Incidence and mechanisms of cerebral ischemia in early clinical head injury

Neuromonitoring for Traumatic Brain Injury in Neurosurgical Intensive Care

The primary aim of neuromonitoring in patients with traumatic brain injury is early detection of secondary brain insults so that timely interventions can be instituted to prevent or treat secondary brain injury. Intracranial pressure monitoring has been a stalwart in neuromonitoring and is still very much the main parameter to guide therapy in brain injured patients in many centres. Cerebral oxygenation is also established as an important parameter for monitoring: global cerebral oxygenation is reliably measured using jugular venous oxygen saturation while brain tissue oxygen tension measurement allows focal brain oxygenation to be monitored. Near-infrared spectroscopy allows a non-invasive option for monitoring of regional cerebral oxygenation. Cerebral microdialysis makes focal measurements of markers of cellular metabolism and cellular injury and death possible, and it is in transition from being a research tool to being an important clinical tool in neuromonitoring. Multimodal monitoring allows different parameters of brain physiology and function to be monitored and can improve identification and prediction of secondary cerebral insults. Multimodal monitoring can potentially improve outcomes in patients with traumatic brain injury by promoting customised treatment strategies for individual patients in place of the commonplace practice of strict adherence to achieving the same standard physiological targets for every patient.

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.

Practical aspects of bedside cerebral hemodynamics monitoring in pediatric TBI

INTRODUCTION

Disturbances in cerebral hemodynamics may have a profound influence on secondary injury after traumatic brain injury (TBI), and many therapies in the neurocritical care unit may adversely affect cerebral blood flow. However, the clinician is often unaware of this when it occurs because practical methods for monitoring cerebral hemodynamics by the bedside have been lacking. Current imaging studies only provide a snapshot of the brain at one point in time, giving limited information about a dynamic condition.

DISCUSSION

This review will focus on key pathophysiological concepts required to understand changes in cerebral hemodynamics after TBI and the principles, potential benefits, and limitations of currently available bedside monitoring techniques, including transcranial Doppler, autoregulation, and local/regional cerebral blood flow.

Intracranial pressure monitoring for traumatic brain injury in the modern era

INTRODUCTION

Intracranial pressure (ICP) has become a cornerstone of care in adult and pediatric patients with traumatic brain injury (TBI).

DISCUSSION

Despite the fact that continuous monitoring of ICP in TBI was described almost 60 years ago, there are no randomized trials confirming the benefit of ICP monitoring and treatment in TBI. There is, however, a large body of clinical evidence showing that ICP monitoring influences treatment and leads to better outcomes if part of protocol-driven therapy. However, treatment of ICP has adverse effects, and there are several questions about ICP management that have yet to be definitively answered, particularly in pediatric TBI. This review examines the history and evolution of ICP monitoring, pathophysiological concepts that influence ICP interpretation, ongoing controversies, and the place of ICP monitoring in modern neurocritical care.

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.

Spreading depolarizations and late secondary insults after traumatic brain injury

Here we investigated the incidence of cortical spreading depolarizations (spreading depression and peri-infarct depolarization) after traumatic brain injury (TBI) and their relationship to systemic physiologic values during neurointensive care. Subdural electrode strips were placed on peri-contusional cortex in 32 patients who underwent surgical treatment for TBI. Prospective electrocorticography was performed during neurointensive care with retrospective analysis of hourly nursing chart data. Recordings were 84 hr (median) per patient and 2,503 hr in total. In 17 patients (53%), 280 spreading depolarizations (spreading depressions and peri-infarct depolarizations) were observed. Depolarizations occurred in a bimodal pattern with peak incidence on days 1 and 7. The probability of a depolarization occurring increased significantly as a function of declining mean arterial pressure (MAP; R(2) = 0.78; p < 0.001) and cerebral perfusion pressure (R(2) = 0.85; p < 0.01), and increasing core temperature (R(2) = 0.44; p < 0.05). Depolarization probability was 7% for MAP values of >100 mm Hg but 33% for MAP of < or =70 mm Hg. Temperatures of < or =38.4 degrees C were associated with a 21% depolarization risk, compared to 63% for >38.4 degrees C. Intracranial pressures were higher in patients with depolarizations (18.3 +/- 9.3 vs. 13.5 +/- 6.7 mm Hg; p < 0.001). We conclude that depolarization phenomena are a common cortical pathology in TBI. Their association with lower perfusion levels and higher temperatures suggests that the labile balance of energy supply and demand is an important determinant of their occurrence. Monitoring of depolarizations might serve as a functional measure to guide therapeutic efforts and their blockade may provide an additional line of defense against the effects of secondary insults.

