The Partition of Trace Amounts of Xenon Between Human Blood and Brain Tissues at 37$deg$C

Methods to measure, model and manipulate fluid flow in brain

The brain consists of a complex network of cells and matrix that is cushioned and nourished by multiple types of fluids: cerebrospinal fluid, blood, and interstitial fluid. The movement of these fluids through the tissues has recently gained more attention due to implications in Alzheimer's Disease and glioblastoma. Therefore, methods to study these fluid flows are necessary and timely for the current study of neuroscience. Imaging modalities such as magnetic resonance imaging have been used clinically and pre-clinically to image flows in healthy and diseased brains. These measurements have been used to both parameterize and validate models of fluid flow both computational and in vitro. Both of these models can elucidate the changes to fluid flow that occur during disease and can assist in linking the compartments of fluid flow with one another, a difficult challenge experimentally. In vitro models, though in limited use with fluid flow, allow the examination of cellular responses to physiological flow. To determine causation, in vivo methods have been developed to manipulate flow, including both physical and pharmacological manipulations, at each point of fluid movement of origination resulting in exciting findings in the preclinical setting. With new targets, such as the brain-draining lymphatics and glymphatic system, fluid flow and tissue drainage within the brain is an exciting and growing research area. In this review, we discuss the methods that currently exist to examine and test hypotheses related to fluid flow in the brain as we attempt to determine its impact on neural function.

Pharmacokinetic analysis of the chronic administration of the inert gases Xe and Ar using a physiological based model

Background

New gas therapies using inert gases such as xenon and argon are being studied, which would require chronically administered repeating doses. The pharmacokinetics of this type of administration has not been addressed in the literature.

Methods

A physiologically based pharmacokinetics (PBPK) model for humans, pigs, mice, and rats has been developed to investigate the unique aspects of the chronic administration of inert gas therapies. The absorption, distribution, metabolism and excretion (ADME) models are as follows: absorption in all compartments is assumed to be perfusion limited, no metabolism of the gases occurs, and excretion is only the reverse process of absorption through the lungs and exhaled.

Results

The model has shown that there can be a residual dose, equivalent to constant administration, for chronic repeated dosing of xenon in humans. However, this is not necessarily the case for small animals used in pre-clinical studies.

Conclusions

The use of standard pharmacokinetics parameters such as area under the curve would be more appropriate to assess the delivered dose of chronic gas administration than the gas concentration in the delivery system that is typically reported in the scientific literature because species and gas differences can result in very different delivered doses.

Determination of natural in vivo noble-gas concentrations in human blood

Although the naturally occurring atmospheric noble gases He, Ne, Ar, Kr, and Xe possess great potential as tracers for studying gas exchange in living beings, no direct analytical technique exists for simultaneously determining the absolute concentrations of these noble gases in body fluids in vivo. In this study, using human blood as an example, the absolute concentrations of all stable atmospheric noble gases were measured simultaneously by combining and adapting two analytical methods recently developed for geochemical research purposes. The partition coefficients determined between blood and air, and between blood plasma and red blood cells, agree with values from the literature. While the noble-gas concentrations in the plasma agree rather well with the expected solubility equilibrium concentrations for air-saturated water, the red blood cells are characterized by a distinct supersaturation pattern, in which the gas excess increases in proportion to the atomic mass of the noble-gas species, indicating adsorption on to the red blood cells. This study shows that the absolute concentrations of noble gases in body fluids can be easily measured using geochemical techniques that rely only on standard materials and equipment, and for which the underlying concepts are already well established in the field of noble-gas geochemistry.

A generic biokinetic model for noble gases with application to radon

To facilitate the estimation of radiation doses from intake of radionuclides, the International Commission on Radiological Protection (ICRP) publishes dose coefficients (dose per unit intake) based on reference biokinetic and dosimetric models. The ICRP generally has not provided biokinetic models or dose coefficients for intake of noble gases, but plans to provide such information for (222)Rn and other important radioisotopes of noble gases in a forthcoming series of reports on occupational intake of radionuclides (OIR). This paper proposes a generic biokinetic model framework for noble gases and develops parameter values for radon. The framework is tailored to applications in radiation protection and is consistent with a physiologically based biokinetic modelling scheme adopted for the OIR series. Parameter values for a noble gas are based largely on a blood flow model and physical laws governing transfer of a non-reactive and soluble gas between materials. Model predictions for radon are shown to be consistent with results of controlled studies of its biokinetics in human subjects.

Measurement of blood flow and xenon solubility coefficient in the human liver by xenon-enhanced computed tomography

PURPOSE

The goal of this work was to develop a method of calculating blood flow and xenon solubility coefficient (λ) in the hepatic tissue by xenon-enhanced computed tomography (Xe-CT) and to demonstrate λ can be used as a measure of fat content in the human liver.

METHODS

A new blood supply model is introduced which incorporates both arterial blood and portal venous blood which join and together flow into hepatic tissue. We applied Fick's law to the model. It was theoretically derived that the time course of xenon concentration in the inflow blood (the mixture of the arterial blood and the portal venous blood) can be approximated by a monoexponential function. This approximation made it possible to obtain the time-course change rate (K(I)) of xenon concentration in the inflow blood using the time course of xenon concentration in the hepatic tissue by applying the algorithm we had reported previously. K(I) was used to calculate blood flow and λ for each pixel in the CT image of the liver. Twenty-six patients (49.2 ± 18.3 years) with nonalcoholic steatohepatitis underwent Xe-CT abdominal studies and liver biopsies. Steatosis of the liver was evaluated using the biopsy specimen and its severity was divided into ten grades according to the fat deposition percentage [(severity 1) ≤ 10%, 10 % <(severity 2) ≤ 20%, [ellipsis (horizontal)], 90% < (severity 10) ≤ 100%]. For each patient, blood flow and λ maps of the liver were created, and the average λ value (λ) was compared with steatosis severity and with the CT value ratio of the liver to the spleen (liver∕spleen ratio).

