Indicator Of Anesthetic Depth Research Articles

The Auditory Steady-State Response Is Not a Suitable Monitor of Anesthesia

Previous studies show that the human 40-Hz auditory steady-state response (ASSR) disappears on induction of general anesthesia, suggesting that it may be a good candidate for a monitor of anesthesia. In this study, we aimed to learn whether all normal alert adults display ASSRs with adequate signal-to-noise ratio. Clicks were presented at a series of frequencies between 35 and 70 Hz and electroencephalographic records taken at the vertex were Fourier transformed. ASSRs were observable as sharp peaks in the electroencephalograph spectrum at the frequency of the clicks. Initial results showed that a discernible ASSR could not be obtained from about half the subjects studied at any click frequency used. Further investigation revealed that in subjects whose ASSR was undetectable in the alert state, induction of a drowsy mental state resulted in appearance of an observable ASSR. This was attributable to an increase in signal in the drowsy state, not to a decrease in noise. We conclude that, because a significant proportion of subjects do not display easily recordable ASSRs when alert, it is not practical to use disappearance of the ASSR as a routine test for adequacy of anesthesia. Auditory steady-state responses (ASSRs) are brain waves evoked by auditory stimuli. Because they reportedly disappear under general anesthesia, they have been suggested as potential indicators of anesthetic depth. However, in this study, we show that about half our normal adult subjects did not produce measurable ASSRs when awake. This suggests that ASSRs are not good candidates for use in monitoring anesthetic depth during surgery.

Optimizing the acquisition time of the middle latency response using maximum length sequences and chirps.

The middle latency response (MLR) may be used as an indicator of anesthetic depth but has been criticized due to its long acquisition time. This study explores methods for optimizing recording of the MLR to maximize signal-to-noise ratio (SNR) and hence reduce acquisition time. The first experiment investigates the effects of increasing stimulation rate beyond conventional values and also using higher rates by means of maximum length sequences (MLS). The second experiment compares the use of click and chirp stimuli to elicit the MLR, both at conventional and MLS stimulation rates. For all conditions total recording duration is fixed at 185 s and stimulation level is fixed at 60 dB SL. It was found that SNR increases progressively with rate using conventional click stimulation until the theoretical rate limit is reached at the reciprocal of the response duration. The SNR improvement is equivalent to increasing test speed by a factor of 3. Using MLS stimulation, the SNR increases further until a maximum is reached at a rate of 167 clicks/s, equivalent to a fivefold test speed improvement relative to a conventional recording at 5 clicks/s. The use of chirp stimuli designed to compensate for the frequency dependent cochlear traveling wave delay produces an increase in wave V-Na amplitude at all recording rates. For the later latency waves of the response an increase in amplitude is seen for MLS, but not for conventional chirp trains. The optimum SNR was obtained using chirp stimuli at a MLS rate of 167 opportunities/s. It is concluded that the combination of chirps and MLS can reduce acquisition time to less than one-tenth of that required for conventional stimulation at 5 clicks/s for the same SNR. This would confer material benefits for estimating anesthetic depth using MLR.

Prediction of movement following noxious stimulation during 1 minimum alveolar anesthetic concentration isoflurane/nitrous oxide anesthesia by means of middle latency auditory evoked responses

This paper investigates the applicability of generalized dynamic neural networks for the design of a two-valued anesthetic depth indicator during isoflurane/nitrous oxide anesthesia. The indicator construction is based on the processing of middle latency auditory evoked responses (MLAER) in combination with the observation of the patient's movement reaction to skin incision. The framework of generalized dynamic neural networks does not require any data preprocessing, visual data inspection or subjective feature extraction. The study is based on a data set of 106 patients scheduled for elective surgery under isoflurane/nitrous oxide anesthesia. The processing of the measured MLAER is performed by a recurrent neural network that transforms the MLAER signals into signals having a very uncomplex structure. The evaluation of these signals is self-evident, and yields to a simple threshold classifier. Using only evoked potentials before the pain stimulus, the patient's reaction could be predicted with a probability of 81.5%. The MLAER is closely associated to the patient's reaction to skin incision following noxious stimulation during 1 minimum alveolar anesthetic concentration isoflurane/nitrous oxide anesthesia. In combination with other parameters, MLAER could contribute to an objective and trustworthy movement prediction to noxious stimulation.

