Regulation of local tissue PO2 of the brain cortex at different arterial O2 pressures

The Impact of Short-Term Hyperoxia on Cerebral Metabolism: A Systematic Review and Meta-Analysis

BACKGROUND

Cerebral ischemia due to hypoxia is a major cause of secondary brain injury and is associated with higher morbidity and mortality in patients with acute brain injury. Hyperoxia could improve energetic dysfunction in the brain in this setting. Our objectives were to perform a systematic review and meta-analysis of the current literature and to assess the impact of normobaric hyperoxia on brain metabolism by using cerebral microdialysis.

METHODS

We searched Medline and Scopus, following the recommendations of the Preferred Reporting Items for Systematic Reviews and Meta-Analyses statement; we searched for retrospective and prospective observational studies, interventional studies, and randomized clinical trials that performed a hyperoxia challenge in patients with acute brain injury who were concomitantly monitored with cerebral microdialysis. This study was registered in PROSPERO (CRD420211295223).

RESULTS

We included a total of 17 studies, with a total of 311 patients. A statistically significant reduction in cerebral lactate values (pooled standardized mean difference [SMD] - 0.38 [- 0.53 to - 0.23]) and lactate to pyruvate ratio values (pooled SMD - 0.20 [- 0.35 to - 0.05]) was observed after hyperoxia. However, glucose levels (pooled SMD - 0.08 [- 0.23 to 0.08]) remained unchanged after hyperoxia.

CONCLUSIONS

Normobaric hyperoxia may improve cerebral metabolic disturbances in patients with acute brain injury. The clinical impact of such effects needs to be further elucidated.

Living High and Feeling Low: Altitude, Suicide, and Depression

After participating in this activity, learners should be better able to:• Assess epidemiologic evidence that increased altitude of residence is linked to increased risk of depression and suicide• Evaluate strategies to address hypoxia-related depression and suicidal ideation ABSTRACT: Suicide and major depressive disorder (MDD) are complex conditions that almost certainly arise from the influences of many interrelated factors. There are significant regional variations in the rates of MDD and suicide in the United States, suggesting that sociodemographic and environmental conditions contribute. Here, we review epidemiological evidence that increases in the altitude of residence are linked to the increased risk of depression and suicide. We consider the possibility that chronic hypobaric hypoxia (low blood oxygen related to low atmospheric pressure) contributes to suicide and depression, which is suggested by animal models, short-term studies in humans, and the effects of hypoxic medical conditions on suicide and depression. We argue that hypobaric hypoxia could promote suicide and depression by altering serotonin metabolism and brain bioenergetics; both of these pathways are implicated in depression, and both are affected by hypoxia. Finally, we briefly examine treatment strategies to address hypoxia-related depression and suicidal ideation that are suggested by these findings, including creatine monohydrate and the serotonin precursors tryptophan and 5-hydroxytryptophan.

Understanding a role for hypoxia in lesion formation and location in the deep and periventricular white matter in small vessel disease and multiple sclerosis

The deep and periventricular white matter is preferentially affected in several neurological disorders, including cerebral small vessel disease (SVD) and multiple sclerosis (MS), suggesting that common pathogenic mechanisms may be involved in this injury. Here we consider the potential pathogenic role of tissue hypoxia in lesion development, arising partly from the vascular anatomy of the affected white matter. Specifically, these regions are supplied by a sparse vasculature fed by long, narrow end arteries/arterioles that are vulnerable to oxygen desaturation if perfusion is reduced (as in SVD, MS and diabetes) or if the surrounding tissue is hypoxic (as in MS, at least). The oxygen crisis is exacerbated by a local preponderance of veins, as these can become highly desaturated 'sinks' for oxygen that deplete it from surrounding tissues. Additional haemodynamic deficiencies, including sluggish flow and impaired vasomotor reactivity and vessel compliance, further exacerbate oxygen insufficiency. The cells most vulnerable to hypoxic damage, including oligodendrocytes, die first, resulting in demyelination. Indeed, in preclinical models, demyelination is prevented if adequate oxygenation is maintained by raising inspired oxygen concentrations. In agreement with this interpretation, there is a predilection of lesions for the anterior and occipital horns of the lateral ventricles, namely regions located at arterial watersheds, or border zones, known to be especially susceptible to hypoperfusion and hypoxia. Finally, mitochondrial dysfunction due to genetic causes, as occurs in leucodystrophies or due to free radical damage, as occurs in MS, will compound any energy insufficiency resulting from hypoxia. Viewing lesion formation from the standpoint of tissue oxygenation not only reveals that lesion distribution is partly predictable, but may also inform new therapeutic strategies.

Clinical perspective on oxidative stress in sporadic amyotrophic lateral sclerosis

Sporadic amyotrophic lateral sclerosis (ALS) is one of the most devastating neurological diseases; most patients die within 3 to 4 years after symptom onset. Oxidative stress is a disturbance in the pro-oxidative/antioxidative balance favoring the pro-oxidative state. Autopsy and laboratory studies in ALS indicate that oxidative stress plays a major role in motor neuron degeneration and astrocyte dysfunction. Oxidative stress biomarkers in cerebrospinal fluid, plasma, and urine are elevated, suggesting that abnormal oxidative stress is generated outside of the central nervous system. Our review indicates that agricultural chemicals, heavy metals, military service, professional sports, excessive physical exertion, chronic head trauma, and certain foods might be modestly associated with ALS risk, with a stronger association between risk and smoking. At the cellular level, these factors are all involved in generating oxidative stress. Experimental studies indicate that a combination of insults that induce modest oxidative stress can exert additive deleterious effects on motor neurons, suggesting that multiple exposures in real-world environments are important. As the disease progresses, nutritional deficiency, cachexia, psychological stress, and impending respiratory failure may further increase oxidative stress. Moreover, accumulating evidence suggests that ALS is possibly a systemic disease. Laboratory, pathologic, and epidemiologic evidence clearly supports the hypothesis that oxidative stress is central in the pathogenic process, particularly in genetically susceptive individuals. If we are to improve ALS treatment, well-designed biochemical and genetic epidemiological studies, combined with a multidisciplinary research approach, are needed and will provide knowledge crucial to our understanding of ALS etiology, pathophysiology, and prognosis.

Basic physiology of hyperbaric oxygen in brain

Oxygen is the proverbial 'double-edged sword' in that it is a necessity for life in moderation and toxic and detrimental to life in excess. This too is the dilemma in hyperbaric oxygen (HBO) treatment in cerebral ischemic-anoxic insults such as stroke, head injury, near drowning, asphyxia, cardiac arrest, etc., i.e. the brain at risk, where regions of ischemia are beside regions of marked hyperemia. The natural heterogeneity of normal brain tissue oxygenation compounds the problem with different microvascular brain regions living at various levels of oxygenation from 0 to arterial PO(2) as an added complication. The application of HBO, whether normobaric or hyperbaric, will result in brain tissue oxygenation ranging from normoxic to highly hyperoxic with the latter possibly exacerbating the injury sustained. On this basis, the application of multiple therapeutic interventions may be considered, for example, HBO in combination with free radical scavengers or inhibitors of free radical generating enzymes. Despite these difficulties in moderating oxygen delivery to treat cerebral ischemic-anoxic insults, overwhelming preclinical evidence indicates that HBO administered during or within 2 hours post-insult effectively attenuates the severity of brain damage sustained. The primary disconnection between pre-clinical and clinical efficacy of HBO then appears to be the time of application. Clinically, HBO therapy is applied at the earliest 6 hours post-insult but usually between 12 hours or longer post-insult. Pre-hospital application of HBO may be required for clear-cut demonstration of clinical efficacy.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.