RESULTS

There were good correlations between λ and steatosis severity (r = 0.914, P < 0.0001), and between λ and liver∕spleen ratio (r = -0.881, P < 0.0001). Ostwald solubility for xenon in the hepatic tissue (tissue Xe solubility), which is calculated using λ and the hematocrit value of the patient, also showed a good correlation with steatosis severity (r = 0.910, P < 0.0001). λ ranged from 0.86 to 7.81, and tissue Xe solubility ranged from 0.12 to 1.16. This range of solubility is reasonable considering the reported Ostwald solubility coefficients for xenon in the normal liver and in the fat tissue are 0.10 and 1.3, respectively, at 37 °C. The average blood flow value ranged from 15.3 to 53.5 ml∕100 ml tissue∕min.

CONCLUSIONS

A method of calculating blood flow and λ in the hepatic tissue was developed by means of Xe-CT. This method would be valid even if portosystemic shunts exist; it is shown that λ maps can be used to deduce fat content in the liver. As a noninvasive modality, Xe-CT would be applicable to the quantitative study of fatty change in the human liver.

Liver atrophy, hypertrophy and regenerative hyperplasia in the rat: the relevance of blood flow

Liver perfusion can be measured sequentially by monitoring the clearance of 85krypton injected into the portal circulation. Preliminary studies in dogs show that the method reflects gas clearance from a homogeneously perfused liver parenchyma and correlates well with measurements of total hepatic blood flow. The method has been adapted to measurement of liver blood flow in normal rats, partially hepatectomized rats and after portacaval transposition with and without partial hepatectomy. There is a marked rise of approximately 250% in the first four hours after partial hepatectomy in the rat. After portacaval trans-position, liver blood flow remains within the normal range and no great rise occurs after partial hepatectomy. In animals subjected to portacaval transposition, there is a reduction in relative liver weight (liver weight/body weight ratio) and when partial hepatectomy is performed three weeks after portacaval transposition, the relative liver weight is regained within three weeks, and does not differ from that of non-hepatectomized controls. DNA synthetic activity, studied during the 72 hours after partial hepatectomy performed three weeks after portacaval transposition, shows an uptake of tritiated thymidine into liver DNA of the same magnitude as and contemporaneous with that of controls. Liver atrophy after portal diversion is not a result of a decrease in absolute liver blood flow. Regenerative hyperplasia appears to be independent of a direct supply of portal blood and occurs in the absence of the post-hepatectomy rise in liver blood flow seen in normal rats.

Comparison of cerebral blood flow between perfusion computed tomography and xenon-enhanced computed tomography for normal subjects: territorial analysis

OBJECTIVE

The purpose of this study was to clarify the difference between cerebral blood flow (CBF) by perfusion computed tomography (CT) and that by xenon-enhanced CT (Xe-CT) through simultaneous measurement.

METHODS

Xenon-enhanced CT and perfusion CT were continually performed on 7 normal subjects. Ratios of CBF by perfusion CT (P-CBF) to CBF by Xe-CT (Xe-CBF) were measured for 5 arterial territories; 3 were territories of 3 major arteries (the anterior [ACA], middle [MCA], and posterior [PCA] cerebral arteries), and the other 2 were areas of the thalamus and putamen.

RESULTS

The ratios were 1.30 +/- 0.10, 1.26 +/- 0.15, 1.61 +/- 0.15, 0.801 +/- 0.087, and 0.798 +/- 0.080 for the ACA, MCA, PCA, thalamus, and putamen, respectively. Although a good correlation was observed between P-CBF and Xe-CBF for each territory, the ratios were significantly different (P < 0.0001) between 3 territory groups (group 1: ACA and MCA, group 2: PCA, and group 3: thalamus and putamen).

CONCLUSIONS

The difference in the ratio of P-CBF to Xe-CBF between the 3 territory groups was considered to result principally from the features of P-CBF. To evaluate P-CBF properly, its territorial characteristics should be taken into account.

Quantitative multilevel mapping of hepatic blood flow by xenon computed tomography using aorta

A noninvasive and quantitative technique has been developed to measure human hepatic blood flow by xenon computed tomography (Xe-CT). Accurate data on time-dependent xenon concentrations in the arterial blood are indispensable for Xe-CT to ensure quantitativeness of measured blood flow. A method has been established by our group to use both aorta and end-tidal data to obtain arterial xenon information. Multilevel (3 levels) maps of arterial blood flow (Fa), portal blood flow (Fp), and partition coefficient (lambda) were created for patients with chronic hepatitis. A method to objectively evaluate Fa, Fp, and lambda values for the whole liver has also been developed by our group.

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.

Determination of regional ventilation and perfusion in the lung using xenon and computed tomography

We propose a model to measure both regional ventilation (V) and perfusion (Q) in which the regional radiodensity (RD) in the lung during xenon (Xe) washin is a function of regional V (increasing RD) and Q (decreasing RD). We studied five anesthetized, paralyzed, mechanically ventilated, supine sheep. Four 2.5-mm-thick computed tomography (CT) images were simultaneously acquired immediately cephalad to the diaphragm at end inspiration for each breath during 3 min of Xe breathing. Observed changes in RD during Xe washin were used to determine regional V and Q. For 16 mm(3), Q displayed more variance than V: the coefficient of variance of Q (CV(Q)) = 1.58 +/- 0.23, the CV of V (CV(V)) = 0.46 +/- 0.07, and the ratio of CV(Q) to CV(V) = 3.5 +/- 1.1. CV(Q) (1.21 +/- 0.37) and the ratio of CV(Q) to CV(V) (2.4 +/- 1.2) were smaller at 1,000-mm(3) scale, but CV(V) (0.53 +/- 0.09) was not. V/Q distributions also displayed scale dependence: log SD of V and log SD of Q were 0.79 +/- 0.05 and 0.85 +/- 0.10 for 16-mm(3) and 0.69 +/- 0.20 and 0.67 +/- 0.10 for 1,000-mm(3) regions of lung, respectively. V and Q measurements made with CT and Xe also demonstrate vertically oriented and isogravitational heterogeneity, which are described using other methodologies. Sequential images acquired by CT during Xe breathing can be used to determine both regional V and Q noninvasively with high spatial resolution.

