- 1.2 vs 17.6 +/- 1.3; P = 0.013) and Nb (47.4 vs 7.1 vs 44.7 +/- 7.8; P = 0.048) remained prolonged compared with awake values. In contrast, the amplitudes NaPa and PaNb had regained baseline level. Nine patients had explicit memory for the immediate recovery period. However, there was no difference for any MLAEP component between patients with and without memory at any time.

CONCLUSIONS

The persistent changes of MLAEP latency components Na and Nb indicated impaired auditory signal processing 38 min after isoflurane anesthesia. There was a marked intra- and inter-individual variability during reversal of the anesthetic induced MLAEP changes. This limits the prediction of recovery of consciousness in the individual patient during emergence from anesthesia.

Assessing sedation with regional anesthesia: inter-rater agreement on a modified Wilson sedation scale

A valid and reliable means for measuring sedation during regional anesthesia would be valuable for both research and practice. Current methods of monitoring sedation include machine-, patient-, and observer-based assessment. The reliability of machine-based methods is limited at lower levels of sedation, whereas patient-based methods are impractical at higher levels. Observer-based methods offer the best alternative for assessing sedation during regional anesthesia; however, their reliability has not been adequately documented. We examined the interrater reliability of the Wilson sedation scale. Sedation was assessed by pairs of anesthesia care providers in 100 patients undergoing surgical procedures with regional anesthesia. On the basis of the findings, the scale was modified, and 50 additional patients were assessed. The study protocol called for a series of standardized stimuli administered by a research assistant. Raters were blinded to each other's ratings. Interrater reliability was assessed by using the kappa statistic, a measure of actual agreement beyond agreement by chance. When continuing checks on its operationalization and reliability are included, the modified Wilson scale provides a simple and reliable means by which to assess and monitor intraoperative sedation.

IMPLICATIONS

We evaluated the interrater reliability of the Wilson scale for measuring sedation during regional anesthesia. Paired anesthesia care providers' ratings of patient sedation indicated very good interrater reliability in both the original scale and a modified version. The modified Wilson scale provides a quick noninvasive means of monitoring sedation during regional anesthesia.

A study of bispectral analysis and auditory evoked potential indices during propofol-induced hypnosis in volunteers: the effect of an episode of wakefulness on explicit and implicit memory

The bispectral index (BIS) and the auditory evoked potential (AEP) index as calculated by the new A-line monitor were measured during hypnosis with propofol, which included an episode of wakefulness. Both indices followed a similar pattern during sedation, with values decreasing with sedation and increasing when awake. Baseline AEP values varied between 60 and 98, and BIS values were between 96 and 98. The AEP-index value was at all times 10-20 points lower than the BIS-index. The transition to loss of response occurred at a mean AEP value of 46 and BIS value of 58. The transition to just responding following a period of unconsciousness occurred at a mean AEP value of 46 and BIS 65. Both monitoring techniques, however, displayed large interindividual variations making it impossible to discriminate in real time between subtle changes of clinical state. The new neurophysiological monitors A-line AEP and BIS are interesting tools for creating a better understanding of the anaesthetic effects of drugs; however, further refinements are required before their relative roles can be fully established in the clinical setting.

The auditory evoked response as an awareness monitor during anaesthesia

We investigated the relationship between the latency of the Nb wave of the auditory evoked response (AER) and periods of awareness during propofol anaesthesia. In the anaesthetic room before cardiac surgery the AER was recorded continuously in 14 patients. Awareness was measured by the ability of the patient to respond to command using the isolated forearm technique (IFT). The Nb latencies were shorter when the patients were able to respond than at loss of response (P<0.001). In six patients who repeated this transition from response to loss of response, there was a high and significant correlation between Nb latencies. None of the patients had any recollection of events after the initial induction of anaesthesia as measured by explicit and implicit memory tests. These results suggest that the Nb latency of the AER may represent an indication of awareness in individual patients, but wide inter-patient variability limits its practical usefulness. In addition, because no evidence of memory was demonstrated, even when patients were known to be awake, the relationship between AER and memory processing remains unclear.

Evoked potential monitoring in anaesthesia and analgesia

Electrophysiological monitoring of selected neural pathways of the brain, brainstem, spinal cord and peripheral nervous system has become mandatory in some surgery of the nervous system where preventable neural injury can occur. Evoked potentials are relatively simple methods of testing the integrity of various aspects of the nervous system. This review covers the variety of evoked potentials that can be monitored and outlines the principles of their measurement. Their use in specific situations and how factors such as anaesthesia might affect them is presented.