Hemodynamic and histological effects of traumatic brain injury in eNOS-deficient mice

Microvascular dysfunction in the brain, characterized by vasoconstriction, vascular occlusion, and disruption of the blood brain barrier, may adversely affect outcome following traumatic brain injury (TBI). Because of its vasodilating and antiaggregative properties, nitric oxide (NO) produced by nitric oxide synthase in the endothelium (eNOS) is a key regulator of vascular homeostasis. The objective of the present study was to evaluate the role of eNOS in vascular disturbances and histological outcome in the brain following TBI. Cortical blood flow ([(14)C]-iodoantipyrine technique), number of perfused capillaries (FITC-dextran technique), brain water content (wet vs. dry weight), and the transfer constant (K(i)) for [(51)Cr]-EDTA, reflecting permeability, were analyzed 3 h and 24 h after a controlled cortical impact injury (CCI) in eNOS-deficient (eNOS-KO) and wild-type (WT) mice. Cortical contusion volume and cell count in the hippocampus were evaluated 3 weeks after injury. Blood flow in the injured cortex decreased in both groups following trauma. There were no significant differences between the groups at 3 h, but blood flow was lower in eNOS-KO mice than in WT mice 24 h after trauma. Brain water content was higher in the WT mice than in eNOS-KO mice at 24 h. Number of perfused capillaries, K(i), and histological outcome were similar in both groups. We conclude that eNOS is important for maintenance of cerebral blood flow after trauma and that eNOS promotes edema formation by mechanisms other than increased permeability. The vascular effects of eNOS do not, however, influence histological outcome.

Administration of S-nitrosoglutathione after traumatic brain injury protects the neurovascular unit and reduces secondary injury in a rat model of controlled cortical impact

Background

Traumatic brain injury (TBI) is a major cause of preventable death and serious morbidity in young adults. This complex pathological condition is characterized by significant blood brain barrier (BBB) leakage that stems from cerebral ischemia, inflammation, and redox imbalances in the traumatic penumbra of the injured brain. Once trauma has occurred, combating these exacerbations is the keystone of an effective TBI therapy. Following other brain injuries, nitric oxide modulators such as S-nitrosoglutathione (GSNO) maintain not only redox balance but also inhibit the mechanisms of secondary injury. Therefore, we tested whether GSNO shows efficacy in a rat model of experimental TBI.

Methods

TBI was induced by controlled cortical impact (CCI) in adult male rats. GSNO (50 μg/kg body weight) was administered at two hours after CCI. GSNO-treated injured animals (CCI+GSNO group) were compared with vehicle-treated injured animals (CCI+VEH group) in terms of tissue morphology, BBB leakage, edema, inflammation, cell death, and neurological deficit.

Results

Treatment of the TBI animals with GSNO reduced BBB disruption as evidenced by decreased Evan's blue extravasation across brain, infiltration/activation of macrophages (ED1 positive cells), and reduced expression of ICAM-1 and MMP-9. The GSNO treatment also restored CCI-mediated reduced expression of BBB integrity proteins ZO-1 and occludin. GSNO-mediated improvements in tissue histology shown by reduction of lesion size and decreased loss of both myelin (measured by LFB staining) and neurons (assayed by TUNEL) further support the efficacy of GSNO therapy. GSNO-mediated reduced expression of iNOS in macrophages as well as decreased neuronal cell death may be responsible for the histological improvement and reduced exacerbations. In addition to these biochemical and histological improvements, GSNO-treated injured animals recovered neurobehavioral functions as evaluated by the rotarod task and neurological score measurements.

Conclusion

GSNO is a promising candidate to be evaluated in humans after brain trauma because it not only protects the traumatic penumbra from secondary injury and improves overall tissue structure but also maintains the integrity of BBB and reduces neurologic deficits following CCI in a rat model of experimental TBI.

Variation in cerebral blood flow velocity with cerebral perfusion pressure >40 mm Hg in 42 children with severe traumatic brain injury

OBJECTIVE

: To determine the prevalence of low, normal, and high mean middle cerebral artery flow velocity when cerebral perfusion pressure is >40 mm Hg in children with severe traumatic brain injury. There is no information regarding the relationship between middle cerebral artery flow velocity and cerebral perfusion pressure in pediatric traumatic brain injury.

DESIGN

: Prospective, observational study.

SETTING

: Level I pediatric trauma center.

PATIENTS

: A total of 42 children <17 yrs of age with an admission diagnosis of severe traumatic brain injury (admission Glasgow Coma Scale score of <9), traumatic brain injury on computed tomography scan, tracheal intubation/mechanical ventilation, and intracranial pressure monitoring.

INTERVENTIONS

: None.