Limited role for nitric oxide in mediating cerebrovascular control of newborn piglets

AIMS

To investigate the effects of the nitric oxide (NO) synthase inhibitor L-nitro-arginine methyl ester (L-NAME) on cerebral blood flow, and its response to alterations in arterial carbon dioxide tension (CBF-CO2 reactivity).

METHODS

Cerebral blood flow was measured six times at varying arterial carbon dioxide tension (PaCO2) using the intravenous 133Xenon clearance technique in eight mechanically ventilated piglets of less than 24 hours postnatal age. After the third measurement L-NAME was administered as a bolus (20 mg/kg) and subsequently infused (10 mg/kg/hour).

RESULTS

PaCO2 ranged between 2.7-8.9 kPa. Cerebral blood flow decreased by 14.0% (95% confidence interval 1.9-27.4) after L-NAME. CBF-CO2 reactivity was 18.4% per kPa (95% CI 14.1-22.2) before L-NAME and 15.2%/kPa (95% CI 11.1-19.3) afterwards; the difference between the CBF-CO2 reactivities was 3.2%/kPa (95% CI -0.4-6.8): these were not significantly different.

CONCLUSIONS

Inhibition of nitric oxide synthesis reduces cerebral blood flow no more than a 0.5-1.0 kPa fall in PaCO2. Nitric oxide is not an important mediator of CBF-CO2 reactivity.

Regional differences of cerebral blood flow in the preterm infant

The purpose of our study was to evaluate the regional distribution of the resting cerebral blood flow (CBF) pattern in preterm neonates. Sixty-eight preterm babies with a gestational age of less than 34 weeks and a birth weight of less than 1500 g were enrolled into the study. The CBF was measured by the noninvasive intravenous 133Xenon method at three different times. Depending on the age we classified our measurements into three groups. Group 1: measurement between 2-36 h (n = 46). Group 2: measurement between 36-108 h (n = 39). Group 3: measurement between 108-240 h (n = 41). In all three groups CBF was significantly lower in the occipital region than in the frontal and parietal regions (group 1: frontal region 12.8 +/- 3.5 ml/100 g/min, parietal region 12.8 +/- 3.9 ml/100 mg/min, and occipital region 11.6 +/- 3.18 ml/100 g/min; group 2: frontal region 15.4 +/- 4.2 ml/100 g/min, parietal region 15.3 +/- 4.1 ml/100 g/min, and occipital region 13.4 +/- 3.5 ml/100 g/min; group 3: frontal region 14.6 +/- 3.6 ml/100 g/min, parietal region 14.6 +/- 3.2 ml/100 g/min, and occipital region 12.8 +/- 2.7 ml/100 g/min.). CBF did not differ between the left and the right hemispheres in either of the three measured regions. No gradient was found in infants between 108 h and 240 h of age with periventricular leukomalacia and periventricular haemorrhage. CONCLUSION. In preterm neonates the antero-posterior gradient of CBF is already present. Periventricular leukomalacia as well as periventricular haemorrhage may affect the regional regulation of CBF.

Cerebral blood flow in migraine accompaniments and vertebrobasilar ischemia

BACKGROUND AND PURPOSE

Transient neurological symptoms of brain stem or occipital lobe origin may be caused by transient ischemic attack in the vertebrobasilar territory (VB-TIA) or late-onset (or late-life) migraine accompaniment (LOMA). It is often clinically difficult to distinguish between VB-TIA and LOMA.

METHODS

Cerebral blood flow of 23 patients with VB-TIA, 24 with LOMA, and 28 age-matched control subjects was measured using the 133Xe inhalation regional cerebral blood flow (rCBF) technique.

RESULTS

After adjusting for differences in baseline variables such as blood pressure, hematocrit, and PCO2, patients with VB-TIA had (1) lower mean rCBF than control subjects (P < .003) as measured by the initial slope index method; (2) more frequent anterior rCBF asymmetries than control subjects and patients with LOMA (P < .03 for both comparisons); and (3) higher mean interhemispheric rCBF differences compared with patients with LOMA (P = .08) and control subjects (P < .02).

CONCLUSIONS

Regional CBF patterns in patients with VB-TIA and LOMA differ, with lower rCBF and more asymmetry of the anterior blood flows in patients with VB-TIA, probably reflecting the effects of stroke risk factors on the cerebral circulation. Patients with LOMA have rCBF patterns more closely resembling those of age-matched healthy subjects. rCBF measurements may assist in the clinical diagnosis of VB-TIA and late-onset migrainous events.

Average blood flow and oxygen uptake in the human brain during resting wakefulness: a critical appraisal of the Kety-Schmidt technique

The Kety-Schmidt technique can be regarded as the reference method for measurement of global average cerebral blood flow (average CBF) and global average cerebral metabolic rate of oxygen (average CMRO2). However, in the practical application of the method, diffusion equilibrium for inert gas tracer between the brain and its venous blood is not reached. As a consequence, normal values for CBF and CMRO2 of 54 ml 100 g-1 min-1 and 3.5 ml 100 g-1 min-1 obtained with the Kety-Schmidt technique are an overestimation of the true values. Using the Kety-Schmidt technique we have performed 57 measurements of CBF and CMRO2 during EEG-verified wakeful rest in young normal adults. In order to estimate the equilibrium values for CBF and CMRO2, a simple computer-based simulation model was employed to quantitate the systematic overestimation caused by incomplete tracer equilibrium. When correcting the measured data, we find that the true average values for CBF and CMRO2 in the healthy young adult are approximately 46 ml 100 g-1 min-1 and approximately 3.0 ml 100 g-1 min-1. Previous studies have suggested that some of the variation in CMRO2 values could be ascribed to differences in cerebral venous anatomy. However in the present study, no correlation between CMRO2 and cerebral venous anatomy as imaged by magnetic resonance angiography could be established. Our data show that the interindividual variation of CMRO2 is 11% (coefficient of variation).

