Probing lung physiology with xenon polarization transfer contrast (XTC)

One of the major goals of hyperpolarized-gas MRI has been to obtain (129)Xe dissolved-phase images in humans. So far, this goal has remained elusive, mainly due to the low concentration of xenon that dissolves in tissue. A method is proposed and demonstrated in dogs that allows information about the dissolved phase to be obtained by imaging the gas phase following the application of a series of RF pulses that selectively destroy the longitudinal magnetization of xenon dissolved in the lung parenchyma. During the delay time between consecutive RF pulses, the depolarized xenon rapidly exchanges with the gas phase, thus lowering the gas polarization. It is demonstrated that the resulting contrast in the (129)Xe gas image provides information about the local tissue density. It is further argued that minor pulse-sequence modifications may provide information about the alveolar surface area or lung perfusion.

Minimum alveolar concentration (MAC) of xenon with sevoflurane in humans

BACKGROUND

Although more than 30 yr ago the minimum alveolar concentration (MAC) of xenon was determined to be 71%, that previous study had technological limitations, and no other studies have confirmed the MAC value of xenon since. The current study was designed to confirm the MAC value of xenon in adult surgical patients using more modern techniques.

METHODS

Sixty patients were anesthetized with sevoflurane with or without xenon. They were randomly allocated to one of four groups; patients in group 1 received no xenon, whereas those in groups 2, 3, and 4 received end-tidal concentrations of 20, 40, and 60%, respectively (n = 15 each group). Target end-tidal sevoflurane concentrations were chosen using the "up-and-down" method in each group. After steady state sevoflurane and xenon concentrations were maintained for at least 15 min, each patient was monitored for a somatic response at surgical incision. Somatic response was defined as any purposeful bodily movement. The MAC of sevoflurane and its reduction by xenon was evaluated using the multiple independent variable logistic regression model.

RESULTS

The interaction coefficient of the multiple variable logistic regression was not significantly different from zero (P = 0.143). The MAC of xenon calculated as xenon concentration that would reduce MAC of sevoflurane to 0% was 63.1%.

CONCLUSIONS

The authors could not determine whether interaction in blocking somatic responses in 50% of patients is additive. The MAC of xenon is in the range of the values that were predicted in a previous study.

Dynamic NMR spectroscopy of hyperpolarized (129)Xe in human brain analyzed by an uptake model

Hyperpolarized (129)Xe (HpXe) NMR not only holds promise for functional lung imaging, but for measurements of tissue perfusion as well. To investigate human brain perfusion, several time-series of (129)Xe MR spectra were recorded from one healthy volunteer after HpXe inhalation. The time-dependent amplitudes of the MR spectra were analyzed by using a compartment model for xenon uptake modified to account for the loss of (129)Xe polarization due to RF-excitation and for the breathhold technique used in the experiments. This analysis suggests that the resonances detected at 196.5 +/- 1 ppm and 193 +/- 1 ppm originate from HpXe dissolved in gray and white matter, respectively, and that T(1) relaxation times of HpXe are different in gray and white matter (T(1g) > T(1w)).

Minimum alveolar concentration-awake of Xenon alone and in combination with isoflurane or sevoflurane

BACKGROUND

The minimum alveolar concentration (MAC)-awake is a traditional index of hypnotic potency of an inhalational anesthetic. The MAC-awake of xenon, an inert gas with anesthetic properties (MAC = 71%), has not been determined. It is also unknown how xenon interacts with isoflurane or sevoflurane on the MAC-awake.

METHODS

In the first part of the study, 90 female patients received xenon, nitrous oxide (N2O), isoflurane, or sevoflurane supplemented with epidural anesthesia (n = 36 for xenon and n = 18 per group for other anesthetics). In the second part, 72 additional patients received either xenon or N2O combined with the 0.5 times MAC-awake concentration of isoflurane or sevoflurane (0.2% and 0.3%, respectively, based on the results of the first part; n = 18 per group). During emergence, the concentration of an assigned anesthetic (xenon or N2O only in the second part) was decreased in 0. 1 MAC decrements every 15 min from 0.8 MAC or from 70% in the case of N2O until the patient followed the command to either open her eyes or to squeeze and release the investigator's hand. The concentration midway between the value permitting the first response to command and that just preventing it was defined as the MAC-awake.

