The midlatency auditory evoked potentials predict responsiveness to verbal commands in patients emerging from anesthesia with xenon, isoflurane, and sevoflurane but not with nitrous oxide

Dissociation of Centrally and Peripherally Induced Transcranial Magnetic Stimulation Effects in Nonhuman Primates

Transcranial magnetic stimulation (TMS) is a noninvasive brain stimulation method that is rapidly growing in popularity for studying causal brain-behavior relationships. However, its dose-dependent centrally induced neural mechanisms and peripherally induced sensory costimulation effects remain debated. Understanding how TMS stimulation parameters affect brain responses is vital for the rational design of TMS protocols. Studying these mechanisms in humans is challenging because of the limited spatiotemporal resolution of available noninvasive neuroimaging methods. Here, we leverage invasive recordings of local field potentials in a male and a female nonhuman primate (rhesus macaque) to study TMS mesoscale responses. We demonstrate that early TMS-evoked potentials show a sigmoidal dose-response curve with stimulation intensity. We further show that stimulation responses are spatially specific. We use several control conditions to dissociate centrally induced neural responses from auditory and somatosensory coactivation. These results provide crucial evidence regarding TMS neural effects at the brain circuit level. Our findings are highly relevant for interpreting human TMS studies and biomarker developments for TMS target engagement in clinical applications.SIGNIFICANCE STATEMENT Transcranial magnetic stimulation (TMS) is a widely used noninvasive brain stimulation method to stimulate the human brain. To advance its utility for clinical applications, a clear understanding of its underlying physiological mechanisms is crucial. Here, we perform invasive electrophysiological recordings in the nonhuman primate brain during TMS, achieving a spatiotemporal precision not available in human EEG experiments. We find that evoked potentials are dose dependent and spatially specific, and can be separated from peripheral stimulation effects. This means that TMS-evoked responses can indicate a direct physiological stimulation response. Our work has important implications for the interpretation of human TMS-EEG recordings and biomarker development.

Latency of auditory evoked potential monitoring the effects of general anesthetics on nerve fibers and synapses

Auditory evoked potential (AEP) is an effective index for the effects of general anesthetics. However, it’s unknown if AEP can differentiate the effects of general anesthetics on nerve fibers and synapses. Presently, we investigated AEP latency and amplitude changes to different acoustic intensities during pentobarbital anesthesia. Latency more regularly changed than amplitude during anesthesia. AEP Latency monotonically decreased with acoustic intensity increase (i.e., latency-intensity curve) and could be fitted to an exponential decay equation, which showed two components, the theoretical minimum latency and stimulus-dependent delay. From the latency-intensity curves, the changes of these two components (∆L and ∆I) were extracted during anesthesia. ∆L and ∆I monitored the effect of pentobarbital on nerve fibers and synapses. Pentobarbital can induce anesthesia, and two side effects, hypoxemia and hypothermia. The hypoxemia was not related with ∆L and ∆I. However, ∆L was changed by the hypothermia, whereas ∆I was changed by the hypothermia and anesthesia. Therefore, we conclude that, AEP latency is superior to amplitude for the effects of general anesthetics, ∆L monitors the effect of hypothermia on nerve fibers, and ∆I monitors a combined effect of anesthesia and hypothermia on synapses. When eliminating the temperature factor, ∆I monitors the anesthesia effect on synapses.

Auditory evoked potential index does not correlate with observer assessment of alertness and sedation score during 0.5% bupivacaine spinal anesthesia with nitrous oxide sedation alone

PURPOSE

The aim of this study was to evaluate the auditory evoked potential (AEP) index as a hypnosis monitor during nitrous oxide (N(2)O) sedation added to spinal analgesia.

METHODS

Forty-five patients scheduled to undergo surgery under spinal anesthesia were recruited after giving informed consent. Adequate anesthesia levels were confirmed, and a disposable AEP index sensor (aepEX, Medical Device Management) was placed. A tight facemask was fitted, and a fresh gas flow of 100% oxygen 10 L/min was provided. AEP index monitoring was then initiated, and measurements and observer assessment of alertness/sedation (OAA/S) scores were recorded manually. N(2)O was administered in stepwise increases in the end-tidal concentration of 33%, 50%, and 67%. Paired AEP index and OAA/S scores were obtained immediately before each change in N(2)O concentration.

RESULTS

Sixteen patients were excluded from final analysis because of nausea, vomiting, or abnormal excitatory behaviors. The increases in N(2)O concentration induced significant decreases in OAA/S scores and no substantial AEP index changes. Although OAA/S scores of 1 and 2 were observed in only two and five patients, respectively, a reduction in the OAA/S score from 5 to 1 was associated with a significant decrease in AEP index to the level indicative of moderate sedation.

CONCLUSION

The AEP index might not be a suitable indicator of light hypnosis as defined by an OAA/S score of ≥3 during sedation with N(2)O alone.

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.

