[Monitoring intraoperative awareness. Vegetative signs, isolated forearm technique, electroencephalogram, and acute evoked potentials]

Premedication with fentanyl-midazolam improves sevoflurane anesthesia for surgical intervention in laboratory mice

Balanced anesthesia allows for a reduced dosage of each component, while inducing general anesthesia of sufficient depth with potentially fewer side effects. Here, we compare two anesthetic protocols combining sevoflurane anesthesia with pre-medication (ketamine [K] or fentanyl-midazolam [FM]) to a sevoflurane monoanesthesia (S) concerning their ability to provide reliable anesthesia suitable for moderate surgery in laboratory mice. Twenty-one female C57BL/6J mice assigned randomly to one of three protocols underwent a 50-min anesthesia and a sham embryo transfer. Heart rate and core body temperature were continuously recorded by telemetry intra-operatively and for three days pre- and three days post-surgery. Intra-operative respiratory rate was determined by counting thorax movements. Body weight, food, and water intake were measured daily for three days pre- and three days post-surgery. The heart rate in the KS group remained at baseline level throughout the 50-min of anesthesia and surgery. FMS caused a lower heart rate and S alone caused a higher heart rate compared to baseline values. Intra-operative body temperature was at baseline levels in all groups. A decreased respiratory rate was observed in all groups compared to baseline values obtained from resting mice of the same strain, sex and age-distribution. Surgical stimuli induced no significant changes in heart rate and respiratory rate in the KS or FMS group but significant respiratory alteration in the S group compared to baseline values obtained 10 s before applying the stimulus. Post-operative heart rate was above baseline values in all groups; with a significant deviation in the S group. There were no changes in body weight, food, and water intake. In summary, FMS was superior to KS and S for moderate surgery in laboratory mice resulting in less inter-individual variability in response to painful stimuli. Fentanyl and midazolam reduced the depressant effect of sevoflurane on the respiratory rate and the negative post-anesthetic effects on the heart rate. Impact statement With approximately 65 million animals used per year mice are still the most prevalent laboratory mammal species worldwide. In course of biomedical research projects approximately 40% of mice will undergo one or more short or long-term anesthesia. Sufficient anesthetic depth, cardiovascular stability, adequate analgesia, and short recovery times are essential requirements of anesthetic protocols to meet animal welfare. Anesthesia in mice and rats are only to be performed by personnel with appropriate basic training and experience. However, more and more adapted and advanced anesthetic protocols, required to answer very specific scientific questions, often exceed the skills acquired through basic training and present a major challenge to researchers. It is therefore of great importance to further develop and evaluate safe and reliable anesthetic protocols as presented in this study to provide new perspectives on this challenging problem.

ICU Sedation: A Review of Its Pharmacology and Assessment

The need for appropriate sedation in the intensive care unit is paramount. Critically ill patients are exposed to multiple adverse stimuli stemming from both their illness and their environment. If left unchecked, these stimuli may often produce potentially harmful physiologic sequelae in patients who already have compromised physiologic reserve. The most useful sedative agents in such circumstances are those which are readily titratable and have manageable side effects. This typically focuses discussion on the intravenous administration of analgesic sedatives (opioids), anxiolytic and amnestic sedatives (benzodiazepines, barbiturates, etomidate, propofol), dissociative sedatives (ketamine), and the antipsychotic sedatives (butyrophenones). With ready titratability, though, comes the need for efficient monitoring and assessment of the degree of sedation. While no measure is without bias, this can effectively be done via the subjective means of a sedation scoring scheme or the more objective means of electrophysiologic measurements. It is the combination of pharmacological tools and consistent assessment which will allow the intensivist to readily achieve the desired sedation goal.

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.

Subjective assessment of depth of anaesthesia by experienced and inexperienced anaesthetists

BACKGROUND AND OBJECTIVE

To measure 'depth of anaesthesia', anaesthesiologists use a combination of observable end-points such as immobility and autonomic stability. Unconsciousness and amnesia are not reliably observable end-points, but correlate with parameters derived from the electroencephalogram. We investigated the association of subjective assessment and electroencephalographic measures of anaesthetic depth in a group of experienced (>4 yr of experience) and a group of inexperienced (<2 yr of experience) anaesthesiologists.

