Increased airway resistance during xenon anaesthesia in pigs is attributed to physical properties of the gas

Argon reduces the pulmonary vascular tone in rats and humans by GABA-receptor activation

Argon exerts neuroprotection. Thus, it might improve patients’ neurological outcome after cerebral disorders or cardiopulmonary resuscitation. However, limited data are available concerning its effect on pulmonary vessel and airways. We used rat isolated perfused lungs (IPL) and precision-cut lung slices (PCLS) of rats and humans to assess this topic. IPL: Airway and perfusion parameters, oedema formation and the pulmonary capillary pressure (P_cap) were measured and the precapillary and postcapillary resistance (R_post) was calculated. In IPLs and PCLS, the pulmonary vessel tone was enhanced with ET-1 or remained unchanged. IPLs were ventilated and PCLS were gassed with argon-mixture or room-air. IPL: Argon reduced the ET-1-induced increase of P_cap, R_post and oedema formation (p < 0.05). PCLS (rat): Argon relaxed naïve pulmonary arteries (PAs) (p < 0.05). PCLS (rat/human): Argon attenuated the ET-1-induced contraction in PAs (p < 0.05). Inhibition of GABA_B-receptors abolished argon-induced relaxation (p < 0.05) in naïve or ET-1-pre-contracted PAs; whereas inhibition of GABA_A-receptors only affected ET-1-pre-contracted PAs (p < 0.01). GABA_A/B-receptor agonists attenuated ET-1-induced contraction in PAs and baclofen (GABA_B-agonist) even in pulmonary veins (p < 0.001). PLCS (rat): Argon did not affect the airways. Finally, argon decreases the pulmonary vessel tone by activation of GABA-receptors. Hence, argon might be applicable in patients with pulmonary hypertension and right ventricular failure.

Xenon-helium gas mixture at equimolar concentration of 37.5% protects against oxygen and glucose deprivation-induced injury and inhibits tissue plasminogen activator

Xenon (Xe) is considered to be the golden standard neuroprotective gas. However, Xe has a higher molecular weight and lower thermal conductivity and specific heat than those of nitrogen, the main diluent of oxygen in air. These physical characteristics could impair or at least reduce the intrinsic neuroprotective action of Xe by increasing the patient's respiratory workload and body temperature. In contrast, helium (He) is a cost-efficient gas with a lower molecular weight and higher thermal conductivity and specific heat than those of nitrogen, but is far less potent than Xe. In this study, we hypothesized that mixing Xe and He could allow obtaining a neuroprotective gas mixture with advantageously reduced molecular weight and increased thermal conductivity. We found that Xe and He at the equimolar concentration of 37.5% reduced oxygen-glucose deprivation-induced increase in lactate dehydrogenase in brain slices, an ex vivo model of acute ischemic stroke. These results together with the effects of Xe-He on the thrombolytic efficiency of tissue plasminogen activator are discussed.

A method for calculating the gas volume proportions and inhalation temperature of inert gas mixtures allowing reaching normothermic or hypothermic target body temperature in the awake rat

The noble gases xenon (Xe) and helium (He) are known to possess neuroprotective properties. Xe is considered the golden standard neuroprotective gas. However, Xe has a higher molecular weight and lower thermal conductivity and specific heat than those of nitrogen, the main diluent of oxygen (O2) in air, conditions that could impair or at least reduce the intrinsic neuroprotective properties of Xe by increasing the critical care patient's respiratory workload and body temperature. In contrast, He has a lower molecular weight and higher thermal conductivity and specific heat than those of nitrogen, but is unfortunately far less potent than Xe at providing neuroprotection. Therefore, combining Xe with He could allow obtaining, depending on the gas inhalation temperature and composition, gas mixtures with neutral or hypothermic properties, the latter being advantageous in term of neuroprotection. However, calculating the thermal properties of a mixture, whatever the substances - gases, metals, rubbers, etc. - is not trivial. To answer this question, we provide a graphical method to assess the volume proportions of Xe, He and O2 that a gas mixture should contain, and the inhalation temperature to which it should be administered to allow a clinician to maintain the patient at a target body temperature.

Argon and xenon ventilation during prolonged ex vivo lung perfusion

BACKGROUND

Evidence supports the use of ex vivo lung perfusion (EVLP) as a platform for active reconditioning before transplantation to increase the potential donor pool and to reduce the incidence of primary graft dysfunction. A promising reconditioning strategy is the administration of inhaled noble gases based on their organoprotective effects. Our aim was to validate a porcine warm ischemic lung injury model and investigate postconditioning with argon (Ar) or xenon (Xe) during prolonged EVLP.

