Measurement of brain tissue oxygenation performed using positron emission tomography scanning to validate a novel monitoring method

Management Strategies Based on Multi-Modality Neuromonitoring in Severe Traumatic Brain Injury

Secondary brain injury after neurotrauma is comprised of a host of distinct, potentially concurrent and interacting mechanisms that may exacerbate primary brain insult. Multimodality neuromonitoring is a method of measuring multiple aspects of the brain in order to understand the signatures of these different pathomechanisms and to detect, treat, or prevent potentially reversible secondary brain injuries. The most studied invasive parameters include intracranial pressure (ICP), cerebral perfusion pressure (CPP), autoregulatory indices, brain tissue partial oxygen tension, and tissue energy and metabolism measures such as the lactate pyruvate ratio. Understanding the local metabolic state of brain tissue in order to infer pathology and develop appropriate management strategies is an area of active investigation. Several clinical trials are underway to define the role of brain tissue oxygenation monitoring and electrocorticography in conjunction with other multimodal neuromonitoring information, including ICP and CPP monitoring. Identifying an optimal CPP to guide individualized management of blood pressure and ICP has been shown to be feasible, but definitive clinical trial evidence is still needed. Future work is still needed to define and clinically correlate patterns that emerge from integrated measurements of metabolism, pressure, flow, oxygenation, and electrophysiology. Pathophysiologic targets and precise critical care management strategies to address their underlying causes promise to mitigate secondary injuries and hold the potential to improve patient outcome. Advancements in clinical trial design are poised to establish new standards for the use of multimodality neuromonitoring to guide individualized clinical care.

The Role of Brain Tissue Oxygenation Monitoring in the Management of Subarachnoid Hemorrhage: A Scoping Review

Monitoring of brain tissue oxygenation (PbtO2) is an important component of multimodal monitoring in traumatic brain injury. Over recent years, use of PbtO2 monitoring has also increased in patients with poor-grade subarachnoid hemorrhage (SAH), particularly in those with delayed cerebral ischemia. The aim of this scoping review was to summarize the current state of the art regarding the use of this invasive neuromonitoring tool in patients with SAH. Our results showed that PbtO2 monitoring is a safe and reliable method to assess regional cerebral tissue oxygenation and that PbtO2 represents the oxygen available in the brain interstitial space for aerobic energy production (i.e., the product of cerebral blood flow and the arterio-venous oxygen tension difference). The PbtO2 probe should be placed in the area at risk of ischemia (i.e., in the vascular territory in which cerebral vasospasm is expected to occur). The most widely used PbtO2 threshold to define brain tissue hypoxia and initiate specific treatment is between 15 and 20 mm Hg. PbtO2 values can help identify the need for or the effects of various therapies, such as hyperventilation, hyperoxia, induced hypothermia, induced hypertension, red blood cell transfusion, osmotic therapy, and decompressive craniectomy. Finally, a low PbtO2 value is associated with a worse prognosis, and an increase of the PbtO2 value in response to treatment is a marker of good outcome.

Mismatch between Tissue Partial Oxygen Pressure and Near-Infrared Spectroscopy Neuromonitoring of Tissue Respiration in Acute Brain Trauma: The Rationale for Implementing a Multimodal Monitoring Strategy

The brain tissue partial oxygen pressure (PbtO₂) and near-infrared spectroscopy (NIRS) neuromonitoring are frequently compared in the management of acute moderate and severe traumatic brain injury patients; however, the relationship between their respective output parameters flows from the complex pathogenesis of tissue respiration after brain trauma. NIRS neuromonitoring overcomes certain limitations related to the heterogeneity of the pathology across the brain that cannot be adequately addressed by local-sample invasive neuromonitoring (e.g., PbtO₂ neuromonitoring, microdialysis), and it allows clinicians to assess parameters that cannot otherwise be scanned. The anatomical co-registration of an NIRS signal with axial imaging (e.g., computerized tomography scan) enhances the optical signal, which can be changed by the anatomy of the lesions and the significance of the radiological assessment. These arguments led us to conclude that rather than aiming to substitute PbtO₂ with tissue saturation, multiple types of NIRS should be included via multimodal systemic- and neuro-monitoring, whose values then are incorporated into biosignatures linked to patient status and prognosis. Discussion on the abnormalities in tissue respiration due to brain trauma and how they affect the PbtO₂ and NIRS neuromonitoring is given.

Photoluminescent oxygen-release microspheres to image the oxygen release process in vivo

Cell therapy is a promising strategy to treat ischemic diseases, but the efficacy is limited due to high rate of cell death under low oxygen environment of the ischemic tissues. Sustained release of oxygen to continuously oxygenate the transplanted cells may augment cell survival and improve therapeutic efficacy. We have shown previously that oxygen released from oxygen-release microspheres stimulated cell survival in ischemic tissue [1]. To understand how oxygen is released in vivo and duration of release, it is attractive to image the process of oxygen release. Herein, we have developed photoluminenscent oxygen-release microspheres where the in vivo oxygen release can be non-invasively and real-time monitored by an In Vivo Imaging System (IVIS). In the oxygen-release microspheres, a complex of polyvinylpyrrolidone, H₂O₂ and a fluorescent drug hypericin (HYP) was used as core, and poly(N-isopropylacrylamide-co-acrylate-oligolactide-co-hydroxyethyl methacrylate-co-N-acryloxysuccinimide) conjugated with catalase was used as shell. To distinguish fluorescent signal change for different oxygen release kinetics, the microspheres with various release profiles were developed by using the shell with different degradation rates. In vitro, the fluorescent intensity gradually decreased during the 21-day oxygen release period, consistent with oxygen release kinetics. The released oxygen significantly augmented mesenchymal stem cell (MSC) survival under hypoxic condition. In vivo, the oxygen release rate was faster. The fluorescent signal can be detected for 17 days for the microspheres with the slowest oxygen release kinetics. The implanted microspheres did not induce substantial inflammation. The above results demonstrate that the developed microspheres have potential to monitor the in vivo oxygen release.