RESULTS

The MAC-awake were as follows: xenon, 32.6 +/- 6.1% (mean +/- SD) or 0.46 +/- 0.09 MAC; N2O, 63.3 +/- 7.1% (0.61 +/- 0.07 MAC); isoflurane, 0.40 +/- 0.07% (0.35 +/- 0.06 MAC); and sevoflurane, 0.59 +/- 0.10% (0.35 +/- 0.06 MAC). Addition of the 0.5 MAC-awake concentrations of isoflurane and sevoflurane reduced the MAC-awake of xenon to 0.50 +/- 0.15 and 0.51 +/- 0.16 times its MAC-awake as a sole agent, but that of N2O to the values significantly greater than 0.5 times its MAC-awake as a sole agent (0.68 +/- 0.12 and 0.66 +/- 0.14 times MAC-awake; P < 0.01, analysis of variance and Dunnett's test).

CONCLUSIONS

The MAC-awake of xenon is 33% or 0.46 times its MAC. In terms of the MAC-fraction, this is smaller than that for N2O but greater than those for isoflurane and sevoflurane. Unlike N2O, xenon interacts additively with isoflurane and sevoflurane on MAC-awake.

Diffusion of Xenon and Nitrous Oxide into the Bowel

Background

Nitrous oxide diffuses easily from blood into air filled spaces. Xenon is also a relatively insoluble gas, like nitrous oxide. Therefore, the authors measured xenon diffusion into obstructed bowel segments during xenon anesthesia and compared this with nitrous oxide and nitrogen diffusion.

Methods

Twenty-one pentobarbital-anesthetized pigs were randomly assigned to three groups to receive either xenon-oxygen, nitrous oxide-oxygen, or nitrogen-oxygen (75%-25%), respectively. In each animal four bowel segments of 15-cm length were isolated. A pressure-measuring catheter was inserted into the lumen, and 30 ml of room air was injected into the segments. Anesthesia with the selected gas mixture was performed for 4 h. Pressure in the segments was measured continuously. The volume of gaseous bowel content was measured on completion of the study.

Results

The median volume of bowel gas in animals breathing nitrous oxide was 88.0 ml as compared with 39.0 ml with xenon anesthesia and 21.5 ml in the nitrogen-oxygen group. After 4 h of anesthesia, the intraluminal pressures in the nitrous oxide group were found to be significantly greater than in the control group and in the xenon group.

Conclusions

The amount of diffused gas was significantly lower during xenon anesthesia than with nitrous oxide anesthesia but greater than with controls. Blood solubility can therefore be regarded as an important factor influencing gas diffusion into air filled cavities.

The pharmacokinetics of hyperpolarized xenon: implications for cerebral MRI

In this work, a compartmental model to predict the concentration of hyperpolarized xenon (Xe) in the brain is developed based on the well established kinetics of Xe and estimated T1 values for the compartments. For the gaseous compartments, T1 was set to 12 seconds. For the tissue compartments, T1 was set to 6 seconds. Three gas delivery techniques were modeled: hyperventilation followed by breath-hold, continual breathing, and hyperventilation followed by continual breathing. Based on Xe CT, it is estimated that the maximum concentration of Xe that could be breathed is 80%. Based on this value and the estimated maximum polarization of 50%, the peak gray matter concentration of hyperpolarized Xe is calculated to be .036 mM. This leads to an estimated signal-to-noise ratio (SNR), at 2 T, for hyperpolarized Xe that is a factor of 50 lower than the SNR for proton MRI. The peak concentration of hyperpolarized Xe was also calculated over a wide range of gas and tissue T1 values. This model also predicts that the arterial blood will have a concentration of hyperpolarized Xe that is 10 times greater than the concentration in gray matter. An interactive version of the model can be found on the World Wide Web at http:(/)/ric.uthscsa.edu/staff /charlesmartinphd.html.