METHODS

Domestic pigs were divided in four groups (n = 5 per group). In the negative control group, lungs were flushed immediately. In the positive control (PC) and treatment (Ar, Xe) groups, lungs were flushed after a warm ischemic interval of 2-h in situ. All grafts were evaluated and treated during normothermic EVLP for 6 h. In the control groups, lungs were ventilated with 70% N2/30% O2 and in the treatment groups with 70% Ar/30% O2 or 70% Xe/30% O2, respectively. Outcome parameters were physiological variables (pulmonary vascular resistance, peak airway pressures, and PaO2/FiO2), histology, wet-to-dry weight ratio, bronchoalveolar lavage, and computed tomography scan.

RESULTS

A significant difference between negative control and PC for pulmonary vascular resistance, peak airway pressures, PaO2/FiO2, wet-to-dry weight ratio, histology, and computed tomography-imaging was observed. No significant differences between the injury group (PC) and the treatment groups (Ar, Xe) were found.

CONCLUSIONS

We validated a reproducible prolonged 6-h EVLP model with 2 h of warm ischemia and described the physiological changes over time. In this model, ventilation during EVLP with Ar or Xe administered postinjury did not improve graft function.

Xenon ventilation during therapeutic hypothermia in neonatal encephalopathy: a feasibility study

BACKGROUND AND OBJECTIVES

Therapeutic hypothermia has become standard of care in newborns with moderate and severe neonatal encephalopathy; however, additional interventions are needed. In experimental models, breathing xenon gas during cooling offers long-term additive neuroprotection. This is the first xenon feasibility study in cooled infants. Xenon is expensive, requiring a closed-circuit delivery system.

METHODS

Cooled newborns with neonatal encephalopathy were eligible for this single-arm, dose-escalation study if clinically stable, under 18 hours of age and requiring less than 35% oxygen. Xenon duration increased stepwise from 3 to 18 hours in 14 subjects; 1 received 25% xenon and 13 received 50%. Respiratory, cardiovascular, neurologic (ie, amplitude-integrated EEG, seizures), and inflammatory (C-reactive protein) effects were examined. The effects of starting or stopping xenon rapidly or slowly were studied. Three matched control subjects per xenon treated subject were selected from our cooling database. Follow-up was at 18 months using mental developmental and physical developmental indexes of the Bayley Scales of Infant Development II.

RESULTS

No adverse respiratory or cardiovascular effects, including post-extubation stridor, were seen. Xenon increased sedation and suppressed seizures and background electroencephalographic activity. Seizures sometimes occurred during rapid weaning of xenon but not during slow weaning. C-reactive protein levels were similar between groups. Hourly xenon consumption was 0.52 L. Three died, and 7 of 11 survivors had mental and physical developmental index scores ≥70 at follow-up.

CONCLUSIONS

Breathing 50% xenon for up to 18 hours with 72 hours of cooling was feasible, with no adverse effects seen with 18 months' follow-up.

The effect of high doses of remifentanil in brain near-infrared spectroscopy and in electroencephalographic parameters in pigs

OBJECTIVE

To study the effects of a high remifentanil bolus dose on pig's electroencephalographic indices and on brain regional and global oxygenation.

STUDY DESIGN

Prospective experimental study.

ANIMALS

Twelve healthy Large-White male pigs, age 3 months and weight 26.2 ± 3.6 kg.

METHODS

Anaesthesia was induced with intravenous propofol 4 mg kg⁻¹, then maintained with constant rate infusions of propofol (15 mg kg⁻¹ hour⁻¹) and remifentanil (0.3 μg kg⁻¹ minute⁻¹). Following instrumentation, all pigs received a 5 μg kg⁻¹ remifentanil bolus. The responses of jugular venous oxygen saturation, cardiac output and cerebral oxygen saturation to the remifentanil bolus were studied. The Bispectral index, spectral edge frequency 95%, total power, approximate entropy and permutation entropy were also studied. Repeated measures anova and Pearson correlation were used to analyze the effect of remifentanil bolus on these variables until 5 minutes after the bolus.