Changes in Pressure, Hemodynamics, and Metabolism within the Spinal Cord during the First 7 Days after Injury Using a Porcine Model

Traumatic spinal cord injury (SCI) triggers many perturbations within the injured cord, such as decreased perfusion, reduced tissue oxygenation, increased hydrostatic pressure, and disrupted bioenergetics. While much attention is directed to neuroprotective interventions that might alleviate these early pathophysiologic responses to traumatic injury, the temporo-spatial characteristics of these responses within the injured cord are not well documented. In this study, we utilized our Yucatan mini-pig model of traumatic SCI to characterize intraparenchymal hemodynamic and metabolic changes within the spinal cord for 1 week post-injury. Animals were subjected to a contusion/compression SCI at T10. Prior to injury, probes for microdialysis and the measurement of spinal cord blood flow (SCBF), oxygenation (in partial pressure of oxygen; PaPO₂), and hydrostatic pressure were inserted into the spinal cord 0.2 and 2.2 cm from the injury site. Measurements occurred under anesthesia for 4 h post-injury, after which the animals were recovered and measurements continued for 7 days. Close to the lesion (0.2 cm), SCBF levels decreased immediately after SCI, followed by an increase in the subsequent days. Similarly, PaPO₂ plummeted, where levels remained diminished for up to 7 days post-injury. Lactate/pyruvate (L/P) ratio increased within minutes. Further away from the injury site (2.2 cm), L/P ratio also gradually increased. Hydrostatic pressure remained consistently elevated for days and negatively correlated with changes in SCBF. An imbalance between SCBF and tissue metabolism also was observed, resulting in metabolic stress and insufficient oxygen levels. Taken together, traumatic SCI resulted in an expanding area of ischemia/hypoxia, with ongoing physiological perturbations sustained out to 7 days post-injury. This suggests that our clinical practice of hemodynamically supporting patients out to 7 days post-injury may fail to address persistent ischemia within the injured cord. A detailed understanding of these pathophysiological mechanisms after SCI is essential to promote best practices for acute SCI patients.

The Role of Multimodal Invasive Monitoring in Acute Traumatic Brain Injury

This article reviews the role of modalities that directly monitor brain parenchyma in patients with severe traumatic brain injury. The physiology monitored involves compartmental and perfusion pressures, tissue oxygenation and metabolism, quantitative blood flow, pressure autoregulation, and electrophysiology. There are several proposed roles for this multimodality monitoring, such as to track, prevent, and treat the cascade of secondary brain injury; monitor the neurologically injured patient; integrate various data into a composite, patient-specific, and dynamic picture; apply protocolized, pathophysiology-driven intensive care; use as a prognostic marker; and understand pathophysiologic mechanisms involved in secondary brain injury to develop preventive and abortive therapies, and to inform future clinical trials.

Brain tissue oxygenation, lactate-pyruvate ratio, and cerebrovascular pressure reactivity monitoring in severe traumatic brain injury: systematic review and viewpoint

BACKGROUND

Prevention and detection of secondary brain insults via multimodality neuromonitoring is a major goal in patients with severe traumatic brain injury (TBI).

OBJECTIVE

Explore the underlying pathophysiology and clinical outcome correlates as it pertains to combined monitoring of ≥2 from the following variables: partial brain tissue oxygen tension (PbtO(2)), pressure reactivity index (PRx), and lactate pyruvate ratio (LPR).

METHODS

Data sources included Medline, EMBASE, and evidence-based databases (Cochrane DSR, ACP Journal Club, DARE, and the Cochrane Controlled Trials Register). The PRISMA recommendations were followed. Two authors independently selected articles meeting inclusion criteria. Studies enrolled adults who required critical care and monitoring in the setting of TBI. Included studies reported on correlations between the monitored variables and/or reported on correlations of the variables with clinical outcomes.

RESULTS

Thirty-four reports were included (32 observational studies and 2 randomized controlled trials) with a mean sample size of 34 patients (range 6-223), and a total of 1,161 patient-observations. Overall methodological quality was moderate. Due to inter-study heterogeneity in outcomes of interest, study design, and in both number and type of covariates included in multivariable analyses, quantitative synthesis of study results was not undertaken.

CONCLUSION

Several literature limitations were identified including small number of subjects, lack of clinical outcome correlations, inconsistent probe location, and overall moderate quality among the included studies. These limitations preclude any firm conclusions; nevertheless we suggest that the status of cerebrovascular reactivity is not only important for cerebral perfusion pressure optimization but should also inform interpretation and interventions targeted on PbtO(2) and LPR. Assessment of reactivity can be the first step in approaching the relations among cerebral blood flow, oxygen delivery, demand, and cellular metabolism.

Monitoring of brain and systemic oxygenation in neurocritical care patients

Maintenance of adequate oxygenation is a mainstay of intensive care, however, recommendations on the safety, accuracy, and the potential clinical utility of invasive and non-invasive tools to monitor brain and systemic oxygenation in neurocritical care are lacking. A literature search was conducted for English language articles describing bedside brain and systemic oxygen monitoring in neurocritical care patients from 1980 to August 2013. Imaging techniques e.g., PET are not considered. A total of 281 studies were included, the majority described patients with traumatic brain injury (TBI). All tools for oxygen monitoring are safe. Parenchymal brain oxygen (PbtO2) monitoring is accurate to detect brain hypoxia, and it is recommended to titrate individual targets of cerebral perfusion pressure (CPP), ventilator parameters (PaCO2, PaO2), and transfusion, and to manage intracranial hypertension, in combination with ICP monitoring. SjvO2 is less accurate than PbtO2. Given limited data, NIRS is not recommended at present for adult patients who require neurocritical care. Systemic monitoring of oxygen (PaO2, SaO2, SpO2) and CO2 (PaCO2, end-tidal CO2) is recommended in patients who require neurocritical care.