MEASUREMENTS AND MAIN RESULTS

: Bilateral middle cerebral arteries were insonated using transcranial Doppler ultrasonography to calculate mean middle cerebral artery flow velocity after traumatic brain injury. Low mean middle cerebral artery flow velocity was defined as middle cerebral artery flow velocity <2 standard deviation and high was defined as mean middle cerebral artery flow velocity >2 standard deviation. Patients were grouped by age (0.8-2.9, 3-5.9, 6-9.9, and 10-16.9 yrs) and gender to examine the relationship between cerebral perfusion pressure and low, high, or normal mean middle cerebral artery flow velocity. Potential confounders of the relationship between cerebral perfusion pressure and mean middle cerebral artery flow velocity (intracranial pressure, PaCO2, hematocrit, sedation, fever,and impaired autoregulation were examined). Most children (n = 33; 79%) had normal mean middle cerebral artery flow velocity but four patients (9%) had low mean middle cerebral artery flow velocity and five children (12%) had high mean middle cerebral artery flow velocity despite cerebral perfusion pressure >40 mm Hg. There was no difference in potential confounders of the relationship between cerebral perfusion pressure and mean middle cerebral artery flow velocity except for hematocrit, which was lower (25 +/- 4%; range = 21-30%) in children with high mean middle cerebral artery flow velocity. An inverse relationship between mean middle cerebral artery flow velocity and hematocrit was also found in boys aged 10 to 16.9 yrs.

CONCLUSIONS

: Both low and/or high mean middle cerebral artery flow velocity occur with cerebral perfusion pressure >40 mm Hg in severe pediatric traumatic brain injury. Of the potential confounders considered, only lower hematocrit was associated with high mean middle cerebral artery flow velocity.

Cerebrovascular pathophysiology in pediatric traumatic brain injury

BACKGROUND

Traumatic brain injury (TBI) is the leading cause of traumatic morbidity and mortality in children. Although there is increasing information concerning TBI in adults and experimental animal models, relatively little is known regarding cerebrovascular pathophysiology specific to children.

MATERIALS

A review of the pertinent medical literature.

RESULTS

Systemic and cerebral hemodynamic factors such as hypotension, hypoxia, hyperglycemia, and fever are associated with poor outcome in pediatric TBI. Similarly, cerebral autoregulation is often impaired after TBI and may adversely affect outcome, especially if systemic hemodynamics are altered. Furthermore, CO2 vasoreactivity may be altered after pediatric TBI and lead to either cerebral ischemia or hyperemia.

CONCLUSIONS

Understanding the effect of pediatric TBI on the cerebral circulation is needed to potentially develop protocols to improve outcome in this vulnerable population. Specifically, changes in pediatric cerebrovascular physiology and pathophysiology, including CO2 vasoreactivity and pressure autoregulation, must be understood and their mechanism elucidated.

[Study of a brain microcirculation in cranioencephalic trauma using the Side Stream Field (SDF) system]

Posttraumatic tissular hypoxia can be due to multiple causes, including microcirculation disturbances, which can be studied with the SDF (Side Stream Dark Field) system. This system is based on a small hand-held microscope that eliminates directly reflected green polarised light from an organ surface using an orthogonal analyser. It offers clear images of red and white blood cells flow through microcirculation. Specific software is later used to determine the length and density of microvessels. We present a case of a TBI patient who required surgical evacuation of a brain contusion. Images of the microcirculatory bed were recorded with the SDF microscope and compared with a normal pattern obtained from another patient who was operated on for an unruptured cerebral aneurysm. Both imaging and quantitative analyses showed significant differences in the cerebral microcirculatory status in these patients. Total length and density of vessels were markedly reduced in the TBI patient. SDF imaging allows direct and non-invasive in vivo observation of cerebral microcirculation, and may allow us to deepen our knowledge of the pathophysiology of posttraumatic brain ischemia.

Comparative study of decompressive craniectomy after mass lesion evacuation in severe head injury

OBJECTIVE

This study was conducted to evaluate outcome after decompressive craniectomy (DC) in the setting of mass evacuation with or without intracranial pressure (ICP) monitoring.

METHODS

Over a 48-month period (March 2000 to March 2004), 54 of 967 consecutive head injury patients underwent DC for evacuation of a mass lesion. DC was performed without ICP monitoring in 27 patients who required urgent decompression (group A) and in 27 patients who did not require urgent surgery and who had their ICP monitored for 1 to 14 days before surgery (group B).

RESULTS

In group A, the mean Glasgow Coma Scale score was 6.0; 80% had computed tomographic evidence of a shift greater than 5 mm; and 25 patients underwent DC immediately after resuscitation. In group B, the mean Glasgow Coma Scale score was 7.3; 40% had computed tomographic evidence of shift; and 75% underwent DC 24 hours or longer after presentation. Overall, 22 patients died (12 in group A and 10 in group B), 11 remained vegetative or severely disabled (3 in group A and 8 in group B), and 19 had good recovery (11 in group A and 8 in group B). Two patients were lost to follow-up. In 18 group B patients with ICP greater than 20 mm Hg before mass evacuation, ICP dropped an average of 13 mm Hg (P < 0.001). A mass lesion greater than 50 mL (odds ratio [OR], 2.86; 95% confidence interval [CI], 1.04-7.89) and evidence of low attenuation on computed tomography before (OR, 3.3; 95% CI, 1.1-10.3) or after (OR, 2.92; 95% CI, 1.02-8.34) DC were predictors of death. A good outcome occurred in 42% of patients with and in 63% of patients without delayed traumatic injury (OR, 0.3; 95% CI, 0.1-1.1). Outcome was favorable in 78.6% of patients who had no ICP monitoring before DC versus 47.1% of patients with ICP monitoring (OR, 0.2; 95% CI, 0.1-1.2).

