Xenon: no stranger to anaesthesia

Critical dynamics in spontaneous EEG predict anesthetic-induced loss of consciousness and perturbational complexity

Consciousness has been proposed to be supported by electrophysiological patterns poised at criticality, a dynamical regime which exhibits adaptive computational properties, maximally complex patterns and divergent sensitivity to perturbation. Here, we investigate dynamical properties of the resting-state electroencephalogram (EEG) of healthy subjects undergoing general anesthesia with propofol, xenon or ketamine. Importantly, all participants were unresponsive under anesthesia, while consciousness was retained only during ketamine anesthesia (in the form of vivid dreams), enabling an experimental dissociation between unresponsiveness and unconsciousness. For each condition, we measure (i) avalanche criticality, (ii) chaoticity, and (iii) criticality-related metrics, revealing that states of unconsciousness are characterized by a distancing from both avalanche criticality and the edge of chaos. We then ask whether these same dynamical properties are predictive of the perturbational complexity index (PCI), a TMS-based measure that has shown remarkably high sensitivity in detecting consciousness independently of behavior. We successfully predict individual subjects' PCI values with considerably high accuracy from resting-state EEG dynamical properties alone. Our results establish a firm link between perturbational complexity and criticality, and provide further evidence that criticality is a necessary condition for the emergence of consciousness.

A metabolic profile of xenon and metabolite associations with 6-month mortality after out-of-hospital cardiac arrest: A post-hoc study of the randomised Xe-Hypotheca trial

PURPOSE

Out-of-hospital cardiac arrest (OHCA) carries a relatively poor prognosis and requires multimodal prognostication to guide clinical decisions. Identification of previously unrecognized metabolic routes associated with patient outcome may contribute to future biomarker discovery. In OHCA, inhaled xenon elicits neuro- and cardioprotection. However, the metabolic effects remain unknown.

MATERIALS AND METHODS

In this post-hoc study of the randomised, 2-group, single-blind, phase 2 Xe-Hypotheca trial, 110 OHCA survivors were randomised 1:1 to receive targeted temperature management (TTM) at 33°C with or without inhaled xenon during 24 h. Blood samples for nuclear magnetic resonance spectroscopy metabolic profiling were drawn upon admission, at 24 and 72 h.

RESULTS

At 24 h, increased lactate, adjusted hazard-ratio 2.25, 95% CI [1.53; 3.30], p<0.001, and decreased branched-chain amino acids (BCAA) leucine 0.64 [0.5; 0.82], p = 0.007, and valine 0.37 [0.22; 0.63], p = 0.003, associated with 6-month mortality. At 72 h, increased lactate 2.77 [1.76; 4.36], p<0.001, and alanine 2.43 [1.56; 3.78], p = 0.001, and decreased small HDL cholesterol ester content (S-HDL-CE) 0.36 [0.19; 0.68], p = 0.021, associated with mortality. No difference was observed between xenon and control groups.

CONCLUSIONS

In OHCA patients receiving TTM with or without xenon, high lactate and alanine and decreased BCAAs and S-HDL-CE associated with increased mortality. It remains to be established whether current observations on BCAAs, and possibly alanine and lactate, could reflect neural damage via their roles in the metabolism of the neurotransmitter glutamate. Xenon did not significantly alter the measured metabolic profile, a potentially beneficial attribute in the context of compromised ICU patients.

TRIAL REGISTRATION

Trial Registry number: ClinicalTrials.gov Identifier: NCT00879892.

Neurobiological basis of emergence from anesthesia

The suppression of consciousness by anesthetics and the emergence of the brain from anesthesia are complex and elusive processes. Anesthetics may exert their inhibitory effects by binding to specific protein targets or through membrane-mediated targets, disrupting neural activity and the integrity and function of neural circuits responsible for signal transmission and conscious perception/subjective experience. Emergence from anesthesia was generally thought to depend on the elimination of the anesthetic from the body. Recently, studies have suggested that emergence from anesthesia is a dynamic and active process that can be partially controlled and is independent of the specific molecular targets of anesthetics. This article summarizes the fundamentals of anesthetics' actions in the brain and the mechanisms of emergence from anesthesia that have been recently revealed in animal studies.

Criticality of resting-state EEG predicts perturbational complexity and level of consciousness during anesthesia

Consciousness has been proposed to be supported by electrophysiological patterns poised at criticality, a dynamical regime which exhibits adaptive computational properties, maximally complex patterns and divergent sensitivity to perturbation. Here, we investigated dynamical properties of the resting-state electroencephalogram of healthy subjects undergoing general anesthesia with propofol, xenon or ketamine. We then studied the relation of these dynamic properties with the perturbational complexity index (PCI), which has shown remarkably high sensitivity in detecting consciousness independent of behavior. All participants were unresponsive under anesthesia, while consciousness was retained only during ketamine anesthesia (in the form of vivid dreams)., enabling an experimental dissociation between unresponsiveness and unconsciousness. We estimated (i) avalanche criticality, (ii) chaoticity, and (iii) criticality-related measures, and found that states of unconsciousness were characterized by a distancing from both the edge of activity propagation and the edge of chaos. We were then able to predict individual subjects' PCI (i.e., PCImax) with a mean absolute error below 7%. Our results establish a firm link between the PCI and criticality and provide further evidence for the role of criticality in the emergence of consciousness.