Cerebral oxygenation in awake rats during acclimation and deacclimation to hypoxia: an in vivo electron paramagnetic resonance study

Exposure to high altitude or hypobaric hypoxia results in a series of metabolic, physiologic, and genetic changes that serve to acclimate the brain to hypoxia. Tissue Po(2) (Pto(2)) is a sensitive index of the balance between oxygen delivery and utilization and can be considered to represent the summation of such factors as cerebral blood flow, capillary density, hematocrit, arterial Po(2), and metabolic rate. As such, it can be used as a marker of the extent of acclimation. We developed a method using electron paramagnetic resonance (EPR) to measure Pto(2) in unanesthetized subjects with a chronically implanted sensor. EPR was used to measure rat cortical tissue Pto(2) in awake rats during acute hypoxia and over a time course of acclimation and deacclimation to hypobaric hypoxia. This was done to simulate the effects on brain Pto(2) of traveling to altitude for a limited period. Acute reduction of inspired O(2) to 10% caused a decline from 26.7 ± 2.2 to 13.0 ± 1.5 mmHg (mean ± SD). Addition of 10% CO(2) to animals breathing 10% O(2) returned Pto(2) to values measured while breathing 21% O(2,) indicating that hypercapnia can reverse the effects of acute hypoxia. Pto(2) in animals acclimated to 10% O(2) was similar to that measured preacclimation when breathing 21% O(2). Using a novel, individualized statistical model, it was shown that the T(1/2) of the Pto(2) response during exposure to chronic hypoxia was approximately 2 days. This indicates a capacity for rapid adaptation to hypoxia. When subjects were returned to normoxia, there was a transient hyperoxygenation, followed by a return to lower values with a T(1/2) of deacclimation of 1.5 to 3 days. These data indicate that exposure to hypoxia results in significant improvements in steady-state oxygenation for a given inspired O(2) and that both acclimation and deacclimation can occur within days.

Cerebral oxygenation is reduced during hyperthermic exercise in humans

AIM

Cerebral mitochondrial oxygen tension (P(mito)O(2)) is elevated during moderate exercise, while it is reduced when exercise becomes strenuous, reflecting an elevated cerebral metabolic rate for oxygen (CMRO(2)) combined with hyperventilation-induced attenuation of cerebral blood flow (CBF). Heat stress challenges exercise capacity as expressed by increased rating of perceived exertion (RPE).

METHODS

This study evaluated the effect of heat stress during exercise on P(mito)O(2) calculated based on a Kety-Schmidt-determined CBF and the arterial-to-jugular venous oxygen differences in eight males [27 +/- 6 years (mean +/- SD) and maximal oxygen uptake (VO(2max)) 63 +/- 6 mL kg(-1) min(-1)].

RESULTS

The CBF, CMRO(2) and P(mito)O(2) remained stable during 1 h of moderate cycling (170 +/- 11 W, approximately 50% of VO(2max), RPE 9-12) in normothermia (core temperature of 37.8 +/- 0.4 degrees C). In contrast, when hyperthermia was provoked by dressing the subjects in watertight clothing during exercise (core temperature 39.5 +/- 0.2 degrees C), P(mito)O(2) declined by 4.8 +/- 3.8 mmHg (P < 0.05 compared to normothermia) because CMRO(2) increased by 8 +/- 7% at the same time as CBF was reduced by 15 +/- 13% (P < 0.05). During exercise with heat stress, RPE increased to 19 (19-20; P < 0.05); the RPE correlated inversely with P(mito)O(2) (r(2) = 0.42, P < 0.05).

CONCLUSION

These data indicate that strenuous exercise in the heat lowers cerebral P(mito)O(2), and that exercise capacity in this condition may be dependent on maintained cerebral oxygenation.

Methods of monitoring brain oxygenation

INTRODUCTION

Posttraumatic brain ischemia or hypoxia is a major potential cause of secondary injury that may lead to poor outcome. Avoidance, or amelioration, of this secondary injury depends on early diagnosis and intervention before permanent injury occurs. However, tools to monitor brain oxygenation continuously in the neuro-intensive care unit have been lacking.

DISCUSSION

In recent times, methods of monitoring aspects of brain oxygenation continuously by the bedside have been evaluated in several experimental and clinical series and are potentially changing the way we manage head-injured patients. These monitors have the potential to alert the clinician to possible secondary injury and enable intervention, help interpret pathophysiological changes (e.g., hyperemia causing raised intracranial pressure), monitor interventions (e.g., hyperventilation for increased intracranial pressure), and prognosticate. This review focuses on jugular venous saturation, brain tissue oxygen tension, and near-infrared spectroscopy as practical methods that may have an important role in managing patients with brain injury, with a particular focus on the available evidence in children. However, to use these monitors effectively and to understand the studies in which these monitors are employed, it is important for the clinician to appreciate the technical characteristics of each monitor, as well as respective strengths and limitations of each. It is equally important that the clinician understands relevant aspects of brain oxygen physiology and head trauma pathophysiology to enable correct interpretation of the monitored data and therefore to direct an appropriate therapeutic response that is likely to benefit, not harm, the patient.

Adaptive response of brain tissue oxygenation to environmental hypoxia in non-sedated, non-anesthetized arctic ground squirrels

The present study examined the physiological mechanisms of the responses of brain tissue oxygen partial pressure (P(t)O(2)), brain temperature (T(brain)), global oxygen consumption (V(O2)), and respiratory frequency (f(R)) to hypoxia in non-sedated and non-anesthetized arctic ground squirrels (Spermophilus parryii, AGS) and rats. We found that (1) in contrast to oxygen partial pressure in blood (P(a)O(2)), the baseline value of P(t)O(2) in summer euthermic AGS is significantly higher than in rats; (2) both P(t)O(2) and P(a)O(2) are dramatically reduced by inspired 8% O(2) in AGS and rats, but AGS have a greater capacity in P(t)O(2) to cope with environmental hypoxia; (3) metabolic rate before, during, and after hypoxic exposure is consistently lower in AGS than in rats; (4) the respiratory responding patterns to hypoxia in the two species differ in that f(R) decreases in AGS but increases in rats. These results suggest that (1) AGS have special mechanisms to maintain higher P(t)O(2) and lower P(a)O(2,) and these levels in AGS represent a typical pattern of adaptation of heterothermic species to and a brain protection from hypoxia; (2) AGS brain responds to hypoxia through greater decreases in P(t)O(2) and decreased f(R) and ventilation. In contrast, rat brain responds to hypoxia by less reduction in P(t)O(2) and increased f(R) and ventilation.

Brain tissue oxygen concentration measurements

Brain function depends exquisitely on oxygen for energy metabolism. Measurements of brain tissue oxygen tension, by a variety of quantitative and qualitative techniques, going back for >50 years, have led to a number of significant conclusions. These conclusions have important consequences for understanding brain physiology as it is now being explored by techniques such as blood-oxygen-level-dependent functional magnetic resonance imaging (BOLD fMRI) and near-infrared spectroscopy (NIRS). It has been known for some time that most of the measured oxygen tensions are less than venous pO2 and are distributed in a spatially and temporally heterogeneous manner on a microregional scale. Although certain large-scale methods can provide reproducible average brain pO2 measurements, no useful concept of a characteristic oxygen tension or meaningful average value for brain tissue oxygen can be known on a microregional level. Only an oxygen field exists with large local gradients due to local tissue respiration, and the most useful way to express this is with a pO2 distribution curve or histogram. The neurons of the brain cortex normally exist in a low-oxygen environment and on activation are oxygenated by increases in local capillary blood flow that lead to increases in hemoglobin saturation and tissue oxygen.

Chronic hypoxia and the cerebral circulation

Exposure to mild hypoxia elicits a characteristic cerebrovascular response in mammals, including humans. Initially, cerebral blood flow (CBF) increases as much as twofold. The blood flow increase is blunted somewhat by a decreasing arterial Pco2 as a result of the hypoxia-induced hyperventilatory response. After a few days, CBF begins to fall back toward baseline levels as the blood oxygen-carrying capacity is increasing due to increasing hemoglobin concentration and packed red cell volume as a result of erythropoietin upregulation. By the end of 2 wk of hypoxic exposure, brain capillary density has increased with resultant decreased intercapillary distances. The relative time courses of these changes suggest that they are adjusted by different control signals and mechanisms. The CBF response appears linked to the blood oxygen-carrying capacity, whereas the hypoxia-induced brain angiogenesis appears to be in response to tissue hypoxia.