Signal analysis of noninvasive Xenon-133 cerebral blood flow measurements

An anatomical model in conjunction with experimentally determined absorption data provides a framework to simulate signals as obtained from the noninvasive Xenon-133 cerebral blood flow technique. The contribution of individual tissue compartments to the total signal as well as the effect on the computed results were investigated under normal conditions. The introduction of physiological abnormalities into the model allowed the determination of sensitivity of the technique with respect to size, position, and perfusion level of the lesion. In addition, effects of cross-talk between hemispheres and signal overlap of adjacent detectors were quantified. It was found that the change of externally measured blood flow is proportional to the decrement/increment of flow in the lesion. Contrary to earlier reports, the effects of cross-talk and signal overlap were not found to be serious limitations in identifying lesions.

[Methods for measuring spinal cord blood flow]

This study aimed to review the techniques used most currently for measuring spinal cord blood blow flow (SCBF) in animals, i.e. the hydrogen clearance, labelled microspheres, 133Xe clearance and 14C-antipyrine autoradiographic methods. All four techniques may only be used in animals, because of their invasiveness. Flow figures varied greatly with the method, the spinal level at which measurements were carried out, and the species of animal. However, results tend to suggest that SCBF is very similar to cerebral blood flow in that it is controlled by chemical, autoregulatory and metabolic factors. Approaches to measuring SCBF in man may be made using stable xenon-enhanced computed tomographic imaging (Xes-CT) in the same way as for measuring cerebral blood flow. The calculation of SCBF is based on Fick's principle transformed by Kety and Schmidt. After a reference CT section has been obtained, twelve 8 mm thick sections are carried out whilst the patient breathes a 30% xenon-70% air/oxygen mixture. This series of views enables the SCBF to be calculated in four steps. Quantitative analysis in eight human subjects gave a mean SCBF of 58.8 +/- 5.96 ml x 100 g-1 x min-1. However, this method has a low signal to noise ratio. Moreover, the qualitative analysis of the parametric views of flow demonstrate tissue heterogeneity, partly due to the patient's movements (breathing movements). However, the method is non invasive, safe, and reproducible. As it can measure very low values of blood flow, the study of ischaemic spinal lesions is made possible, although some technical and software improvements are still required.

Prevention of endotoxin-induced increase of cerebral oxygen consumption in dogs by propranolol pretreatment

Cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) were studied in experimental endotoxic shock in dogs. Eight animals were pretreated with a beta-adrenoceptor blocking agent, propranolol (PPL), per os 12 mg/kg once a day for 7 days. Ten animals served as controls. After an intravenous injection of endotoxin, 1 mg/kg, CBF decreased in both groups, with no significant differences between the groups. CMRO2 increased in the control animals by about 18% from the baseline value both 1 and 2 h after the injection of endotoxin. CMRO2 in the PPL-pretreated animals was unchanged after endotoxin. The CMRO2-reactions to endotoxin in control and PPL animals were significantly different after both 1 and 2 h (P less than 0.05). The present results indicate that the increase in CMRO2 following intravenous endotoxin is mediated via beta-adrenoceptors.

Effect of hemodilution on maximal oxygen consumption, blood lactate response to exercise and cerebral blood flow in subjects with a high-affinity hemoglobin

10 subjects with Hb Linköping (beta 36 Pro greater than Thr), a high-affinity hemoglobin variant, with a P50 of 2.2 kPa (16.5 mm Hg) were investigated before and 3-4 days after normovolemic hemodilution. Blood hemoglobin concentration decreased from 176 +/- 13 (SD) to 146 +/- 15 g l-1 and the red cell volume from 2.77 +/- 0.83 to 2.23 +/- 0.67 l. Maximal oxygen consumption decreased slightly by 2.7 +/- 3.8 ml min-1 kg-1 and maximal exercise power by 11 +/- 23 W; these changes were, however, not statistically significant. Maximal heart rate was unchanged (-1.1 +/- 6.4 beats min-1) while submaximal heart rate was consistently increased on comparable loads after hemodilution compared to before. The exercise ECG was normal both before and after hemodilution. Capillary lactate levels at exercise were always higher after hemodilution than before. Cerebral blood flow was normal both before and after hemodilution in all subjects but one who had a high flow. The grey matter blood flow increased slightly but significantly by 8.6 +/- 10.3 ml min-1 100 g-1 from before to after hemodilution. The results indicate that subjects with Hb Linköping have only limited benefit from their increased blood hemoglobin concentration.

Role of thalamus and white matter in cognitive outcome after head injury

Local CBF (LCBF) and local partition coefficients (L lambda) were measured by xenon-enhanced computed tomography among 15 patients with remote cerebral trauma resulting from severe head injury. Results were compared with similar measures among age-matched normal volunteers (N = 20). The patients were divided into two groups according to different outcomes based on serial cognitive testing: Group I (N = 10) improved but Group D (N = 5) deteriorated throughout a mean interval of 10 years of follow-up. Initial LCBF measurements were performed at mean intervals of 6.8 years after injury. Cortical LCBF values were decreased in frontal (p less than 0.01) and temporal (p less than 0.05) regions among both groups, but only in Group D were flow values decreased in putamen and thalamus (p less than 0.05). L lambda values were reduced in frontotemporal cortex among both groups but in the thalamus only among Group D (p less than 0.05). Mean white matter flow values were normal in Group I but were reduced in Group D (p less than 0.05). Mean partition coefficients for white matter were reduced in both groups (p less than 0.01) but were lower in Group D (p less than 0.05). Reduced perfusion of frontotemporal gray matter is consonant with neuropathological reports following severe brain trauma of neuronal atrophy, gliosis, and infarction affecting these regions. Group comparisons between patients who cognitively improved versus those that deteriorated demonstrate an association between reductions of CBF in putamen, thalamus and subcortical white matter and impaired cognition after severe head injury.