Distribution of hyperpolarized xenon in the brain following sensory stimulation: preliminary MRI findings

In hyperpolarized xenon magnetic resonance imaging (HP (129)Xe MRI), the inhaled spin-1/2 isotope of xenon gas is used to generate the MR signal. Because hyperpolarized xenon is an MR signal source with properties very different from those generated from water-protons, HP (129)Xe MRI may yield structural and functional information not detectable by conventional proton-based MRI methods. Here we demonstrate the differential distribution of HP (129)Xe in the cerebral cortex of the rat following a pain stimulus evoked in the animal's forepaw. Areas of higher HP (129)Xe signal corresponded to those areas previously demonstrated by conventional functional MRI (fMRI) methods as being activated by a forepaw pain stimulus. The percent increase in HP (129)Xe signal over baseline was 13-28%, and was detectable with a single set of pre and post stimulus images. Recent innovations in the production of highly polarized (129)Xe should make feasible the emergence of HP (129)Xe MRI as a viable adjunct method to conventional MRI for the study of brain function and disease.

Plasma concentration of fentanyl with xenon to block somatic and hemodynamic responses to surgical incision

BACKGROUND

Although anesthesia with xenon has been supplemented with fentanyl, its requirement has not been established. This study was conducted to determine the plasma concentrations of fentanyl necessary to suppress somatic and hemodynamic responses to surgical incision in 50% patients in the presence of 0.7 minimum alveolar concentration (MAC) xenon.

METHODS

Twenty-five patients were allocated randomly to predetermined fentanyl concentration between 0.5 and 4.0 ng/ml during 0.7 MAC xenon anesthesia. Fentanyl was administered using a pharmacokinetic model-driven computer-assisted continuous infusion device. At surgical incision each patient was monitored for somatic and hemodynamic responses. A somatic response was defined as any purposeful bodily movement. A positive hemodynamic response was defined as a more than 15% increase in heart rate or mean arterial pressure more than the preincision value. The concentrations of fentanyl to prevent somatic and hemodynamic responses in 50% of patients were calculated using logistic regression.

RESULTS

The concentration of fentanyl to prevent a somatic response to skin incision in 50% of patients in the presence of 0.7 MAC xenon was 0.72 +/- 0.07 ng/ml and to prevent a hemodynamic response was 0.94 +/- 0.06 ng/ml.

CONCLUSIONS

Comparing these results with previously published results in the presence of 70% nitrous oxide, the fentanyl requirement in xenon anesthesia is smaller than that in the equianesthetic nitrous oxide anesthesia.

Development of hyperpolarized noble gas MRI

Magnetic resonance imaging using the MR signal from hyperpolarized noble gases 129Xe and 3He may become an important new diagnostic technique. Alex Pines (adapting the hyperpolarization technique pioneered by William Happer) presented MR spectroscopy studies using hyperpolarized 129Xe. The current authors recognized that the enormous enhancement in the delectability of 129Xe, promised by hyperpolarization, would solve the daunting SNR problems impeding their attempts to use 129Xe as an in vivo MR probe, especially in order to study the action of general anesthetics. It was hoped that hyperpolarized 129Xe MRI would yield resolutions equivalent to that achievable with conventional 1H2O MRI, and that xenon's solubility in lipids would facilitate investigations of lipid-rich tissues that had as yet been hard to image. The publication of hyperpolarized 129Xe images of excised mouse lungs heralded the emergence of hyperpolarized noble-gas MRI. Using hyperpolarized 3He, researchers have obtained images of the lung gas space of guinea pigs and of humans. Lung gas images from patients with pulmonary disease have recently been reported. 3He is easier to hyperpolarize than 129Xe, and it yields a stronger MR signal, but its extremely low solubility in blood precludes its use for the imaging of tissue. Xenon, however, readily dissolves in blood, and the T1, of dissolved 129Xe is long enough for sufficient polarization to be carried by the circulation to distal tissues. Hyperpolarized 129Xe dissolved-phase tissue spectra from the thorax and head of rodents and humans have been obtained, as have chemical shift 129 Xe images from the head of rats. Lung gas 129Xe images of rodents, and more recently of humans, have been reported. Hyperpolarized 129Xe MRI (HypX-MRI) may elucidate the link between the structure of the lung and its function. The technique may also be useful in identifying ventilation-perfusion mismatch in patients with pulmonary embolism, in staging and tracking the success of therapeutic approaches in patients with chronic obstructive airway diseases, and in identifying candidates for lung transplantation or reduction surgery. The high lipophilicity of xenon may allow MR investigations of the integrity and function of excitable lipid membranes. Eventually, HypX-MRI may permit better imaging of the lipid-rich structures of the brain. Cortical brain function is one perfusion-dependent phenomena that may be explored with hyperpolarized 129Xe MR. This leads to the exciting possibility of conducting hyperpolarized 129Xe functional MRI (HypX-fMRI) studies.