The oxygen sensing signal cascade under the influence of reactive oxygen species

Structural and functional integrity of organ function profoundly depends on a regular oxygen and glucose supply. Any disturbance of this supply becomes life threatening and may result in severe loss of organ function. Particular reductions in oxygen availability (hypoxia) caused by respiratory or blood circulation irregularities cannot be tolerated for longer periods due to an insufficient energy supply by anaerobic glycolysis. Complex cellular oxygen sensing systems have evolved to tightly regulate oxygen homeostasis. In response to variations in oxygen partial pressure (PO2), these systems induce adaptive and protective mechanisms to avoid or at least minimize tissue damage. These various responses might be based on a range of oxygen sensing signal cascades including an isoform of the neutrophil NADPH oxidase, different electron carrier units of the mitochondrial chain such as a specialized mitochondrial, low PO2 affinity cytochrome c oxidase (aa3) and a subfamily of 2-oxoglutarate dependent dioxygenases termed HIF (hypoxia inducible factor) prolyl-hydroxylase and HIF asparaginyl hydroxylase called factor-inhibiting HIF (FIH-1). Thus, specific oxygen sensing cascades involving reactive oxygen species as second messengers may by means of their different oxygen sensitivities, cell-specific and subcellular localization help to tailor various adaptive responses according to differences in tissue oxygen availability.

Cellular oxygen sensing need in CNS function: physiological and pathological implications

Structural and functional integrity of brain function profoundly depends on a regular oxygen and glucose supply. Any disturbance of this supply becomes life threatening and may result in severe loss of brain function. In particular, reductions in oxygen availability (hypoxia) caused by systemic or local blood circulation irregularities cannot be tolerated for longer periods due to an insufficient energy supply to the brain by anaerobic glycolysis. Hypoxia has been implicated in central nervous system pathology in a number of disorders including stroke, head trauma, neoplasia and neurodegenerative disease. Complex cellular oxygen sensing systems have evolved for tight regulation of oxygen homeostasis in the brain. In response to variations in oxygen partial pressure (PO2) these induce adaptive mechanisms to avoid or at least minimize brain damage.

A significant advance in our understanding of the hypoxia response stems from the discovery of the hypoxia inducible factors (HIF), which act as key regulators of hypoxia-induced gene expression. Depending on the duration and severity of the oxygen deprivation, cellular oxygen-sensor responses activate a variety of short- and long-term energy saving and cellular protection mechanisms. Hypoxic adaptation encompasses an immediate depolarization block by changing potassium, sodium and chloride ion fluxes across the cellular membrane, a general inhibition of protein synthesis, and HIF-mediated upregulation of gene expression of enzymes or growth factors inducing angiogenesis, anaerobic glycolysis, cell survival or neural stem cell growth. However, sustained and prolonged activation of the HIF pathway may lead to a transition from neuroprotective to cell death responses. This is reflected by the dual features of the HIF system that include both anti- and proapoptotic components.

These various responses might be based on a range of oxygen-sensing signal cascades, including an isoform of the neutrophil NADPH oxidase, different electron carrier units of the mitochondrial chain such as a specialized mitochondrial, low PO2 affinity cytochrome c oxidase (aa3) and a subfamily of 2-oxoglutarate dependent dioxygenases termed HIF prolyl-hydroxylase (PHD) and HIF asparaginyl hydroxylase, known as factor-inhibiting HIF (FIH-1). Thus specific oxygen-sensing cascades, by means of their different oxygen sensitivities,cell-specific and subcellular localization, may help to tailor various adaptive responses according to differences in tissue oxygen availability.

Oxygen tension under hyperbaric conditions in healthy pig brain

OBJECTIVE

To investigate the effect of hyperbaric conditions on brain oxygenation, intracranial pressure and brain glucose/lactate levels in healthy non-brain-traumatized animals.

DESIGN AND SETTING

Prospective animal study in a hyperbaric chamber.

SUBJECTS

Twelve adult Landrace/Yorkshire pigs.

INTERVENTIONS

The animals were normoventilated in a pressure-controlled mode according to the open lung concept first at normobaric pressures (FiO2 of 0.4 and 1.0) and subsequently in the hyperbaric chamber at 1.9 and 2.8 bar (both at an FiO2 of 1.0). Under these conditions brain oxygen tension and intracranial pressure were recorded and brain glucose/lactate levels were measured by microdialysis.

RESULTS

At normobaric conditions, increasing the FiO2 from 0.4 (baseline) to 1.0 resulted in a significant increase in brain oxygen tension from 33 +/- 14 to 63 +/- 28 mmHg (P<0.05). Compared with baseline, both hyperbaric conditions (at an FiO2 of 1.0) led to a significant increase in brain oxygen tension to 151 +/- 65 mmHg (P<0.001) at 1.9 bar and to 294 +/- 134 mmHg (P<0.001) at 2.8 bar.

CONCLUSIONS

If there is a need for increased oxygenation in the brain, then one way to achieve this is to apply hyperbaric conditions at 100% oxygen. Compared with an atmospheric pressure with a FiO2 of 0.4, a nine-fold increase (900%) in PbrO2 values can be reached by increasing the FiO2 to 1.0 and the pressure to 2.8 bar. In this study, hyperbaric oxygen pressure in the brain did not lead to changes in intracranial pressure or in brain glucose/lactate levels.

Cerebral blood flow responses to changes in oxygen and carbon dioxide in humans

This study characterized cerebral blood flow (CBF) responses in the middle cerebral artery to PCO2 ranging from 30 to 60 mmHg (1 mmHg = 133.322 Pa) during hypoxia (50 mmHg) and hyperoxia (200 mmHg). Eight subjects (25 +/- 3 years) underwent modified Read rebreathing tests in a background of constant hypoxia or hyperoxia. Mean cerebral blood velocity was measured using a transcranial Doppler ultrasound. Ventilation (VE), end-tidal PCO2 (PETCO2), and mean arterial blood pressure (MAP) data were also collected. CBF increased with rising PETCO2 at two rates, 1.63 +/- 0.21 and 2.75 +/- 0.27 cm x s(-1) x mmHg(-1) (p < 0.05) during hypoxic and 1.69 +/- 0.17 and 2.80 +/- 0.14 cm x s(-1) x mmHg(-1) (p < 0.05) during hyperoxic rebreathing. VE also increased at two rates (5.08 +/- 0.67 and 10.89 +/- 2.55 L min(-1) m mHg(-1) and 3.31 +/- 0.50 and 7.86 +/- 1.43 L x min(-1) x mmHg(-1)) during hypoxic and hyperoxic rebreathing. MAP and PETCO2 increased linearly during both hypoxic and hyperoxic rebreathing. The breakpoint separating the two-component rise in CBF (42.92 +/- 1.29 and 49.00 +/- 1.56 mmHg CO2 during hypoxic and hyperoxic rebreathing) was likely not due to PCO2 or perfusion pressure, since PETCO2 and MAP increased linearly, but it may be related to VE, since both CBF and VE exhibited similar responses, suggesting that the two responses may be regulated by a common neural linkage.

Cerebral microvascular changes in permeability and tight junctions induced by hypoxia-reoxygenation

Cerebral microvessel endothelial cells that form the blood-brain barrier (BBB) have tight junctions (TJ) that are critical for maintaining brain homeostasis and low permeability. Both integral (claudin-1 and occludin) and membrane-associated zonula occluden-1 and -2 (ZO-1 and ZO-2) proteins combine to form these TJ complexes that are anchored to the cytoskeletal architecture (actin). Disruptions of the BBB have been attributed to hypoxic conditions that occur with ischemic stroke, pathologies of decreased perfusion, and high-altitude exposure. The effects of hypoxia and posthypoxic reoxygenation in cerebral microvasculature and corresponding cellular mechanisms involved in disrupting the BBB remain unclear. This study examined hypoxia and posthypoxic reoxygenation effects on paracellular permeability and changes in actin and TJ proteins using primary bovine brain microvessel endothelial cells (BBMEC). Hypoxia induced a 2.6-fold increase in [(14)C]sucrose, a marker of paracellular permeability. This effect was significantly reduced (~58%) with posthypoxic reoxygenation. After hypoxia and posthypoxic reoxygenation, actin expression was increased (1.4- and 2.3-fold, respectively). Whereas little change was observed in TJ protein expression immediately after hypoxia, a twofold increase in expression was seen with posthypoxic reoxygenation. Furthermore, immunofluorescence studies showed alterations in occludin, ZO-1, and ZO-2 protein localization during hypoxia and posthypoxic reoxygenation that correlate with the observed changes in BBMEC permeability. The results of this study show hypoxia-induced changes in paracellular permeability may be due to perturbation of TJ complexes and that posthypoxic reoxygenation reverses these effects.