Inhaled nonimmobilizers do not alter the middle latency auditory-evoked response of rats

General anesthetics cause surgical immobility and oblivion (unconsciousness and amnesia). A class of compounds known as "nonimmobilizers" were predicted to be anesthetic, based on their physiochemical properties, but found to cause only amnesia. In humans, cerebrocortical electrical activity after auditory stimulation is depressed by concentrations of anesthetics which impair auditory recall. We sought to use these evoked responses to characterize the effects of the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (2N) and conventional inhaled anesthetics on early sensory processing in rats. Unrestrained rats with chronically implanted epidural silver screw electrodes were put into a chamber. On separate days, the same population of rats were exposed to isoflurane, desflurane, nitrous oxide, or 2N, each at several subminimum alveolar concentration of anesthetic required to eliminate movement in response to a noxious stimulus concentrations. After equilibration at each concentration, auditory-evoked responses were obtained. The behavioral state (activity and righting reflex) and electroencephalogram were also examined. 2N did not significantly change the middle latency auditory-evoked response, whereas the anesthetics all slowed conduction and depressed amplitude in a dose-dependent fashion. 2N neither depressed the righting reflex, nor induced epileptiform activity.

IMPLICATIONS

Although the nonimmobilizer 1,2-dichlorohexafluorocyclobutane (2N) suppresses learning, we find that 2N does not depress middle latency auditory-evoked responses. This suggests that 2N may suppress learning by depressing transmission through rostral subcortical structures, such as the amygdala, rather than by acting on the brainstem or neocortical structures.

Hemispheric-synchronisation during anaesthesia: a double-blind randomised trial using audiotapes for intra-operative nociception control

The possible antinociceptive effect of hemispheric-synchronised sounds, classical music and blank tape were investigated in patients undergoing surgery under general anaesthesia. The study was performed on 76 patients, ASA 1 or 2, aged 18-75 years using a double-blind randomised design. Each of the three tapes was allocated to the patients according to a computer-generated random number table. General anaesthesia was standardised and consisted of propofol, nitrous oxide 66%/oxygen 33%, isoflurane and fentanyl. Patients breathed spontaneously through a laryngeal mask and the end-tidal isoflurane concentration was maintained near to its minimum alveolar concentration value of 1.2%. Fentanyl was given intravenously sufficient to keep the intra-operative heart rate and arterial blood pressure within 20% of pre-operative baseline values and the fentanyl requirements were used as a measure of nociception control. Patients to whom hemispheric-synchronised sounds were played under general anaesthesia required significantly less fentanyl compared with patients listening to classical music or blank tape (mean values: 28 microgram, 124 microgram and 126 microg, respectively) (p < 0.001). This difference remained significant when regression analysis was used to control for the effects of age and sex.

Isoflurane indirectly depresses middle latency auditory evoked potentials by action in the spinal cord in the goat

PURPOSE

The auditory evoked potential (AEP) has been proposed as a method to determine depth of anesthesia, as AEPs are generated in the thalamus and cerebral cortex. Because general anesthesia, in part, relies on anesthetic action in the spinal cord, we hypothesized that isoflurane would have indirect depressant effects on the AEP because of its action in the spinal cord.

METHODS

Six goats were anesthetized with isoflurane and the jugular veins and carotid arteries isolated to permit cerebral bypass and differential delivery of isoflurane to the head and torso. The AEPs were determined by delivering binaural clicks at 9 Hz, and measuring the evoked response from electrodes placed in the skull and scalp. Isoflurane was maintained at 0.8% in the head while the torso isoflurane was alternated between 0.3% and 1.3%, and the AEP determined at each torso concentration.

RESULTS

At isoflurane 0.3% delivered to the torso, the peak-to-trough amplitude of the mid-latency AEP waves designated as N14/P24 was 0.337+/-0.185 microV and the N36 wave amplitude was 0.115+/-0.054 microV. When torso isoflurane was 1.3%, these waves decreased to 0.297+/-0.186 microV and 0.066+/-0.037 microV, respectively (P<0.05). The latency of the N14 wave increased slightly (from 13.7+/-2.6 msec to 14.4+/-2.8 msec, P<0.05) but the latencies of the other waves were unchanged.

CONCLUSIONS

Isoflurane action in the spinal cord indirectly alters the AEP, however, the effect is small. These data suggest that isoflurane can depress the transmission of afferent information from the spinal cord to thalamus, midbrain and cerebral cortex.