Noncovalent chemistry of xenon opens the door for anesthetic xenon recovery using Bio-MOFs

Designing an inexpensive and highly efficient recovery process for xenon (Xe) is gaining importance in the development of sustainable applications. Using metal organic frameworks (MOFs) for separating Xe from anesthetic gas mixtures has been a recent topic studied rarely and superficially in the literature. We theoretically investigated Xe recovery performances of 43 biological MOFs (Bio-MOFs) formed by biocompatible metal cations and biological endogenous linkers. Xe uptakes and Xe permeabilities in its binary mixtures with CO2, O2, and N2 were investigated by applying Grand Canonical Monte Carlo and Molecular Dynamics simulations. Materials with metalloporphyrin, hexacarboxylate, triazine, or pyrazole ligands, dimetallic paddlewheel units, relatively large pore sizes (PLD > 5 Å and LCD > 10 Å), large void fractions (≈0.8), and large surface areas (>2900 m2 g-1) have been determined as top performing Bio-MOFs for Xe recovery. By applying Density Functional Theory simulations and generating electron density difference maps, we determined that Xe-host interactions in the top performing Bio-MOFs are maximized mainly due to noncovalent interactions of Xe, such as charge-induced dipole and aerogen-π interactions. Polarized Xe atoms in the vicinity of cations/anions as well as π systems are fingerprints of enhanced guest-host interactions. Our results show examples of rarely studied aerogen interactions that play a critical role in selective adsorption of Xe in nanoporous materials.

Complexes of HXeY with HX (Y, X = F, Cl, Br, I): Symmetry-Adapted Perturbation Theory Study and Anharmonic Vibrational Analysis

A comprehensive analysis of the intermolecular interaction energy and anharmonic vibrations of 41 structures of the HXeY⋯HX (X, Y = F, Cl, Br, I) family of noble-gas-compound complexes for all possible combinations of Y and X was conducted. New structures were identified, and their interaction energies were studied by means of symmetry-adapted perturbation theory, up to second-order corrections: this provided insight into the physical nature of the interaction in the complexes. The energy components were discussed, in connection to anharmonic frequency analysis. The results show that the induction and dispersion corrections were the main driving forces of the interaction, and that their relative contributions correlated with the complexation effects seen in the vibrational stretching modes of Xe-H and H-X. Reasonably clear patterns of interaction were found for different structures. Our findings corroborate previous findings with better methods, and provide new data. These results suggest that the entire group of the studied complexes can be labelled as "naturally blueshifting", except for the complexes with HI.

Effect of xenon on brain injury, neurological outcome, and survival in patients after aneurysmal subarachnoid hemorrhage-study protocol for a randomized clinical trial

BACKGROUND

Aneurysmal subarachnoid hemorrhage (aSAH) is a neurological emergency, affecting a younger population than individuals experiencing an ischemic stroke; aSAH is associated with a high risk of mortality and permanent disability. The noble gas xenon has been shown to possess neuroprotective properties as demonstrated in numerous preclinical animal studies. In addition, a recent study demonstrated that xenon could attenuate a white matter injury after out-of-hospital cardiac arrest.

METHODS

The study is a prospective, multicenter phase II clinical drug trial. The study design is a single-blind, prospective superiority randomized two-armed parallel follow-up study. The primary objective of the study is to explore the potential neuroprotective effects of inhaled xenon, when administered within 6 h after the onset of symptoms of aSAH. The primary endpoint is the extent of the global white matter injury assessed with magnetic resonance diffusion tensor imaging of the brain.

DISCUSSION

Despite improvements in medical technology and advancements in medical science, aSAH mortality and disability rates have remained nearly unchanged for the past 10 years. Therefore, new neuroprotective strategies to attenuate the early and delayed brain injuries after aSAH are needed to reduce morbidity and mortality.

TRIAL REGISTRATION

ClinicalTrials.gov NCT04696523. Registered on 6 January 2021. EudraCT, EudraCT Number: 2019-001542-17. Registered on 8 July 2020.

Hyperpolarized 129 Xe imaging of the brain: Achievements and future challenges

Hyperpolarized (HP) xenon-129 (¹²⁹ Xe) brain MRI is a promising imaging modality currently under extensive development. HP ¹²⁹ Xe is nontoxic, capable of dissolving in pulmonary blood, and is extremely sensitive to the local environment. After dissolution in the pulmonary blood, HP ¹²⁹ Xe travels with the blood flow to the brain and can be used for functional imaging such as perfusion imaging, hemodynamic response detection, and blood-brain barrier permeability assessment. HP ¹²⁹ Xe MRI imaging of the brain has been performed in animals, healthy human subjects, and in patients with Alzheimer's disease and stroke. In this review, the overall progress in the field of HP ¹²⁹ Xe brain imaging is discussed, along with various imaging approaches and pulse sequences used to optimize HP ¹²⁹ Xe brain MRI. In addition, current challenges and limitations of HP ¹²⁹ Xe brain imaging are discussed, as well as possible methods for their mitigation. Finally, potential pathways for further development are also discussed. HP ¹²⁹ Xe MRI of the brain has the potential to become a valuable novel perfusion imaging technique and has the potential to be used in the clinical setting in the future.

Can Ultrasound-Guided Xenon Delivery Provide Neuroprotection in Traumatic Brain Injury?

Traumatic brain injury (TBI) is associated with high mortality and morbidity in children and adults. Unfortunately, there is no effective management for TBI in the acute setting. Rodent studies have shown that xenon, a well-known anesthetic gas, can be neuroprotective when administered post-TBI. Gas inhalation therapy, however, the approach typically used for administering xenon, is expensive, inconvenient, and fraught with systemic side effects. Therapeutic delivery to the brain is minimal, with much of the inhaled gas cleared by the lungs. To bridge major gaps in clinical care and enhance cerebral delivery of xenon, this study introduces a novel xenon delivery technique, utilizing microbubbles, in which a high impulse ultrasound signal is used for targeted cerebral release of xenon. Briefly, an ultrasound pulse is applied along the carotid artery at the level of the neck on intravenous injection of xenon microbubbles (XeMBs) resulting in release of xenon from microbubbles into the brain. This delivery technique employs a hand-held, portable ultrasound system that could be adopted in resource-limited environments. Using a high-fidelity porcine model, this study demonstrates the neuroprotective efficacy of xenon microbubbles in TBI for the first time.

Xenon-Enhanced Dynamic Dual-Energy CT Is Able to Quantify Sinus Ventilation Using Laminar and Pulsating Air-/Gas Flow Before and After Surgery: A Pilot Study in a Cadaver Model

Background

Chronic rhinosinusitis is a common disease with a significant impact on the quality of life. Topical drug delivery to the paranasal sinuses is not efficient to prevent sinus surgery or expensive biologic treatment in a lot of cases as the affected mucosa is not reached. More efficient approaches for topical drug delivery are, therefore, necessary. In the current study, dual-energy CT (DECT) imaging was used to examine sinus ventilation before and after sinus surgery using a pulsating xenon gas ventilator in a cadaver head.