Post-traumatic multimodal brain monitoring: response to hypertonic saline

Emerging evidence suggests that hypertonic saline (HTS) is efficient in decreasing intracranial pressure (ICP). However there is no consensus about its interaction with brain hemodynamics and oxygenation. In this study, we investigated brain response to HTS bolus with multimodal monitoring after severe traumatic brain injury (TBI). We included 18 consecutive TBI patients during 10 days after neurocritical care unit admission. Continuous brain monitoring applied included ICP, tissue oxygenation (PtO2) and cerebral blood flow (CBF). Cerebral perfusion pressure (CPP), cerebrovascular resistance (CVR), and reactivity indices related to pressure (PRx) and flow (CBFx) were calculated. ICM+software was used to collect and analyze monitoring data. Eleven of 18 (61%) patients developed 99 episodes of intracranial hypertension (IHT) greater than 20 mm Hg that were managed with 20% HTS bolus. Analysis over time was performed with linear mixed-effects regression modelling. After HTS bolus, ICP and CPP improved over time (p<0.001) following a quadratic model. From baseline to 120 min, ICP had a mean decrease of 6.2 mm Hg and CPP a mean increase of 3.1 mmHg. Mean increase in CBF was 7.8 mL/min/100 g (p<0.001) and mean decrease in CVR reached 0.4 mm Hg*min*100 g/mL (p=0.01). Both changes preceded pressures improvement. PtO2 exhibited a marginal increase and no significant models for time behaviour could be fitted. PRx and CBFx were best described by a linear decreasing model showing autoregulation recover after HTS (p=0.01 and p=0.04 respectively). During evaluation, CO2 remained constant and sodium level did not exhibit significant variation. In conclusion, management of IHT with 20% HTS significantly improves cerebral hemodynamics and cerebrovascular reactivity with recovery of CBF appearing before rise in CPP and decrease in ICP. In spite of cerebral hemodynamic improvement, no significant changes in brain oxygenation were identified.

Physiological monitoring of the severe traumatic brain injury patient in the intensive care unit

Traumatic brain injury (TBI) is a major cause of morbidity and mortality worldwide. Despite encouraging animal research, pharmacological agents and neuroprotectants have disappointed in the clinical environment. Current TBI management therefore is directed towards identification, prevention, and treatment of secondary cerebral insults that are known to exacerbate outcome after injury. This strategy is based on a variety of monitoring techniques that include the neurological examination, imaging, laboratory analysis, and physiological monitoring of the brain and other organ systems used to guide therapeutic interventions. Recent clinical series suggest that TBI management informed by multimodality monitoring is associated with improved patient outcome, in part because care is provided in a patient-specific manner. In this review we discuss physiological monitoring of the brain after TBI and the emerging field of neurocritical care bioinformatics.

Brain tissue oxygen monitoring and hyperoxic treatment in patients with traumatic brain injury

Cerebral ischemia is a well-recognized contributor to high morbidity and mortality after traumatic brain injury (TBI). Standard of care treatment aims to maintain a sufficient oxygen supply to the brain by avoiding increased intracranial pressure (ICP) and ensuring a sufficient cerebral perfusion pressure (CPP). Devices allowing direct assessment of brain tissue oxygenation have showed promising results in clinical studies, and their use was implemented in the Brain Trauma Foundation Guidelines for the treatment of TBI patients in 2007. Results of several studies suggest that a brain tissue oxygen-directed therapy guided by these monitors may contribute to reduced mortality and improved outcome of TBI patients. Whether increasing the oxygen supply to supraphysiological levels has beneficial or detrimental effects on TBI patients has been a matter of debate for decades. The results of trials of hyperbaric oxygenation (HBO) have failed to show a benefit, but renewed interest in normobaric hyperoxia (NBO) in the treatment of TBI patients has emerged in recent years. With the increased availability of advanced neuromonitoring devices such as brain tissue oxygen monitors, it was shown that some patients might benefit from this therapeutic approach. In this article, we review the pathophysiological rationale and technical modalities of brain tissue oxygen monitors, as well as its use in studies of brain tissue oxygen-directed therapy. Furthermore, we analyze hyperoxia as a treatment option in TBI patients, summarize the results of clinical trials, and give insights into the recent findings of hyperoxic effects on cerebral metabolism after TBI.

[Brain tissue oxygen pressure, for what, for whom?]

The main purpose of neurointensive care is to fight against cerebral ischaemia. Ischaemia is the cell energy failure following inadequacy between supply of glucose and oxygen and demand. Ischemia monitoring starts with a global approach, especially with cerebral perfusion pressure (CPP) determined by mean arterial pressure and intracranial pressure (ICP). However, global monitoring is insufficient to detect "regional" ischaemia, leading to development of local monitoring such as brain oxygen partial pressure (PtiO(2)). PtiO(2) is measured on a volume of a few mm(3) from a probe implanted in the cerebral tissue. The normal value is classically included between 25 and 35 mmHg and critical ischemic threshold is 10 mmHg. Understanding what exactly is PtiO(2) is still a matter of debate. PtiO(2) is more an indicator of oxygen diffusion depending of oxygen arterial pressure (PaO(2)) and local cerebral blood flow (CBF). Increase PaO(2) to treat PtiO(2) would hide information about local CBF. PtiO(2) is useful for the detection of low local CBF even when ICP is low as in hypocapnia-induced vasoconstriction. PtiO(2)-guided management could lead to a continuous optimization of arterial oxygen transport for an optimal cerebral tissue oxygenation. Finally, PtiO(2) has probably a global prognostic value because studies showed that hypoxic values for a long period of time lead to an unfavourable neurologic outcome. In conclusion, PtiO(2) provides additional information for regional monitoring of cerebral ischaemia and deserves more intensive use to better understand it and probably improve neurointensive care management.

Multimodal intracranial monitoring: implications for clinical practice

This article presents an overview of intracranial monitoring techniques during the perioperative and intensive care management of neurologic patients. Various regional and global brain monitors are available; some modalities are well established whereas others are new to the clinical arena and their indications are still being evaluated. Indications for monitoring are reviewed, modalities critically evaluated, and future directions identified.

Microenvironment changes in mild traumatic brain injury

Traumatic brain injury (TBI) is a major public-health problem for which mild TBI (MTBI) makes up majority of the cases. MTBI is a poorly-understood health problem and can persist for years manifesting into neurological and non-neurological problems that can affect functional outcome. Presently, diagnosis of MTBI is based on symptoms reporting with poor understanding of ongoing pathophysiology, hence precluding prognosis and intervention. Other than rehabilitation, there is still no pharmacological treatment for the treatment of secondary injury and prevention of the development of cognitive and behavioural problems. The lack of external injuries and absence of detectable brain abnormalities lend support to MTBI developing at the cellular and biochemical level. However, the paucity of suitable and validated non-invasive methods for accurate diagnosis of MTBI poses as a substantial challenge. Hence, it is crucial that a clinically useful evaluation and management procedure be instituted for MTBI that encompasses both molecular pathophysiology and functional outcome. The acute microenvironment changes post-MTBI presents an attractive target for modulation of MTBI symptoms and the development of cognitive changes later in life.