Cerebral oxygen metabolism and cerebral blood flow in man during light sleep (stage 2)

We measured cerebral blood flow (CBF) and cerebral metabolic rate of oxygen (CMRO2) during light sleep (stage 2) in 8 young healthy volunteers using the Kety-Schmidt technique with 133Xe as the inert gas. Measurements were performed during wakefulness and light sleep as verified by standard polysomnography. Unlike our previous study in man showing a highly significant 25% decrease in CMRO2 during deep sleep (stage 3-4) we found a modest but statistically significant decrease of 5% in CMRO2 during stage 2 sleep. Deep and light sleep are both characterized by an almost complete lack of mental activity. They differ in respect of arousal threshold as a stronger stimulus is required to awaken a subject from deep sleep as compared to light sleep. Our results suggest that during non-rapid eye movement sleep cerebral metabolism and thereby cerebral synaptic activity is correlated to cerebral readiness rather than to mental activity.

Cardiopulmonary perfusion and cerebral blood flow in bilateral carotid artery disease

The fear of cerebral complications after cardiopulmonary bypass in patients with heart disease and severe carotid artery disease has led many authors to suggest combined approaches in these patients. The pathogenetic mechanism for stroke is based partly on the stenotic narrowing of the carotid artery. A diameter reduction of 75% is frequently considered hemodynamically significant and indicative of an increased risk for neurological morbidity. We studied the cerebral blood flow in 7 patients undergoing coronary artery bypass grafting who also had severe bilateral carotid disease. The results were compared with the results in 17 patients without carotid disease who had bypass grafting. The cerebral blood flow was measured by xenon 133 washout technique before, during, and after cardiopulmonary bypass with moderate hypothermia. Acid-base regulation was according to the alpha-stat theory, and blood pressure was kept greater than 50 mm Hg. The cerebral blood flow levels (mL.100g-1.min-1) before, during, and after cardiopulmonary bypass in the study group (30 +/- 11, 31 +/- 8, 47 +/- 20) (mean +/- standard deviation) were almost identical to those in the control group (30 +/- 11, 28 +/- 8, 47 +/- 12). The cerebral blood flow levels for the left and right hemispheres in the group with carotid disease were comparable and within normal ranges. In 2 patients, slight differences were noted between hemispheres, and this finding may indicate an increased risk for ischemia. These patients, however, did not show any signs of postoperative deficit. The flow limitations of critical carotid stenoses do not seem to imply a risk for cerebral hypoperfusion if cardiopulmonary perfusion is performed in a controlled manner.(ABSTRACT TRUNCATED AT 250 WORDS)

Cerebral vasoreactivity to carbon dioxide during cardiopulmonary perfusion at normothermia and hypothermia

With the pH-stat acid-base regulation strategy during hypothermic cardiopulmonary bypass (CPB), carbon dioxide (CO2) is generally administered to maintain the partial pressure of arterial CO2 at a higher level than with the alpha-stat method. With preserved CO2 vasoreactivity during CPB, this induction of "respiratory acidosis" can lead to a much higher cerebral blood flow level than is motivated metabolically. To evaluate CO2 vasoreactivity, cerebral blood flow was measured using a xenon 133 washout technique before, during, and after CPB at different CO2 levels in patients who were undergoing coronary artery bypass grafting with perfusion at either hypothermia or normothermia. The overall CO2 reactivity was 1.2 mL/100 g/min/mm Hg. There was no difference between the groups. The CO2 reactivity was not affected by temperature or CPB. The induced hemodilution resulted in higher cerebral blood flow levels during CPB, although this was counteracted by the temperature-dependent decrease in the hypothermia group. After CPB, a transient increase in cerebral blood flow was noted in the hypothermia group, the reason for which remains unclear. The study shows that manipulation of the CO2 level at different temperatures results in similar changes in cerebral blood flow irrespective of the estimated metabolic demand. This finding further elucidates the question of whether alpha-stat or pH-stat is the most physiological way to regulate the acid-base balance during hypothermic CPB.

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.

Imaging local cerebral blood flow by Xenon-enhanced computed tomography--technical optimization procedures

Methods are described for non-invasive, computer-assisted serial scanning throughout the human brain during eight minutes of inhalation of 27%-30% Xenon gas in order to measure local cerebral blood flow (LCBF). Optimized Xenon-enhanced computed tomography (XeCT) was achieved by 5-second scanning at one-minute intervals utilizing a state-of-the-art CT scanner and rapid delivery of Xenon gas via a face mask. Values for local brain-blood partition coefficients (L lambda) measured in vivo were utilized to calculate LCBF values. Previous methods assumed L lambda values to be normal, introducing the risk of systematic errors, because L lambda values differ throughout normal brain and may be altered by disease. Color-coded maps of L lambda and LCBF values were formatted directly onto CT images for exact correlation of function with anatomic and pathologic observations (spatial resolution: 26.5 cubic mm). Results were compared among eight normal volunteers, aged between 50 and 88 years. Mean cortical gray matter blood flow was 46.3 +/- 7.7, for subcortical gray matter was 50.3 +/- 13.2 and for white matter was 18.8 +/- 3.2. Modern CT scanners provide stability, improved signal to noise ratio and minimal radiation scatter. Combining these advantages with rapid Xenon saturation of the blood provides correlations of L lambda and LCBF with images of normal and abnormal brain in a safe, useful and non-invasive manner.