Methods

Xenon gas was administered to the nasal cavity of a cadaver head with a laminar flow of 7 L/min and with pulsating xenon-flow (45 Hz frequency, 25 mbar amplitude). Nasal cavity and paranasal sinuses were imaged by DECT. This procedure was repeated after functional endoscopic sinus surgery (FESS). Based on the enhancement levels in the different sinuses, regional xenon concentrations were calculated.

Results

Xenon-related enhancement could not be detected in most of the sinuses during laminar gas flow. By superimposing laminar flow with pulsation, DECT imaging revealed a xenon wash-in and wash-out in the sinuses. After FESS, xenon enhancement was immediately seen in all sinuses and reached higher concentrations than before surgery.

Conclusion

Xenon-enhanced DECT can be used to visualize and quantify sinus ventilation. Pulsating air-/gas flow was superior to laminar flow for the administration of xenon to the paranasal sinuses. FESS leads to successful ventilation of all paranasal sinuses.

Nitrous oxide as an adjunctive therapy in major depressive disorder: a randomized controlled double-blind pilot trial

OBJECTIVE

Major depressive disorder (MDD) is related to glutamatergic dysfunction. Antagonists of glutamatergic N-methyl-D-aspartate receptor (NMDAR), such as ketamine, have antidepressant properties. Nitrous oxide (N2O) is also a NMDAR antagonist. Thus, this study aimed to evaluate the effects of augmenting antidepressant treatment with N2O.

METHODS

This double blind, placebo-controlled randomized parallel pilot trial was conducted from June 2016 to June 2018 at the Hospital das Clínicas, Faculdade de Medicina de Ribeirão Preto, Universidade de São Paulo. Twenty-three subjects with MDD (aged 18 to 65, on antidepressants, with a score > 17 on the 17-item-Hamilton Depression Rating Scale [HAM-D17]) received 50% N2O (n=12; 37.17±13.59 years) or placebo (100% oxygen) (n=11; 37.18±12.77 years) for 60 minutes twice a week for 4 weeks. The primary outcome was changes in HAM-D17 from baseline to week 4.

RESULTS

Depressive symptoms improved significantly in the N2O group (N2O: from 22.58±3.83 to 5.92±4.08; placebo: from 22.44±3.54 to 12.89±5.39, p < 0.005). A total of 91.7% and 75% of the N2O group subjects achieved response (≥ 50% reduction in HAM-D17 score) and remission (HAM-D17 < 7), respectively. The predominant adverse effects of N2O treatment were nausea, vomiting, and headache.

CONCLUSION

N2O treatment led to a statistically significant reduction in HAM-D17 scores compared to placebo.

CLINICAL TRIAL REGISTRATION

Brazilian Register of Clinical Trials, RBR-5rz5ch.

Ab initio relativistic potential energy surfaces of benzene-Xe complex with application to intermolecular vibrations

The benzene-Xe (BXe) complex in its electronic ground state is studied using ab initio methods. Since this complex contains the heavy Xe atom, the relativistic effects cannot be neglected. We test two different approaches that describe the scalar relativistic effects in the framework of the coupled-cluster level of theory with single, double, and perturbative triple excitations, used for the interaction energy calculations. The first one is based on the small core pseudopotential (PP), and the second one is based on the explicit treatment of scalar relativistic effects using the Douglas-Kroll-Hess (DKH) Hamiltonian. A few basis sets are tested with the PP and DKH, and for each one, the analytical potential energy surface (PES) is constructed. It is shown that the difference between PESs determined with PP and DKH methods is small, if the orbitals of the 4d subshell in Xe are correlated. We select the most appropriate approach for the calculation of the potential energy surface of BXe, with respect to accuracy and computational cost. The optimal level of theory includes a small Dunning's basis set for the benzene monomer and a larger PP basis set for Xe supplemented by midbond functions. The PES obtained using such an approach provides a reasonable accuracy when compared to the empirical one derived from the microwave spectra of BXe. The empirical and the theoretical values of intermolecular vibrational energies agree within 0.5 cm^-1 up to second overtones. The vibrational energy level pattern of BXe is characterized by a distinct polyad structure.

Beneficial effects of xenon inhalation on behavioral changes in a valproic acid-induced model of autism in rats

BACKGROUND

Xenon (Xe) is a noble gas that has been used for the last several decades as an anesthetic during surgery. Its antagonistic effect on glutamate subtype of NMDA (N-methyl-D-aspartate) receptors resulted in evaluation of this gas for treatment of CNS pathologies, including psychoemotional disorders. The aim of this study was to assess the behavioral effects of acute inhalation of subanesthetic concentrations of Xe and to study the outcomes of Xe exposure in valproic acid (VPA)-induced rodent model of autism.

METHODS

We have conducted two series of experiments with a battery of behavioral tests aimed to evaluate locomotion, anxiety- and depression-like behavior, and social behavior in healthy, VPA-treated and Xe-exposed young rats.

RESULTS

We have shown that in healthy animals Xe exposure resulted in acute and delayed decrease of exploratory motivation, partial decrease in risk-taking and depressive-like behavior as well as improved sensorimotor integration during the negative geotaxis test. Acute inhalations of Xe in VPA-exposed animals led to improvement in social behavior, decrease in exploratory motivation, and normalization of behavior in forced-swim test.

CONCLUSION

Behavioral modulatory effects of Xe are probably related to its generalized action on excitatory/inhibitory balance within the CNS. Our data suggest that subanesthetic short-term exposures to Xe have beneficial effect on several behavioral modalities and deserves further investigation.