CONCLUSION

In this study, mortality after DC for mass lesion was greater than expected, and outcome did not differ between patients with or without ICP monitoring.

Early metabolic characteristics of lesion and nonlesion tissue after head injury

We defined lesion and structurally normal regions using magnetic resonance imaging at follow-up in patients recovering from head injury. Early metabolic characteristics in these regions of interest (ROIs) were compared with physiology in healthy volunteers. Fourteen patients with severe head injury had positron emission tomography within 72 h, and magnetic resonance imaging at 3 to 18 months after injury. Cerebral blood flow (CBF), oxygen utilization (CMRO(2)), and oxygen extraction fraction (OEF) were all lower in lesion ROIs, compared with nonlesion and control ROIs (P<0.001); however, there was substantial overlap in physiology. Control ROIs showed close coupling among CBF, blood volume (CBV), and CMRO(2), whereas relationships within lesion and nonlesion ROIs were abnormal. The relationship between CBF and CMRO(2) generally remained coupled but the slope was reduced; that for CBF and OEF was variable; whereas that between CBF and CBV was highly variable. There was considerable heterogeneity between and within patients. Although irreversibly damaged tissue is characterized by marked derangements in physiology, a more detailed analysis shows acute changes in physiology and physiologic relationships within regions of the brain that appear structurally normal at follow-up. Such pathophysiological derangements may result in selective neuronal loss and impact on functional outcome.

Lactate and glucose as energy substrates and their role in traumatic brain injury and therapy

Traumatic brain injury is a leading cause of disability and mortality worldwide, but no new pharmacological treatments are clinically available. A key pathophysiological development in the understanding of traumatic brain injury is the energy crisis derived from decreased cerebral blood flow, increased energy demand and mitochondrial dysfunction. Although still controversial, new findings suggest that brain cells try to cope in these conditions by metabolizing lactate as an energy substrate ‘on-demand’ in lieu of glucose. Experimental and clinical data suggest that lactate, at least when exogenously administered, is transported from astrocytes to neurons for neuronal utilization, essentially bypassing the slow, catabolizing glycolysis process to quickly and efficiently produce ATP. Treatment strategies using systemically applied lactate have proved to be protective in various experimental traumatic brain injury studies. However, lactate has the potential to elevate oxygen consumption to high levels and, therefore, could potentially impose a danger for tissue-at-risk with low cerebral blood flow. The present review outlines the experimental basis of lactate in energy metabolism under physiological and pathophysiological conditions and presents arguments for lactate as a new therapeutical tool in human head injury.

Early derangements in oxygen and glucose metabolism following head injury: the ischemic penumbra and pathophysiological heterogeneity

INTRODUCTION

Conclusive evidence of cerebral ischemia following head injury has been elusive. We aimed to use (15)O and (18)Fluorodeoxyglucose positron emission tomography (PET) to investigate pathophysiological derangements following head injury.

RESULTS

Eight patients underwent PET within 24 h of injury to map cerebral blood flow (CBF), cerebral oxygen metabolism (CMRO2), oxygen extraction fraction (OEF), and cerebral glucose metabolism (CMRglc). Physiological regions of interest (ROI) were generated for each subject using a range of OEF values from very low (<10), low (10-30), normal range (30-50), high (50-70), and critically high (> or =70%). We applied these ROIs to each subject to generate data that would examine the balance between blood flow and metabolism across the injured brain independent of structural injury.

DISCUSSION

Compared to the normal range, brain regions with higher OEF demonstrate a progressive CBF reduction (P < 0.01), CMRO2 increase (P < 0.05), and no change in CMRglc, while regions with lower OEF are associated with reductions in CBF, CMRO2, and CMRglc (P < 0.01). Although all subjects demonstrate a decrease in CBF with increases in OEF > 70%, CMRO2 and CMRglc were generally unchanged. One subject demonstrated a reduction in CBF and small fall in CMRO2 within the high OEF region (>70%), combined with a progressive increase in CMRglc.

CONCLUSIONS

The low CBF and maintained CMRO2 in the high OEF ROIs is consistent with classical cerebral ischemia and the presence of an 'ischemic penumbra' following early head injury, while the metabolic heterogeneity that we observed suggests significant pathophysiological complexity. Other mechanisms of energy failure are clearly important and further study is required to delineate the processes involved.

