Recording Brain Electromagnetic Activity During the Administration of the Gaseous Anesthetic Agents Xenon and Nitrous Oxide in Healthy Volunteers

Published: January 13, 2018

doi:

Andria Pelentritou, Levin Kuhlmann, John Cormack, Will Woods, Jamie Sleigh, David Liley

¹Centre for Human Psychopharmacology,Swinburne University of Technology, ²Department of Anaesthesia and Pain Management,St. Vincent’s Hospital Melbourne, ³Brain and Psychological Science Research Centre,Swinburne University of Technology, ⁴Department of Anaesthesiology,University of Auckland

Summary

Simultaneous magnetoencephalography and electroencephalography provides a useful tool to search for common and distinct macro-scale mechanisms of reductions in consciousness induced by different anesthetics. This paper illustrates the empirical methods underlying the recording of such data from healthy humans during N-Methyl-D-Aspartate-(NMDA)-receptor-antagonist-based anesthesia during inhalation of nitrous oxide and xenon.

Abstract

Anesthesia arguably provides one of the only systematic ways to study the neural correlates of global consciousness/unconsciousness. However to date most neuroimaging or neurophysiological investigations in humans have been confined to the study of γ-Amino-Butyric-Acid-(GABA)-receptor-agonist-based anesthetics, while the effects of dissociative N-Methyl-D-Aspartate-(NMDA)-receptor-antagonist-based anesthetics ketamine, nitrous oxide (N₂O) and xenon (Xe) are largely unknown. This paper describes the methods underlying the simultaneous recording of magnetoencephalography (MEG) and electroencephalography (EEG) from healthy males during inhalation of the gaseous anesthetic agents N₂O and Xe. Combining MEG and EEG data enables the assessment of electromagnetic brain activity during anesthesia at high temporal, and moderate spatial, resolution. Here we describe a detailed protocol, refined over multiple recording sessions, that includes subject recruitment, anesthesia equipment setup in the MEG scanner room, data collection and basic data analysis. In this protocol each participant is exposed to varying levels of Xe and N₂O in a repeated measures cross-over design. Following relevant baseline recordings participants are exposed to step-wise increasing inspired concentrations of Xe and N₂O of 8, 16, 24 and 42%, and 16, 32 and 47% respectively, during which their level of responsiveness is tracked with an auditory continuous performance task (aCPT). Results are presented for a number of recordings to highlight the sensor-level properties of the raw data, the spectral topography, the minimization of head movements, and the unequivocal level dependent effects on the auditory evoked responses. This paradigm describes a general approach to the recording of electromagnetic signals associated with the action of different kinds of gaseous anesthetics, which can be readily adapted to be used with volatile and intravenous anesthetic agents. It is expected that the method outlined can contribute to the understanding of the macro-scale mechanisms of anesthesia by enabling methodological extensions involving source space imaging and functional network analysis.

Introduction

There is good consensus between pre-clinical and clinical neuroscientific evidence suggesting that the phenomenon of human consciousness depends on the integrity of explicit neural circuits. The observation that such circuits are systematically influenced by the descent into unconsciousness has substantiated the need for neuroimaging techniques to be utilized during anesthesia and enable 'navigating' the search for the neural correlates of consciousness. With the possible exception of sleep, anesthesia represents the only method by which one can, in a controlled, reversible and reproducible fashion, perturb, and thus dissect, the mechanisms that sub-serve consciousness, especially at the macroscopic scale of global brain dynamics. Clinically, general anesthesia can be defined as a state of hypnosis/unconsciousness, immobility and analgesia and remains one of the most abundantly used and safest medical interventions. Despite the clarity and efficiency in the end result, there remains great uncertainty regarding the mechanisms of action of the various types of agents giving rise to anesthetic induced unconsciousness¹.

Anesthetics can be divided into intravenous agents notably propofol and the barbiturates, or volatile/gaseous agents such as sevoflurane, isoflurane, nitrous oxide (N₂O) and xenon (Xe). The pharmacology of anesthesia has been well established with multiple cellular targets identified as linked to anesthetic action. Most agents studied to date act principally via the agonism of γ-Amino-Butyric-Acid-(GABA) receptor mediated activity. In contrast, the dissociative agents ketamine, Xe and N₂O are believed to exert their effects by primarily targeting N-Methyl-D-Aspartate-(NMDA) glutamatergic receptors²^,³. Other important pharmacological targets include potassium channels, acetylcholine receptors and the remnant glutamate receptors, AMPA and kainate, however the extent of their contribution to anesthetic action remains elusive (for a comprehensive review see ⁴).