A method for measuring brain partial pressure of oxygen in unanesthetized unrestrained subjects: the effect of acute and chronic hypoxia on brain tissue PO(2)

The level of tissue oxygenation provides information related to the balance between oxygen delivery, oxygen utilization, tissue reactivity and morphology during physiological conditions. Tissue partial pressure of oxygen (PtO(2)) is influenced by the use of anesthesia or restraint. These factors may impact the absolute level of PtO(2). In this study we present a novel fiber optic method to measure brain PtO(2). This method can be used in unanesthetized, unrestrained animals, provides absolute values for PO(2), has a stable calibration, does not consume oxygen and is MRI compatible. Brain PtO(2) was studied during acute hypoxia, as well as before and after 28 days of high altitude acclimatization. A sensor was chronically implanted in the frontal cortex of eight Wistar rats. It is comprised of a fiber optic probe with a tip containing material that fluoresces with an oxygen dependent lifetime. Brain PtO(2) declines by 80% and 76% pre- and post-acclimatization, respectively, when the fraction of inspired oxygen declines from 0.21 to 0.08. In addition, a linear relationship between brain PtO(2) and inspired O(2) levels was demonstrated r(2)=0.98 and r(2)=0.99 (pre- and post-acclimatization). Hypoxia acclimatization resulted in an increase in the overall brain PtO(2) by approximately 35%. This paper demonstrates the use of a novel chronically implanted fiber optic based sensor for measuring absolute PtO(2). It shows a very strong linear relationship in awake animals between inspired O(2) and tissue O(2), and shows that there is a proportional increase in PtO(2) over a range of inspired values after exposure to chronic hypoxia.

PET Imaging for Traumatic Brain Injury

Traumatic brain injury represents a substantial public health problem for which clinicians have limited treatment avenues. Traditional FDG-positron emission tomography (PET) brain imaging has provided unique insights into this disease including prognostic information. With the advent and implementation of novel tracers as well as improvement in instrumentation, molecular brain imaging using PET can further illustrate traumatic brain injury pathophysiology and point to novel treatment strategies.

The effect of increased inspired fraction of oxygen on brain tissue oxygen tension in children with severe traumatic brain injury

BACKGROUND

This study examines the effect of an increase in the inspired fraction of oxygen (FiO2) on brain tissue oxygen (PbO2) in children with severe traumatic brain injury (TBI).

METHODS

A prospective observational study of patients who underwent PbO2 monitoring and an oxygen challenge test (temporary increase of FiO2 for 15 min) was undertaken. Pre- and post-test values for arterial partial pressure of oxygen (PaO2), PbO2, and arterial oxygen content (CaO2) were examined while controlling for any changes in arterial carbon dioxide tension and cerebral perfusion pressure during the test. Baseline transcranial Doppler studies were done. Outcome was assessed at 6 months.

RESULTS

A total of 43 tests were performed in 28 patients. In 35 tests in 24 patients, the PbO2 monitor was in normal-appearing white matter and in eight tests in four patients, the monitor was in a pericontusional location. When catheters were pericontusional or in normal white matter the baseline PbO2/PaO2 ratio was similar. PaO2 (P < 0.0001) and PbO2 (P < 0.0001) significantly increased when FiO2 was increased. The magnitude of the PbO2 response (PbO2) was correlated with PaO2 (P < 0.0001, R(2) = 0.37) and CaO2 (P = 0.001, R(2) = 0.23). The PbO2/PaO2 ratio (oxygen reactivity) varied between patients, was related to the baseline PbO2 (P = 0.001, r = 0.54) and was inversely related to outcome (P = 0.02, confidence interval 0.03-0.78).

CONCLUSION

Normobaric hyperoxia increases PbO2 in children with severe TBI, but the response is variable. The magnitude of this response is related to the change in PaO2 and the baseline PbO2. A greater response appears to be associated with worse outcome.

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.

Periprocedural MRI perfusion imaging to assess and monitor the hemodynamic impact of intracranial angioplasty and stenting for symptomatic atherosclerotic stenosis

We aimed to assess the clinical value of MRI perfusion imaging in the periprocedural management of intracranial atherosclerosis, analyzing if changes in mean transit time (MTT), cerebral blood volume (CBV) and cerebral blood flow (CBF) correlated with angiographic outcomes. Pre-procedural and post-procedural MRI perfusion was performed on six patients who underwent angioplasty and/or stenting for symptomatic intracranial atherosclerosis. MTT, CBV and CBF were analyzed and graded. In 83% of patients, perfusion imaging correlated with angiographic outcomes. Perfusion parameters improved to normal in two patients. Two showed marked improvement and one showed mild improvement. In one patient, the results of the post-procedural MRI perfusion prompted an angiogram, which confirmed stent occlusion. Semi-quantitative scores of MTT and CBF changed over time (p=0.05, p=0.03) whereas CBV did not change significantly (p>0.05). We conclude that MRI perfusion appears a promising technique for analyzing the impact of intracranial stenosis on cerebral hemodynamics before and after treatment.

Management guided by brain tissue oxygen monitoring and outcome following severe traumatic brain injury

Object

The authors sought to describe changes in clinical management associated with brain tissue oxygen (PbO2) monitoring and how these changes affected outcomes and resource utilization.

Methods

The cohort study comprised 629 patients admitted to a Level I trauma center with a diagnosis of severe traumatic brain injury over a period of 3 years. Hospital mortality rate, neurological outcome, and resource utilization of 123 patients who underwent both PbO2 and intracranial pressure (ICP) monitoring were compared with the same measures in 506 patients who underwent ICP monitoring only. The main outcomes were hospital mortality rate, functional independence at hospital discharge, duration of mechanical ventilation, hospital length of stay, and hospital cost. Multivariable regression with robust variance was used to estimate the adjusted differences in the main outcome measures between patient groups. The models were adjusted for patient age, severity of injury, and pathological features seen on head CT scan at admission.

Results

On average, patients who underwent ICP/PbO2 monitoring were younger and had more severe injuries than patients who received ICP monitoring alone. Relatively more patients treated with PbO2 monitoring received osmotic therapy, vasopressors, and prolonged sedation. After adjustment for baseline characteristics, the hospital mortality rate was, if anything, slightly higher in patients undergoing PbO2-guided management than in patients monitored with ICP only (adjusted mortality difference 4.4%, 95% CI −3.9 to 13%). Patients who underwent PbO2-guided management also had lower adjusted functional independence scores at hospital discharge (adjusted score difference −0.75, 95% CI −1.41 to −0.09). There was a 27% relative increase (95% CI 6–53%) in the median hospital length of stay when the PbO2 group was compared with the ICP-only group.