Middle latency auditory evoked responses and electroencephalographic derived variables do not predict movement to noxious stimulation during 1 minimum alveolar anesthetic concentration isoflurane/nitrous oxide anesthesia

The electroencephalogram (EEG) and middle latency auditory evoked responses (MLAER) have been proposed for assessment of the depth of anesthesia. However, a reliable monitor of the adequacy of anesthesia has not yet been defined. In a multicenter study, we tested whether changes in the EEG and MLAER after a tetanic stimulus applied to the wrist could be used to predict subsequent movement in response to skin incision in patients anesthetized with 1 minimum alveolar anesthetic concentration (MAC) isoflurane in N2O. We also investigated whether the absolute values of any of these variables before skin incision was able to predict subsequent movement. After the induction of anesthesia with propofol and facilitation of tracheal intubation with succinylcholine, 82 patients received 1 MAC isoflurane (0.6%) in N2O 50% without an opioid or muscle relaxant. Spontaneous EEG and MLAER to auditory click-stimulation were recorded from a single frontoparietal electrode pair. MLAER were severely depressed at 1 MAC isoflurane. At least 20 min before skin incision, a 5-s tetanic stimulus was applied at the wrist, and the changes in EEG and MLAER were recorded. EEG and MLAER values were evaluated before and after skin incision for patients who did not move in response to tetanic stimulation. Twenty patients (24%) moved after tetanic stimulation. The changes in the EEG or MLAER variables were unable to predict which patients would move in response to skin incision. Preincision values were not different between patients who did and did not move in response to skin incision for any of the variables. MLAER amplitude increased after skin incision. We conclude that it is unlikely that linear EEG measures or MLAER variables can be of practical use in titrating isoflurane anesthesia to prevent movement in response to noxious stimulation.

IMPLICATIONS

Reliable estimation of anesthetic adequacy remains a challenge. Changes in spontaneous or auditory evoked brain activity after a brief electrical stimulus at the wrist could not be used to predict whether anesthetized patients would subsequently move at the time of surgical incision.

Middle latency auditory evoked potentials during total intravenous anesthesia with droperidol, ketamine and fentanyl

We investigated whether total intravenous anesthesia with ketamine, fentanyl and droperidol would affect middle latency auditory evoked potentials and explicit memory, and whether dreams during the anesthesia are related to plasma concentrations of fentanyl and the infusion technique. A total number of 40 patients were the subjects for this study. Twenty patients (group A) were maintained with intravenous ketamine 2 mg kg-1 hr-1 and fentanyl 5 micrograms kg-1 hr-1 for the first 60 min and 3 micrograms kg-1 hr-1 for the next 90 min, and droperidol 0.1 mg kg-1. The remaining 20 patients (group B) were maintained with intravenous ketamine 2 mg kg-1 hr-1, droperidol 0.1 mg kg-1 and fentanyl 50-100 micrograms in a bolus intermittently as needed by vital signs such as increases in heart rate and arterial blood pressure. Middle latency auditory evoked potentials, plasma fentanyl and ketamine levels were measured; explicit memory and dreams were also estimated. There were no patients who recollected explicit memories of intraoperative events in both groups. The middle latency auditory evoked potentials were not significantly changed during the anesthesia in both groups. We could find no significant differences in latencies and amplitudes of the middle latency auditory evoked potentials between the both groups. Plasma fentanyl levels of group B patients were significantly lower than those of group A patients and the incidence of the dreams was significantly higher in group B patients. We conclude that the anesthesia with ketamine, fentanyl and droperidol is not associated with the explicit memories, though the middle latency auditory evoked potentials were not significantly changed as compared with those in the waking state. In addition, dreams during the anesthesia may correlate with plasma fentanyl concentrations or the infusion technique.

Midlatency auditory-evoked potentials in the rat: effects of interventions that modulate arousal

The vertex-recorded P13 midlatency auditory-evoked potential in the rat shows the same characteristics as the P1 potential in the human, namely, sleep-state dependence, rapid habituation and blockade by the cholinergic antagonist scopolamine. The P13 potential appears to be generated, at least in part, by projections of the pedunculopontine nucleus, the cholinergic arm of the reticular activating system. On the other hand, the auditory cortex-recorded P7 potential appears to be of primary cortical origin. Simultaneous recordings from the vertex and the auditory cortex showed that (1) the P13 potential was suppressed by administration of the anesthetics ketamine, pentobarbital or halothane in a dose-dependent manner, but the P7 potential was not; (2) the P13 potential was suppressed by intragastric injections of ethanol in a dose-dependent manner, but the P7 potential was not; (3) the amplitude of the P13 potential was negatively correlated with blood ethanol levels; (4) both the P13 and P7 potentials were still present following injections of the neuromuscular blocker pancuronium bromide; and (5) both the P13 and P7 potentials were decreased by diffuse brain injury induced by a weight-drop device in a weight-dependent manner. These findings suggest that the P13 potential is more sensitive than the P7 potential to changes in arousal and that the P13 and P7 potentials are not of myogenic but of neural origin.