The extent of variability in the mechanism of action and the observed physiological and neural effects of the various types of agents renders the derivation of general conclusions on their influence on conscious processing difficult. Loss of consciousness (LOC) induced by GABAergic agents is typically characterized by a global change in brain activity. This is evident in the emergence of high-amplitude, low-frequency delta (δ, 0.5-4 Hz) waves and the reduction in high frequency, gamma (γ, 35-45Hz) activity in the electroencephalogram (EEG), similar to slow wave sleep⁵^,⁶ as well as the widespread reductions in cerebral blood flow and glucose metabolism⁵^,⁶^,⁷^,⁸^,⁹^,¹⁰^,¹¹^,¹². Boveroux et al.¹³ added to such observations by demonstrating a significant decrease in resting state functional connectivity under propofol anesthesia using functional magnetic resonance imaging (fMRI). In contrast, dissociative anesthetics yield a less clear profile of effects on brain activity. In some cases, they are associated with increases in cerebral blood flow and glucose metabolism¹⁴^,¹⁵^,¹⁶^,¹⁷^,¹⁸^,¹⁹^,²⁰^,²¹ while studies by Rex and colleagues²² and Laitio and colleagues²³^,²⁴ looking at the effects of Xe provided evidence of both increased and decreased brain activity. A similar irregularity can be seen in the effects on the EEG signals²⁵^,²⁶^,²⁷^,²⁸. Johnson et al.²⁹ demonstrated an increase in total power of the low frequency bands delta and theta as well as in the higher frequency band gamma in a high density EEG study of Xe anesthesia while opposing observations were made for N₂O in the delta, theta and alpha frequency bands³⁰^,³¹ and for Xe in the higher frequencies³². Such variability in the effects of Xe on the electrical scalp activity can be observed in the alpha and beta frequency ranges also with both increases³³ and reductions³⁴ being reported.

In spite of the discrepancies mentioned above, the picture starts to become more consistent across agents when one attempts to look at alterations in functional connectivity between brain areas. Such measures however, have been predominantly restricted to modalities that necessarily make concessions with respect to either spatial or temporal resolution. While studies using the EEG appear to reveal clear, and to some extent consistent, changes in the topological structure of functional networks during anesthesia/sedation with propofol³⁵, sevoflurane³⁶ and N₂O³⁷, the widely spaced sensor level EEG data has insufficient spatial resolution to meaningfully define and delineate the vertices of the corresponding functional networks. Conversely, studies utilizing the superior spatial resolution of fMRI and positron emission tomography (PET), find similar topological alterations in large-scale functional connectivity to that of EEG¹³^,³⁸^,³⁹^,⁴⁰^,⁴¹, however possess insufficient temporal resolution to characterize phase-amplitude coupling in the alpha (8-13 Hz) EEG band and other dynamical phenomena that are emerging as important signatures of anesthetic action¹²^,⁴². Moreover, these measures do not directly evaluate electromagnetic neural activity⁴³.

Therefore, in order to meaningfully advance the understanding of the macroscopic processes associated with the action of anesthetics, the limitations of the previously mentioned investigations need to be addressed; the restricted coverage of anesthetic agents and the insufficient spatio-temporal resolution of the non-invasive measurements. On this basis, the authors outline a method to simultaneously record magnetoencephalogram (MEG) and EEG activity in healthy volunteers that has been developed for the administration of the gaseous dissociative anesthetic agents, Xe and N₂O.

The MEG is utilized as it is the only non-invasive neurophysiological technique other than the EEG that has a temporal resolution in the millisecond range. EEG has the problem of blurring of electrical fields by the skull, which acts as a low-pass filter on cortically generated activity, while MEG is much less sensitive to this issue and the issue of volume conduction⁴⁴. It can be argued that MEG has higher spatial and source localization accuracy than EEG ⁴⁵^,⁴⁶. EEG does not allow true reference-free recording³⁷^,⁴⁷, however MEG does. MEG systems also typically record cortical activity in a much wider frequency range than EEG, including high gamma⁴⁸(typically 70 – 90 Hz), which have been suggested to be involved in the hypnotic effects of anesthetic agents including Xe²⁹ and N₂O²⁸. The MEG offers neurophysiological activity that compliments that conveyed by EEG, as EEG activity relates to extracellular electrical currents whereas MEG mainly reflects the magnetic fields generated by intracellular currents⁴⁶^,⁴⁹. Furthermore, MEG is particularly sensitive to electrophysiological activity tangential to the cortex, while EEG mostly records extracellular activity radial to the cortex⁴⁹. Thus combining MEG and EEG data has super-additive advantages⁵⁰.