Oxygen dependency and precision of cytochrome oxidase signal from full spectral NIRS of the piglet brain

Oxidation changes of the copper A (Cu(A)) center of cytochrome oxidase in the brain were measured during brief anoxic swings at both normocapnia and hypercapnia (arterial PCO(2) approximately 55 mmHg). Hypercapnia increased total hemoglobin from 37.5 +/- 9.1 to 50.8 +/- 12.9 micromol/l (means +/- SD; n = 7), increased mean cerebral saturation (Smc(O(2))) from 65 +/- 4 to 77 +/- 3%, and oxidized Cu(A) by 0.43 +/- 0.23 micromol/l. During the onset of anoxia, there were no significant changes in the Cu(A) oxidation state until Smc(O(2)) had fallen to 43 +/- 5 and 21 +/- 6% at normocapnia and hypercapnia, respectively, and the maximum reduction during anoxia was not significantly different at hypercapnia (1.49 +/- 0.40 micromol/l) compared with normocapnia (1.53 +/- 0.44 micromol/l). Residuals of the least squares fitting algorithm used to convert near-infrared spectra to concentrations are presented and shown to be small compared with the component of attenuation attributed to the Cu(A) signal. From these observations, we conclude that there is minimal interference between the hemoglobin and Cu(A) signals in this model, the Cu(A) oxidation state is independent of cerebral oxygenation at normoxia, and the oxidation after hypercapnia is not the result of increased cerebral oxygenation.

Critical oxygen tension in rat brain: a combined (31)P-NMR and EPR oximetry study

The relationship between cerebral interstitial oxygen tension (Pt(O(2))) and cellular energetics was investigated in mechanically ventilated, anesthetized rats during progressive acute hypoxia to determine whether there is a "critical" brain Pt(O(2)) for maintaining steady-state aerobic metabolism. Cerebral Pt(O(2)), measured by electron paramagnetic resonance oximetry, decreased proportionately to inspired oxygen fraction. (31)P-nuclear magnetic resonance measurements revealed no changes in P(i), phosphocreatine (PCr)/P(i) ratio, or intracellular pH when arterial blood oxygen tension (Pa(O(2))) was reduced from 145.1 +/- 11.7 to 56.5 +/- 4.4 mmHg (means +/- SE). Intracellular acidosis, a sharp rise in P(i), and a decline in the PCr/P(i) ratio developed when Pa(O(2)) was reduced further to 40.7 +/- 2.3 mmHg. The corresponding Pt(O(2)) values were 15.1 +/- 1.8, 8.8 +/- 0.4, and 6.8 +/- 0.3 mmHg. We conclude that over a range of decreasing oxygen tensions, cerebral oxidative metabolism is not sensitive to oxygen concentration. Oxygen becomes a regulatory substrate, however, when Pt(O(2)) is decreased to a critical level.

Increased inspired oxygen concentration as a factor in improved brain tissue oxygenation and tissue lactate levels after severe human head injury

OBJECT

Early impairment of cerebral blood flow in patients with severe head injury correlates with poor brain tissue O2 delivery and may be an important cause of ischemic brain damage. The purpose of this study was to measure cerebral tissue PO2, lactate, and glucose in patients after severe head injury to determine the effect of increased tissue O2 achieved by increasing the fraction of inspired oxygen (FiO2).

METHODS

In addition to standard monitoring of intracranial pressure and cerebral perfusion pressure, the authors continuously measured brain tissue PO2, PCO2, pH, and temperature in 22 patients with severe head injury. Microdialysis was performed to analyze lactate and glucose levels. In one cohort of 12 patients, the PaO2 was increased to 441+/-88 mm Hg over a period of 6 hours by raising the FiO2 from 35+/-5% to 100% in two stages. The results were analyzed and compared with the findings in a control cohort of 12 patients who received standard respiratory therapy (mean PaO2 136.4+/-22.1 mm Hg). The mean brain PO2 levels increased in the O2-treated patients up to 359+/-39% of the baseline level during the 6-hour FiO2 enhancement period, whereas the mean dialysate lactate levels decreased by 40% (p < 0.05). During this O2 enhancement period, glucose levels in brain tissue demonstrated a heterogeneous course. None of the monitored parameters in the control cohort showed significant variations during the entire observation period.

CONCLUSIONS

Markedly elevated lactate levels in brain tissue are common after severe head injury. Increasing PaO2 to higher levels than necessary to saturate hemoglobin, as performed in the O2-treated cohort, appears to improve the O2 supply in brain tissue. During the early period after severe head injury, increased lactate levels in brain tissue were reduced by increasing FiO2. This may imply a shift to aerobic metabolism.

Oxygenation strategy and neurologic damage after deep hypothermic circulatory arrest. II. hypoxic versus free radical injury

OBJECTIVES

Laboratory studies suggest that myocardial reperfusion injury is exacerbated by free radicals when pure oxygen is used during cardiopulmonary bypass. In phase I of this study we demonstrated that normoxic perfusion during cardiopulmonary bypass does not increase the risk of microembolic brain injury so long as a membrane oxygenator with an arterial filter is used. In phase II of this study we studied the hypothesis that normoxic perfusion increases the risk of hypoxic brain injury after deep hypothermia with circulatory arrest.

METHODS

With membrane oxygenators with arterial filters, 10 piglets (8-10 kg) underwent 120 minutes of deep hypothermia and circulatory arrest at 15 degrees C, were rewarmed to 37 degrees C, and were weaned from bypass. In 5 piglets normoxia (PaO2 64-181 mm Hg) was used during cardiopulmonary bypass and in 5 hyperoxia (PaO2 400-900 mm Hg) was used. After 6 hours of reperfusion the brain was fixed for histologic evaluation. Near-infrared spectroscopy was used to monitor cerebral oxyhemoglobin and oxidized cytochrome a,a3 concentrations.

RESULTS

Histologic examination revealed a significant increase in brain damage in the normoxia group (score 12.4 versus 8.6, P =.01), especially in the neocortex and hippocampal regions. Cytochrome a,a 3 and oxyhemoglobin concentrations tended to be lower during deep hypothermia and circulatory arrest in the normoxia group (P =.16).

CONCLUSIONS

In the setting of prolonged deep hypothermia and circulatory arrest with membrane oxygenators, normoxic cardiopulmonary bypass significantly increases histologically graded brain damage with respect to hyperoxic cardiopulmonary bypass. Near-infrared spectroscopy suggests that the mechanism is hypoxic injury, which presumably overwhelms any injury caused by increased oxygen free radicals.

Brain tissue oxygenation during hemorrhagic shock, resuscitation, and alterations in ventilation

OBJECTIVES

Recently developed polarographic microelectrodes permit continuous, reliable monitoring of oxygen tension in brain tissue (PbrO2). The aim of this study was to investigate the feasibility and utility of directly monitoring PbrO2 in cerebral tissue during changes in oxygenation or ventilation and during hemorrhagic shock and resuscitation. We also sought to develop a model in which treatment protocols could be evaluated using PbrO2 as an end point.

METHODS

Licox Clark-type polarographic probes were inserted in the brain tissue of 16 swine to monitor PbrO2. In eight swine, changes in PbrO2 were observed over a range of fractional concentrations of inspired O2 (FiO2) as well as during periods of hyperventilation and hypoventilation. In eight other swine, PbrO2 was monitored during a graded hemorrhage of up to 70% estimated blood volume and during the resuscitation period.

RESULTS

When FiO2 was elevated to 100%, PbrO2 increased from a baseline of 15+/-2 mm Hg to 36+/-11 mm Hg. Hyperventilation while breathing 100% oxygen resulted in a 40% decrease in PbrO2 (p < 0.05), whereas hypoventilation increased PbrO2 to 88 mm Hg (p < 0.01). A graded hemorrhage to 50% estimated blood volume significantly reduced PbrO2, mean arterial pressure, and intracranial pressure (p < 0.01). Continued hemorrhage to 70% estimated blood volume resulted in a PbrO2 of 2.9+/-1.5 mm Hg. After resuscitation, PbrO2 was significantly elevated, reaching 65+/-13 mm Hg (p < 0.01), whereas mean arterial pressure and cerebral perfusion pressure simply returned to baseline.

CONCLUSION

Directly measured PbrO2 was highly responsive to changes in FiO2, ventilatory rate, and blood volume in this experimental model. In particular, hypoventilation significantly increased PbrO2, whereas hyperventilation had the opposite effect. The postresuscitation increase in PbrO2 may reflect changes in both O2 delivery and O2 metabolism. These experiments set the stage for future investigations of a variety of resuscitation protocols in both normal and injured brain.