METHODS

One hundred ASA I or II patients were assigned to either group. Anaesthesiologists assessed 'anaesthetic depth' using an 11-point numeric and a 5-point verbal scale. Bispectral index and spectral entropy were recorded as electroencephalogram parameters. The association between the subjective assessment and the electroencephalogram parameters was calculated using the prediction probability, PK.

RESULTS

Association between subjective assessment and electroencephalographic parameters showed a tendency to a better prediction probability in the experienced group. The difference was significant (P < 0.05) for the bispectral index (PK 0.76 +/- 0.01 for experienced and 0.71 +/- 0.01 for inexperienced anaesthesiologists). In both groups, a large percentage of the data points recorded during surgery showed bispectral index values above the recommended value of 60 (13.2% in the experienced and 34.3% in the inexperienced group) despite a subjective assessment of 'deep' or 'very deep' anaesthetic depth.

CONCLUSION

The study demonstrates that the association between subjectively assigned values of anaesthetic depth and electroencephalographic parameters of anaesthetic depth is better for anaesthesiologists with more clinical experience. However, in the 'inexperienced' as well as 'experienced' group a high percentage of bispectral index and entropy values above 60 occurred despite a subjective assessment of adequate anaesthetic depth. Although there was no evidence for explicit memory, this may indicate a risk for memory formation.

Pitfalls and challenges when assessing the depth of hypnosis during general anaesthesia by clinical signs and electronic indices

The objective of this article was to review the present methods used for validating the depth of hypnosis. We introduce three concepts, the real depth of hypnosis (DHreal), the observed depth of hypnosis (DHobs), and the electronic indices of depth of hypnosis (DHel-ind). The DHreal is the real state of hypnosis that the patient has in a given moment during the general anaesthesia. The DHobs is the subjective assessment of the anaesthesiologist based on clinical signs. The DHel-ind is any estimation of the depth of hypnosis given by an electronic device. The three entities DHreal, DHobs and DHel-ind should in the ideal situation be identical. However, this is rarely the case. The correlation between the DHobs and the DHel-ind can be affected by a number of factors such as the stimuli used for the assessment of the level of consciousness or the administration of analgesic agents or neuro muscular blocking agents. Opioids, for example, can block the response to tactile and noxious stimuli, and even the response to verbal command could vanish, hence deeming the patient in a lower depth of hypnosis than the real patient state. The DHel-ind can be disturbed by the presence of facial muscular activity. In conclusion, although several monitors and clinical scoring scales are available to assess the depth of hypnosis during general anaesthesia, care should be taken when interpreting their results.

La neuro-anesthésie assistée par l’électroencéphalogramme

The electrophysiology can be considered as one of the most reliable assessment of the nervous system function during anaesthesia. For instance, the electroencephalogram (EEG) can be relatively easily simplified and adapted to the current practice of the neuro-anaesthesia or neuro-intensive medicine. Since approximately 10 years, some extremely simplified devices have been introduced, but they allowed only inaccurate assessment because of their too global analysis of the brain function. The monitoring of the true digitised EEG, based on a simplified brain mapping montage combined to the analysis in the frequency and the time domains corresponds to a new investigation tool: the simplified EEG or sEEG, which can be included in our usual monitors or anaesthesia and intensive care machines. This new sort of monitoring could give us valuable information about, first, the control of the peroperative consciousness and the nociceptive or not afferent inputs due to surgical stimulations, and, second, the detection of several particular physiopathologic states during neuro-anaesthesia (brain ischaemia, non-convulsive epilepsy, cerebral protection em leader ). These different theoretical and practical topics are reviewed to try to define the possible preliminary rules of the use of the sEEG.

Performance of the rapidly extracted auditory evoked potentials index to detect the recovery and loss of wakefulness in anesthetized and paralyzed patients

BACKGROUND

The rapidly extracted auditory evoked potentials index (A-lineTM ARX Index or AAI) has been proposed as a method to measure the depth of anesthesia. A prospective study was designed to assess the performance of AAI to detect the recovery and loss of wakefulness in anesthetized and paralyzed patients.

