Analyses of cerebral microdialysis in patients with traumatic brain injury: relations to intracranial pressure, cerebral perfusion pressure and catheter placement

<p>Abstract</p> <p>Background</p> <p>Cerebral microdialysis (MD) is used to monitor local brain chemistry of patients with traumatic brain injury (TBI). Despite an extensive literature on cerebral MD in the clinical setting, it remains unclear how individual levels of real-time MD data are to be interpreted. Intracranial pressure (ICP) and cerebral perfusion pressure (CPP) are important continuous brain monitors in neurointensive care. They are used as surrogate monitors of cerebral blood flow and have an established relation to outcome. The purpose of this study was to investigate the relations between MD parameters and ICP and/or CPP in patients with TBI.</p> <p>Methods</p> <p>Cerebral MD, ICP and CPP were monitored in 90 patients with TBI. Data were extensively analyzed, using over 7,350 samples of complete (hourly) MD data sets (glucose, lactate, pyruvate and glycerol) to seek representations of ICP, CPP and MD that were best correlated. MD catheter positions were located on computed tomography scans as pericontusional or nonpericontusional. MD markers were analyzed for correlations to ICP and CPP using time series regression analysis, mixed effects models and nonlinear (artificial neural networks) computer-based pattern recognition methods.</p> <p>Results</p> <p>Despite much data indicating highly perturbed metabolism, MD shows weak correlations to ICP and CPP. In contrast, the autocorrelation of MD is high for all markers, even at up to 30 future hours. Consequently, subject identity alone explains 52% to 75% of MD marker variance. This indicates that the dominant metabolic processes monitored with MD are long-term, spanning days or longer. In comparison, short-term (differenced or Δ) changes of MD vs. CPP are significantly correlated in pericontusional locations, but with less than 1% explained variance. Moreover, CPP and ICP were significantly related to outcome based on Glasgow Outcome Scale scores, while no significant relations were found between outcome and MD.</p> <p>Conclusions</p> <p>The multitude of highly perturbed local chemistry seen with MD in patients with TBI predominately represents long-term metabolic patterns and is weakly correlated to ICP and CPP. This suggests that disturbances other than pressure and/or flow have a dominant influence on MD levels in patients with TBI.</p

Climate change impacts on groundwater recharge- uncertainty, shortcomings, and the way forward?

An integrated approach to assessing the regional impacts of climate and socio- economic change on groundwater recharge is described from East Anglia, UK. Many factors affect future groundwater recharge including changed precipitation and temperature regimes, coastal flooding, urbanization, woodland establishment, and changes in cropping and rotations. Important sources of uncertainty and shortcomings in recharge estimation are discussed in the light of the results. The uncertainty in, and importance of, socio-economic scenarios in exploring the consequences of unknown future changes are highlighted. Changes to soil properties are occurring over a range of time scales, such that the soils of the future may not have the same infiltration properties as existing soils. The potential implications involved in assuming unchanging soil properties are described. To focus on the direct impacts of climate change is to neglect the potentially important role of policy, societal values and economic processes in shaping the landscape above aquifers. If the likely consequences of future changes of groundwater recharge, resulting from both climate and socio-economic change, are to be assessed, hydrogeologists must increasingly work with researchers from other disciplines, such as socio-economists, agricultural modellers and soil scientists

A Neurological Wake-Up Test in the Neurointensive Care Unit: Pros and Cons

Traumatic brain injury (TBI) induces a marked systemic biochemical stress response with the release of several stress-related hormones including cortisol and the catecholamines. A major aim of using continuous sedation in the neurointensive care unit (NIC) unit is to attenuate the TBI-induced stress response via reduction of the cerebral energy metabolic demands. In the era of modern multimodality monitoring and neuroimaging for patients with severe TBI, what is the role for neurological evaluation, a neurological wake-up test (NWT), of patients on continuous sedation and mechanical ventilation? In particular, does the information obtained by the NWT outweigh the risk of inducing a substantial stress response? The additional use of NWTs in NIC is controversial and is not mentioned in any recent TBI guidelines. Although daily interruption of continuous sedation is suggested for patients in general intensive care, reasons for not using the NWT in NIC may be a fear of an NWT-induced stress response and uncertainty to the additional value of NWTs in patients monitored with multimodality monitoring and frequent neuroradiological examinations. A recent survey showed that use of NWT varies markedly in Scandinavians’ NIC units where half of the evaluated centres never use the NWT, whereas others use the NWT up to six times daily. In a series of studies characterising the NWT-induced stress response, the NWT was found to induce a significant increase in ICP and CPP in severe TBI patients on controlled ventilation. Additionally, the NWT caused an increase in adrenocorticotrophic (ACTH) hormone, catecholamine and cortisol levels. In the absolute majority of patients, the ICP and CPP changes were modest and transient and the absolute increases in stress hormone levels were small. However, the stress response was marked in a small subset of patients. These studies suggest that the NWT is safe in the majority of patients but that the test should be individualised and avoided in patients reacting with markedly increased ICP and/or decreased CPP. Although important clinical information may be obtained from the NWT, future studies need to evaluate the risk-benefit ratio of the NWT in TBI management

Spatial organization of bacterial populations in response to oxygen and carbon counter-gradients in pore networks

Spatial organisation of bacteria could contribute to the persistence of anoxic hotspots in soils, however such processes are difficult to observe directly. Here, the authors develop an experimental platform mimicking resource gradients postulated in soil aggregates to assess bacterial spatial organisation