RESULTS

Cardiac output and cerebral oxygen saturation decreased significantly after the remifentanil bolus from 4.6 ± 0.9 to 3.8 ± 1.0 L minute⁻¹ and from 65 ± 6 to 62 ± 1% (p < 0.05), respectively. No significant changes were observed in the jugular venous oxygen saturation (p > 0.05) nor in any of the electroencephalogram derived indices (p > 0.05). Correlation analysis revealed strong positive significant correlations between cerebral oxygen saturation and cardiac output (r = 0.82, p < 0.001) and between cerebral oxygen saturation and approximate entropy (r = 0.65, p < 0.001).

CONCLUSIONS AND CLINICAL RELEVANCE

The effect caused by the remifentanil bolus on the brain oxygenation seems to be better reflected by the cerebral oxygen saturation than the jugular venous oxygen saturation. The effect of remifentanil on the electroencephalogram may not be reflected in indices derived from the electroencephalogram, but the potential of the approximate entropy in reflecting changes caused by opioids on the electroencephalogram should be further investigated.

A xenon recirculating ventilator for the newborn piglet: developing clinical applications of xenon for neonates

CONTEXT

The clinical applications of xenon for the neonate include both anaesthesia and neuroprotection. However, due to the limited natural availability of xenon, special equipment is required to administer and recapture the gas to develop xenon as a therapeutic agent.

OBJECTIVE

In order to test the xenon recirculating ventilator for the application of neuroprotection in a preclinical trial, our primary objective was to test the efficiency, reliability and safety of administering 50% xenon for 24 h in hypoxic ischaemic piglets.

DESIGN

A prospective observational study.

SETTING

Institute for Women's Health, University College London, January 2008 to March 2008.

ANIMALS

Four anaesthetised male piglets, less than 24 h old, underwent a global hypoxic ischaemic insult for approximately 25 min prior to switching to the xenon recirculating ventilator.

INTERVENTION

Between 2 and 26 h after hypoxic ischaemia, anaesthetised piglets were administered a mixture of 50% xenon, air, oxygen and isoflurane.

MAIN OUTCOME MEASURES

The primary outcome measure was blood gas PaCO2 (kPa) and secondary outcome measure was xenon gas use (l h), over the 24-h duration of xenon administration.

RESULTS

The xenon recirculating ventilator provided effective ventilation, automated control of xenon/air gas mixtures, and stable blood gas PaCO2 (4.5 to 6.3 kPa) for 24 h of ventilation with the xenon recirculating ventilator. Total xenon use was minimal at approximately 0.6 l h at a cost of approximately &OV0556;8 h. Additional features included an isoflurane scavenger and bellows height alarm.

CONCLUSION

Stable gas delivery to a piglet with minimal xenon loss and analogue circuitry made the xenon recirculating ventilator easy to use and it could be modified for other large animals and noble gas mixtures. The technologies, safety and efficiency of xenon delivery in this preclinical system have been taken forward in the development of neonatal ventilators for clinical use in phase II clinical trials for xenon-augmented hypothermia and for xenon anaesthesia.

Pancuronium dose refinement in experimental pigs used in cardiovascular research

OBJECTIVE

To quantify the dose of pancuronium required to obtain moderate neuromuscular blockade as monitored by acceleromyography (NMB(mod) : train-of-four count of ≤2) as a part of a balanced anaesthetic protocol in pigs used in cardiovascular research.

STUDY DESIGN

Prospective cross-sectional study.

ANIMALS

Five pigs (median body weight: 60 (range 60-63) kg).

METHODS

Anaesthesia was induced with xylazine, ketamine, atropine and midazolam and maintained with isoflurane in O(2) :air and fentanyl. Pigs received 0.1 mg kg(-1) pancuronium initial bolus to reach NMB(mod) followed by 0.1 mg kg(-1) hour(-1) constant rate infusion (CRI). During anaesthesia a twitch count of 3 or measureable T4/T1 ratio indicated unsatisfactory NMB. In this case additional 0.4 mg boluses of pancuronium were administered IV to effect in addition to the CRI. Descriptive statistical analysis was performed to express the median and range of the bolus and CRI dose of pancuronium in pigs. Cardiovascular parameters were analyzed at selected time points with Friedman Repeated Measures Analysis on Ranks. Spearman Rank test was used to evaluate correlation between parameters.

RESULTS

Acceleromyographic monitoring of NMB is feasible in anaesthetized pigs. The median initial dose and rate of pancuronium required to achieve NMB(mod) were 0.10 (range 0.10-0.13) mg kg(-1) and 0.11 (range 0.10-0.21) mg kg(-1) hour(-1) , respectively. The administration rate showed considerable individual variation.