Specific blood flow reducing effects of hyperoxaemia on high flow capillaries in the pig brain

The mechanisms behind oxygen mediated changes in tissue blood flow remain unsettled. Today these are thought to (from experiments on separate vessels and other tissues than the brain) operate through the vessels themselves, probably by involvement of the endothelium in the distal parts of the vascular tree. The aim of this study was to investigate how hyperoxaemia affects the cerebrocortical capillary blood flow distribution in order to gain further knowledge of oxygen mediated blood flow regulating mechanisms. The experiments were performed on seven ventilated anaesthetized pigs. A multiwire Clark-type microelectrode, placed on the brain surface (motor cortex), was used for capillary blood flow (hydrogen clearance) and oxygen pressure measurements, both of which were made at normoxaemia (arterial PO2 14.4 kPa) and hyperoxaemia (arterial PO2 50.4 kPa)(the animals serving as their own control). Blood pressure, arterial PCO2 and pH remained unchanged throughout the experiments. During hyperoxaemia a 11% reduction in the cerebrocortical capillary blood flow was found (P < 0.001). This flow reduction was seen mainly in two capillary blood flow classes (6/7 animals). In parallel a heterogeneous increase in the cerebrocortical oxygen pressures from 4.5 to 10.1 kPa (mean) (P < 0.001) was found. These results show that hyperoxaemia causes a selective reduction in capillary blood flow affecting capillaries at specific flow levels. A finding that suggests, for the brain, that both the oxygen sensor and effect mechanism is situated distally, in the vascular tree.

Clinical experience with 118 brain tissue oxygen partial pressure catheter probes

OBJECTIVE

We assessed the technical and diagnostic reliability of partial pressure of oxygen (PO2) of brain tissue (P(ti)O2) monitoring. The monitoring system and the catheter probes were tested in vitro, and clinical experiences obtained with 118 brain P(ti)O2 catheter probes, used in 101 patients, are reported.

METHODS

The polarographic (LICOX; Medical Systems Corp., Greenvale, NY) P(ti)O2 catheter probe lies 22 to 27 mm below the dura level; its PO2-sensitive surface is 7.1 mm2. For 10 patients, the adaptation time (with initially unreliable signals after insertion) was determined. For 27 patients, the probe was removed in a stepwise fashion (three increments of 5 mm) and the heterogeneity of brain P(ti)O2 levels was investigated. After removal of the catheter probes, their PO2 and zero display error values were determined and compared with probe performance data obtained in vitro with unused PO2 catheter probes.

RESULTS

Small iatrogenic hematomas were observed for two patients (1.7%). No infection occurred after 6.7 +/- 3.9 days (mean +/- standard deviation) of monitoring. The technical complication (dislocation or defect) rate was 13.6%. The mean adaptation time was 79.0 +/- 51.7 min. A flow chart is presented, which helps to rule out artifacts. The mean P(ti)O2 measured at 22 to 27 mm below the dura was 23.8 +/- 8.1 mm Hg, at 17 to 22 mm was 25.7 +/- 8.3 mm Hg, at 12 to 17 mm was 33.0 +/- 13.3 mm Hg (P < 0.01, compared with the initial value), and at 7 to 12 mm was 33.3 +/- 13.3 mm Hg (P < 0.01). Recent catheter probe versions exhibited a PO2 display error of -1.2 +/- 5.1% (mean +/- standard deviation, n = 38) and a mean zero display error of 1.1 +/- 0.9 mm Hg (n = 34). The greatest PO2 display errors were measured during the first 4 days of continuous monitoring. In the in vitro test (of 12 unused catheter probes), the maximal probe display error was 1.07 +/- 2.14%, tested at temperatures between 22 degrees C and 37 degrees C and tested at oxygen pressures of 0, 44, and 150 mm Hg. In vitro, the zero display error was -0.21 +/- 0.25 mm Hg.

CONCLUSION

Brain P(ti)O2 monitoring, reflecting an area 17 to 27 mm below the dura, is a safe and reliable technique for monitoring cerebral oxygenation. Excluding the first 1 hour after insertion, data are reliable, with almost 100% good data quality.

Brain parenchyma/pO2 catheter interface: a histopathological study in the rat

Local cerebral oxygenation can be monitored continuously using an intraparenchymal Clark-type pO2 sensitive catheter. Measured values of brain tissue pO2 (PbrO2) not only depend on the clinically interesting balance between oxygen offer and demand, but also on catheter properties and characteristics of the probe tissue interface. Microdamage surrounding pO2-sensitive needles, inserted into various tissues, has been reported; we evaluated histologic changes at the probe tissue interface after insertion of pO2 probes, suitable for clinical use, in the rat brain. The effect of insertion of the probe itself (mechanical damage), the application of micropotential during the measurements, and the effect of time was evaluated using digital image analysis of H&E-stained histological slices. Surrounding the probe tract, a zone of edema with an average radius of 126.8 microm was seen; microhemorrhages with an average surface area of 56.2 x 10(3) microm2 were observed in nearly all cases. The area of edema and the presence of microhemorrhages were not influenced by performed measurements or by time. Intraventricular blood was observed in 10 of 19 rats studied. Measured low PbrO2 values were related to the presence of a microhemorrhage in either probe tract or ventricles. Tissue damage due to the measurements is negligible, and the amount of edema itself does not influence the accuracy or response time of the pO2 probe. Low PbrO2 readings, however, could be caused by local microhemorrhages, undetectable on CT or MRI.

A model for the regulation of cerebral oxygen delivery

On the basis of the assumption that oxygen delivery across the endothelium is proportional to capillary plasma PO2, a model is presented that links cerebral metabolic rate of oxygen utilization (CMRO2) to cerebral blood flow (CBF) through an effective diffusivity for oxygen (D) of the capillary bed. On the basis of in vivo evidence that the oxygen diffusivity properties of the capillary bed may be altered by changes in capillary PO2, hematocrit, and/or blood volume, the model allows changes in D with changes in CBF. Choice in the model of the appropriate ratio of Omega identical with (DeltaD/D)/(DeltaCBF/CBF) determines the dependence of tissue oxygen delivery on perfusion. Buxton and Frank (J. Cereb. Blood Flow. Metab. 17: 64-72, 1997) recently presented a limiting case of the present model in which Omega = 0. In contrast to the trends predicted by the model of Buxton and Frank, in the current model when Omega > 0, the proportionality between changes in CBF and CMRO2 becomes more linear, and similar degrees of proportionality can exist at different basal values of oxygen extraction fraction. The model is able to fit the observed proportionalities between CBF and CMRO2 for a large range of physiological data. Although the model does not validate any particular observed proportionality between CBF and CMRO2, generally values of (DeltaCMRO2/CMRO2)/(DeltaCBF/CBF) close to unity have been observed across ranges of graded anesthesia in rats and humans and for particular functional activations in humans. The model's capacity to fit the wide range of data indicates that the oxygen diffusivity properties of the capillary bed, which can be modified in relation to perfusion, play an important role in regulating cerebral oxygen delivery in vivo.

Comparison between continuous brain tissue pO2, pCO2, pH, and temperature and simultaneous cerebrovenous measurement using a multisensor probe in a porcine intracranial pressure model

Local brain tissue oxygenation (p(ti)O2) and global cerebrovenous hemoglobin saturation (SjO2) are increasingly used to continuously monitor patients after severe head injury (SHI). In patients, simultaneous local and global oxygen measurements of these types have shown different results regarding the comparability of the findings during changes in CPP and ICP. This is in contrast to theoretical expectations. The aim of this study was to compare p(ti)O2 measurement with cerebrovenous oxygen partial pressure measurement (p(cv)O2) in an animal intracranial pressure model. To this end, a multisensor probe was placed in the left frontoparietal white matter to measure p(ti)O2, pCO2 (p(ti)CO2), pH (pH[ti]), and temperature (t[ti]) while simultaneously measuring these same parameters (p(cv)O2, p(cv)CO2 pH(cv), t[cv]) in the sagittal sinus of 9 pigs under general anesthesia. By stepwise inflating a balloon catheter, placed in supracerebellar infratentorial compartment, ICP was increased and CPP was decreased. The baseline levels of p(ti)O2, p(ti)CO2, and pH(ti) in the noninjured brain tissue showed more heterogeneity compared to the findings in cerebrovenous blood. Both, p(ti)O2 and p(cv)O2 were significantly correlated to the induced CPP decrease. PCO2 was inversely correlated to the course of CPP in both measurement compartments. Temperature measurement showed a positive correlation with CPP in both compartments. These findings demonstrate that brain tissue oximetry and cerebrovenous PO2 measurement are sensitive to CPP changes. The newly available continuous parameters in multisensor probes could be helpful in interpreting findings of cerebral oxygen measurement in man by analyzing the interrelationship of these parameters.