A comparison of the rate of clearance of xenon (133Xe) and pertechnetate ion (99mTcO4-) in murine tumors and normal leg muscles

Xenon-133 is a widely used tracer for clearance measurements to estimate nutritive blood flow. There are however certain difficulties in handling and obtaining the radionuclide. Pertechnetate ion, 99mTcO4-, is used extensively as a radiotracer in nuclear medicine studies and is easily available. To assess the usefulness of pertechnetate ion for blood flow studies, we have measured the rate of clearance following a local injection of pertechnetate ion and xenon simultaneously in normal mouse leg muscle and in murine tumors (fibrosarcoma and mammary carcinoma) of different sizes. The partition coefficients for the two tracers were also measured. The results indicated that the clearance halftime for pertechnetate ion was consistently greater than that for xenon in the tumors but similar in normal resting muscle. The estimated blood flow values assessed using the two tracers were similar in all cases except for large tumors (greater than 1.5 g). In this case, pertechnetate ion gave lower values than xenon. The difference could be explained by the large intracapillary distances which occur in poorly vascularized regions of large tumors since pertechnetate is a charged ion and hence might be expected to have limited diffusion capacity. The results indicate that caution is warranted when pertechnetate ion is used as a tracer for measurements of blood flow and for other biological determinations which depend on tissue transport for properties.

Regional variations in human newborn cerebral blood flow

Regional differences in the local cerebral metabolic rate of glucose have been reported in newborn infants. This study was performed to determine if comparable differences exist in neonatal regional cerebral blood flow (rCBF). In 21 infants, rCBF was measured with a modified xenon 133 (133Xe) clearance technique by means of eight extracranial detectors positioned over four homologous regions (frontal, parietal, temporal, and occipital). The rCBF was lowest in the frontal region, higher in the parietal region, and highest in the temporal and occipital regions. Regional differences in rCBF may be caused by regional differences in brain development and function.

Comparison of the effects of infusion with hydroxyethyl starch and low molecular weight dextran on cerebral blood flow and hemorheology in normal baboons

Cerebral blood flow (CBF) and hemorheological parameters, such as hematocrit, plasma viscosity, and erythrocyte aggregation, were measured before and up to 7 h after 60-min infusions with 10% hydroxyethyl starch (HES), or 0.9% NaCl solution and 10% low molecular weight dextran (LMWD) in a total of 12 normal baboons. Infusion of HES increased CBF up to 48% from the resting level, and decreased hematocrit without an increase in plasma viscosity. Infusion of LMWD decreased hematocrit with an increase in CBF of up to 9.6%, but increased plasma viscosity at the same time. The disaggregating effect for erythrocytes was rather more obvious with LMWD than with HES but without significant difference between them. These data show different rheological effects with infusions of HES and LMWD on the physiological conditions of normal baboons.

The effect of hematocrit and systolic blood pressure on cerebral blood flow in newborn infants

The effects of hematocrit and systolic blood pressure on cerebral blood flow were measured in 15 stable, low birth weight babies. CBF was measured with a modification of the xenon-133 (133Xe) clearance technique, which uses an intravenous bolus of 133Xe, an external chest detector to estimate arterial 133Xe concentration, eight external cranial detectors to measure cephalic 133Xe clearance curves, and a two-compartmental analysis of the cephalic 133Xe clearance curves to estimate CBF. There was a significant inverse correlation between hematocrit and CBF, presumably due to alterations in arterial oxygen content and blood viscosity. Newborn CBF varied independently of systolic blood pressure between 60 and 84 mm Hg, suggesting an intact cerebrovascular autoregulatory mechanism. These results indicate that at least two of the factors that affect newborn animal CBF are operational in human newborns and may have important clinical implications.

Mechanisms of cerebrovascular dilation by ether in monkeys

We hypothesized that when the depth of ether anesthesia is increased from 2 to 5%, cerebral vessels dilate secondary to circulating catecholamine stimulation of cerebral metabolism. Cerebral blood flow (CBF) by 133Xe clearance and cerebral metabolic rate for oxygen (CMRO2) were measured on 2% and then 5% ether in air in two groups of seven monkeys each during mechanical ventilation. Propranolol, 0.5 mg/kg i.v., was infused over 5 min in one group, and the other received saline. All measurements were repeated on 5% and 2% ether. Cerebrovascular resistance (CVR) fell by 30%, from 2.28 +/- 0.61 (mean +/- SD) to 1.51 +/- 0.28 mm Hg ml-1 100 g-1 min-1 (p less than 0.01), with the increase in ether from 2 to 5%. CBF and CMRO2 were unaltered from values of about 45 ml 100 g-1 min-1 and 2.3 ml 100 g-1 min-1, respectively. During 5% ether anesthesia, propranolol had no effect on CBF, CMRO2, or CVR. On 2% ether, it increased CVR twofold, from 1.5 +/- 0.30 to 3.0 +/- 1.0 mm Hg ml-1 100 g-1 min-1, and decreased CBF by 33%, from 48 +/- 8 to 32 +/- 10 ml 100 g-1 min-1. Plasma epinephrine was two-fold higher on 2% compared to 5% ether, both before and after saline or propranolol infusion. In monkeys, cerebrovascular dilation by ether at 5% compared to 2% is not secondary to catecholamine stimulation of CMRO2. It may result from a direct effect of either plasma catecholamines or ether on the cerebrovasculature.

Cerebral blood flow and autoregulation during hypothermic cardiopulmonary bypass

Mean hemispheric cerebral blood flow (CBF) was studied following intravenous or intraarterial administration of xenon-133, in 10 men admitted for coronary artery bypass grafting. Repeated CBF measurements were performed to evaluate autoregulation before, during, and after cardiopulmonary bypass (CPB). During CPB mean CBF remained unchanged compared with the pre-CPB level, without evidence of cerebral hyperemia or impairment of autoregulation. A marked increase in CBF occurred after CPB and was followed by a time-dependent reduction toward the pre-CPB level. The data support the alpha-stat regulation theory but cannot explain the cerebral vasodilation observed after CPB.