CONCLUSIONS AND CLINICAL RELEVANCE

These pancuronium doses can be used as a guideline to achieve NMB(mod) in pigs as part of a balanced anaesthetic protocol. Instrumental NMB monitoring is essential because of individual kinetic variations and compliance to monitoring guidelines.

Property value estimation for inhaled therapeutic binary gas mixtures: He, Xe, N2O, and N2 with O2

Background

The property values of therapeutic gas mixtures are important in designing devices, defining delivery parameters, and in understanding the therapeutic effects. In the medical related literature the vast majority of articles related to gas mixtures report property values only for the pure substances or estimates based on concentration weighted averages. However, if the molecular size or structures of the component gases are very different a more accurate estimate should be considered.

Findings

In this paper estimates based on kinetic theory are provided of density, viscosity, mean free path, thermal conductivity, specific heat at constant pressure, and diffusivity over a range of concentrations of He-O₂, Xe-O₂, N₂O-O₂ and N₂-O₂ mixtures at room (or normal) and body temperature, 20 and 37°C, respectively and at atmospheric pressure.

Conclusions

Property value estimations have been provided for therapeutic gas mixtures and compared to experimental values obtained from the literature where possible.

[Benefits and indications of xenon anaesthesia]

OBJECTIVE

To analyze the current knowledge related to xenon anaesthesia.

DATA SOURCES

References were obtained from computerized bibliographic research (Medline), recent review articles, the library of the service and personal files.

STUDY SELECTION

All categories of articles on this topic have been selected.

DATA EXTRACTION

Articles have been analyzed for biophysics, pharmacology, toxicity and environmental effects, clinical effects and using prospect.

DATA SYNTHESIS

The noble gas xenon has anaesthetic properties that have been recognized 50 years ago. Xenon is receiving renewed interest because it has many characteristics of an ideal anaesthetic. In addition to its lack of effects on cardiovascular system, xenon has a low solubility enabling faster induction of and emergence from anaesthesia than with other inhalational agents. Nevertheless, at present, the cost and rarity of xenon limits widespread use in clinical practice. The development of closed rebreathing system that allowed recycling of xenon and therefore reducing its waste has led to a recent interest in this gas.

CONCLUSION

Reducing its cost will help xenon to find its place among anaesthetic agents and extend its use to severe patients with specific pathologies.

Ventral recumbency is crucial for fast and safe orotracheal intubation in laboratory swine

The aim of this study was to find the fastest, easiest and safest method of achieving orotracheal intubation for general anaesthesia in laboratory pigs. Twenty-one Yorkshire × Landrace crossbreed male castrated pigs (32.9 ± 4.8 kg) were investigated. Dorsal and ventral recumbency are the alternatives most frequently described for animal positioning during intubation procedures. Based on standardized induction of general anaesthesia using pentobarbital and remifentanil, the dorsoventral and ventrodorsal positions were compared with regard to the time needed, changes in oxygenation and circulatory response. Positioning was found to be crucial for fast orotracheal intubation. The time required for safe intubation is significantly shorter with the ventrodorsal position (17.3 s) in comparison with the dorsoventral position (58.4 s; P < 0.001). Hypoxia did not occur in either group. A significant drop in systolic blood pressure was observed in both groups. Diastolic and mean arterial pressures were not influenced by intubation. A significant increase in heart rate was observed in pigs intubated in ventral recumbency, but not after intubation in the dorsal position. Preoxygenation before intubation is vitally important for preventing hypoxia. With regard to clinical practice, the haemodynamic changes observed in this investigation do not appear to be relevant, as the mean arterial pressure was not altered and heart rates only increased moderately. It may be concluded that the ventrodorsal position can be recommended for orotracheal intubation in pigs as the first choice for providing a smooth and fast airway.

Efficacy of plasmin enzymes and chondroitinase ABC in creating posterior vitreous separation in the pig: a masked, placebo-controlled in vivo study

BACKGROUND

Induction of posterior vitreous detachment (PVD) during vitrectomy helps to prevent proliferative complications, but can be traumatic to the retina, particularly in young patients. Adjunct enzymes have been proposed to facilitate PVD. We investigated the efficacy of enzymes in creating PVD as an adjunct to vitrectomy in the pig.