The minimum alveolar concentration of xenon in the elderly is sex-dependent

BACKGROUND

The minimum alveolar concentration (MAC) of xenon in the elderly has not been determined. Moreover, because xenon inhibits the activity of the N-methyl-D-aspartate receptors, and because N-methyl-D-aspartate receptor antagonists such as ketamine and MK-801 exert sex-dependent actions, we hypothesized that the MAC of xenon would also be sex-dependent.

METHODS

Forty-eight patients of both sexes (24 patients of each sex), who were aged 65 yr or older and were undergoing elective laparotomy, were anesthetized with inhalational induction of xenon. Those who demonstrated marked agitation received supplemental propofol intravenously. After tracheal intubation, the end-tidal concentration of xenon was maintained at 45 (women only), 50, 55, 60, 65, 70, or 75% (men only) for at least 15 min before skin incision. These concentrations were randomly allocated to four patients of each sex. Each patient was monitored for the presence or absence of any purposeful bodily movement for 1 min following skin incision. The MAC of xenon was calculated separately for men and women using logistic regression analysis.

RESULTS

The MAC of xenon was 69.3% (95% CI, 63.0-75.6%) for men and 51.1% (44.6-57.6%) for women. The two 95% confidence intervals did not overlap, indicating a statistically significant difference (P < 0.05).

CONCLUSIONS

The MAC of xenon in the elderly is higher in men than in women.

Unequal distribution of blood flow in exercising muscle of the dog

The incomplete O2 extraction in stimulated muscle preparations as well as in exercising muscles has been attributed to diffusion limitation. In particular, the constancy of the ratio maximum O2 uptake-venous PO2 with varied O2 supply conditions is in agreement with predictions on the basis of simple perfusion-diffusion models. But several methods (inert gas washout, local xenon clearance, microsphere embolization) have revealed presence of a considerable degree of inhomogeneity of muscle blood flow and shunting both in supramaximally stimulated and in naturally exercising muscle. This inhomogeneity must be taken into account when estimating diffusion limitation in O2 supply.

Delivery of xenon-containing echogenic liposomes inhibits early brain injury following subarachnoid hemorrhage

Xenon (Xe), a noble gas, has promising neuroprotective properties with no proven adverse side-effects. We evaluated neuroprotective effects of Xe delivered by Xe-containing echogenic liposomes (Xe-ELIP) via ultrasound-controlled cerebral drug release on early brain injury following subarachnoid hemorrhage (SAH). The Xe-ELIP structure was evaluated by ultrasound imaging, electron microscopy and gas chromatography-mass spectroscopy. Animals were randomly divided into five groups: Sham, SAH, SAH treated with Xe-ELIP, empty ELIP, or Xe-saturated saline. Treatments were administrated intravenously in combination with ultrasound application over the common carotid artery to trigger Xe release from circulating Xe-ELIP. Hematoma development was graded by SAH scaling and quantitated by a colorimetric method. Neurological evaluation and motor behavioral tests were conducted for three days following SAH injury. Ultrasound imaging and electron microscopy demonstrated that Xe-ELIP have a unique two-compartment structure, which allows a two-stage Xe release profile. Xe-ELIP treatment effectively reduced bleeding, improved general neurological function, and alleviated motor function damage in association with reduced apoptotic neuronal death and decreased mortality. Xe-ELIP alleviated early SAH brain injury by inhibiting neuronal death and bleeding. This novel approach provides a noninvasive strategy of therapeutic gas delivery for SAH treatment.