Hemodilution and hyperoxia locally change distribution of regional pulmonary perfusion in dogs

In seven anesthetized dogs, the effects of acute normovolemic hemodilution (ANH) to a hematocrit of 20 and 8% and the effects of hyperoxic ventilation (100% oxygen) on distribution of regional pulmonary blood flow (rPBF; radioactive microspheres) were investigated. Normovolemia was monitored with blood volume measurements (indocyanine green dilution kinetics). Before ANH, fractal dimension (D) of rPBF in the whole lung was 1.19 +/- 0.09 (mean +/- SD). Spatial correlation (rho) of rPBF in the whole lung was 0.6 +/- 0.08. D is a resolution-independent measure for global rPBF distribution, and rho is the averaged flow relationship of directly neighboring lung samples. With regard to the entire lung, neither ANH nor hyperoxia changed D or rho. With regard to horizontal, isogravitational planes, ANH induced opposite changes of rPBF heterogeneity depending on the vertical location of the plane and the parameter used. In ventral planes, a change in relative dispersion (SD/mean) indicated decreased homogeneity. However, rho suggested more homogeneous perfusion. Hyperoxia restored baseline rPBF distribution. Our data suggest that ANH causes different alterations of heterogeneity of rPBF depending on location within the lung.

Continuous monitoring of cerebral substrate delivery and clearance: initial experience in 24 patients with severe acute brain injuries

OBJECTIVE

Current neuromonitoring techniques in severe human head injury often fail to detect the causes of clinical deterioration. A sensor is now available for continuous monitoring of brain oxygen tension, carbon dioxide tension, and pH values. In this study, brain tissue oxygen tension was used to differentiate patients at risk for brain ischemia and to predict outcome.

METHODS

The multiparameter sensor was inserted into brain tissue, along with a standard ventriculostomy catheter and a microdialysis probe, in 24 patients. Lactate and glucose were measured by high-pressure liquid chromatography in hourly dialysate samples.

RESULTS

Patients who experienced a good recovery (n = 8) sustained a mean brain partial oxygen pressure of 39 +/- 4 mm Hg, brain partial carbon dioxide pressure (PCO2) of 50 +/- 8 mm Hg, and a brain pH of 7.14 +/- 0.12. Patients with moderate to severe disability (n = 6) sustained a mean brain partial oxygen pressure of 31 +/- 5 mm Hg, brain PCO2 of 47 +/- 2 mm Hg, and a brain pH of 7.11 +/- 0.12. Ten patients who died or remained vegetative sustained a mean brain partial oxygen pressure of 19 +/- 8 mm Hg, a brain PCO2 of 64 +/- 21 mm Hg, and a brain pH of 6.85 +/- 0.41. Mean brain PCO2 levels of 90 to 150 mm Hg were consistently observed after cerebral circulatory arrest or brain death. Dialysate lactate and glucose were less clearly correlated to outcome than brain oxygen tension. Dialysate glucose was extremely low in all patients and zero in most patients who died.

CONCLUSION

Brain oxygen pressure, brain carbon dioxide pressure, and brain pH measurements, as well as a microdialysis probe for glucose and lactate analysis, may optimize the management of comatose neurosurgical patients by allowing a fuller understanding of the dynamic factors affecting brain metabolism.

Membrane potentials and microenvironment of rat dorsal vagal cells in vitro during energy depletion

1. Brainstem slices were taken from mature rats. In the dorsal vagal nucleus (DVNX), membrane potentials (Em) of neurons (DVNs) and glia, as well as extracellular oxygen, K+ and pH (Po2, aKo, pHo), were analysed during metabolic disturbances. 2. Postsynaptic potentials of DVNs, elicited by repetitive electrical stimulation of the solitary tract (TS), led to a secondary glial depolarization of up to 25 mV, a fall in Po2 of up to 150 mmHg, a rise in extracellular aKo of up to 9 mM, and a fall in pHo of about 0.2 pH units. 3. Hypoxic superfusates produced tissue anoxia, leading to an aKo increase of less than 2 mM and a pHo fall of 0.24 +/- 0.04 pH units (mean +/- S.D.). Glucose-free solution evoked, after a delay of more than 8 min, a slow rise in aKo of 1.9 +/- 0.8 mM, accompanied by a mean increase in pHo of 0.24 +/- 0.13 pH units. After pre-incubation in glucose-free solution, anoxia elevated aKo by up to 15 mM, whereas the anoxia-induced pHo decrease was completely blocked. 4. In 45 of 118 DVNs, anoxia elicited a persistent hyperpolarization of 15.6 +/- 5.0 mV. In the remaining DVNs, anoxic exposure either did not produce a change in Em (37%) or led to a depolarization of less than 10 mV (25%). A stable depolarization of 9 +/- 3.8 mV was detected in glial cells during anoxia. Similar responses were revealed in oxygenated glucose-free solution after a delay of 12-60 min. 5. The metabolism-related hyperpolarizations were blocked by 100-500 microM tolbutamide or 20-100 microM glibenclamide, leading to recovery of spontaneous (0.5-6 Hz) spike discharge. In these cells, 400-500 microM diazoxide evoked hyperpolarizations and blockade of spontaneous activity. 6. In DVNs and glial cells, a progressive depolarization of up to 40 mV in amplitude developed during anoxic exposure after pre-incubation in glucose-free solution. 7. The results show that oxygen or glucose depletion does not impair the viability of DVNX cells. The contribution of neuronal ATP-sensitive K+ (KATP) channels to this tolerance is discussed.

Brain tissue pO2 in relation to cerebral perfusion pressure, TCD findings and TCD-CO2-reactivity after severe head injury

As a reliable continuous monitoring of cerebral blood flow and/or cerebral oxygen metabolism is necessary to prevent secondary ischaemic events after severe head injury (SHI) the authors introduced brain tissue pO2 (ptiO2) monitoring and compared this new parameter with TCD-findings, cerebral perfusion pressure (CPP) and CO2-reactivity over time on 17 patients with a SHI. PtiO2 reflects the balance between the oxygen offered by the cerebral blood flow and the oxygen consumption by the brain tissue. According to TCD-CO2-reactivity PtiO2-CO2-reactivity was introduced. After initially (day 0) low mean values (ptiO2 7.7 +/- 2.6 mmHg, TCD 60.5 +/- 32.0 cm/sec and CPP 64.5 +/- 16.0 mmHg/, ptiO2 increased together with an increase in blood flow velocity of the middle cerebral artery and CPP. The relative hyperaemic phase on days 3 and 4 was followed by a decrease of all three parameters. Although TCD-CO2-reactivity was except for day 0 (1.4 +/- 1.5%), sufficient, ptiO2-CO2-reactivity sometimes showed so-called paradox reactions from day 0 till day 3, meaning an increase of ptiO2 on hyperventilation. Thereafter ptiO2-CO2-reactivity increased, increasing the risk of inducing ischaemia by hyperventilation. The authors concluded that ptiO2-monitoring might become an important tool in our treatment regime for patients requiring haemodynamic monitoring.

Evaluation of brain tissue O2 supply based on results of PO2 measurements with needle and surface microelectrodes

Tissue PO2 distribution was measured in rat cerebral cortex during arterial normoxia and arterial hypoxia. The study was designed to examine the relationship between the PO2 histograms determined with surface electrodes in the superficial cortical cells and with needle electrodes in the cortical laminae below. Under normoxic as well as under hypoxic conditions the PO2 distributions of the compared brain regions were in close agreement. The results indicate that PO2 measurements in the brain cortex with surface microelectrodes give a correct view over the tissue oxygenation in cortical regions up to 500 microns below the measuring field.