Anesthesia and the electrophysiology of auditory consciousness

Empirical work is reviewed which correlates the presence or absence of various parts of the auditory evoked potential with the disappearance and reemergence of auditory sensation during induction of and recovery from anesthesia. As a result, the hypothesis is generated that the electrophysiological correlate of auditory sensation is whatever neural activity generates the middle latency waves of the auditory evoked potential. This activity occurs from 20 to 80 ms poststimulus in the primary and secondary areas of the auditory cortex. Evidence is presented suggesting that earlier or later waves in the auditory evoked potential do not covary with auditory sensation (as opposed to auditory perception) and it is therefore suggested that they are possibly not the electrophysiological correlates of sensation.

Cortical responses to auditory stimuli during isoflurane burst suppression anaesthesia

The cortical responses to auditory stimuli were studied in 12 patients during isoflurane anaesthesia producing burst suppression (ETisof (SD) 1.4 (0.2) vol.%). Earphones were used to give 3-s trains of auditory click stimuli (60 clicks, 20 clicks per second, 80 dB, 0.1 ms) at irregular intervals. In 10 patients, the electroencephalography (EEG) showed a burst suppression pattern consisting of high-amplitude activity intermingled with suppressed background activity. In eight patients with burst suppression patterns, there was a strong cortical reactivity to the termination, not to the beginning, of auditory stimuli: 80 (20)% of all stimuli presented during EEG suppression evoked high amplitude cortical response, offset-burst. The latency of these auditory offset evoked bursts was 540 (60) ms. Auditory offset evoked bursts suggest that in spite of cortical suppression during deep anaesthesia the brain retains its ability to respond to changes in the acoustic environment.

Cigarette smoking does not affect thiopentone pharmacodynamic or pharmacokinetic behaviour

PURPOSE

Smoking affects the pharmacodynamic and pharmacokinetic behaviour of several drugs. In smokers, induction of anaesthesia is often stormy. In this study we have determined whether cigarette smoking affected thiopentone pharmacodynamic or pharmacokinetic behaviour curing induction of anaesthesia.

METHODS

Fifteen smokers and 15 non-smokers, scheduled for elective surgery, were studied. Heart rate, invasive arterial pressures and middle latency auditory evoked potentials were recorded awake and during thiopentone induction (9 mg.kg-1 lean body mass), before and after tracheal intubation. Blood was sampled up to 24 hr after induction to measure thiopentone plasma concentrations and to calculate pharmacokinetic parameters.

RESULTS

Anaesthesia was adequate in all patients, although haemodynamic intubation response was not blunted. Latencies or amplitudes of middle latency auditory evoked potentials (MLAEP) did not differ between the groups. The postintubation latencies of Nb waves were 48.9 +/- 8.1 msec (mean +/- SD) in smokers and 48.1 +/- 8.5 msec in non-smokers. Pharmacokinetic data showed no differences between smokers and non-smokers. Clearance of thiopentone was 2.9 +/- 1.1 ml.min-1.kg-1 in smokers and 3.3 +/- 1.0 ml.min-1.kg-1 in non-smokers and elimination half life of thiopentone was 12.5 +/- 6.3 hr in smokers and 10.7 +/- 3.1 hr in non-smokers, respectively. The haemodynamic response after the induction dose of thiopentone and after tracheal intubation were similar in smokers and non-smokers. Mean postintubation systolic arterial pressures were 192 +/- 35 vs 189 +/- 20 mmHg and mean postintubation heart rates were 103 +/- 12 vs 102 +/- 17 beat per minute (bpm) in smokers and non-smokers, respectively.

CONCLUSION

We conclude, that cigarette smoking does not affect the pharmacodynamic or pharmacokinetic behaviour of thiopentone.