XTC MRI: Sensitivity improvement through parameter optimization

Xenon polarization Transfer Contrast (XTC) MRI pulse sequences permit the gas exchange of hyperpolarized xenon-129 in the lung to be measured quantitatively. However, the pulse sequence parameter values employed in previously published work were determined empirically without considering the now-known gas exchange rates and the underlying lung physiology. By using a theoretical model for the consumption of magnetization during data acquisition, the noise intensity in the computed gas-phase depolarization maps was minimized as a function of the gas-phase depolarization rate. With such optimization the theoretical model predicted an up to threefold improvement in precision. Experiments in rabbits demonstrated that for typical imaging parameter values the optimized XTC pulse sequence yielded a median noise intensity of only about 3% in the depolarization maps. Consequently, the reliable detection of variations in the average alveolar wall thickness of as little as 300 nm can be expected. This improvement in the precision of the XTC MRI technique should lead to a substantial increase in its sensitivity for detecting pathological changes in lung function.

Evaluation of carrier agents for hyperpolarized xenon MRI

Several biocompatible carrier agents, in which xenon is highly soluble and has a long T(1), were tested, and injected in living rats. These included saline, Intralipid suspension, perfluorocarbon emulsion and (129)Xe gas-filled liposomes. The T(1) of (129)Xe in these compounds ranged from 47 to 116 s. Vascular injection of these carrier agents was tolerated well, encouraging their use for further experiments in live animals. In vivo spectra, obtained from gas-filled liposomes and perfluorocarbon solutions, suggest that these carrier agents have potential for use in angiography and perfusion imaging.

LCBF values decline while L lambda values increase during normal human aging measured by stable xenon-enhanced computed tomography

Results of measurements of LCBF and L lambda values utilizing optimal CT-CBF methods under resting conditions are reported among thirty-two neurologically normal volunteers aged between 20 and 88 years. Measurements were made during inhalation of 26-30% stable xenon gas for 8 min and serial scanning utilizing a state-of the-art CT scanner with both eyes closed and ears unplugged. LCBF values for cortical gray matter were lowest in occipital cortex and highest in frontal cortex. Gray matter flow values were also high in subcortical structures with highest values measured in the thalamus. For white matter, highest flow values were measured in the internal capsule. Changes in LCBF and L lambda values were analyzed with respect to advancing age. Significant age-related declines in LCBF values were observed in occipital cortex and frontal white matter. Significant age-related increases in L lambda values were measured in frontal and temporal cortex, caudate nucleus and thalamus. Possible explanations are offered for these age-related increases in L lambda values for gray matter, such as accumulation of lipofuscin in neurons and relative compacting of gray matter with advancing age. The latter increases the numbers of nerve cells sampled per volume of gray matter measured.

Intravenous delivery of hyperpolarized (129)Xe: a compartmental model

There is an increasing interest in the use of hyperpolarized 129-xenon (HpXe) NMR for the measurement of tissue perfusion. In this paper we present a theoretical study designed to assess the merit of intravenous HpXe delivery compared with the existing respiration techniques. A compartmental model was created to describe the behavior of the injected bolus in the circulatory system and in the lungs. The dependence of the tissue concentration on the T(1) and solubility of the Xe in the various compartments, and on injection rate, were evaluated. By this process the critical loss mechanisms are identified. It is shown that the predicted tissue concentrations of HpXe in gray and white matter are comparable using respiration or injection techniques.

Biomedical imaging using hyperpolarized noble gas MRI: pulse sequence considerations

Hyperpolarized noble gas MRI is a new technique for imaging of gas spaces and tissues that have been hitherto difficult to image, making it a promising diagnostic tool. The unique properties of hyperpolarized species, particularly the non-renewability of the large non-equilibrium spin polarization, raises questions about the feasibility of hyperpolarized noble gas MRI methods. In this paper, the critical issue of T1 relaxation is discussed and it is shown that a substantial amount of polarization should reach the targets of interest for imaging. We analyse various pulse sequence designs, and point out that total scan times can be decreased so that they are comparable or shorter than tissue T1 values. Pulse sequences can be optimized to effectively utilize the non-renewable hyperpolarization, to enhance the SNR, and to eliminate image artifacts. Hyperpolarized noble gas MRI is concluded to be quite feasible.