Brain tissue oxygen, carbon dioxide, and pH in neurosurgical patients at risk for ischemia

A sensor that measures oxygen pressure (PO2), carbon dioxide pressure (PCO2), and pH was evaluated in brain tissue of patients at risk for ischemia. The sensor is 0.5 mm in diameter and was inserted into cortex tissue in 14 patients undergoing craniotomy for cerebrovascular surgery. A compromised cerebral circulation was identified in 8 of 14 patients by single photon emission computed tomography (SPECT) scan, cerebral angiography, and transient ischemic episodes before surgery. Under baseline conditions with isoflurane anesthesia and normal blood gases, tissue P02 was lower in the eight compromised compared to six noncompromised patients (noncompromised 37 +/- 12 mm Hg, compromised 10 +/- 5 mm Hg; P < 0.05), PCO2 was increased (noncompromised 49 +/- 5 mm Hg, compromised 72 +/- 23 mm Hg; P < 0.05), and pH was decreased (noncompromised 7.16 +/- 0.08, compromised 6.82 +/- 0.21; P < 0.05). Critical tissue values for the identification of ischemia were a P02 of 20 mm Hg, PCO2 of 60 mm Hg, and a pH of 7.0. These results suggest that brain tissue measures of P02, PCO2, and pH provide information on the adequacy of cerebral perfusion in neurosurgical patients.

Brain oxygen, CO2, pH, and temperature monitoring: evaluation in the feline brain

Currently, no ideal method exists for monitoring the injured brain. Recently, a single, compact, fiberoptic sensor has become available for measuring oxygen, CO2, pH and temperature in blood. We have adapted this instrument for continuous use in brain tissue to measure oxygen tension, carbon dioxide tension (pCO2), pH, and temperature. To evaluate this new technique, we produced hypercapnia, hypocapnia, intracranial pressure increase, and hypoxemia in seven normal cats. In an additional six animals, sensors were placed within a zone of focal brain ischemia induced by occluding the left middle cerebral artery. The sensor readings were compared with cerebral blood flow measurements, intracranial pressure, and brain histological findings. An in vitro experiment was also performed using human blood to test the accuracy of the sensor over a wide range of pCO2 and oxygen tension values. After careful precalibration and rigid cranium fixation, stable measurements could be obtained throughout the 6- to 8-hour experiments. In normal animals, brain oxygen was 42 +/- 9 mm Hg, brain CO2 was 59 +/- 14 mm Hg, brain pH was 7.0 +/- 0.2, and brain temperature was 36.7 +/- 0.7 degrees C. Hypocapnia and hypoxemia produced a significant decline in tissue oxygen (< or = 30 +/- 3 mm Hg; P < 0.001), whereas hypercapnia caused by hypoventilation and intracranial pressure increase produced a significant increase in tissue CO2 (> or = 74 +/- 4 mm Hg; P < 0.001). Focal ischemia produced a rapid 42% decline in brain oxygen (25 +/- 7 mm Hg) and a 25% increase in tissue pCO2 (71 +/- 23 mm Hg). Brain oxygen further decreased to 19 +/- 6 mm Hg toward the end of the experiment, 4 hours later. After middle cerebral artery occlusion, the regional cerebral blood flow decreased to 10 +/- 5 ml per 100 g per minute, within the 1st hour, from a baseline value of 65 +/- 15 ml per 100 g per minute. It then gradually increased to 15 +/- 5 ml per 100 g per minute by the end of the 4-hour experiment. Brain pH was closely and inversely related to brain CO2. The brain temperature in the focally ischemic tissue decreased from 36.7 +/- 0.7 to 35.5 +/- 1.6 degrees C by the end of the experiment. The in vitro experiment demonstrated good linear correlation between the sensor readings and the blood gas analysis. Continuous monitoring of oxygen, CO2, pH, and temperature in damaged or at-risk brain tissue using a single sensor is now feasible and will, thus, allow improved continuous monitoring of neurosurgical patients who are at risk of significant secondary brain damage.

Comparison of growth rates of bovine retinal and brain microvascular pericytes in different oxygen concentrations in vitro

BACKGROUND

The hyperoxic injury of the microcirculation in the central nervous system appears to be specific to the retina in premature mammals. Oxygen tensions in normal adult mammalian retina and brain vary between nearly 0 and 90 mmHg. This study sought to compare the in vitro replication of retinal and brain microvascular pericytes in normal glucose medium and in 1%, 5% and 20% oxygen (equivalent to 15 mmHg, 35 mmHg and 150 mmHg, respectively).

METHODS

A preliminary study, using oxygen microelectrodes, confirmed that the pericellular oxygen tension of pericytes, cultured in medium under air, was within 13 mmHg of the tension of the gas phase above the media. Pericytes were highly enriched by magnetic antibody cell sorting with the anti-pericyte monoclonal antibody (3G5) to 95% to 99% purity, to remove cell contaminants which may have invalidated the mitogenic assay.

RESULTS

Mitogenic assays showed that brain pericytes replicated faster than their counterparts from retina (P < 0.0001, averaged for data from all culture conditions using three-way ANOVA). Reduction of oxygen tension from 150 to 15 mmHg led to significantly increased replication of retinal pericytes (P = 0.01), but an insignificant increase for brain pericytes.

CONCLUSIONS

We have found that pericytes from the brain and retina cultured conventionally in fetal calf serum consume a relatively low amount of oxygen. Decreasing the oxygen tension to 1% (15 to 20 mmHg) increased the replication of retinal pericytes but not brain pericytes in normal glucose concentrations and in fetal calf serum. That retinal pericyte replication is sensitive to variation in oxygen tensions, indicates that the retinal microvascular cells have a unique biological response. This growth sensitivity to oxygen may be important in the pathogenesis of retinopathy of prematurity.

Studies of tissue PO2 in normal and pathological human brain cortex

Brain cortex PO2 was measured after craniotomy and opening of the dura mater in 26 patients. We determined the brain tissue PO2 under standard narcotic conditions and after changing arterial PO2 and PCO2. Patients were divided into two groups (normal and pathological), depending on the aspect of their cortex on Ct/MRI and intraoperative appearance of the cortex. No statistical significantly difference was seen between tissue PO2 of the normal and the pathological group. A significant difference was seen only between the normal group and a subgroup with brain swelling (p = 0.0344). In the normal group no correlation was seen between tissue PO2 and arterial PO2 (r = 0.1541, p = 0.3076), whereas in the pathological group and especially in the oedema subgroup there was a highly significant correlation between tissue PO2 and PaO2 (r = 0.754, p = 0.0015 and r = 0.888, p = 0.0007). Breathing 100% oxygen changed tissue PO2 to 137.8 or 352 mmHg in the normal or the pathological group, respectively. Again, there was no correlation between tissue PO2 and PaO2 in the normal group (r = 0.1071, p = 0.392), whereas this correlation was significant in the pathological and the oedema subgroup (r = 0.6291, p = 0.0473 and r = 0.8385, p = 0.0185). This is evidence for regulatory mechanisms of tissue PO2. During hyperventilation no significant difference in tissue PO2 between the normal and the pathological group was seen. Low tissue PO2 values, however, indicate a risk for inducing ischemia.

Sciatic nerve stimulation does not increase endogenous adenosine production in sensory-motor cortex

Adenosine participates in the coupling of cerebral blood flow to oxygen consumption in the brain during such stimuli as hypoxia, ischemia, and seizures. It has been suggested that it also participates in the regulation of cerebral blood flow during somatosensory stimulation, a condition during which cerebral blood flow and oxygen consumption appear to be uncoupled. Interstitial adenosine was estimated by the microdialysis technique and cerebral blood flow was measured by hydrogen clearance in the hindlimb sensory-motor cortex during sciatic nerve stimulation. Cerebral blood flow increased from 102 to 188 ml min-1 100 g-1 (p less than 0.001) in the cortex contralateral to the stimulated leg without an associated increase in interstitial adenosine (baseline 0.624 microM, stimulation 0.583 microM). Infusion of the adenosine antagonist 8-sulfophenyltheophylline failed to block an increase in cerebral blood flow during central sciatic nerve stimulation, but decreased basal cerebral blood flow (69 ml min-1 100 g-1). These results suggest that adenosine does not mediate changes in cerebral blood flow during somatosensory stimulation, but may participate in the regulation of cerebral blood flow in the basal state.