Development of a hybrid CFD-PBPK model to predict the transport of xenon gas around a human respiratory system to systemic regions

Administering incorrect doses of conventional anesthetic agents through the pulmonary route can cause potential health risks such as blood coagulation, platelet dysfunction, and deteriorating organ function. As an alternative, xenon can minimize the impact on the cardiovascular system and provide the neuroprotective effect, hemodynamic stability, and fast recovery. However, the inhalation pattern still needs to be carefully monitored and controlled to avoid health risks caused by over administering xenon to patients during unconsciousness. Thus, high-resolution lung absorption and whole-body translocation data are critically needed to fully understand how to administer the gas and coordinate with the patient to accurately control the dose. Clinical studies are not able to provide accurate dosimetry data due to their limited operational flexibility and imaging resolution. Therefore, a computational fluid dynamics (CFD) model was employed in this study to simulate the transport and absorption of the inhaled xenon which is connected with a physiologically based pharmacokinetic (PBPK) model to predict the translocation into the systemic regions. To study the effects of different breathing patterns on xenon transport dynamics in the human body, a realistic breathing waveform and two steady-state flow rates with inhalation durations of 2 and 1.5 seconds were selected. For the realistic breathing cycle, the inhalation-exhalation periods are defined for a human at rest and the other two cases have a fixed volumetric flow rate of 15 L/min. As the two latter cases only simulate the inspiratory phase, a 1-second holding time was applied to represent the missing periods of the full breathing time. Simulations were performed in a subject-specific human upper airway configuration from mouth to G6. Numerical results show that with the accurate lung uptake predictions obtained from the CFD model, the hybrid CFD-PBPK model with TRANSIT compartments generates more precise and breath-specific trends compared to simple PBPK models. Numerical results demonstrate that breathing pattern can significantly influence the xenon uptake in the human body, which can be utilized as a critical factor to be coordinated by clinicians to achieve the optimized xenon dose. Furthermore, parametric analyses were performed for the influence of breathing patterns on local airflow distributions, gas species translocations, and lung elimination mechanisms followed by species diffusion into the systemic regions.

The actions of volatile anesthetics: a new perspective

This article reviews recent work in applying neutron and X-ray scattering towards the elucidation of the molecular mechanisms of volatile anesthetics. Experimental results on domain mixing in ternary lipid mixtures, and the influence of volatile anesthetics and hydrostatic pressure are placed in the contexts of ion-channel function and receptor trafficking at the postsynaptic density.

Xenon-Protein Interactions: Characterization by X-Ray Crystallography and Hyper-CEST NMR

The physiological activity of xenon has long been recognized, though the exact nature of its interactions with biomolecules remains poorly understood. Xe is an inert noble gas, but can act as a general anesthetic, most likely by binding internal hydrophobic cavities within proteins. Understanding Xe-protein interactions, therefore, can provide crucial insight regarding the mechanism of Xe anesthesia and potentially other general anesthetic agents. Historically, Xe-protein interactions have been studied primarily through X-ray crystallography and nuclear magnetic resonance (NMR). In this chapter, we first describe our methods for preparing Xe derivatives of protein crystals and identifying Xe-binding sites. Second, we detail our procedure for ¹²⁹Xe hyper-CEST NMR spectroscopy, a versatile NMR technique well suited for characterizing the weak, transient nature of Xe-protein interactions.

The noble gas xenon provides protection and trophic stimulation to midbrain dopamine neurons

Despite its low chemical reactivity, the noble gas xenon possesses a remarkable spectrum of biological effects. In particular, xenon is a strong neuroprotectant in preclinical models of hypoxic‐ischemic brain injury. In this study, we wished to determine whether xenon retained its neuroprotective potential in experimental settings that model the progressive loss of midbrain dopamine (DA) neurons in Parkinson's disease. Using rat midbrain cultures, we established that xenon was partially protective for DA neurons through either direct or indirect effects on these neurons. So, when DA neurons were exposed to l‐trans‐pyrrolidine‐2,4‐dicarboxylic acid so as to increase ambient glutamate levels and generate slow and sustained excitotoxicity, the effect of xenon on DA neurons was direct. The vitamin E analog Trolox also partially rescued DA neurons in this setting and enhanced neuroprotection by xenon. However, in the situation where DA cell death was spontaneous, the protection of DA neurons by xenon appeared indirect as it occurred through the repression of a mechanism mediated by proliferating glial cells, presumably astrocytes and their precursor cells. Xenon also exerted trophic effects for DA neurons in this paradigm. The effects of xenon were mimicked and improved by the N‐methyl‐d‐aspartate glutamate receptor antagonist memantine and xenon itself appeared to work by antagonizing N‐methyl‐d‐aspartate receptors. Note that another noble gas argon could not reproduce xenon effects. Overall, present data indicate that xenon can provide protection and trophic support to DA neurons that are vulnerable in Parkinson's disease. This suggests that xenon might have some therapeutic value for this disorder.

Feasibility, tolerability and safety of pediatric hyperpolarized 129Xe magnetic resonance imaging in healthy volunteers and children with cystic fibrosis

BACKGROUND

Hyperpolarized ¹²⁹Xe is a promising contrast agent for MRI of pediatric lung function, but its safety and tolerability in children have not been rigorously assessed.

OBJECTIVE

To assess the feasibility, safety and tolerability of hyperpolarized ¹²⁹Xe gas as an inhaled contrast agent for pediatric pulmonary MRI in healthy control subjects and in children with cystic fibrosis.

MATERIALS AND METHODS

Seventeen healthy control subjects (ages 6-15 years, 11 boys) and 11 children with cystic fibrosis (ages 8-16 years, 4 boys) underwent ¹²⁹Xe MRI, receiving up to three doses of ¹²⁹Xe gas prepared by either a commercially available or a homebuilt ¹²⁹Xe polarizer. Subject heart rate and SpO₂ were monitored for 2 min post inhalation and compared to resting baseline values. Adverse events were reported via follow-up phone call at days 1 and 30 (range ±7 days) post-MRI.