The rat vertex-Middle Latency Auditory-Evoked Potential as indicator of anaesthetic depth: a comparison with evoked-reflex testing

We investigated whether components from the rat Vx-MLAEP could be used to assess depth of anaesthesia induced by propofol. Propofol decreased MLAEP amplitudes and increased latencies. We propose that the P 16–N 22 wave in the rat MLAEP is similar to the human P1, and that recovery of this wave during propofol anaesthesia correlates with behavioural measures of the regaining of consciousness.

Bispectral index as an indicator of anaesthetic depth during isoflurane anaesthesia in the pig

The objective of the study was to examine the relationship between the ‘depth’ of anaesthesia - as determined by clinical signs - and the bispectral index (BIS). Electroencephalograms (EEG)s were recorded in 8 female and 8 castrated male, healthy Norwegian landrace pigs undergoing isoflurane anaesthesia, from which the bispectral index (BIS) was calculated. Isoflurane was delivered in pure oxygen at end-tidal concentrations of 1.6, 1.9, 2.2 and 2.5%, in randomised order, for 30 min after which the EEG was recorded over a 5 min period. Anaesthetic depth was evaluated on a visual analogue scale (VAS) by an experienced anaesthetist. The 95% confidence interval for the mean correlation coefficient between BIS and VAS was calculated to be -0.52–0.30. Confidence intervals (95%) for the mean change in the BIS obtained during the conscious state and that obtained during anaesthesia at different isoflurane concentrations was also calculated. There was a significant decrease in the BIS recorded during consciousness and after 1.6% isoflurane anaesthesia, and between readings after inhalation of 2.2% and 2.5% isoflurane. This indicates that BIS does not accurately reflect ‘depth’ at surgical levels of isoflurane anaesthesia in the pig, and is of no use for this purpose.

EEG evaluation of reflex testing as assessment of depth of pentobarbital anaesthesia in the rat.

Electroencephalography (EEG) was applied to evaluate the validity of the paw pinch reflex as an indicator of anaesthetic depth in rats which are anaesthetized with a single intraperitoneal dose of pentobarbital. After induction of the anaesthesia, characterized by the rapid loss of the animals' ability to maintain upright posture, the EEG of 10 out of 11 rats was dominated by paroxysmal (burst suppression) activity, associated with unconsciousness. In seven out of 11 rats, the paw pinch reflex was lost after onset of paroxysmal electroencephalographic activity. However, the paw pinch reflex remained present in four out of 11 animals, demonstrating that the response is independent of cortical activity. In five out of seven rats, the EEG still showed paroxysmal activity when the paw pinch reflex was regained. However, in two other rats the EEG returned to a pattern similar to that shown by awake animals, 4 and 21 min respectively, before the reflex was regained. These data indicate that in the pentobarbital-anaesthetized rat, presence of the paw pinch reflex is not related to the level of depression of electrical activity in the cerebral cortex, and consequently is probably not related to the level of consciousness. Based upon these findings it is concluded that the paw pinch reflex is unreliable as a sole indicator of anaesthetic depth.