Conclusions

The mortality rate in patients with traumatic brain injury whose clinical management was guided by PbO2 monitoring was not reduced in comparison with that in patients who received ICP monitoring alone. Brain tissue oxygen monitoring was associated with worse neurological outcome and increased hospital resource utilization.

Physiologic and functional outcome correlates of brain tissue hypoxia in traumatic brain injury

OBJECTIVE

Assess the prevalence of brain tissue hypoxia in patients with severe traumatic brain injuries (TBI), and to characterize the relationship between brain tissue hypoxia and functional outcome.

DESIGN

Retrospective review of severe TBI patients.

SETTING

Intensive care unit of a level I trauma center.

PATIENTS

Twenty-seven patients with severe TBI requiring intracranial pressure (ICP) monitoring. Median age was 22 yrs, and a majority (63%) had traumatic subarachnoid hemorrhage.

INTERVENTIONS

Hourly assessments of ICP, brain tissue oxygen, mean arterial pressure, fraction of inspired oxygen (FiO2), partial pressure of arterial carbon dioxide (PaCO2), and hemoglobin concentration (hemoglobin) were recorded. Outcome was assessed 6-9 months postinjury.

MEASUREMENTS AND MAIN RESULTS

Mean (SD) ICP and BTpO2 were 13.7 (6.6) cm H2O and 30.8 (13.6) mm Hg. A total of 13.5% (379) of the BTpO2 values recorded were < 20 mm Hg, only 86 of which were associated with ICP > or = 20 cm H2O. This prevalence was comparable with episodes of ICP elevations above 20 cm H2O (14.1%, 397). Hypoxic episodes were more common when cerebral perfusion pressure was below 60 mm Hg (relative risk = 3.0, p < 0.0001). We did not find an association in hypoxic risk and hemoglobin in the range of 7-12 g/dL or PaCO2 in the range of 25-40 mm Hg. Subjects with hourly episodes (epochs) of hypoxia > 20% of the time had poorer scores on outcome measures compared with those with fewer hypoxic epochs.

CONCLUSIONS

Hypoxic episodes are common after severe TBI, and most are independent of ICP elevations. Most episodes of hypoxia occur while cerebral perfusion pressure and mean arterial pressure are within the accepted target range. There is no clear association between PaCO2 and hemoglobin with BTpO2. The young age and high prevalence of traumatic subarachnoid hemorrhage in this cohort may limit its generalizability. Increased frequency of hypoxic episodes is associated with poor functional outcome.

Perioperative uses of transcranial perfusion monitoring

Transcranial perfusion monitoring provides early warning of impending brain ischemia and may be used to guide management of cerebral perfusion and oxygenation. The monitoring options include measurement of intracranial and cerebral perfusion pressures, assessment of cerebral blood flow, and assessment of the adequacy of perfusion by measurement of cerebral oxygenation and brain tissue biochemistry. Some monitoring techniques are well established, whereas others are relatively new to the clinical arena and their indications are still being evaluated. Currently available monitoring techniques are reviewed and their appropriateness and application to the perioperative period is discussed.

NICEM consensus on neurological monitoring in acute neurological disease

This manuscript summarises the consensus on neuromonitoring in neuro-intensive care promoted and organised by the Neuro-Intensive Care and Emergency Medicine (NICEM) Section of the European Society of Intensive Care Medicine (ESICM). It is expected that continuous monitoring using multi-modal techniques will help to overcome the limitations of each individual method and will provide a better diagnosis. More specific treatment can then be applied; however, it remains to be determined which combination of parameters is optimal. The questions discussed and addressed in this manuscript are: (1) Who should have ICP monitoring and for how long? (2) What ICP technologies are available and what are their relative advantages/disadvantages? (3) Should CPP monitoring and autoregulation testing be used? (4) When should brain tissue oxygen tension (PbrO(2)) be monitored? (5) Should structurally normal or abnormal tissue be monitored with PbrO(2)? (6) Should microdialysis be considered in complex cases? It is hoped that this document will prove useful to clinicians working in NICU and also to those developing specialist NICU services within their hospital practice.

Applications of nitroimidazole in vivo hypoxia imaging in ischemic stroke

BACKGROUND AND PURPOSE

Nitroimidazole imaging is a promising contender for noninvasive in vivo mapping of brain hypoxia after stroke. However, there is a dearth of knowledge about the behavior of these compounds in the various pathophysiologic situations encountered in ischemic stroke. In this article we report the findings from a systematic review of the literature on the use of the nitroimidazoles to map hypoxia after stroke.

SUMMARY OF REVIEW

We describe the characteristics of nitroimidazoles as imaging tracers, their pharmacology, and results of both animal and clinical studies during and after focal cerebral ischemia. Findings in brain tumors are also presented to the extent that they enlighten results in stroke. Early results from application of kinetic modeling for quantitative measurement of tracer binding are briefly discussed.

CONCLUSIONS

Based on this literature review, nitroimidazole hypoxia imaging agents are of considerable interest in stroke because they appear, both in animal models and in humans, to specifically detect the severely hypoxic viable tissue, but not the reperfused nor the necrotic tissue. To fully realize this potential in stroke, however, formal validation by concurrent measurement of tissue oxygen tension, together with development of novel ligands with faster distribution kinetics, faster clearance from normal tissue, and well-defined trapping mechanisms, are important goals for future investigations.

Multimodality monitoring in neurocritical care

Multimodality monitoring of cerebral physiology encompasses the application of different monitoring techniques and integration of several measured physiologic and biochemical variables into assessment of brain metabolism, structure, perfusion, and oxygenation status. Novel monitoring techniques include transcranial Doppler ultrasonography, neuroimaging, intracranial pressure, cerebral perfusion, and cerebral blood flow monitors, brain tissue oxygen tension monitoring, microdialysis, evoked potentials, and continuous electroencephalogram. Multimodality monitoring enables immediate detection and prevention of acute neurologic injury as well as appropriate intervention based on patients' individual disease states in the neurocritical care unit. Real-time analysis of cerebral physiologic, metabolic, and cardiovascular parameters simultaneously has broadened knowledge about complex brain pathophysiology and cerebral hemodynamics. Integration of this information allows for more precise diagnosis and optimization of management of patients with brain injury.

