Cerebral microdialysis: research technique or clinical tool

Cerebral microdialysis is a well-established laboratory tool that is increasingly used as a bedside monitor to provide on-line analysis of brain tissue biochemistry during neuro-intensive care. This review describes the principles of cerebral microdialysis and the rationale for its use in the clinical setting, including discussion of the most commonly used microdialysis biomarkers of acute brain injury. Potential clinical applications are reviewed and future research applications identified. Microdialysis has the potential to become an established part of mainstream multimodality monitoring during the management of acute brain injury but at present should be considered a research tool for use in specialist centres

Non-invasive near infrared spectroscopy: A tool for measuring cerebral oxygenation and metabolism in patients with traumatic brain injury.

Traumatic brain injury (TBI) causes significant morbidity and mortality. Modern neurocritical care management utilises several monitoring techniques to identify or predict secondary insults but many of these techniques have significant limitations. The ideal cerebral monitor would be a non-invasive system able to provide realtime quantitative haemodynamic and metabolic information at multiple sites with high temporal and spatial resolution. Near infrared spectroscopy (NIRS) fulfils many of these requirements and has great potential as a cerebral monitoring tool. In addition to measurements of oxy- and deoxy-haemoglobin concentration, NIRS can monitor changes in mitochondrial redox state by measuring changes in oxidised cytochrome c oxidase (oxCCO) concentration. This is an attractive monitoring target as it is intimately involved in adenosine triphosphate synthesis and cellular homeostasis, yet few studies exist which investigate oxCCO concentration changes in either the normal or injured adult human brain. This thesis explores the use of NIRS, and in particular measurement of oxCCO concentration changes, for monitoring patients with TBI. Cerebral metabolism in both health and TBI is described and current monitoring tools and treatment strategies used in TBI are discussed. Studies investigating NIRS measurements in the brains of healthy volunteers during changes in arterial oxygen and carbon dioxide tension are described in order to determine the ability of NIRS to detect these physiological perturbations and to characterise the resulting metabolic changes. Spectroscopic data are analysed to investigate the methodology used to calculate oxCCO concentration changes. NIRS is used to monitor patients with TBI during normobaric hyperoxygenation and non-invasively measured NIRS variables are compared with those acquired using invasive cerebral monitoring devices. Correlations are shown between non-invasive measures of mitochondrial redox state and invasive measures of cellular redox state. NIRS can monitor changes in cerebral physiology after TBI and has the potential to guide neuroprotective strategies on the neurocritical care unit

Relationship between physical attributes, clay fraction mineralogy and aluminum forms in soil

O alumínio (Al) é um dos elementos mais abundantes em massa nos solos brasileiros, estando presente em diferentes minerais e formas. Considerando a carência de informações sobre a sua participação no comportamento físico do solo, buscou-se, no presente estudo, avaliar a relação entre a mineralogia da fração argila, formas de Al e atributos físicos de diferentes solos. Para tanto, foram coletadas amostras deformadas e indeformadas dos horizontes A e B de solos representativos das Zonas Fisiográficas Sul e Campos das Vertentes do estado de Minas Gerais. As amostras foram submetidas às caracterizações física, química e mineralógica. Estudos de correlação mostraram que os atributos físicos dos solos estudados estavam associados às diferentes formas de Al presentes e à mineralogia da fração argila: as formas de Al amorfas e menos cristalinas associaram-se mais aos atributos físicos relacionados com a agregação das partículas do solo, podendo esta associação ser atribuída à afinidade deste elemento com a matéria orgânica, enquanto os componentes cristalinos associaram-se mais aos atributos físicos relacionados com a organização estrutural dos solos._________________________________________________________________________________ ABSTRACT: Aluminum (Al) is one of the most abundant elements in weight in Brazilian soils, found in different minerals and forms. Considering the lack of information about its participation in the physical behavior of soil, the present study aimed to evaluate the relationship between the mineralogy of the clay fraction, different forms of Al and physical attributes of different soils. Disturbed and undisturbed samples of A and B horizons of representative soils from the South and Campos das Vertentes physiographic regions in the State of Minas Gerais were therefore collected. The samples were subjected to physical, chemical and mineralogical characterizations. Correlation studies performed with the resulting data from the different characterizations showed that the physical attributes of the different studied soils are associated to the different forms of Al and to the mineralogy of the clay fraction: the amorphous and less crystalline forms of Al were more associated to physical attributes related with particle aggregation, such association being ascribed to the affinity of this element with organic matter, while the crystalline components were more associated to physical attributes related with the structural organization of the soils

Dual role of cerebral blood flow in regional brain temperature control in the healthy newborn infant.