HEART SOUNDS DURING PEDIATRIC SURGERY

S218 INTRODUCTION: Although continous ausculation is often used during anesthesia, heart sounds in anesthetized children have never been studied. Phonocardiographic studies on awake patients have shown close correlations between heart sounds and hemodynamics. We hypothesized that amplitude and frequency characteristics of heart sounds could be used as indicators of cardiovascular status and anesthetic depth in children. METHODS: After IRB approval and informed consent was obtained, heart sounds of 11 ASA class I-II children aged 1-10 years undergoing general anesthesia and surgery were continuously recorded to digital audio tape (DAT). Patients with congenital heart disease were excluded. Heart rate and blood pressure (noninvasive) were recorded using a computerized automated record keeper. At five portions of each case anesthetic depth was estimated using criteria other than heart sounds (eg. movement, tachycardia upon surgical stimulation). Anesthetic level was graded 1-4, 1 being almost awake (emergence) and 4 being "deep". Retrospectively, heart sounds at the corresponding portions were transferred to a computer and analyzed for amplitude (root mean squared) and frequency (peak) characteristics. Linear regression was used to determine relationships between apparent anesthetic depth, hemodynamics, and heart sound characteristics. RESULTS: In 10 of the 11 cases lighter anesthesia was associated with louder first heart sound (S1) (R2 =0.80 +/-.04). In all cases there was a positive correlation between heart rate and S1 amplitude (R2 =0.86 +/-.04); similarly, between systolic blood pressure and second heart sound (S2) amplitude, except during extreme excitement (Depth=1). Characteristic phonocardiographic traces for "light" and "deep" anesthesia are shown below. No correlation between spectral pattern and hemodynamics was detected. (Figure 1)Figure 1DISCUSSION: In awake adults S1 amplitude has been shown to correlate closely with sympathetic nervous system activity and cardiac contractility [1], and S2 amplitude has been shown to correlate with systolic blood pressure [2]. This study indicates that S1 amplitude may be used to monitor cardiac performance, and, possibly, anesthetic depth in pediatric patients. Advanced digital technology allows rapid and inexpensive analysis of phonocardiographic data, thus providing new opportunities for monitoring technique and research.

Measuring the Performance of Anesthetic Depth Indicators

An appropriate measure of performance is needed to identify anesthetic depth indicators that are promising for use in clinical monitoring. To avoid misleading results, the measure must take into account both desired indicator performance and the nature of available performance data. Ideally, anesthetic depth indicator value should correlate perfectly with anesthetic depth along a lighter-deeper anesthesia continuum. Experimentally, however, a candidate anesthetic depth indicator is judged against a "gold standard" indicator that provides only quantal observations of anesthetic depth. The standard anesthetic depth indicator is the patient's response to a specified stimulus. The resulting observed anesthetic depth scale may consist only of patient "response" versus "no response," or it may have multiple levels. The measurement scales for both the candidate anesthetic depth indicator and observed anesthetic depth are no more than ordinal; that is, only the relative rankings of values on these scales are meaningful. Criteria were established for a measure of anesthetic depth indicator performance and the performance measure that best met these criteria was found. The performance measure recommended by the authors is prediction probability PK, a rescaled variant of Kim's dy.x measure of association. This performance measure shows the correlation between anesthetic depth indicator value and observed anesthetic depth, taking into account both desired performance and the limitations of the data. Prediction probability has a value of 1 when the indicator predicts observed anesthetic depth perfectly, and a value of 0.5 when the indicator predicts no better than a 50:50 chance. Prediction probability avoids the shortcomings of other measures. For example, as a nonparametric measure, PK is independent of scale units and does not require knowledge of underlying distributions or efforts to linearize or to otherwise transform scales. Furthermore, PK can be computed for any degree of coarseness or fineness of the scales for anesthetic depth indicator value and observed anesthetic depth; thus, PK fully uses the available data without imposing additional arbitrary constraints, such as the dichotomization of either scale. And finally, PK can be used to perform both grouped- and paired-data statistical comparisons of anesthetic depth indicator performance. Data for comparing depth indicators, however, must be gathered via the same response-to-stimulus test procedure and over the same distribution of anesthetic depths. Prediction probability PK is an appropriate measure for evaluating and comparing the performance of anesthetic depth indicators.

Halothane decreases pontine acetylcholine release and increases EEG spindles.

This study tested the hypothesis that halothane anesthesia would cause decreased acetylcholine (ACh) release within the medial pontine reticular formation (mPRF). ACh was collected by microdialysis and measured by high pressure liquid chromatography during wakefulness and during halothane-induced anesthesia. The electroencephalogram (EEG) showed that spindles were a reliable indicator of anesthetic depth. There was a statistically significant disease in ACh release during halothane anesthesia compared with ACh release during wakefulness. Spindles always disappeared during noxious stimulation and during emergence from anesthesia when pontine ACh levels began to increase. These results are consistent with previous data concerning brain stem cholinergic influences on thalamocortical spindle generation, and suggest that similar mechanisms generate cortical spindles during natural sleep and halothane anesthesia.