RESULTS

All children tolerated multiple doses of ¹²⁹Xe, and no children withdrew from the study. Relative to baseline, most children who received a full dose of gas for imaging (10 of 12 controls and 8 of 11 children with cystic fibrosis) experienced a nadir in SpO₂ (mean -6.0 ± standard deviation 7.2%, P≤0.001); however within 2 min post inhalation SpO₂ values showed no significant difference from baseline (P=0.11). There was a slight elevation in heart rate (mean +6.6 ± 13.9 beats per minute [bpm], P=0.021), which returned from baseline within 2 min post inhalation (P=0.35). Brief side effects related to the anesthetic properties of xenon were mild and quickly resolved without intervention. No serious or severe adverse events were observed; in total, four minor adverse events (14.3%) were reported following ¹²⁹Xe MRI, but all were deemed unrelated to the study.

CONCLUSION

The feasibility, safety and tolerability of ¹²⁹Xe MRI has been assessed in a small group of children as young as 6 years. SpO₂ changes were consistent with the expected physiological effects of a short anoxic breath-hold, and other mild side effects were consistent with the known anesthetic properties of xenon and with previous safety assessments of ¹²⁹Xe MRI in adults. Hyperpolarized ¹²⁹Xe is a safe and well-tolerated inhaled contrast agent for pulmonary MR imaging in healthy children and in children with cystic fibrosis who have mild to moderate lung disease.

Structural Basis for Xenon Inhibition in a Cationic Pentameric Ligand-Gated Ion Channel

GLIC receptor is a bacterial pentameric ligand-gated ion channel whose action is inhibited by xenon. Xenon has been used in clinical practice as a potent gaseous anaesthetic for decades, but the molecular mechanism of interactions with its integral membrane receptor targets remains poorly understood. Here we characterize by X-ray crystallography the xenon-binding sites within both the open and "locally-closed" (inactive) conformations of GLIC. Major binding sites of xenon, which differ between the two conformations, were identified in three distinct regions that all belong to the trans-membrane domain of GLIC: 1) in an intra-subunit cavity, 2) at the interface between adjacent subunits, and 3) in the pore. The pore site is unique to the locally-closed form where the binding of xenon effectively seals the channel. A putative mechanism of the inhibition of GLIC by xenon is proposed, which might be extended to other pentameric cationic ligand-gated ion channels.

Comparison of recovery parameters for xenon versus other inhalation anesthetics: systematic review and meta-analysis

STUDY OBJECTIVE

To summarize and evaluate the available data describing the recovery parameters of xenon anesthesia.

DESIGN

Systematic review and meta-analysis.

SETTING

Anesthesia for elective surgeries.

PATIENTS

Systematic review of randomized controlled trials (RCTs) from databases including Medline (1964-2013), the Cochrane Central Register of Controlled Trials (CENTRAL, 1990-2012), and Google Scholar (1966-2013).

INTERVENTIONS

Inhalation of xenon or other anesthetics was administered in elective surgery.

MEASUREMENTS

Recovery parameters (time to recovery, alertness/sedation scale scores at "eye opening," bispectral index at "reaction on demand," time to extubation, and time to orientation).

MAIN RESULTS

Eleven RCTs (N = 661 patients) met the inclusion criteria. Recovery from xenon anesthesia was significantly faster in terms of the time to eye opening (mean difference [MD], -4.18 minutes; 95% confidence interval [CI], -5.03 to -3.32 minutes; P < .00001), the time to reaction on demand (MD, -5.35 minutes; 95% CI, -6.59 to -4.11 minutes; P < .00001), the time to extubation (MD, -4.49 minutes; 95% CI, -5.40 to -3.58 minutes; P < .00001), and the time to orientation (MD, -4.99 minutes; 95% CI, -6.45 to -3.52 minutes; P < .00001).

CONCLUSIONS

This meta-analysis confirmed that recovery from xenon anesthesia is faster than other inhalation anesthesia.

Potent Inhalational Anesthetics for Dentistry

Nitrous oxide and the volatile inhalational anesthetics have defined anxiety and pain control in both dentistry and medicine for over a century. From curious experimentation to spectacular public demonstrations, the initial work of 2 dentists, Horace Wells and William T. G. Morton, persists to this day in modern surgery and anesthesia. This article reviews the history, similarities, differences, and clinical applications of the most popular inhalational agents used in contemporary dental surgical settings.

Xenon does not increase heart rate-corrected cardiac QT interval in volunteers and in patients free of cardiovascular disease

BACKGROUND

Impaired cardiac repolarization, indicated by prolonged QT interval, may cause critical ventricular arrhythmias. Many anesthetics increase the QT interval by blockade of rapidly acting potassium rectifier channels. Although xenon does not affect these channels in isolated cardiomyocytes, the authors hypothesized that xenon increases the QT interval by direct and/or indirect sympathomimetic effects. Thus, the authors tested the hypothesis that xenon alters the heart rate-corrected cardiac QT (QTc) interval in anesthetic concentrations.

METHODS

The effect of xenon on the QTc interval was evaluated in eight healthy volunteers and in 35 patients undergoing abdominal or trauma surgery. The QTc interval was recorded on subjects in awake state, after their denitrogenation, and during xenon monoanesthesia (FetXe > 0.65). In patients, the QTc interval was recorded while awake, after anesthesia induction with propofol and remifentanil, and during steady state of xenon/remifentanil anesthesia (FetXe > 0.65). The QTc interval was determined from three consecutive cardiac intervals on electrocardiogram printouts in a blinded manner and corrected with Bazett formula.

RESULTS

In healthy volunteers, xenon did not alter the QTc interval (mean difference: +0.11 ms [95% CI, -22.4 to 22.7]). In patients, after anesthesia induction with propofol/remifentanil, no alteration of QTc interval was noted. After propofol was replaced with xenon, the QTc interval remained unaffected (417 ± 32 ms vs. awake: 414 ± 25 ms) with a mean difference of 4.4 ms (95% CI, -4.6 to 13.5).

CONCLUSION

Xenon monoanesthesia in healthy volunteers and xenon/remifentanil anesthesia in patients without clinically relevant cardiovascular disease do not increase QTc interval.