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.

Quantifying the effect of intravascular perfluorocarbon on xenon elimination from canine muscle

Intravenous infusions of perfluorocarbon (PFC) may improve decompression sickness outcome in animals by accelerating inert gas elimination from tissue, but any such effect has not been quantified experimentally. In this study we used an animal model of tissue Xe kinetics to test this hypothesis and to quantify the effect of PFC. Eight dogs were ventilated with dilute 133Xe in air for 4 h of Xe uptake. Four dogs were then given an infusion (20 ml/kg iv) of a 40% (vol/vol) perfluorodecalin-glycerol emulsion, and four control dogs were given only isotonic glycerol. All were then switched to open-circuit air breathing for 4 h of Xe elimination. During this time Xe radioactivity-time curves were recorded from two intact hind leg muscles, and the Xe mean residence times during elimination were estimated using an analysis by moments and compared by group. Tissue blood flows were measured using microspheres once during Xe uptake and twice during Xe elimination, and cardiac outputs were measured by thermodilution at 30-min intervals. In the PFC group the measured circulating PFC fraction increased the calculated Xe solubility by an average factor of 1.77 and so was expected to increase the Xe elimination rate by 77%. The observed Xe mean residence times on elimination for the PFC group averaged 33.5 min [95% confidence interval (CI) 19.5-47.6] compared with the glycerol control average of 70.1 min (95% CI 56.1-84.2), representing an increase in the rate of Xe elimination by a factor of 2.09 or 109%.(ABSTRACT TRUNCATED AT 250 WORDS)

MAC of xenon and halothane in rhesus monkeys

Local cerebral blood flow (LCBF) maps produced by 33% xenon-enhanced computed tomographic scanning (Xe/CT LCBF) are useful in the clinical diagnosis and management of patients with cerebrovascular disorders. However, observations in humans that 25-35% xenon (Xe) inhalation increases cerebral blood flow (CBF) have raised concerns that Xe/CT LCBF measurements may be inaccurate and that Xe inhalation may be hazardous in patients with decreased intracranial compliance. In contrast, 33% Xe does not increase CBF in rhesus monkeys. To determine whether this interspecies difference in the effect of Xe on CBF correlates with an interspecies difference in the anesthetic potency of Xe, we measured the minimum alveolar concentration (MAC) of Xe preventing movement to a tail-clamp stimulus in rhesus monkeys. Using a standard protocol for the determination of MAC in animals, we first measured the MAC of halothane (n = 5), and then used a combination of halothane and Xe to measure the MAC of Xe (n = 7). The halothane MAC was 0.99 +/- 0.12% (M +/- SD), and the Xe MAC was 98 +/- 15%. These results suggest that the MAC of Xe in rhesus monkeys is higher than the reported human Xe MAC value of 71%. Thus the absence of an effect of 33% Xe on CBF in the rhesus monkey may be related to its lower anesthetic potency.

Utility of ventilation and perfusion scan in the diagnosis of young military recruits with an incidental finding of hyperlucent lung

Swyer-James-MacLeod syndrome (SJMS) is considered to be a relatively uncommon disease presenting with unilateral hyperlucent lung due to hypoplasia of a pulmonary artery and bronchiectasis of the affected lung. In this report, we describe the ventilation-perfusion (V/Q) scan findings of nine male recruits (aged 20-29 years, mean 24.4+/-2.96 years) with SJMS in whom the diagnosis was first established in adulthood. V/Q scan findings of all patients were compared with those on planar radiographs, pulmonary function studies, high resolution computed tomography (HRCT) and digital subtraction angiography (DSA). The ventilation (133Xe) and perfusion (99Tcm-macro-aggregated albumin) scans showed the characteristic pattern of a matched V/Q defect and marked air trapping on the washout phase on 133Xe scintigraphy. HRCT displayed hypodense lung with integrity of main airways, and markedly diminished vasculature on the affected side in all patients. A smaller pulmonary artery on the affected side with poor peripheral vasculature was observed with DSA in all patients. All patients had features of obstructive airway disease in varying degrees on pulmonary function studies. In contrast to other imaging methods, bronchiectasis as an etiological factor was displayed on HRCT. Some pulmonary areas, which were normal on HRCT and planar radiographs, showed air trapping on V/Q scan. Although a V/Q scan was more helpful in determining the extent of the disease and correlates well with conventional imaging methods, HRCT was the most valuable imaging method for the evaluation of aetiology in unilateral hyperlucent lung.