Unique challenges in clinical trials in traumatic brain injury

Clinical trials in traumatic brain injury have shown little success in providing an evidence base for the introduction of successful new therapies into clinical practice. In addition to the problems that are common to all such studies in critical illness, trials in traumatic brain injury are complicated by the extremely short temporal window for intervention, failure of many candidate drugs to cross the blood-brain barrier, ethical and regulatory obstacles associated with research in subjects who cannot provide consent, the tendency to use small sample sizes in anticipation of unrealistic treatment benefits, and difficulty in translating experimental success into clinical practice. This article reviews the potential causes of these problems and suggests some solutions. These include the changes in regulatory frameworks that are making waived consent an acceptable strategy once more, and an increasing trend toward appropriately large trials. Other encouraging developments include the increasing use of human experimental medicine strategies before phase III trials to assess blood-brain barrier penetration and dose ranging, and provide proof of concept and proof of mechanism. Novel approaches to trial design, such as sliding dichotomy, coupled with robust outcome prediction models, can increase statistical power and improve trial design.

Does adherence to treatment targets in children with severe traumatic brain injury avoid brain hypoxia? A brain tissue oxygenation study

OBJECTIVE

Most physicians rely on conventional treatment targets for intracranial pressure, cerebral perfusion pressure, systemic oxygenation, and hemoglobin to direct management of traumatic brain injury (TBI) in children. In this study, we used brain tissue oxygen tension (PbtO2) monitoring to examine the association between PbtO2 values and outcome in pediatric severe TBI and to determine the incidence of compromised PbtO2 in patients for whom acceptable treatment targets had been achieved.

METHODS

In this prospective observational study, 26 children with severe TBI and a median postresuscitation Glasgow Coma Scale score of 5 were managed with continuous PbtO2 monitoring. The relationships between outcome and the 6-hour period of lowest PbtO2 values and the length of time that PbtO2 was less than 20, 15, 10, and 5 mmHg were examined. The incidence of reduced PbtO2 for each threshold was evaluated where the following targets were met: intracranial pressure less than 20 mmHg, cerebral perfusion pressure greater than 50 mmHg, arterial oxygen tension greater than 60 mmHg (and peripheral oxygen saturation > 90%), and hemoglobin greater than 8 g/dl.

RESULTS

There was a significant association between poor outcome and the 6-hour period of lowest PbtO2 and length of time that PbtO2 was less than 15 and 10 mmHg. Multiple logistic regression analysis showed that low PbtO2 had an independent association with poor outcome. Despite achieving the management targets described above, 80% of patients experienced one or more episodes of compromised PbtO2 (< 20 mmHg), and almost one-third experienced episodes of brain hypoxia (PbtO2 < 10 mmHg).

CONCLUSION

Reduced PbtO2 is associated with poor outcome in pediatric severe TBI. In addition, many patients experience episodes of compromised PbtO2 despite achieving acceptable treatment targets.

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.

A combined microdialysis and FDG-PET study of glucose metabolism in head injury

BACKGROUND

Microdialysis continuously monitors the chemistry of a small focal volume of the cerebral extracellular space. Positron emission tomography (PET) establishes metabolism of the whole brain but only for the scan's duration. This study's objective was to apply these techniques together, in patients with traumatic brain injury, to assess the relationship between microdialysis (extracellular glucose, lactate, pyruvate, and the lactate/pyruvate (L/P) ratio as a marker of anaerobic metabolism) and PET parameters of glucose metabolism using the glucose analogue [(18)F]-fluorodeoxyglucose (FDG). In particular, we aimed to determine the fate of glucose in terms of differential metabolism to pyruvate and lactate.

MATERIALS AND METHODS

Microdialysis catheters (CMA70 or CMA71) were inserted into the cerebral cortex of 17 patients with major head injury. Microdialysis was performed during FDG-PET scans with regions of interest for PET analysis defined by the location of the gold-tipped microdialysis catheter. Microdialysate analysis was performed on a CMA600 analyser.

FINDINGS

There was significant linear relationship between the PET-derived parameter of glucose metabolism (regional cerebral metabolic rate of glucose; CMRglc) and levels of lactate (r = 0.778, p < 0.0001) and pyruvate (r = 0.799, p < 0.0001), but not with the L/P ratio.

CONCLUSION

The results suggest that in this population of patients, glucose was metabolised to both lactate and pyruvate, but was not associated with an increase in the L/P ratio. This suggests an increase in glucose metabolism to both lactate and pyruvate, as opposed to a shift towards anaerobic metabolism.

Differential influence of arterial blood glucose on cerebral metabolism following severe traumatic brain injury

INTRODUCTION

Maintaining arterial blood glucose within tight limits is beneficial in critically ill patients. Upper and lower limits of detrimental blood glucose levels must be determined.