METHODS

Fourteen adult patients undergoing elective surgery were anesthetized with propofol 1.5 mg kg-1, vecuronium 0.1 mg kg-1 and further propofol 1.0 mg kg-1. Wakefulness was measured by the ability of the patient to respond to command using the isolated forearm technique (IFT). After the patient responded, propofol was infused at 10 mg kg-1. h-1 until wakefulness (responsiveness) was lost. The AAI was recorded continuously throughout the study and analyzed off-line.

RESULTS

The AAI showed a significant difference between the values registered during, 30 s before and 30 s after the recovery, and also between 30 s before and 30 s after the loss of wakefulness. The prediction probability (Pk) values for AAI were 0.786 and 0.864 during the transitions from unresponsiveness to responsiveness and from responsiveness to unresponsiveness. The area under the receiver operating characteristic curve for the responsive and unresponsive values was 0.926 (SE 0.002, 95% CI 0.922-0.931), and the AAI values of approximately 5%, 50% and 95% predicted probability of wakefulness were 19, 29 and 39, respectively.

CONCLUSION

The AAI may be a good predictor of recovery and loss of wakefulness for anesthetized and paralyzed patients.

Combining median electroencephalography frequency and sympathetic activity in an index to evaluate opioid detoxification in patients

During rapid opioid detoxification, increased sympathetic activity and a greater median frequency (MF) of activity on electroencephalography (EEG) have been reported. The purpose of this study was to evaluate a new index for detoxification that combines sympathetic activity and MF data. After informed consent was obtained, eight patients were sedated with propofol. The MF of EEG activity derived from frontal electrodes was determined. Heart rate variability was evaluated in 256-second segments by power spectral analysis, and sympathetic activity was determined by the low frequency component. The Hoffman Index for narcotic detoxification was weighted 70% to sympathetic activity and 30% to MF to normalize the difference in scales and to provide adequate weight to the sympathetic component. Opioid detoxification was produced by infusion of 25 mg naloxone for 30 minutes, followed by a 24-hour infusion of 1 mg per hour. The MF showed a rapid increase during high-dose infusion of naloxone, but the peak response occurred 1 to 2 hours later. Sympathetic activation and the Hoffman Index increased more slowly after the start of naloxone infusion, but peak increases in all components occurred at approximately the same time. The peak increases in Hoffman Index (110% of baseline), MF (260%), and sympathetic activity (304%) during administration of naloxone were significant and correlated with respect to time (r = 0.89-0.94). The Hoffman Index showed an early increase related to MF and a well-defined peak response indicative of sympathetic and MF activity. The behavior of the Hoffman Index in relation to the MF and sympathetic activity more clearly indicated the onset of opioid detoxification and the maximum response to opioid reversal than did MF or sympathetic activity alone.

On-line analysis of middle latency auditory evoked potentials (MLAEP) for monitoring depth of anaesthesia in laboratory rats

In laboratory animals as well as in human beings a depth of anaesthesia, where the subject has no pain or recall of events from the surgery, should be provided. Haemodynamic parameters such as heart rate and blood pressure are not a guarantee for an optimal depth of anaesthesia, especially when using neuromuscular blocking agents (NMBA). A number of studies suggest that the Middle Latency Auditory Evoked Potentials (MLAEP) contain information about the state of consciousness in humans. The purpose of this study was to examine whether the AEP could serve as an indicator of depth of anaesthesia in rats. The AEP was elicited with a click stimulus and monitored in an 80 ms window synchronised to the stimulus. The AEP was extracted applying an Auto Regressive Model with Exogenous Input (ARX-model) from which a Depth of Anaesthesia Index (DAI) was calculated. DAI was normalised to 100 while awake and decreasing gradually to a level between 50 and 20 as the rat was anaesthetised. Nine rats were anaesthetised and included in the study. Four doses of Hypnorm vet. and Dormicum were given as a total, each with 5 minutes interval. Clinical signs of the level of anaesthesia were observed simultaneously with the AEP. The results showed that in four rats DAI decreased to a level below 30 while anaesthetised. In the remaining five rats the AEP was only decreased to a level below 45. The results indicated that a simple dosing regimen based on weight was unable to give the same depth of anaesthesia in individual rats. The decrease in the DAI correlated well with the loss of stimulus response. In conclusion, MLAEP could be used as an indicator of depth of anaesthesia in rats during Hypnorm vet. and Dormicum administration. However studies applying other anaesthetic drugs should be carried out, before a conclusion of the general utility of the method can be made.