Minimum alveolar concentration (MAC) of xenon in intubated swine

BACKGROUND

The minimum alveolar concentration (MAC) is a traditional index of the hypnotic potency of an inhalational anaesthetic. To investigate the anaesthetic as well as the unwanted effects of xenon (Xe) in a swine model, it is useful to know MAC(Xe).

METHODS

The study was performed using ten swine (weight 27.8-35.4 kg) anaesthetized with halothane and Xe 0, 15, 30, 40, 50 and 65% in oxygen. With each Xe concentration, various concentrations of halothane were administered in a step-by-step design. For each combination, a supramaximal pain stimulus (claw clamp) was applied and the appearance of a withdrawal reaction was recorded. The MAC(Xe) with halothane was calculated using a logistic regression model.

RESULTS

During stable ventilation, haemodynamics and temperature, MAC(Xe) value was determined as 119 vol. % (95% confidence limits 103-135).

CONCLUSION

MAC(Xe) in swine was calculated by extrapolation of a logistic regression model. Its theoretical value is 119 vol. %.

Cerebral hemodynamics in patients with moyamoya disease and in patients with atherosclerotic occlusion of the major cerebral arterial trunks

To determine the difference in cerebral hemodynamics between Moyamoya disease and atherosclerotic occlusion of the major cerebral arterial trunks, we measured local cerebral blood flow (CBF) and local CO2 reactivity (CO2R) by xenon-enhanced computed tomography (CT). A total of 11 adult patients with Moyamoya disease (mean age, 39.6 +/- 7.8 years) and eight patients with atherosclerotic occlusion of the major arterial trunks (mean age, 62.4 +/- 15.4 years) were studied. Regions of interest were frontal, temporal and occipital cortex, caudate, putamen and thalamus in each hemisphere. In patients with Moyamoya disease, local CBF values in the internal carotid artery territory (frontal and temporal cortex, caudate, putamen) were significantly higher than those in the occluded side of patients with atherosclerotic occlusion. Local CO2R values in the caudate and putamen were significantly higher than those in the occluded side of patients with atherosclerotic occlusion. These results suggest that the cerebral hemodynamics of Moyamoya disease differ from those of atherosclerotic occlusion of the major cerebral arterial trunks, and may be a result of the abundant collateral circulation through basal 'Moyamoya' vessels.

Quantitative cerebral blood flow calculation method using white matter lambda in xenon CT

The objective of this work is to propose a quantitative cerebral blood flow (CBF) calculation method for xenon CT (Xe-CT) by logically estimating the time course change rate (rate constant) of the arterial xenon concentration from that of end-tidal xenon concentration. A single factor, gamma (gamma), which is considered to reflect the diffusing capacity of the lung for xenon, was introduced to correlate the end-tidal rate constant (Kend-tidal) with the arterial rate constant (Karterial). When an appropriate value is given to gamma, it is possible to calculate the arterial rate constant (calculated Karterial) from Kend-tidal. A procedure was developed to determine the gamma value utilizing the characteristics of white matter lambda (lambda). This procedure was applied to three healthy volunteers. The gamma gammaalues for the three subjects were consistent with those directly calculated from end-tidal and arterial (abdominal aorta) xenon data. Hemispheric CBF values with use of calculated Karterial (47.3 +/- 10.3 ml/100 g/min) were close to the reported normative values. We conclude this method could make current Xe-CT examinations substantially reliable and quantitative in measuring CBF.