The Electroencephalographs Effects of Desflurane in Humans

The electroencephalographic (EEG) effects of a new inhaled anesthetic are of interest because of the potential of such agents to produce excitatory (convulsant) activity and because of the potential usefulness of the EEG as an indicator of anesthetic depth and cerebral activity. Accordingly, we examined the EEG in 12 healthy, young male volunteers during desflurane anesthesia. Each subject had a baseline recording and then steady-state exposure to 6, 9, and 12% (0.83, 1.24, and 1.66 MAC) desflurane in O2 alone, and to 3, 6, and 9% desflurane in O2 with 60% N2O. The sequence of doses and the presence of N2O were randomized. We used mechanical ventilation to maintain normocapnia at each dose level. We also tested the effects of hypercapnia secondary to spontaneous ventilation. Additionally, at 1.24 MAC, subjects' lungs were hyperventilated to a PCO2 of 25.8 +/- 0.7 mmHg and exposed to rhythmic, loud clapping to attempt to provoke excitatory phenomena. Finally, after at least 6 h exposure to desflurane, we repeated measurements at 0.83 and 1.66 MAC to assess possible tolerance. Four channels of EEG were monitored visually, and at each dose, a quantitative EEG analysis was performed. Desflurane produced EEG changes comparable to those observed with equipotent levels of isoflurane. No epileptiform activity was seen. Desflurane significantly suppressed EEG activity; prominent burst suppression was seen at 1.24 MAC and higher. Substitution of N2O for 0.42 MAC desflurane reduced the degree of EEG suppression relative to the equipotent administration of desflurane and O2. Quantitative EEG measures for the early doses and for the later, repeated exposures did not differ.

Effets de l'isoflurane et de l'halothane sur les potentiels évoqués auditifs et somesthésiques

The effects of isoflurane and halothane anaesthesia on brainstem auditory (BEAR) and somatosensory evoked potentials (SEP) were recorded in 15 patients ; eight received isoflurane and seven halothane. Atropine alone was given as premedication. After induction of anaesthesia with thiopentone, it was maintained with 50 % nitrous oxide in oxygen. Ventilation was controlled. Isoflurane and halothane concentrations were gradually increased to 3 %. Recordings were made at 1 %, 2.5 % and 3 %. There were statistically significant differences in the latencies of SEP and BEAR with increasing concentrations of anaesthetic. The BEAR waves significantly affected were wave V and the latency between peaks 1 to V (p < 0.001) ; for the SEP, it was wave N20. SEP central conduction time was prolonged with both anaesthetics (p < 0.001). The effects of isoflurane and halothane were similar. These results could not be explained by changes in central body temperature or end-tidal carbon dioxide concentration. The study showed a dose-related direct effect of both anaesthetic agents on the brainstem auditory and somatosensory evoked responses. It may be that the measurement of changes in evoked potentials could be a useful indicator of anaesthetic depth.

EVOKED POTENTIALS DURING ISOFLURANE ANAESTHESIA

Somatosensory, visual and brainstem auditory evoked potentials were recorded in 10 unpremedicated patients anaesthetized with isoflurane in oxygen. Recordings were made at 0.5%, 1.1% and 1.65% (six patients) end-tidal isoflurane concentration. There were statistically significant increases in the latencies of the somatosensory (N20), visual and brainstem auditory potentials (waves III and V) with increasing concentrations of isoflurane. The central conduction time was prolonged. Amplitudes of the somatosensory and visual potentials were reduced with increasing concentrations of isoflurane. The effects of isoflurane on evoked potentials are similar to those of halothane and enflurane. It is possible that changes in evoked potential measurements may be useful as a neurophysiological indicator of anaesthetic depth.