Oxygen distribution and consumption in the cat retina during normoxia and hypoxemia

Oxygen tension (PO2) was measured with microelectrodes within the retina of anesthetized cats during normoxia and hypoxemia (i.e., systemic hypoxia), and photoreceptor oxygen consumption was determined by fitting PO2 measurements to a model of steady-state oxygen diffusion and consumption. Choroidal PO2 fell linearly during hypoxemia, about 0.64 mmHg/mmHg decrease in arterial PO2 (PaO2). The choroidal circulation provided approximately 91% of the photoreceptors' oxygen supply under dark-adapted conditions during both normoxia and hypoxemia. In light adaptation the choroid supplied all of the oxygen during normoxia, but at PaO2's less than 60 mmHg the retinal circulation supplied approximately 10% of the oxygen. In the dark-adapted retina the decrease in choroidal PO2 caused a large decrease in photoreceptor oxygen consumption, from approximately 5.1 ml O2/100 g.min during normoxia to 2.6 ml O2/100 g.min at a PaO2 of 50 mmHg. When the retina was adapted to a rod saturating background, normoxic oxygen consumption was approximately 33% of the dark-adapted value, and hypoxemia caused almost no change in oxygen consumption. This difference in metabolic effects of hypoxemia in light and dark explains why the standing potential of the eye and retinal extracellular potassium concentration were previously found to be more affected by hypoxemia in darkness. Frequency histograms of intraretinal PO2 were used to characterize the oxygenation of the vascularized inner half of the retina, where the oxygen distribution is heterogeneous and simple diffusion models cannot be used. Inner retinal PO2 during normoxia was relatively low: 18 +/- 12 mmHg (mean and SD; n = 8,328 values from 36 profiles) in dark adaptation, and significantly lower, 13 +/- 6 mmHg (n = 4,349 values from 19 profiles) in light adaptation. Even in the dark-adapted retina, 30% of the values were less than 10 mmHg. The mean PO2 in the inner (i.e., proximal) half of the retina was well regulated during hypoxemia. In dark adaptation it was significantly reduced only at PaO2's less than 45 mmHg, and it was reduced less at these PaO2's in light adaptation.

Depth profile of local oxygen tension and blood flow in rat cerebral cortex, white matter and hippocampus

Microregional oxygenation and blood flow were measured in rat cerebral cortical lamina, subcortical white matter and hippocampus. This was done to examine relationships between oxygenation and blood flow at a local level, to determine effects of craniotomy, and to consider whether flow/oxygenation relationships might be predictive of selective vulnerability known to accompany anoxia or ischemia. Blood flow and oxygen tension were measured with closely apposed polarographic microelectrodes. Oxygen tension was highest in white matter, lower in the region of cortical laminae IV-V and lowest in the hippocampus. Blood flow in the hippocampus was higher than that in white matter or laminae IV-V of cerebral cortex. Ratios of blood flow to oxygenation were similar throughout the cortex, higher in white matter but oxygenation in hippocampus was significantly less than expected from measurement of hippocampal blood flow reflecting increased oxygen consumption or relative hypoxia due to increased diffusion distances for oxygen in hippocampus. Comparison of data from closed vs opened skull animals indicated that diffusion of oxygen and hydrogen influenced data to approximately 1000 micron below the cerebral surface.

Tissue pO2 of human brain cortex--method, basic results and effects of pentoxifylline

A polarographic multiwire surface electrode was used for measurement of local oxygen partial pressure (pO2) on human brain cortex during neurosurgical operations. The two major problems encountered in this application of the electrode involved sterility of the equipment and mounting of the electrode. The described method of sterilization does not alter the electrical properties of the electrode. A special mount was designed to allow free three-dimensional placement of the electrode without exerting pressure on the cortex. Basic results of this technique demonstrated that it is possible to distinguish different pO2 distribution patterns displayed in pO2 histograms for various types of brain tumors and edematous brain tissue. In patients with arteriovenous malformations (AVMs) of the brain, an increase of tissue pO2 in cortical areas adjacent to the AVM was the result of extirpation of the lesion. The effect of intravenously administered pentoxifylline was studied during extraintracranial bypass operations in patients with cerebrovascular disease. In 7 patients a consistent shift of the pO2 histograms to the right, i.e., to higher pO2 values, could be demonstrated. Mean pO2 values increased statistically significantly by 16 +/- 7 mmHg as early as ten minutes after infusion of pentoxifylline. The rapid improvement of tissue oxygenation of human brain cortex is thought to be the result of an improvement of microcirculation, for other parameters influencing tissue pO2 showed no significant alterations if any.

Effect of carotid artery ligation and infusion of fluosol FC-43 emulsion on brain surface oxygen tensions

In eight rabbits, the common carotid artery was ligated and multiple estimations of brain surface oxygen tension performed using a seven barrelled mini-electrode. In five rabbits ligation of the carotid artery resulted in impairment of cortical oxygenation. The remaining three rabbits showed no impairment in the supply of oxygen to the cerebral cortex after carotid occlusion. In the five rabbits who displayed a reduction in oxygen supply after carotid ligation, ventilation with 33% oxygen after the infusion of 15 ml/kg of Fluosol FC-43 produced an improvement in cortical oxygenation in only three of the five rabbits. When these animals were ventilated with 100% oxygen after carotid ligation and Fluosol infusion, oxygen supply in all five was commensurate with or greater than that during control conditions.

Effects of increases in the inspired oxygen fraction on brain surface oxygen pressure fields in pig and man

In six patients undergoing neurosurgical operation, brain surface oxygen pressure was studied during an increase of the inspired oxygen fraction (FiO2). The eight-channel oxygen surface electrode (MDO-electrode) was placed directly on the brain cortex. FiO2 was increased to four levels, from baseline level 0.21 to 0.3, 0.5, 0.7 and 1.0, respectively. During these four stages and FiO2 0.21, brain surface oxygen pressure (PtO2) was measured. The physiological variables such as blood pressure, PaCO2, pH and temperature were stable throughout the study. The results are presented as mean values +/- s.d. and a PtO2 histogram for each FiO2-level. Already at an FiO2 of 0.3 (at a PaO2 of 16.3 +/- 3.4 kPa) scattered histograms were seen in five of six patients. A scattered histogram indicates disturbed microcirculation. At the FiO2 levels of 0.5, 0.7 and 1.0, all histograms were scattered. The PtO2 values did not increase proportionally to PaO2 at FiO2 levels 0.3, 0.5 or 0.7. But at FiO2 1.0 four patients had normal mean PtO2 values and two patients very high mean PtO2 values. It is possible that the four patients with normal PtO2 values succeeded in regulating the cerebral microcirculation as a response to the high FiO2 leading to a high PaO2 (60.1 +/- 6.4 kPa). The same study was initially done on six pigs in which the regional cerebral blood flow (rCBF) was also measured. MDO-electrode measurements at different FiO2-levels gave the same results as in the patients. rCBF decreased when FiO2 was increased.

The influence of hemorrhagic hypotension on spinal cord tissue oxygen tension

In order to investigate the spinal cord surface PO2 (sPO2) reaction to hypovolemic hypotension, nine female pigs (25 kg bw) were premedicated, anesthetized, intubated and artificially ventilated with N2O:O2 = 3:1. Following laminectomy from L3 to L5, sPO2 was measured on the dorsal side of the exposed spinal cord using six gold cathodes (luminal diameter 15 microns) while MAP (mean +/- SD) was lowered in steps of about 10 Torr by bleeding into a reservoir from 69.3 +/- 10.1 Torr to extreme low values of 13.3 +/- 3.1 Torr. Only a slight decrease of lumbar sPO2 (mean +/- SD) from 33.5 +/- 7.2 Torr to 27.6 +/- 4.8 Torr was evaluated from the data in response to MAP reduction to 50.2 +/- 3.5 Torr. Below that value, a marked proportional decrease of sPO2 and MAP was observed (0.65 Torr/Torr) corresponding to pressure passive flow behavior of the Hagen Poisseuille Law ("loss of autoregulation"). Four to five minutes after start of reinfusion both sPO2 and MAP showed an overshoot with maximum values of 54.0 +/- 11.1 Torr resp. 102.6 +/- 18 Torr. Initial values were approximated about 15 min. later. Histograms plotted from the individual sPO2 values of all pigs and of all the different experimental stages showed signs of severe hypoxia only if MAP was reduced below about 30 Torr. In general, this situation was reversible within reinfusion, only one of the nine pigs did not tolerate hemorrhagic hypoxia induced by MAP reduction to less than 30 Torr for at least 5-10 min. Nevertheless, the experiments showed a considerable circulatory stability in the investigated pigs.