Cerebral blood flow in preterm infants during the first week of life

Forty-two preterm infants of 28-33 weeks of gestation were studied once during the first week of life by 133-Xenon clearance after intravenous injection to estimate global cerebral blood flow. Count rates detected over the chest were corrected for chest wall contribution and used as arterial input function. A neonatal blood-brain partition coefficient of Xenon was used for the calculation of a mean flow estimator (CBF-infinity). The technique was internally validated by use of differently obtained arterial input functions. In 11 infants without respiratory distress, CBF-infinity was 19.8 ml/100 g/min +/- 5.3 SD. In 24 infants treated with mechanical ventilation CBF-infinity was 11.8 ml/100 g/min +/- 3.2 SD. In 7 infants treated with continuous positive airway pressure CBF-infinity was 21.3 ml/100 g/min +/- 12.0 SD. When the reduction of CBF-infinity associated with mechanical ventilation was taken into account, the 9 infants with subependymal/intraventricular haemorrhage had increased CBF-infinity. The effects of gestational age, birthweight, mode of delivery, postnatal age, mean arterial blood pressure, PaCO2, blood haemoglobin and phenobarbitone medication were also analysed and found inconsistent. In conclusion, CBF was lower than expected and in infants requiring mechanical ventilation the values were lower still.

Regional cerebral blood flow utilizing the gamma camera and xenon inhalation: reproducibility and clinical applications

A modified collimator and standard gamma camera have been used to measure regional cerebral blood flow following inhalation of radioactive xenon. The collimator and a simplified analysis technique enables excellent statistical accuracy to be achieved with acceptable precision in the measurement of grey matter blood flow. The validity of the analysis was supported by computer modelling and patient measurements. Sixty-one patients with subarachnoid haemorrhage, cerebrovascular disease or dementia were retested to determine the reproducibility of our method. The measured coefficient of variation was 6.5%. Of forty-six patients who had a proven subarachnoid haemorrhage, 15 subsequently developed cerebral ischaemia. These showed a CBF of 42 +/- 6 ml X minute-1 X 100 g brain-1 compared with 49 +/- 11 ml X minute-1 X 100 g brain-1 for the remainder. There is evidence that decreasing blood flow and low initial flow correlate with the subsequent onset of cerebral ischaemia.

Selected noble-gas partition coefficients

The Ostwald solubility of 41Ar, 127,133Xe, N13N and CH3 18F in water, saline, blood, plasma, lipids, benzene and bone were measured in vitro. In addition the bone-blood partition coefficients for these gases were determined. For 41Ar, the bone-blood partition function is found to be 1.1 +/- 0.3, whereas for xenon the bone-blood partition coefficient is 0.41 +/- 0.1.

Locally applied 133Xenon for the measurement of regional cerebral blood flow (rCBF): an experimental study in the pig

A method for measuring regional cerebral blood flow has been developed. On pig brain cortex a 1 cm2 size polyester film was placed, under which 0.6-1.3 mCi of 133Xenon in 2-4 microliter of saline was applied atraumatically. The wash-out process was registered with an external detector, and can be described as a sum of monoexponential functions. The first component of the curve, obtained by curve resolution, indicates blood flow in grey matter and the second slow component indicates blood flow in white matter. When total ischaemia was induced, there was no wash-out of the isotope. Freezing the brain after isotope application at different stages during the wash-out showed isotope in both grey and white matter. The isotope did not diffuse into the polyester film. This technique was also used in studies on the spinal medulla where white matter is outermost and grey innermost. The wash-out curve obtained consisted of only one monoexponential function; blood flow from grey matter was not present in the wash-out curve. For calculation of cerebral blood flow a modified two-compartment model was used. It is concluded that this method measures local cerebral blood flow in both grey and white matter. The method can be used clinically to measure the local cerebral blood flow during neurosurgical operations.

Regulation of cerebral blood flow in response to changes in blood viscosity

Cerebral blood flow (CBF) was measured by the non-invasive xenon-133 technique in patients with increased blood viscosity as a result of paraproteinaemia or leukaemia. A highly significant inverse relation was found between CBF and arterial oxygen content in 59 paraproteinaemic patients. There was no significant correlation between CBF and whole blood viscosity, and no significant difference between CBF in paraproteinaemic patients and a matched group of anaemic patients. 7 leukaemic patients with up to threefold increases in whole blood viscosity were also found to have CBF appropriate to their degree of anaemia. The effects of treatment to reduce blood viscosity were studied in 7 paraproteinaemic and 5 leukaemic patients; changes in CBF were significantly related to changes in arterial oxygen content but not to changes in blood viscosity. These studies confirm the importance of arterial oxygen content in the determination of CBF, and demonstrate that regulatory mechanisms can maintain normal cerebral oxygen transport despite increased plasma and whole blood viscosity.

Effects of xenon and krypton on regional cerebral blood flow in the rat

The effects of high inspired concentrations of xenon and krypton on regional CBF (rCBF) were assessed in the rat using [14C]iodoantipyrine and quantitative autoradiography. Inhalation of 80% xenon for 1 or 2 min and inhalation of 40% xenon for 2 min were found to have significant effects on rCBF, including average increases of 75-96% in cerebral neocortical regions. Inhalation of 40% xenon for 1 min and of 80% krypton for 2 min had no significant effect on rCBF in most brain regions studied. If xenon inhalation produces effects on rCBF in humans similar to those observed in the rat, such effects could be an important source of error in xenon computed tomography rCBF studies.

What is the correct value for the brain--blood partition coefficient for water?

A knowledge of the brain-blood partition coefficient (lambda) for water is usually required for the measurement of CBF with [15O]water. The currently accepted value for whole-brain lambda, 0.95-0.96 ml/g, calculated from brain and blood water content data, is incorrect because in the calculation, the blood water content was not adjusted for the density of blood. The correct value is 0.90 ml/g. Variations in brain or blood water content affect lambda. Thus, lambda changes during development of the brain and varies regionally in it, even among different gray matter structures, owing to variation in brain water content. In addition, lambda would be expected to vary with the hematocrit, owing to changes in blood water content. The impact of using an incorrect value for lambda will depend on the sensitivity of the CBF measurement technique used to errors in lambda.