METHODS

In 69 patients with severe traumatic brain injury (TBI), cerebral metabolism was monitored by assessing changes in arterial and jugular venous blood at normocarbia (partial arterial pressure of carbon dioxide (paCO2) 4.4 to 5.6 kPa), normoxia (partial arterial pressure of oxygen (paO2) 9 to 20 kPa), stable haematocrit (27 to 36%), brain temperature 35 to 38 degrees C, and cerebral perfusion pressure (CPP) 70 to 90 mmHg. This resulted in a total of 43,896 values for glucose uptake, lactate release, oxygen extraction ratio (OER), carbon dioxide (CO2) and bicarbonate (HCO3) production, jugular venous oxygen saturation (SjvO2), oxygen-glucose index (OGI), lactate-glucose index (LGI) and lactate-oxygen index (LOI). Arterial blood glucose concentration-dependent influence was determined retrospectively by assessing changes in these parameters within pre-defined blood glucose clusters, ranging from less than 4 to more than 9 mmol/l.

RESULTS

Arterial blood glucose significantly influenced signs of cerebral metabolism reflected by increased cerebral glucose uptake, decreased cerebral lactate production, reduced oxygen consumption, negative LGI and decreased cerebral CO2/HCO3 production at arterial blood glucose levels above 6 to 7 mmol/l compared with lower arterial blood glucose concentrations. At blood glucose levels more than 8 mmol/l signs of increased anaerobic glycolysis (OGI less than 6) supervened.

CONCLUSIONS

Maintaining arterial blood glucose levels between 6 and 8 mmol/l appears superior compared with lower and higher blood glucose concentrations in terms of stabilised cerebral metabolism. It appears that arterial blood glucose values below 6 and above 8 mmol/l should be avoided. Prospective analysis is required to determine the optimal arterial blood glucose target in patients suffering from severe TBI.

Blood Pressure Management in Acute Head Injury

Head injury remains a major cause of preventable death and serious morbidity in young adults. Based on the available evidence, it appears that a cerebral perfusion pressure of 50 to 70 mm Hg is generally adequate to ensure cerebral oxygen delivery and prevent ischemia. However, evidence suggests that perfusion requirements may not only vary across the injured brain but also differ depending on the time since injury. Such heterogeneity, both within and between subjects, suggests that individualized therapy may be an appropriate treatment strategy. Future studies should aim to assess which groups of patients, and what regional pathophysiological derangements, may benefit with improvements in functional outcome from therapeutic increases or decreases in cerebral perfusion pressure beyond these proposed limits. Such functional improvements may be of immense importance to patients and require formal neurocognitive assessments to discriminate improvements.

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.

Simultaneous measurement of brain tissue oxygen partial pressure, temperature, and global oxygen consumption during hibernation, arousal, and euthermy in non-sedated and non-anesthetized Arctic ground squirrels

This study reports an online temperature correction method for determining tissue oxygen partial pressure P(tO2) in the striatum and a novel simultaneous measurement of brain P(tO2) and temperature (T(brain)) in conjunction with global oxygen consumption V(O2) in non-sedated and non-anesthetized freely moving Arctic ground squirrels (AGS, Spermophilus parryii). This method fills an important research gap-the lack of a suitable method for physiologic studies of tissue P(O2) in hibernating or other cool-blooded species. P(tO2) in AGS brain during euthermy (21.22+/-2.06 mmHg) is significantly higher (P=0.016) than during hibernation (13.21+/-0.46 mmHg) suggests brain oxygenation in the striatum is normoxic during euthermy and hypoxic during hibernation. These results in P(tO2) are different from blood oxygen partial pressure P(aO2) in AGS, which are significantly lower during euthermy than during hibernation and are actually hypoxic during euthermy and normoxic during hibernation in our previous study. This intriguing difference between the P(O2) of brain tissue and blood during these two physiological states suggests that regional mechanisms in the brain play a role in maintaining tissue oxygenation and protect against hypoxia during hibernation.

In vitro norepinephrine significantly activates isolated platelets from healthy volunteers and critically ill patients following severe traumatic brain injury

Introduction

Norepinephrine, regularly used to increase systemic arterial blood pressure and thus improve cerebral perfusion following severe traumatic brain injury (TBI), may activate platelets. This, in turn, could promote microthrombosis formation and induce additional brain damage.

Methods

The objective of this study was to investigate the influence of norepinephrine on platelets isolated from healthy volunteers and TBI patients during the first two post-traumatic weeks. A total of 18 female and 18 male healthy volunteers of different age groups were recruited, while 11 critically ill TBI patients admitted consecutively to our intensive care unit were studied. Arterial and jugular venous platelets were isolated from norepinephrine-receiving TBI patients; peripheral venous platelets were studied in healthy volunteers. Concentration-dependent functional alterations of isolated platelets were analyzed by flow cytometry, assessing changes in surface P-selectin expression and platelet-derived microparticles before and after in vitro stimulation with norepinephrine ranging from 10 nM to 100 μM. The thrombin receptor-activating peptide (TRAP) served as a positive control.