Effects of increasing sevoflurane MAC levels on mid-latency auditory evoked potentials in infants, schoolchildren, and the elderly

BACKGROUND

Detection of mid-latency auditory evoked potentials (MLAEPs) is a technology to monitor central nervous structures. As seen in adults and children, general anaesthesia influences the MLAEP latencies. MLAEP detection seems to be a promising tool to assess different levels of anaesthesia depth in adults and children.

METHODS

MLAEPs were recorded in 10 infants (2 months-3 yr), 12 schoolchildren (6-14 yr), and 10 elderly (75-89 yr) under general anaesthesia with increasing concentrations of sevoflurane at steady state. In addition, MLAEPs were detected before and after the application of sufentanil.

RESULTS

At all different ages, MLAEP latencies increased significantly with higher volume percentages of sevoflurane. These results were also detectable when MAC values of sevoflurane were compared with MLAEP peaks. An age-dependent effect could be displayed as elderly people need lower absolute sevoflurane concentrations to achieve the same MLAEP peak increase. Overall, the application of sufentanil under steady-state sevoflurane application at 1 MAC did not importantly affect the MLAEP latencies.

CONCLUSIONS

MLAEP latencies increase at the influence of sevoflurane in a dose-dependent manner and in relation to age. These results imply that MLAEP detection is a reasonable tool for monitoring hypnotic effects at all ages. Further studies are required to standardize MLAEP alterations related to effects of medication used for general anaesthesia at all different ages.

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.

Special cases: Ketamine, nitrous oxide and xenon

Most general anaesthetic agents produce anaesthesia by increasing the activity of inhibitory gamma-aminobutyric acid type A receptors. The effects of ketamine, xenon and nitrous oxide on these receptors are, however, negligible. These anaesthetic agents potently inhibit excitatory N-methyl-D-aspartate receptors. Although these anaesthetic agents display some similar clinical features, such as potent analgesic effects, there are some important differences. Ketamine and nitrous oxide produce sympathomimetic effects, whereas xenon produces a sympatholytic effect. In addition, these anaesthetic agents return differential signals on clinical available anaesthetic depth monitors such as the bispectral index and mid-latency auditory evoked potential. Ketamine and nitrous oxide do not per se decrease the bispectral index. However, xenon decreases the bispectral index in a concentration-dependent manner. Similarly, ketamine and nitrous oxide do not suppress the mid-latency auditory evoked potential whereas xenon does. Thus, anaesthetic depth monitors fail to describe consciousness accurately when ketamine and nitrous oxide are used.

Inert gases as the future inhalational anaesthetics?

Of all the inert gases, only xenon has considerable anaesthetic properties under normobaric conditions. Its very low blood/gas partition coefficient makes induction of and emergence from anaesthesia more rapid compared with other inhalational anaesthetics. In experimental and clinical studies the safety and efficiency of xenon as an anaesthetic has been demonstrated. Xenon causes several physiological changes, which mediate protection of the brain or myocardium. The use of xenon might therefore be beneficial in certain clinical situations, as in patients at high risk for neurological or cardiac damage.

Cardiovascular effects of xenon and nitrous oxide in patients during fentanyl-midazolam anaesthesia*

Xenon anaesthesia appears to have minimal haemodynamic effects. The purpose of this randomised prospective study was to compare the cardiovascular effects of xenon and nitrous oxide in patients with known ischaemic heart disease. In 20 patients who were due to undergo coronary artery bypass graft surgery, 30 min following induction of anaesthesia with fentanyl 30 microg x kg(-1) and midazolam 0.1 mg x kg(-1) but prior to the start of surgery, xenon or nitrous oxide 60% was administered for 15 min. The results showed that xenon caused a minimal decrease in the mean arterial pressure (from 81 (7) to 75 (8) mmHg, mean (SD)), but did not affect the systolic function of the left ventricle, as demonstrated by unchanged left ventricular stroke work index (LVSWI) and the fractional area change of the left ventricle (FAC) derived from transoesophageal echocardiography (TOE). However, in contrast, nitrous oxide was found to decrease the mean arterial pressure (from 81 (8) to 69 (7) mmHg), the LVSWI, and the FAC. The cardiac index, central venous and pulmonary artery occlusion pressures, systemic and pulmonary vascular resistances, and the TOE-derived E/A ratio through the mitral valve were unchanged by xenon or nitrous oxide. We conclude that xenon provides improved haemodynamic stability compared with nitrous oxide, conserving the left ventricular systolic function.

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.

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.

The effects of nitrous oxide on the auditory evoked potential index during sevoflurane anaesthesia

We have studied the effects of nitrous oxide on the auditory evoked response index (AAI-index) derived from the A-line monitoring device during sevoflurane anaesthesia in 21 patients undergoing minor ambulatory surgery. During sevoflurane anaesthesia with an AAI-index < 30, the addition or withdrawal of nitrous oxide in a concentration of 66% end tidal did not show any linear dose dependent change in AAI-index . However, comparing nitrous oxide > 40% to nitrous oxide < 10% end tidal concentration the AAI-index did decrease, p < 0.05. The AAI-index is either non-linear at deeper anaesthetic levels or is insensitive to the anaesthetic effects of nitrous oxide in terms of MAC-multiples.

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.