METHODS

Five groups of 8 pigs received a masked intravitreal injection of chondroitinase (1 IU), human (0.4 or 1.3 activity units [AU]) or porcine plasmin (0.18 AU or 0.47 AU) into one eye, and osmolarity adjusted control into the other. After incubation, a core vitrectomy was performed on each eye at low suction, without vitreous peeling. The occurrence of spontaneous PVD and its extent were graded. Eyes were investigated using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Vitreous remnants on the retina were quantified in SEM. Data were analyzed using McNemar's test for paired observations and Wilcoxon paired signed rank test.

RESULTS

Spontaneous PVD occurred more frequently in human plasmin-treated eyes (p<0.025) and all plasmin eyes (p<0.025) than in placebo controls. The extent of PVD appeared larger in human plasmin (p<0.025) and all plasmin-treated eyes (p<0.025). In plasmin-treated eyes, SEM morphometry showed a significant reduction in the vitreous-covered retina areas. Chondroitinase failed to produce an effect.

CONCLUSIONS

Plasmin may prove a useful adjunct to conventional vitrectomy.

Differences in the time course of proximal and distal airway response to inhaled histamine studied by synchrotron radiation CT

We studied the kinetics of proximal and distal bronchial response to histamine aerosol in healthy anesthetized and mechanically ventilated rabbits up to 60 min after histamine administration using a novel xenon-enhanced synchrotron radiation computed tomography imaging technique. Individual proximal airway constriction was assessed by measuring the luminal cross-sectional area. Distal airway obstruction was estimated by measuring the ventilated alveolar area after inhaled xenon administration. Respiratory system conductance was assessed continuously. Proximal airway cross-sectional area decreased by 57% of the baseline value by 20 min and recovered gradually but incompletely within 60 min. The ventilated alveolar area decreased immediately after histamine inhalation by 55% of baseline value and recovered rapidly thereafter. The results indicate that the airway reaction to inhaled histamine and the subsequent recovery are significantly slower in proximal than in distal bronchi in healthy rabbit. The findings suggest that physiological reaction mechanisms to inhaled histamine in the airway walls of large and small bronchi are not similar.

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.

Differences in regional wash-in and wash-out time constants for xenon-CT ventilation studies

Xenon-enhanced computed tomography (Xe-CT) has been used to measure regional ventilation by determining the wash-in (WI) and wash-out (WO) rates of stable Xe. We tested the common assumption that WI and WO rates are equal by measuring WO-WI in different anatomic lung regions of six anesthetized, supine sheep scanned using multi-detector-row computed tomography (MDCT). We further investigated the effect of tidal volume, image gating (end-expiratory EE versus end-inspiratory EI), local perfusion, and inspired Xe concentration on this phenomenon.

RESULTS

WO time constant was greater than WI in all lung regions, with the greatest differences observed in dependent base regions. WO-WI time constant difference was greater during EE imaging, smaller tidal volumes, and with higher Xe concentrations. Regional perfusion did not correlate with WI-WO. We conclude that Xe-WI rate can be significantly different from the WO rate, and the data suggest that this effect may be due to a combination of anatomic and fluid mechanical factors such as Rayleigh-Taylor instabilities set up at interfaces between two gases of different densities.

Effect of xenon anaesthesia on accuracy of cardiac output measurement using partial CO2rebreathing

Cardiac output (CO) determination based on partial CO(2) rebreathing has recently been introduced into clinical practice. The determination of flow is crucial for exact CO readings and the physical properties of xenon, i.e. high density and viscosity, may influence flow readings. This study compared echocardiography-derived CO measurements with the partial rebreathing method during total intravenous (TIVA) vs. xenon-based anaesthesia. Thirty-nine patients ASA physical status III undergoing aortic reconstruction were randomly assigned to receive either xenon (Xe, n = 20) or TIVA (T, n = 19) based general anaesthetic. Paired measurements were taken before xenon administration, after xenon administration, before and after clamping of the abdominal aorta and after declamping and at corresponding time points in the TIVA group. Data were analysed with a Bland-Altmann plot. Bias and precision were acceptable and comparable before xenon administration (T 0.54 +/- 0.92 l.min(-1) vs. Xe 0.11 +/- 1.1 l.min(-1)), but after xenon administration CO was largely overestimated by partial CO(2) rebreathing (T 0.04 +/- 0.91 l.min(-1) vs. Xe -4.0 +/- 2.1 l.min(-1)). In the TIVA group, bias and precision after declamping increased significantly (P < 0.01) compared to all time points except baseline. In its current application, the NICO cardiac output monitor appears to be inappropriate for determination of CO during xenon based anaesthesia.

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.