Results

During the first week following TBI, norepinephrine-mediated stimulation of isolated platelets was significantly reduced compared with volunteers (control). In the second week, the number of P-selectin- and microparticle-positive platelets was significantly decreased by 60% compared with the first week and compared with volunteers. This, however, was associated with a significantly increased susceptibility to norepinephrine-mediated stimulation, exceeding changes observed in volunteers and TBI patients during the first week. This pronounced norepinephrine-induced responsiveness coincided with increased arterio-jugular venous difference in platelets, reflecting intracerebral adherence and signs of cerebral deterioration reflected by elevated intracranial pressure and reduced jugular venous oxygen saturation.

Conclusion

Clinically infused norepinephrine might influence platelets, possibly promoting microthrombosis formation. In vitro stimulation revealed a concentration- and time-dependent differential level of norepinephrine-mediated platelet activation, possibly reflecting changes in receptor expression and function. Whether norepinephrine should be avoided in the second post-traumatic week and whether norepinephrine-stimulated platelets might induce additional brain damage warrant further investigations.

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.

Cerebral blood flow and autoregulation after pediatric traumatic brain injury

Traumatic brain injury is a global health concern and is the leading cause of traumatic morbidity and mortality in children. Despite a lower overall mortality than in adult traumatic brain injury, the cost to society from the sequelae of pediatric traumatic brain injury is very high. Predictors of poor outcome after traumatic brain injury include altered systemic and cerebral physiology, including altered cerebral hemodynamics. Cerebral autoregulation is often impaired after traumatic brain injury and may adversely impact the outcome. Although altered cerebrovascular hemodynamics early after traumatic brain injury may contribute to disability in children, there is little information regarding changes in cerebral blood flow and cerebral autoregulation after pediatric traumatic brain injury. This review addresses normal pediatric cerebral physiology and cerebrovascular pathophysiology after pediatric traumatic brain injury.

Effect of hyperoxia on regional oxygenation and metabolism after severe traumatic brain injury: preliminary findings

OBJECTIVE

To determine the effect of normobaric hyperoxia on cerebral metabolism in patients with severe traumatic brain injury.

DESIGN

Prospective clinical investigation.

SETTING

Neurosciences critical care unit of a university hospital.

PATIENTS

Eleven patients with severe traumatic brain injury.

INTERVENTIONS

Cerebral microdialysis, brain tissue oximetry (PbO2), and oxygen-15 positron emission tomography (15O-PET) were undertaken at normoxia and repeated at hyperoxia (FiO2 increase of between 0.35 and 0.50).

MEASUREMENTS AND MAIN RESULTS

Established models were used to image cerebral blood flow, blood volume, oxygen metabolism, and oxygen extraction fraction. Physiology was characterized in a focal region of interest (surrounding the microdialysis catheter) and correlated with microdialysis and oximetry. Physiology was also characterized in a global region of interest (including the whole brain), and a physiologic region of interest (defined using a critical cerebral metabolic rate of oxygen threshold). Hyperoxia increased mean +/- sd PbO2 from 28 +/- 21 mm Hg to 57 +/- 47 mm Hg (p = .015). Microdialysate lactate and pyruvate were unchanged, but the lactate/pyruvate ratio showed a statistically significant reduction across the study population (34.1 +/- 9.5 vs. 32.5 +/- 9.0, p = .018). However, the magnitude of reduction was small, and its clinical significance doubtful. The focal region of interest and global 15O-PET variables were unchanged. "At-risk" tissue defined by the physiologic region of interest, however, showed a universal increase in cerebral metabolic rate of oxygen from a median (interquartile range) of 23 (22-25) micromol x 100 mL(-1) x min(-1) to 30 (28-36) micromol x 100 mL(-1) x min(-1) (p < .01).

CONCLUSIONS

In severe traumatic brain injury, hyperoxia increases PbO2 with a variable effect on lactate and lactate/pyruvate ratio. Microdialysis does not, however, predict the universal increases in cerebral metabolic rate of oxygen in at-risk tissue, which imply preferential metabolic benefit with hyperoxia.

Metabolic disturbance without brain ischemia in traumatic brain injury: a positron emission tomography study

BACKGROUND

Cerebral ischemia is believed to be an important mechanism of secondary neuronal injury in traumatic brain injury (TBI).

METHODS

In this study, we performed 15O2 positron emission tomography (PET) studies to measure the cerebral blood flow (CBF) and oxygen metabolism (CMRO2) in the pericontusional region in a total of 15 patients (11 males, 4 females, aged 15-81 years) who sustained TBI with contusional hematoma. PET studies were performed a mean of 13.5 +/- 9.1 days (range 2-33 days) after TBI occurred.