Molecular imaging: reporter gene imaging

Non-invasive in-vivo molecular genetic imaging developed over the past decade and predominantly utilises radiotracer (PET, gamma camera, autoradiography), magnetic resonance and optical imaging technology. Molecular genetic imaging has its roots in both molecular biology and cell biology. The convergence of these disciplines and imaging modalities has provided the opportunity to address new research questions, including oncogenesis, tumour maintenance and progression, as well as responses to molecular-targeted therapy. Three different imaging strategies are described: (1) "bio-marker" or "surrogate" imaging; (2) "direct" imaging of specific molecules and pathway activity; (3) "indirect" reporter gene imaging. Examples of each imaging strategy are presented and discussed. Several applications of PET- and optical-based reporter imaging are demonstrated, including signal transduction pathway monitoring, oncogenesis in genetic mouse models, endogenous molecular genetic/biological processes and the response to therapy in animal models of human disease. Molecular imaging studies will compliment established ex-vivo molecular-biological assays that require tissue sampling by providing a spatial and a temporal dimension to our understanding of disease development and progression, as well as response to treatment. Although molecular imaging studies are currently being performed primarily in experimental animals, we optimistically expect they will be translated to human subjects with cancer and other diseases in the near future.

Brain tissue oxygen tension response to induced hyperoxia reduced in hypoperfused brain

Object

Increasing PaO2 can increase brain tissue PO2 (PbtO2). Nevertheless, the small increase in arterial O2 content induced by hyperoxia does not increase O2 delivery much, especially when cerebral blood flow (CBF) is low, and the effectiveness of hyperoxia as a therapeutic intervention remains controversial. The purpose of this study was to examine the role of regional (r)CBF at the site of the PO2 probe in determining the response of PbtO2 to induced hyperoxia.

Methods

The authors measured PaO2 and PbtO2 at baseline normoxic conditions and after increasing inspired O2 concentration to 100% on 111 occasions in 83 patients with severe traumatic brain injury in whom a stable xenon–enhanced computed tomography measurement of CBF was available. The O2 reactivity was calculated as the change in PbtO2 × 100/change in PaO2.

Results

The O2 reactivity was significantly different (p < 0.001) at the 5 levels of rCBF (<10, 11–15, 16–20, 21–40, and > 40 ml/100 g/min). When rCBF was < 20 ml/100 g/min, the increase in PbtO2 induced by hyperoxia was very small compared with the increase that occurred when rCBF was > 20 ml/100 g/min.

Conclusions

Although the level of CBF is probably only one of the factors that determines the PbtO2 response to hyperoxia, it is apparent from these results that the areas of the brain that would most likely benefit from improved oxygenation are the areas that are the least likely to have increased PbtO2.

Barbiturate therapy for patients with refractory intracranial hypertension following severe traumatic brain injury: its effects on tissue oxygenation, brain temperature and autoregulation

The aim of this study was to explore the effects of barbiturate coma on cerebral tissue oxygen tension and cerebrovascular pressure reactivity (PRx), as an index of cerebral autoregulation in severe head injury patients. This was a prospective observational clinical study of 12 patients with severe traumatic brain injury, carried out at a tertiary-level neurosurgical intensive care unit between April 2002 and May 2005. All patients received standard neurosurgical intensive care and monitoring. Probes for intracranial pressure (ICP), brain temperature (BT) and brain tissue oxygenation (PTiO2) were inserted into (noncontused) normal-looking white matter. Cerebrovascular PRx was measured as a moving correlation between ICP and arterial blood pressure. Barbiturate coma was instituted when ICP became refractory (ICP>20 mmHg). All data from the multimodal monitoring were digitally extracted and statistically analysed. The mean ICP decreased with barbiturate coma in eight of the 12 patients (75% of the patients), but only four achieved a value below 20 mmHg. Of eight patients with prebarbiturate PTiO2 levels above 10 mmHg, six had a further improvement in oxygenation. Thus, concordant favourable changes in ICP, PRx and PTiO2 with barbiturate coma were seen in those who survived. Effective response to barbiturates can be detected by improved PTiO2 and autoregulation (PRx) in severe head injury patients.

A comparative evaluation of EPR and OxyLite oximetry using a random sampling of pO(2) in a murine tumor

Methods currently available for the measurement of oxygen concentrations (oximetry) in viable tissues differ widely from each other in their methodological basis and applicability. The goal of this study was to compare two novel methods, particulate-based electron paramagnetic resonance (EPR) and OxyLite oximetry, in an experimental tumor model. EPR oximetry uses implantable paramagnetic particulates, whereas OxyLite uses fluorescent probes affixed on a fiber-optic cable. C3H mice were transplanted with radiation-induced fibrosarcoma (RIF-1) tumors in their hind limbs. Lithium phthalocyanine (LiPc) microcrystals were used as EPR probes. The pO(2) measurements were taken from random locations at a depth of approximately 3 mm within the tumor either immediately or 48 h after implantation of LiPc. Both methods revealed significant hypoxia in the tumor. However, there were striking differences between the EPR and OxyLite readings. The differences were attributed to the volume of tissue under examination and the effect of needle invasion at the site of measurement. This study recognizes the unique benefits of EPR oximetry in terms of robustness, repeatability and minimal invasiveness.

Perioperative uses of transcranial perfusion monitoring

Transcranial perfusion monitoring provides early warning of impending brain ischemia and may be used to guide management of cerebral perfusion and oxygenation. The monitoring options include measurement of intracranial and cerebral perfusion pressures, assessment of cerebral blood flow, and assessment of the adequacy of perfusion by measurement of cerebral oxygenation and brain tissue biochemistry. Some monitoring techniques are well established, whereas others are relatively new to the clinical arena and their indications are still being evaluated. Currently available monitoring techniques are reviewed and their appropriateness and application to the perioperative period is discussed.

Multimodal monitoring in traumatic brain injury: current status and future directions

Traumatic brain injury (TBI) remains a major cause of morbidity and mortality, particularly in young people. Despite encouraging animal studies, human trials assessing the use of pharmacological agents after TBI have all failed to show efficacy. Current management strategies are therefore directed towards providing an optimal physiological environment in order to minimize secondary insults and maximize the body's own regenerative processes. Modern neurocritical care management utilizes a host of monitoring techniques to identify or predict the occurrence of secondary insults and guide subsequent therapeutic interventions in an attempt to minimize the resulting secondary injury. Recent data suggest that the use of protocolized management strategies, informed by multimodality monitoring, can improve patient outcome after TBI. Developments in multimodality monitoring have allowed a movement away from rigid physiological target setting towards an individually tailored, patient-specific, approach. The wealth of monitoring information available provides a challenge in terms of data integration and accessibility and modern software applications may aid this process.