Urine analysis concerning xenon for doping control purposes

RATIONALE

On September 1(st) 2014, a modified Prohibited List as established by the World Anti-Doping Agency (WADA) became effective featuring xenon as a banned substance categorized as hypoxia-inducible factor (HIF) activator. Consequently, the analysis of xenon from commonly provided doping control specimens such as blood and urine is desirable, and first data on the determination of xenon from urine in the context of human sports drug testing, are presented.

METHODS

In accordance to earlier studies utilizing plasma as doping control matrix, urine was enriched to saturation with xenon, sequentially diluted, and the target analyte was detected as supported by the internal standard d6 -cyclohexanone by means of gas chromatography/triple quadrupole mass spectrometry (GC/MS/MS) using headspace injection. Three major xenon isotopes at m/z 128.9, 130.9 and 131.9 were targeted in (pseudo) selected reaction monitoring mode enabling the unambiguous identification of the prohibited substance. Assay characteristics including limit of detection (LOD), intraday/interday precision, and specificity as well as analyte recovery under different storage conditions were determined. Proof-of-concept data were generated by applying the established method to urine samples collected from five patients before, during and after (up to 48 h) xenon-based general anesthesia.

RESULTS

Xenon was traceable in enriched human urine samples down to the detection limit of approximately 0.5 nmol/mL. The intraday and interday imprecision values of the method were found below 25%, and specificity was demonstrated by analyzing 20 different blank urine samples that corroborated the fitness-for-purpose of the analytical approach to unequivocally detect xenon at non-physiological concentrations in human urine. The patients' urine specimens returned 'xenon-positive' test results up to 40 h post-anesthesia, indicating the limits of the expected doping control detection window.

CONCLUSIONS

Since xenon has been considered a prohibited substance according to WADA regulations in September 2014, its analysis from common specimens of routine sports drug testing is desirable. In previous studies, its traceability in whole blood and plasma was shown, and herein a complementary approach utilizing doping control urine samples for the GC/MS/MS analysis of xenon was reported.

How drugs get into cells: tested and testable predictions to help discriminate between transporter-mediated uptake and lipoidal bilayer diffusion

One approach to experimental science involves creating hypotheses, then testing them by varying one or more independent variables, and assessing the effects of this variation on the processes of interest. We use this strategy to compare the intellectual status and available evidence for two models or views of mechanisms of transmembrane drug transport into intact biological cells. One (BDII) asserts that lipoidal phospholipid Bilayer Diffusion Is Important, while a second (PBIN) proposes that in normal intact cells Phospholipid Bilayer diffusion Is Negligible (i.e., may be neglected quantitatively), because evolution selected against it, and with transmembrane drug transport being effected by genetically encoded proteinaceous carriers or pores, whose “natural” biological roles, and substrates are based in intermediary metabolism. Despite a recent review elsewhere, we can find no evidence able to support BDII as we can find no experiments in intact cells in which phospholipid bilayer diffusion was either varied independently or measured directly (although there are many papers where it was inferred by seeing a covariation of other dependent variables). By contrast, we find an abundance of evidence showing cases in which changes in the activities of named and genetically identified transporters led to measurable changes in the rate or extent of drug uptake. PBIN also has considerable predictive power, and accounts readily for the large differences in drug uptake between tissues, cells and species, in accounting for the metabolite-likeness of marketed drugs, in pharmacogenomics, and in providing a straightforward explanation for the late-stage appearance of toxicity and of lack of efficacy during drug discovery programmes despite macroscopically adequate pharmacokinetics. Consequently, the view that Phospholipid Bilayer diffusion Is Negligible (PBIN) provides a starting hypothesis for assessing cellular drug uptake that is much better supported by the available evidence, and is both more productive and more predictive.

Measuring xenon in human plasma and blood by gas chromatography/mass spectrometry

RATIONALE

Due to the favorable pharmacokinetic properties and minimal side effects of xenon, its use in modern anesthesia has been well accepted, and recent studies further demonstrated the intra- and postoperative neuro-, cardio-, and reno-protective action of the noble gas. Since the production of the hypoxia-inducible factor 1α (HIF-1α) and its downstream effector erythropoietin as well as noradrenalin reuptake inhibition have been found to play key roles in this context, the question arose as to whether the use of xenon is a matter for doping controls and preventive doping research. The aim of the present study was hence to evaluate whether the (ab)use of xenon can be detected from doping control samples with the instrumentation commonly available in sports drug testing laboratories.

METHODS

Plasma was saturated with xenon according to reported protocols, and the target analyte was measured by means of gas chromatography/time-of-flight and triple quadrupole mass spectrometry with headspace injection. Recording the accurate mass of three major xenon isotopes at m/z 128.9048, 130.9045 and 131.9042 allowed for the unequivocal identification of the analyte and the detection assay was characterized concerning limit of detection (LOD), intraday precision, and specificity as well as analyte recovery under different storage conditions.

RESULTS

Xenon was detected in fortified plasma samples with detection limits of approximately 0.5 nmol/mL to 50 nmol/mL, depending on the type of mass spectrometer used. The method characteristics of intraday precision (coefficient of variation <20%) and specificity demonstrated the fitness-for-purpose of the analytical approach to unambiguously detect xenon at non-physiological concentrations in human plasma and blood. Eventually, authentic plasma and blood samples collected pre-, intra-, and post-operative (4, 8, and 24 h) were positively analyzed after storage for up to 30 h, and provided proof-of-concept for the developed assay.

CONCLUSIONS

If relevant to doping controls, xenon can be determined from plasma and blood samples, i.e. common specimens of routine sports drug testing in the context of Athlete Biological Passport (ABP) analyses. Optimization of sampling and analytical procedures will allow the detection limit to be further improved and potentially enable accurate quantification of the anesthetic agent.