Midlatency auditory evoked potentials predict movements during anesthesia with isoflurane or propofol

To determine threshold values, sensitivity, and specificity of midlatency auditory evoked potentials (MLAEP) for prediction of spontaneous intraoperative movements, 40 patients undergoing elective laparotomy were studied. Continuous epidural analgesia was used in all patients. To maintain general anesthesia, the patients in Group 1 (n = 20) received isoflurane (0.4-1.2 vol%), and the patients in Group 2 (n = 20) received propofol (3-5 mg x kg(-1) x h(-1) intravenously). Spontaneous movements were documented intraoperatively. Auditory evoked potentials were recorded continuously until the end of anesthesia. Latencies of the peaks V, Na, Pa, Nb, and P1 (ms) and amplitudes Na/Pa, Pa/Nb, and Nb/P1 (microV) were measured. Changes of MLAEP latencies and amplitudes during anesthesia were similar in both groups. Anesthesia led to statistically significant increases in the latencies of Na, Pa, Nb, and P1 and decreases in the amplitudes of Na/Pa, Pa/Nb, and Nb/P1 compared with the awake state. Before and during spontaneous movement observed intraoperatively or during emergence from anesthesia, the latencies of the peaks Na, Pa, Nb, and P1 decreased, and the amplitudes Na/Pa, Pa/Nb, Nb/P1 increased significantly. A threshold value of 60 ms of Nb proved to be most predictive of movement during anesthesia. MLAEP recording seems to be a promising method to monitor the level of anesthesia as defined by spontaneous movement during anesthesia.

The rationale and utility of neurophysiological investigations in clinical monitoring for brain and spinal cord ischaemia during surgery and intensive care

This review concerns the role of neurophysiological monitoring in prevention of damage to the brain or spinal cord from inadequate supply of oxygenated blood during surgery and intensive care. The spontaneous and evoked electrical activity from neurones depends on oxidative metabolism of glucose, the substrate being delivered via the arterial blood supply and not stored locally. In critical situations electrical activity declines as oxygen delivery fails. Since this happens before neuronal death occurs it can provide early warning of risk. Methods for signal processing of EEG for this purpose must include time domain assessment and recognition of burst suppression pattern, as well as frequency domain information, and provide multivariate pattern recognition. Models for monitoring systems require analysis of data sets based on clinical studies where accurate outcome data is available so that valid comparisons between different approaches can be made. Neurophysiological features must be considered in the context of other relevant clinical parameters in the design of effective warning systems for prevention of brain damage due to inadequate oxygen supply.

Middle latency auditory evoked potentials during repeated transitions from consciousness to unconsciousness

We have investigated the relationship between changes in the middle latency auditory evoked potentials during alternating periods of consciousness and unconsciousness produced by propofol infusion combined with spinal anaesthesia for total knee replacement. Eleven patients completed the study, of whom two had recollection of events after the onset of the anaesthetic. There were no significant differences in heart rate or systolic arterial pressure between any conscious and unconscious period. With the first change from consciousness to unconsciousness, latencies of Na, Pa and Nb increased from mean (SD) starting values of 20.0 (1.4), 31.7 (1.0) and 42.8 (1.6) ms to 22.5 (2.0), 39.3 (2.1) and 57.8 (4.4) ms, respectively. During successive transitions from unconsciousness to consciousness, awake latencies were slightly higher than those of baseline awake, whereas anaesthetised latencies were similar to the ones obtained during the first period of unconsciousness. The consistent changes demonstrated, suggest that the auditory evoked potentials could represent a reliable indicator of potential awareness during anaesthesia.

Midlatency auditory evoked potentials during anaesthesia with increasing endexpiratory concentrations of desflurane

BACKGROUND

Under general anaesthesia with the volatile anaesthetics halothane, enflurane and isoflurane, midlatency auditory evoked potentials (MLAEP) are suppressed dose-dependently. Therefore, MLAEP have been used to measure depth of anaesthesia and to indicate intraoperative awareness. Desflurane is a new volatile anaesthetic and its effect on MLAEP have not been studied previously.

METHODS

We have studied MLAEP during general anaesthesia with increasing endexpiratory concentrations of desflurane in 12 patients scheduled for elective gynaecological surgery. Auditory evoked potentials were recorded in the awake state and during anaesthesia with endexpiratory steady state concentrations of 1.5, 3.0, 4.5 and 6.0 vol % of desflurane on vertex (positive) and mastoids on both sides (negative). Latencies of the peaks V, Na, Pa, Nb, Pl (ms) and amplitudes Na/Pa, Pa/Nb and Nb/Pl (micro V) were measured.

RESULTS

In the awake state, MLAEP had high peak-to-peak amplitudes and a periodic waveform. During general anaesthesia with increasing endexpiratory concentration of desflurane, the latency of the brainstem response V increased only slightly. In contrast, MLAEP showed a marked dose-dependent and statistically significant increase in latencies of Na, Pa, Nb and Pl and decrease in amplitudes of Na/Pa, Pa/Nb and Nb/Pl. Under 6.0 vol % of desflurane MLAEP were severely attenuated or even abolished.

CONCLUSION

Based on these observations, endexpiratory concentrations of > or = 4.5 vol % desflurane should suppress awareness phenomena such as auditory perceptions during anaesthesia.