Multimodality monitoring in severe head injury

PURPOSE OF REVIEW

This review describes recent advances in multimodal neuromonitoring of patients following severe head injury during the period from 2001 to 2002.

RECENT FINDINGS

Monitoring intracranial pressure is considered a standard part of therapy despite a lack of randomized studies comparing patients with and without intracranial pressure monitoring. Jugular oximetry and brain tissue oxygen pressure monitoring are being used more frequently as part of a treatment protocol. Intracerebral microdialysis, despite the widespread use as a research tool, still cannot be considered a standard in clinical monitoring. These new monitoring devices may provide useful insight into the evolution of brain injury.

SUMMARY

Technology is rapidly changing the nature of neuromonitoring. New devices are becoming available which make the monitoring truly multimodal. Studies are needed to determine how to best incorporate these new parameters into effective management protocols.

Neuromonitoring in the intensive care unit. II. Cerebral oxygenation monitoring and microdialysis

BACKGROUND

Monitoring the injured brain is an integral part of the management of severely brain injured patients in intensive care. There is increasing interest in methods to monitor global and regional cerebral oxygenation. There have been significant advances in analysing tissue oxygenation and local metabolites in the injured brain over the past decade.

DISCUSSION

Cerebral oxygenation can be assessed on a global or regional basis by jugular venous oximetry and near infra-red spectroscopy respectively. Techniques of brain tissue oxygenation monitoring and microdialysis are also covered in this review.

CONCLUSIONS

Various modalities are available to monitor oxygenation and the local milieu in the injured brain in the intensive care unit. Use of these modalities helps to optimise brain oxygen delivery and metabolism in patients with acute brain injury.

Future of brain support: multimodality monitoring

Multimodality monitoring of cerebral physiology encompasses the application of different monitoring techniques and integration of several measured physiological and biochemical variables into the assessment of brain metabolism, structure, perfusion and oxygenation status, in addition to clinical evaluation. Novel monitoring techniques include transcranial Doppler ultrasonography, neuroimaging, intracranial pressure, cerebral perfusion and cerebral blood flow monitors, brain tissue oxygen tension monitoring, microdialysis, evoked potentials and continuous electroencephalography. Multimodality monitoring enables the immediate detection and prevention of acute neurological events, as well as appropriate intervention based on a patient’s individual disease state in the neurocritical care unit. Simultaneous real-time analysis of cerebral physiological, metabolic and cardiovascular parameters has broadened knowledge regarding complex brain pathophysiology and cerebral hemodynamics. Integration of this information allows for a more precise diagnosis and optimization of management of patients with brain injury.

The role of tissue oxygen monitoring in patients with acute brain injury

Cerebral ischaemia is implicated in poor outcome after brain injury, and is a very common post-mortem finding. The inability of the brain to store metabolic substrates, in the face of high oxygen and glucose requirements, makes it very susceptible to ischaemic damage. The clinical challenge, however, remains the reliable antemortem detection and treatment of ischaemic episodes in the intensive care unit. Outcomes have improved in the traumatic brain injury setting after the introduction of progressive protocol-driven therapy, based, primarily, on the monitoring and control of intracranial pressure, and the maintenance of an adequate cerebral perfusion pressure through manipulation of the mean arterial pressure. With the increasing use of multi-modal monitoring, the complex pathophysiology of the injured brain is slowly being unravelled, emphasizing the heterogeneity of the condition, and the requirement for individualization of therapy to prevent secondary adverse hypoxic cerebral events. Brain tissue oxygen partial pressure (Pb(O2) monitoring is emerging as a clinically useful modality, and this review examines its role in the management of brain injury.

Glucose metabolism in traumatic brain injury: a combined microdialysis and [18F]-2-fluoro-2-deoxy-D-glucose-positron emission tomography (FDG-PET) study

Following traumatic brain injury, as a consequence of ionic disturbances and neurochemical cascades, glucose metabolism is affected. [18F]-2-Fluoro-2-deoxy-D-glucose (FDG) Positron Emission Tomography (FDG-PET) provides a measure of global and regional cerebral metabolic rate of glucose (rCMRglc), but only during the time of the scan. Microdialysis monitors energy metabolites over extended time periods, but only in a small focal volume of the brain. Our objective in this study is to assess the association of parameters derived from these techniques when applied to patients with traumatic brain injury. Eleven sedated, ventilated patients receiving intracranial pressure monitoring and managed using Addenbrooke's Neurosciences Critical Care Unit protocols were monitored. Dialysate values for glucose, lactate, pyruvate, and glutamate, and the lactate to glucose (L/G), lactate to pyruvate (L/P) and pyruvate to glucose (P/G) ratios were determined and correlated with rCMRglc. FDG-PET scans were performed within 24 hours (five patients), or between 1 and 4 days (two patients) or after 4 days (six patients). Two patients were rescanned 4 and 7 days after their initial scan. A 20 mm region of interest (ROI) was defined on co-registered CT scan on two contiguous slices around the microdialysis catheter. Mean (+/-sd) for rCMRglc was 19.1 +/- 5.5 micromol/100 g/min, and the corresponding microdialysis values were: glucose 1.4 +/- 1.4 mmol/ L; lactate 5.3 +/- 3.6 mmol/L; pyruvate 164.1 +/- 142.3 micromol/L; glutamate 15.0 +/- 14.7 micromol/L; L/G 11.0 +/- 16.0; L/P 27.3 +/- 7.9 and P/G 381 +/- 660. There were significant relations between rCMRglc and dialysate lactate (r = 0.58, P = 0.04); pyruvate (r = 0.57, P = 0.04), L/G (r = 0.55, P = 0.05), and the P/G (r = 0.56, P = 0.05) but not between rCMRglc and dialysate glucose, L/P or glutamate in this data set. The results suggest that increases in glucose utilization as assessed by FDG-PET in these patients albeit in mainly healthy tissue are associated with increases in dialysate lactate, pyruvate, L/G and the P/G ratio perhaps indicating a general rise in metabolism rather than a shift towards non-oxidative metabolism. Further observations are required with regions of interest (microdialysis catheters positioned) adjacent to mass lesions notably contusions.