Determination of the partition coefficient of 133xenon between oral tissues and blood in the dog

Ostwald solubility coefficients for 133Xe in plasma (Sp), red blood cells (Sc), and selected oral tissue homogenates (St) obtained from the dog were determined at 4, 20, 27 and 37 degrees C by counting the 133Xe activity of the air phase and the sample (liquid, red cells or tissue) in the same tube following equilibration. The partition coefficient for xenon (lambda, in ml/g) between red cells and plasma (lambda cp) and that between tissues and plasma (lambda tp) were calculated from the measured Sp, Sc and St. The tissue-blood partition coefficients (lambda tb) of oral tissues for 133Xe were then calculated using lambda cp, lambda tp and the haematocrit value. The calculated values of lambda tb at 37 degrees C and 50 per cent haematocrit were 0.650 for the pulp, 0.674 for the tongue, 0.828 for the submandibular gland and 0.881 for the gingiva of the dog. lambda cp increased and lambda tp decreased as the temperature was reduced from 37 to 4 degrees C.

Superior sagittal sinus blood flow in the dog measured by an electromagnetic flow meter. A comparison with a radioactive gas elimination method

A simple method for continuous measurement of superior sagittal sinus blood flow (SSBF) in dogs, using an electromagnetic flow meter around the intact vessel, is presented. This method is compared with a procedure measuring radioactive gas elimination upon intra-arterial indicator injection. A good correlation between the cerebral blood flow (CBF), calculated from the fast component of the elimination curve and SSBF (r = 0.96) was found within a CBF range of 50-200 ml/100 g min.

Computed tomographic measurement of local cerebral blood flow by xenon enhancement

The present technique was developed to overcome some of the disadvantages of measuring cerebral blood flow by radionuclide methods, such as poor localization of flow values and errors that result if the brain is pathological and local partition coefficients are altered. Serial CT scanning in humans was carried out during and after inhalation of 50 to 70% non-radioactive xenon. This diffusible gas with high atomic number enhanced gray matter first by 19 +/- 4 Hounsfield Units (HU) and later white matter by 24 +/- 4 HU. The regionality of flow values were cursored on CT pictures with a high spatial resolution of 4 X 4 mm (64 pixels) or 0.16cm2 X 0.5cm. In seven normal subjects, blood flow in gray matter was 82 +/- 11 ml/100gm/min and that in white matter 24 +/- 5ml/100gm/min. The partition coefficient (lambda), which is not obtainable in vivo by radionuclide scanning, was 0.9 +/- 0.1 for normal gray matter, 1.4 +/- 0.2 for normal white matter. Reduced flow, 13% in gray matter and 46% in white matter, was found in a large infarct secondary to complete occlusion of middle cerebral artery. In edematous tissue, blood flow was not significantly impaired in gray matter but was reduced to 29 to 54% in white matter. Local lambda values were reduced to 0.6-0.9 in edematous tissue, and 0.3-0.7 in infarction. This method appears to have several advantages over conventional isotope methods of measuring cerebral blood flow and provides useful clinical and research information.

Blood flow changes in arteriovenous malformation during behavioral activation

Striking task-dependent fluctuations were observed in the cerebral blood flow pattern of a patient with a left posterior hemispheric arteriovenous malformation (AVM). Two-dimensional measures of regional cerebral flow in the resting state, using the xenon 133 inhalation technique, revealed a region of high flow coincident with the AVM seen on the patient's arteriograms. In subsequent studies, the AVM stood out as a region of high blood flow during a relaxed state, while it approached normal levels of flow when there was attentional demand. These observations suggest that focal regulatory mechanisms exist at the AVM or else that very substantial redistributions of blood flow are taking place which the flow rate in the AVM reflects only passively. Patients considered for embolic treatment of an AVM would benefit from an assessment of behavioral influences on flow in the AVM.

Noninvasive method of estimating human newborn regional cerebral blood flow

A noninvasive method of estimating regional cerebral blood flow (rCBF) in premature and full-term babies has been developed. Based on a modification of the xenon-133 inhalation rCBF technique, this method uses eight extracranial NaI scintillation detectors and an i.v. bolus injection of xenon-133 (approximately 0.5 mCi/kg). Arterial xenon concentration was estimated with an external chest detector. Cerebral blood flow was measured in 15 healthy, neurologically normal premature infants. Using Obrist's method of two-compartment analysis, normal values were calculated for flow in both compartments, relative weight and fractional flow in the first compartment (gray matter), initial slope of gray matter blood flow, mean cerebral blood flow, and initial slope index of mean cerebral blood flow. The application of this technique to newborns, its relative advantages, and its potential uses are discussed.

Cerebral hemodynamics, oxygen uptake and cerebral arteriovenous differences of catecholamines following E. coli endotoxin in dogs

In experimental endotoxic shock in dogs, a decrease in cerebral blood flow and an increase in cerebral metabolic rate of oxygen (CMRO2) have been shown to occur. In this situation the blood levels of adrenaline and noradrenaline are markedly elevated. The aim of the present study was to investigate whether a cerebral uptake of circulating catecholamines with a possible influence on CMRO2 takes place in the brain. In eight anaesthetized dogs, arterial blood, superior sagittal sinus blood and cerebrospinal fluid were analyzed for the concentrations of adrenaline, noradrenaline and dopamine before and up to 4 h after an injection of E. coli endotoxin 1.0-1.5 mg . kg-1. The blood levels of adrenaline and noradrenaline, but not dopamine, increased in response to the endotoxin. From about 30 min after the endotoxin injection, arteriovenous adrenaline and noradrenaline differences indicating a cerebral uptake were most often seen. Increased concentrations of noradrenaline, adrenaline and dopamine in cerebrospinal fluid were observed. Noradrenaline gave the highest concentrations and these were correlated to the CMRO2. In some animals the blood and cerebrospinal fluid concentrations of adrenaline seemed to be related. These results indicate that catecholamines might be of importance for the development of an increased CMRO2 in endotoxic shock.