Volatile anaesthesia during cardiopulmonary bypass

Since its inception, administering and ensuring anaesthesia during cardiopulmonary bypass has been challenging. Partly because of the difficulty of administering volatile agents during cardiopulmonary bypass, total intravenous anaesthesia has been a popular technique used by cardiac anaesthetists in the last two decades. However, the possibility that volatile agents reduce mortality and the incidence of myocardial infarction by preconditioning the myocardium has stimulated a resurgence of interest in their use for cardiac anaesthesia. The aim of this review is to provide an overview of the administration of volatile anaesthetic agents during cardiopulmonary bypass for the maintenance of anaesthesia and to address some of the practical issues that are involved in doing so.

Xenon and other volatile anesthetics change domain structure in model lipid raft membranes

Inhalation anesthetics have been in clinical use for over 160 years, but the molecular mechanisms of action continue to be investigated. Direct interactions with ion channels received much attention after it was found that anesthetics do not change the structure of homogeneous model membranes. However, it was recently found that halothane, a prototypical anesthetic, changes domain structure of a binary lipid membrane. The noble gas xenon is an excellent anesthetic and provides a pivotal test of the generality of this finding, extended to ternary lipid raft mixtures. We report that xenon and conventional anesthetics change the domain equilibrium in two canonical ternary lipid raft mixtures. These findings demonstrate a membrane-mediated mechanism whereby inhalation anesthetics can affect the lipid environment of transmembrane proteins.

Neuroprotection against Traumatic Brain Injury by Xenon, but Not Argon, Is Mediated by Inhibition at the N-Methyl-d-Aspartate Receptor Glycine Site

Background:

Xenon, the inert anesthetic gas, is neuroprotective in models of brain injury. The authors investigate the neuroprotective mechanisms of the inert gases such as xenon, argon, krypton, neon, and helium in an in vitro model of traumatic brain injury.

Methods:

The authors use an in vitro model using mouse organotypic hippocampal brain slices, subjected to a focal mechanical trauma, with injury quantified by propidium iodide fluorescence. Patch clamp electrophysiology is used to investigate the effect of the inert gases on N-methyl-d-aspartate receptors and TREK-1 channels, two molecular targets likely to play a role in neuroprotection.

Results:

Xenon (50%) and, to a lesser extent, argon (50%) are neuroprotective against traumatic injury when applied after injury (xenon 43 ± 1% protection at 72 h after injury [N = 104]; argon 30 ± 6% protection [N = 44]; mean ± SEM). Helium, neon, and krypton are devoid of neuroprotective effect. Xenon (50%) prevents development of secondary injury up to 48 h after trauma. Argon (50%) attenuates secondary injury, but is less effective than xenon (xenon 50 ± 5% reduction in secondary injury at 72 h after injury [N = 104]; argon 34 ± 8% reduction [N = 44]; mean ± SEM). Glycine reverses the neuroprotective effect of xenon, but not argon, consistent with competitive inhibition at the N-methyl-d-aspartate receptor glycine site mediating xenon neuroprotection against traumatic brain injury. Xenon inhibits N-methyl-d-aspartate receptors and activates TREK-1 channels, whereas argon, krypton, neon, and helium have no effect on these ion channels.

Conclusions:

Xenon neuroprotection against traumatic brain injury can be reversed by increasing the glycine concentration, consistent with inhibition at the N-methyl-d-aspartate receptor glycine site playing a significant role in xenon neuroprotection. Argon and xenon do not act via the same mechanism.

Emergence in elderly patient undergoing general anesthesia with xenon

Introduction. It is a consensus that the postoperative cognitive function is impaired in elderly patients after general anaesthesia, and such category patient takes more time to recover. Xenon is a noble gas with anesthetic properties mediated by antagonism of N-methyl-D-aspartate receptors. With a minimum alveolar concentration of 0.63, xenon is intended for maintaining hypnosis with 30% oxygen. The fast recovery after xenon anaesthesia was hypothesized to be advantageous in this scenario. Case Presentation. We report the case of 99-year-old woman who underwent sigmoid colon carcinoma resection with colorectal anastomosis. We carried out the induction phase by propofol, oxygen, fentanil, and rocuronium bromide, and then we proceeded to a rapid sequence endotracheal intubation consequently. The patient was monitored by IBP, NIBP, ECG, cardiac frequency, respiratory rate, capnometry, TOF Guard, blood gas analysis, and BIS. For maintenance we administrated oxygen, remifentanil, rocuronium bromide, and xenon gas 60-65%. Shortly after the end of surgery the patients started an autonomous respiratory activity, and a high BIS level was also recorded. Decision was made by our team to proceed into the emergence phase. The residual neuromuscular block was antagonized by sugammadex, modified Aldrete score was implicated, and we got our patient fully awake without any cognitive dysfunction or delirium. Conclusion. The rapid emergence to full orientation in very elderly patient who had been anesthetized by xenon shows concordance to the high BIS values and the clinical signs of the depth of anesthesia.

Biophysical changes induced by xenon on phospholipid bilayers

Structural and dynamic changes in cell membrane properties induced by xenon, a volatile anesthetic molecule, may affect the function of membrane-mediated proteins, providing a hypothesis for the mechanism of general anesthetic action. Here, we use molecular dynamics simulation and differential scanning calorimetry to examine the biophysical and thermodynamic effects of xenon on model lipid membranes. Our results indicate that xenon atoms preferentially localize in the hydrophobic core of the lipid bilayer, inducing substantial increases in the area per lipid and bilayer thickness. Xenon depresses the membrane gel-liquid crystalline phase transition temperature, increasing membrane fluidity and lipid head group spacing, while inducing net local ordering effects in a small region of the lipid carbon tails and modulating the bilayer lateral pressure profile. Our results are consistent with a role for nonspecific, lipid bilayer-mediated mechanisms in producing xenon's general anesthetic action.

Xenon preconditioning: molecular mechanisms and biological effects

Xenon is one of noble gases and has been recognized as an anesthetic for more than 50 years. Xenon possesses many of the characteristics of an ideal anesthetic, but it is not widely applied in clinical practice mainly because of its high cost. In recent years, numerous studies have demonstrated that xenon as an anesthetic can exert neuroprotective and cardioprotective effects in different models. Moreover, xenon has been applied in the preconditioning, and the neuroprotective and cardioprotective effects of xenon preconditioning have been investigated in a lot of studies in which some mechanisms related to these protections are proposed. In this review, we summarized these mechanisms and the biological effects of xenon preconditioning.