Brain ischaemia after traumatic brain injury: lessons from 15O2 positron emission tomography

PURPOSE OF REVIEW

To describe the role of O2 positron emission tomography in studies aimed at understanding ischaemia in head injury. It has been difficult to use cerebral blood flow levels to provide a secure definition of cerebral ischaemia in head injury, since primary changes in cerebral metabolism may be responsible for coupled reductions in cerebral blood flow. Further, regional heterogeneity of pathophysiology can confound global measures of adequacy of cerebral oxygen delivery. There is a need for a technique that can provide a comprehensive and quantitative description of cerebral physiology in this setting.

RECENT FINDINGS

O2 positron emission tomography can image cerebral blood flow, cerebral blood volume, cerebral metabolic rate for oxygen and oxygen extraction fraction, and thus allows a robust and specific definition of true ischaemia. When used in combination with other monitoring tools and imaging modalities, positron emission tomography has also been used to validate and refine bedside monitors of cerebrovascular physiology, study the impact of therapeutic interventions and provide clues to novel pathophysiology.

SUMMARY

There is a clear role for O2 positron emission tomography in elucidating pathophysiology in head injury. The technique may provide most information when combined with other imaging and monitoring tools.

Relative importance of hypertension compared with hypervolemia for increasing cerebral oxygenation in patients with cerebral vasospasm after subarachnoid hemorrhage

Object. Hypervolemia and hypertension therapy is routinely used for prophylaxis and treatment of symptomatic cerebral vasospasm at many institutions. Nevertheless, there is an ongoing debate about the preferred modality (hypervolemia, hypertension, or both), the degree of therapy (moderate or aggressive), and the risk or benefit of hypervolemia, moderate hypertension, and aggressive hypertension in patients following subarachnoid hemorrhage.

Methods. Monitoring data and patient charts for 45 patients were retrospectively searched to identify periods of hypervolemia, moderate hypertension, or aggressive hypertension. Measurements of central venous pressure, fluid input, urine output, arterial blood pressure, intracranial pressure, and oxygen partial pressure (PO2) in the brain tissue were extracted from periods ranging from 1 hour to 24 hours. For these periods, the change in brain tissue PO2 and the incidence of complications were analyzed.

During the 55 periods of moderate hypertension, an increase in brain tissue PO2 was found in 50 cases (90%), with complications occurring in three patients (8%). During the 25 periods of hypervolemia, an increase in brain oxygenation was found during three intervals (12%), with complications occurring in nine patients (53%). During the 10 periods of aggressive hypervolemic hypertension, an increase in brain oxygenation was found during six of the intervals (60%), with complications in five patients (50%).

Conclusions. When hypervolemia treatment is applied as in this study, it may be associated with increased risks. Note, however, that further studies are needed to determine the role of this therapeutic modality in the care of patients with cerebral vasospasm. In poor-grade patients, moderate hypertension (cerebral perfusion pressure 80–120 mm Hg) in a normovolemic, hemodiluted patient is an effective method of improving cerebral oxygenation and is associated with a lower complication rate compared with hypervolemia or aggressive hypertension therapy.

Effect of cerebral perfusion pressure augmentation on regional oxygenation and metabolism after head injury*

OBJECTIVE

In this study we have used O positron emission tomography, brain tissue oxygen monitoring, and cerebral microdialysis to assess the effects of cerebral perfusion pressure augmentation on regional physiology and metabolism in the setting of traumatic brain injury.

DESIGN

Prospective interventional study.

SETTING

Neurosciences critical care unit of a university hospital.

PATIENTS

Eleven acutely head-injured patients requiring norepinephrine to maintain cerebral perfusion pressure.

INTERVENTIONS

Using positron emission tomography, we have quantified the response to an increase in cerebral perfusion pressure in a region of interest around a brain tissue oxygen sensor (Neurotrend) and microdialysis catheter. Oxygen extraction fraction and cerebral blood flow were measured with positron emission tomography at a cerebral perfusion pressure of approximately 70 mm Hg and approximately 90 mm Hg using norepinephrine to control cerebral perfusion pressure. All other aspects of physiology were kept stable.

MEASUREMENTS AND MAIN RESULTS

Cerebral perfusion pressure augmentation resulted in a significant increase in brain tissue oxygen (17 +/- 8 vs. 22 +/- 8 mm Hg; 2.2 +/- 1.0 vs. 2.9 +/- 1.0 kPa, p < .001) and cerebral blood flow (27.5 +/- 5.1 vs. 29.7 +/- 6.0 mL/100 mL/min, p < .05) and a significant decrease in oxygen extraction fraction (33.4 +/- 5.9 vs. 30.3 +/- 4.6 %, p < .05). There were no significant changes in any of the microdialysis variables (glucose, lactate, pyruvate, lactate/pyruvate ratio, glycerol). There was a significant linear relationship between brain tissue oxygen and oxygen extraction fraction (r = .21, p < .05); the brain tissue oxygen value associated with an oxygen extraction fraction of 40% (the mean value for oxygen extraction fraction in normal controls) was 14 mm Hg (1.8 kPa). The cerebral perfusion pressure intervention resulted in a greater percentage increase in brain tissue oxygen than the percentage decrease in oxygen extraction fraction; this suggests that the oxygen gradients between the vascular and tissue compartments were reduced by the cerebral perfusion pressure intervention.

CONCLUSIONS

Cerebral perfusion pressure augmentation significantly increased levels of brain tissue oxygen and significantly reduced regional oxygen extraction fraction. However, these changes did not translate into predictable changes in regional chemistry. Our results suggest that the ischemic level of brain tissue oxygen may lie at a level below 14 mm Hg (1.8 kPa); however, the data do not allow us to be more specific.

Brain tissue oxygenation monitoring in acute brain injury

Cerebral ischemia is one of the most important causes of secondary insults following acute brain injury. While intracranial pressure monitoring in the intensive care unit constitutes the cornerstone of neurocritical care monitoring, it does not reflect the state of oxygenation of the injured brain. The holy grail of neuromonitoring is a modality that would reflect accurately real time the status of oxygenation in the tissue of interest, is robust, artefact free and that which provides information that can be used for therapeutic interventions and to improve outcome. Such a device could conceivably be used to augment the sensitivity of current multi-modality monitoring systems in the neurocritical management of brain injured patients. This article examines the availability of data in the literature to support clinical use of local tissue oxygen probes in intensive care.