The gaseous dissociative agents Xe and N₂O have been chosen for the following principle reasons: they are odorless (Xe) or essentially odorless (N₂O) and thus can easily be utilized in the presence of control conditions when employed at sub-clinical concentrations. In addition, they are well suited for remote administration and monitoring in a laboratory environment due to their weak cardio-respiratory depressant effects⁶¹. Xenon and to a lesser extent N₂O, retain a relatively low minimum-alveolar-concentration-(MAC)-awake at which 50% of patients become unresponsive to a verbal command with values of 32.6 ± 6.1%⁵¹ and 63.3 +- 7.1%⁵² respectively. Despite Xe and N₂O both being NMDA receptor antagonists, they modulate the EEG differently – Xe appears to behave more like a typical GABAergic agent when monitored using the Bispectral Index³³^,⁵³^,⁵⁴ (one of several approaches used to electroencephalographically monitor depth of anesthesia). In contrast, N₂O produces a much less apparent electroencephalographic effect in that it is poorly, if at all, monitored using the Bispectral Index²⁶. Because Xe has different reported electroencephalographic properties to the other dissociative agents, but possesses similar characteristics to the more commonly studied GABAergic agents, its electrophysiological study has the potential to reveal important features relating to the neural correlates of consciousness and the corresponding functional network changes. Agents that act at the NMDA receptor are likely to reveal more about the brain networks that subserve normal and altered consciousness, given the critical role that NMDA receptor mediated activity plays in learning and memory and its implicated role in a range of psychiatric disorders that include schizophrenia and depression⁸⁰.

This paper focuses primarily on the demanding and complex data collection procedure associated with the delivery of gaseous anesthetic agents in a non-hospital environment while simultaneously recording MEG and EEG. Basic data analysis at the sensor level is outlined and example data are provided illustrating that high-fidelity recordings can be obtained with minimal head movement. The many potential methods for subsequent source imaging and/or functional connectivity analysis that would be typically performed using this kind of data are not described, as these methods are well described in the literature and demonstrate various options for analysis⁵⁵^,⁵⁶.

Protocol

The study entitled "Effects of inhaled Xe and N2O on brain activity recorded using EEG and MEG" was approved (approval number: 260/12) by the Alfred Hospital and Swinburne University of Technology Ethics Committee and met the requirements of the National Statement on Ethical Conduct in Human Research (2007). 1. Participant Selection and Pre-Study Requirements Conduct an interview to select healthy, right handed, adult males between the ages of 20 and 40 years old…

Representative Results

This section utilizes data obtained from one subject in order to demonstrate the typical features of the simultaneous recordings and the potential of such information to contribute a better understanding of anesthetic induced altered states of consciousness. To simplify the exposition, results are shown for i) recordings of the post-anti-emetic administration baseline (baseline 3), ii) 0.75 equi-MAC-awake peak gas concentrations (level 3) of N2O (47%) and Xe (24%), and iii) Xe …

Discussion

This paper has outlined a comprehensive protocol for the simultaneous recording of MEG and EEG during anesthetic gas delivery with N₂O and Xe. Such a protocol will be valuable for studying the electromagnetic neural correlates of anesthetic-induced reductions in consciousness. The protocol is also expected to generalize to the delivery of other anesthetic gases such as sevoflurane or isoflurane. This will facilitate a greater understanding of the common, specific and distinct macroscopic mechanisms that underl…

Disclosures

The authors have nothing to disclose.

Acknowledgements

The authors would like to thank Mahla Cameron Bradley, Rachel Anne Batty and Johanna Stephens for valuable technical assistance with MEG data collection. Thanks are additionally extended to Dr. Steven Mcguigan for support as a second anesthesiologist. Paige Pappas provided invaluable anesthetic nurse oversight. Markus Stone graciously proffered his time and expertise in editing and filming the protocol. Dr. Suresh Muthukumaraswamy gave specific advice regarding data analysis and the interpretation of results. Finally, Jarrod Gott contributed many a stimulating discussion, helped in the execution of a number of pilot experiments and was central in the design of the foam head brace.