Small shifts in brain temperature after hypoxia-ischaemia affect cell viability. The main determinants of brain temperature are cerebral metabolism, which contributes to local heat production, and brain perfusion, which removes heat. However, few studies have addressed the effect of cerebral metabolism and perfusion on regional brain temperature in human neonates because of the lack of non-invasive cot-side monitors. This study aimed (i) to determine non-invasive monitoring tools of cerebral metabolism and perfusion by combining near-infrared spectroscopy and echocardiography, and (ii) to investigate the dependence of brain temperature on cerebral metabolism and perfusion in unsedated newborn infants. Thirty-two healthy newborn infants were recruited. They were studied with cerebral near-infrared spectroscopy, echocardiography, and a zero-heat flux tissue thermometer. A surrogate of cerebral blood flow (CBF) was measured using superior vena cava flow adjusted for cerebral volume (rSVC flow). The tissue oxygenation index, fractional oxygen extraction (FOE), and the cerebral metabolic rate of oxygen relative to rSVC flow (CMRO2 index) were also estimated. A greater rSVC flow was positively associated with higher brain temperatures, particularly for superficial structures. The CMRO2 index and rSVC flow were positively coupled. However, brain temperature was independent of FOE and the CMRO2 index. A cooler ambient temperature was associated with a greater temperature gradient between the scalp surface and the body core. Cerebral oxygen metabolism and perfusion were monitored in newborn infants without using tracers. In these healthy newborn infants, cerebral perfusion and ambient temperature were significant independent variables of brain temperature. CBF has primarily been associated with heat removal from the brain. However, our results suggest that CBF is likely to deliver heat specifically to the superficial brain. Further studies are required to assess the effect of cerebral metabolism and perfusion on regional brain temperature in low-cardiac output conditions, fever, and with therapeutic hypothermia

Near infrared spectroscopic quantification of changes in the concentration of oxidized cytochrome oxidase in the healthy human brain during hypoxemia

The near infrared cytochrome c oxidase (CCO) signal has potential as a clinical marker of changes in mitochondrial oxygen utilization. We examine the CCO signal response to reduced oxygen delivery in the healthy human brain. We induced a reduction in arterial oxygen saturation from baseline levels to 80% in eight healthy adult humans, whilst minimising changes in end tidal carbon dioxide tension. We measured changes in the cerebral concentrations of oxidized CCO (Δ[oxCCO]), oxy-hemoglobin (Δ[HbO2]) and deoxy-hemoglobin (Δ[HHb]) using broadband near infrared spectroscopy (NIRS), and estimated changes in cerebral oxygen delivery (ecDO2) using pulse oximetry and transcranial Doppler ultrasonography. Results are presented as median (interquartile range). At the nadir of hypoxemia ecDO2 decreased by 9.2(5.4-12.1)% (p<0.0001), Δ[oxCCO] decreased by 0.24(0.06-0.28) micromoles/l (p<0.01), total hemoglobin concentration increased by 2.83(2.27-4.46) micromoles/l (p<0.0001) and change in haemoglobin difference concentration (Δ[Hbdiff]=Δ[HbO2]-Δ[HHb]) decreased by 12.72(11.32-16.84) micromoles/l (p<0.0001). Change in ecDO2 correlated with Δ[oxCCO] (r=0.78, p<0.001), but not with either change in total hemoglobin concentration or Δ[Hbdiff]. This is the first description of cerebral Δ[oxCCO] during hypoxemia in healthy adults. Studies are ongoing to investigate the clinical relevance of this signal in patients with traumatic brain injury