The Hip Fracture Surgery in Elderly Patients (HIPELD) study: protocol for a randomized, multicenter controlled trial evaluating the effect of xenon on postoperative delirium in older patients undergoing hip fracture surgery

Background

Strategies to protect the brain from postoperative delirium (POD) after hip fracture are urgently needed. The development of delirium often is associated with the loss of independence, poor functional recovery, and increased morbidity, as well as increases in length of hospital stay, discharges to nursing facilities, and healthcare costs. We hypothesize that xenon may reduce the burden of POD, (i) by avoiding the need to provide anesthesia with a drug that targets the γ-amino-butyric acid (GABA)_A receptor and (ii) through beneficial anesthetic and organ-protective effects.

Methods and design

An international, multicenter, phase 2, prospective, randomized, blinded, parallel group and controlled trial to evaluate the incidence of POD, diagnosed with the Confusion Assessment Method (CAM), in older patients undergoing hip fracture surgery under general anesthesia with xenon or sevoflurane, for a period of 4 days post surgery (primary outcome) is planned. Secondary objectives are to compare the incidence of POD between xenon and sevoflurane, to evaluate the incidence of POD from day 5 post surgery until discharge from hospital, to determine the time to first POD diagnosis, to evaluate the duration of POD, to evaluate the evolution of the physiological status of the patients in the postoperative period, to evaluate the recovery parameters, to collect preliminary data to evaluate the economical impact of POD in the postoperative period and to collect safety data. Patients are eligible if they are older aged (≥ 75 years) and assigned to a planned hip fracture surgery within 48 h after the hip fracture. Furthermore, patients need to be willing and able to complete the requirements of this study including the signature of the written informed consent. A total of 256 randomized patients in the 10 participating centers will be recruited, that is, 128 randomized patients in each of the 2 study groups (receiving either xenon or sevoflurane).

Trial registration

EudraCT Identifier: 2009-017153-35; ClinicalTrials.gov Identifier: NCT01199276

Diffusive nature of xenon anesthetic changes properties of a lipid bilayer: molecular dynamics simulations

Effects of general anesthesia can be controllable by the ambient pressure. We perform molecular dynamics simulations for a 1-palmitoyl-2-oleoyl phosphatidylethanolamine lipid bilayer with or without xenon molecules by changing the pressure to elucidate the mechanism of the pressure reversal of general anesthesia. According to the diffusive nature of xenon molecules in the lipid bilayer, a decrease in the orientational order of the lipid tails, an increase in the area and volume per lipid molecule, and an increase in the diffusivity of lipid molecules are observed. We show that the properties of the lipid bilayer with xenon molecules at high pressure come close to those without xenon molecules at 0.1 MPa. Furthermore, we find that xenon molecules are concentrated in the middle of the lipid bilayer at high pressures by the pushing effect and that the diffusivity of xenon molecules is suppressed. These results suggest that the pressure reversal originates from a jamming and suppression of the diffusivity of xenon molecules in lipid bilayers.

Ventilation imaging of the paranasal sinuses using xenon-enhanced dynamic single-energy CT and dual-energy CT: a feasibility study in a nasal cast

OBJECTIVE

To show the feasibility of dual-energy CT (DECT) and dynamic CT for ventilation imaging of the paranasal sinuses in a nasal cast.

METHODS

In a first trial, xenon gas was administered to a nasal cast with a laminar flow of 7 L/min. Dynamic CT acquisitions of the nasal cavity and the sinuses were performed. This procedure was repeated with pulsating xenon flow. Local xenon concentrations in the different compartments of the model were determined on the basis of the enhancement levels. In a second trial, DECT measurements were performed both during laminar and pulsating xenon administration and the xenon concentrations were quantified directly.

RESULTS

Neither with dynamic CT nor DECT could xenon-related enhancement be detected in the sinuses during laminar airflow. Using pulsating flow, dynamic imaging showed a xenon wash-in and wash-out in the sinuses that followed a mono-exponential function with time constants of a few seconds. Accordingly, DECT revealed xenon enhancement in the sinuses only after pulsating xenon administration.

CONCLUSION

The feasibility of xenon-enhanced DECT for ventilation imaging was proven in a nasal cast. The superiority of pulsating gas flow for the administration of gas or aerosolised drugs to the paranasal sinuses was demonstrated.

KEY POINTS

• Ventilation of the paranasal sinuses is poorly understood. • Dual-energy CT ventilation imaging has been explored using phantom simulation. • Xenon can be seen in the paranasal sinuses using pulsating xenon flow. • Dual-energy CT uses a lower radiation dose compared with dynamic ventilation CT.

Theoretical predictions of the spectroscopic parameters in noble-gas molecules: HXeOH and its complex with water

We employ state-of-the-art methods and basis sets to study the effect of inserting the Xe atom into the water molecule and the water dimer on their NMR parameters. Our aim is to obtain predictions for the future experimental investigation of novel xenon complexes by NMR spectroscopy. Properties such as molecular structure and energetics have been studied by supermolecular approaches using HF, MP2, CCSD, CCSD(T) and MP4 methods. The bonding in HXeOH···H(2)O complexes has been analyzed by Symmetry-Adapted Perturbation Theory to provide the intricate insight into the nature of the interaction. We focus on vibrational spectra, NMR shielding and spin-spin coupling constants-experimental signals that reflect the electronic structures of the compounds. The parameters have been calculated at electron-correlated and Dirac-Hartree-Fock relativistic levels. This study has elucidated that the insertion of the Xe atom greatly modifies the NMR properties, including both the electron correlation and relativistic effects, the (129)Xe shielding constants decrease in HXeOH and HXeOH···H(2)O in comparison to Xe atom; the (17)O, as a neighbour of Xe, is deshielded too. The HXeOH···H(2)O complex in its most stable form is stabilized mainly by induction and dispersion energies.