This research was supported by a James S. McDonnell collaborative grant #220020419 “Reconstructing Consciousness” awarded to George Mashour, Michael Avidan, Max Kelz and David Liley.

Materials

Neuromag TRIUX 306-channel MEG system	Elekta Oy, Stockholm, SWEDEN	N/A
Polhemus Fastrak 3D system	Polhemus, VT, USA	N/A
MEG compatible ER-1 insert headphones	Etymotic Research Inc., IL, USA	N/A
Low Density foam head cap, MEG compatible	N/A	N/A	Custom made by research team
Harness, MEG compatible	N/A		~3 m long, ~ 5 cm wide, cloth/jute strip to secure participant position on MEG chair
Ambu Neuroline 720 Single Patient Surface Electrodes	Ambu, Copenhagen, Denmark	72015-K10
3.0T TIM Trio MRI system	Siemens AB, Erlangen, GERMANY	N/A
Asalab amplifier system	ANT Neuro, Enschede, NETHERLANDS	N/A	this system is no longer manufactured and has been deprecated to 64 channel eego EEG amplifier
64-channel Waveguard EEG cap, MEG compatible	ANT Neuro, Enschede, NETHERLANDS	CA-138	size Medium
Magnetically shielded cordless battery box	ANT Neuro, Enschede, NETHERLANDS	N/A	Magnetic shielding not provided by manufacturer – Modified by research team
OneStep ClearGel Electrode gel	H+H Medizinprodukte GbR, Munster, GERMANY	154547
Akzent Xe Color Anesthesia Machine	Stephan GmbH, Gackenbach, GERMANY	N/A
Omron M6-Comfort Blood Pressure Monitor	Omron Healthcare, Kyoto, JAPAN	N/A
Xenon gas (99.999% purity)	Coregas, Thomastown, VIC, AUSTRALIA	N/A	we estimate that we use approx 40 L (SATP) per participant
Medical Nitrous Oxide	Coregas, Thomastown, VIC, AUSTRALIA	N/A	x2 G size cylinders
Medical Oxygen	Coregas, Thomastown, VIC, AUSTRALIA	N/A	x2 G size cylinders
Medical Air	Coregas, Thomastown, VIC, AUSTRALIA	N/A	x2 G size cylinders
Filter Respiratory & HMES with Capno Port Hypnobag	Medtronic, MN, USA	352/5805
Yankauer High Adult	Medtronic, MN, USA	8888-502005
Quadralite EcoMask anaesthetic masks	Intersurgical Australia Pty Ltd	7093000/7094000	size 3 and size 4
Suction Canister Disp 1200 mL Medival Guardian	Cardinal Health, OH, USA	65651-212
Catheter Mount Ext 4-13 cm with 90A elbow	Medtronic, MN, USA	330/5667
Catheter IV Optiva 24g x 19 mm Yellow St Su	Smiths Medical, MN, USA	5063-INT
Dexamethasone Mylan Injection Vials (4 mg/1 mL)	Alphapharm Pty Ltd, Sydney, AUSTRALIA	400528517
Ondasetron (4 mg/2 mL)	Alphapharm Pty Ltd, Sydney, AUSTRALIA	400008857
Medical resuscitation cart			The medical resuscitation cart is configured according to the suggested minimal requirements for Adult resuscitation recommended in the document "Standards for Resuscitation: Clinical Practice and Education; June 2014) by the Australian and New Zealand Resuscitation councils and specifically endorsed by multiple professional health care organizations including the Australian and New Zealand College of Anaesthetists. It includes all the necessary airway and circulatory equipment, as well as the associated pharmacuetical agents to enable full cardio-respiratory resuscitation and support in a non-clinical environment. Full details can be found at https://resus.org.au/standards-for-resuscitation-clinical-practice-and-education/
Maxfilter Version 2.2	Elekta Oy, Stockholm, SWEDEN	N/A	Data analysis software provided with Elekta's Neuromag TRIUX MEG system

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Pelentritou, A., Kuhlmann, L., Cormack, J., Woods, W., Sleigh, J., Liley, D. Recording Brain Electromagnetic Activity During the Administration of the Gaseous Anesthetic Agents Xenon and Nitrous Oxide in Healthy Volunteers. J. Vis. Exp. (131), e56881, doi:10.3791/56881 (2018).