FINDINGS

The areas of pericontusional tissues located 10 mm away from the cerebral contusion exhibited mildly decreased CBF (89%) and severely suppressed CMRO2 (67%) when comparison was made with the remote cerebral cortex. Severely suppressed oxygen metabolism in the pericontusional tissue was observed not only in the acute stage, but also in the subacute stage after TBI, whereas blood flow was slightly recovered in the subacute stage. We also compared the PET findings obtained in the acute or subacute stage after TBI and structural abnormalities on late-stage MRI in 5 patients. The area of flow defect on the CBF-PET image developed into irreversible tissue damage (necrosis) in the chronic stage. The area of hypoperfusion surrounding the lesion partly resulted in tissue necrosis: however, a large part of the hypoperfused tissue survived in the chronic stage. Again, a significant portion of oxygen hypometabolism surrounding the lesion did not develop into tissue necrosis.

CONCLUSIONS

We conclude that impaired cerebral blood flow and metabolism in the pericontusional region is observed even in the subacute stage after TBI and is unlikely to cause severe further neuronal damage.

Neuroimaging in trauma

PURPOSE OF REVIEW

Developments in imaging following traumatic brain injury are outlined. Numerous techniques have evolved over the past several years giving us more information about the injury and prognosis for recovery. Some of these techniques are in clinical use while others are used primarily in research but have the potential to become clinically useful.

RECENT FINDINGS

Computed tomography (CT) scanning is the primary imaging technique for acute brain injury, giving rapid information and being part of a general trauma work up in the emergency situation. It has supplanted plain films in the immediate management of brain injury. Following stabilization, MRI is the method of choice for evaluating the full extent of brain injury. Information on diffuse axonal injury is obtained by several MRI sequences. Diffusion tensor imaging is able to show long tract damage and relates to prognosis. There are several techniques which are best suited to research in brain injury, including single photon emission CT, PET and xenon CT.

SUMMARY

CT and MRI are now the imaging techniques for acute and subacute brain injury, respectively. Diffusion tensor imaging is being developed to provide more information on structural damage in brain injury. There are several research techniques available for brain injury, particularly relating to cerebral blood flow and metabolism.

Monitoring brain tissue oxygen tension in brain-injured patients reveals hypoxic episodes in normal-appearing and in peri-focal tissue

OBJECTIVE

We compared brain tissue oxygen tension (PtiO2) measured in peri-focal and in normal-appearing brain parenchyma on computerized tomography (CT) in patients following traumatic brain injury (TBI).

DESIGN

Prospective observational study.

SETTING

Neurointensive care unit.

PATIENTS AND PARTICIPANTS

Thirty-two consecutive TBI patients were subjected to PtiO2 monitoring.

INTERVENTIONS

Peri-focal tissue was identified by the presence of a hypodense area of the contusion and/or within 1 cm from the core of the contusion. The position of the tip of the PtiO2 probe was assessed at follow-up CT scan.

MEASUREMENTS AND RESULTS

Mean PtiO2 in the peri-contusional tissue was 19.7+/-2.1 mmHg and was lower than PtiO2 in normal-appearing tissue (25.5+/-1.5 mmHg, p < 0.05), despite a greater cerebral perfusion pressure (CPP) (73.7+/-2.3 mmHg vs. 67.4+/-1.4 mmHg, p < 0.05). We observed both in peri-focal tissue and in normal-appearing tissue episodes of brain hypoxia (PtiO2 < 20 mmHg for at least 10 min), whose median duration was longer in peri-focal tissue than in normal-appearing tissue (51% vs. 34% of monitoring time, p < 0.01). In peri-focal tissue, we observed a progressive PtiO2 increase from pathologic to normal values (p < 0.01).

CONCLUSIONS

Multiple episodes of brain hypoxia occurred over the first 5 days following severe TBI. PtiO2 was lower in peri-contusional tissue than in normal-appearing tissue. In peri-contusional tissue, a progressive increase of PtiO2 from pathologic to normal values was observed over time, suggestive of an improvement at microcirculatory level.

Cerebral arterio-venous pCO2 difference, estimated respiratory quotient, and early posttraumatic outcome: comparison with arterio-venous lactate and oxygen differences

Arterio-venous pCO2 difference (AVDpCO2) and estimated respiratory quotient, the ratio between AVDpCO2 and arterio-venous O2 difference, may be potentially useful estimators of irreversible posttraumatic global cerebral ischemia. Our aim was to evaluate their relevance, along with arterio-venous lactate difference (AVDL) and lactate oxygen index (LOI), in early outcome prediction. The retrospective study involved 55 patients with severe head injury, admitted consecutively in a multidisciplinary intensive care unit of a general hospital. A retrograde jugular catheter was placed as soon as possible, allowing for 324 simultaneous arterio-jugular samples to be taken throughout the first 48-hour postinjury. Early brain death (within 48 h) was assumed to be due to early global ischemia. A multivariate model including clinical and radiologic descriptors and jugular bulb variables showed that a widening of AVDL and LOI was associated with early brain death. Whereas in the patients who died, a progressive worsening of AVDpCO2 and estimated respiratory quotient, associated with corresponding changes in AVDL and LOI were observed, in patients who survived the widening of AVDpCO2 normalized along with that of arterio-venous O2 difference. These findings suggest that the isolated measurement of widening AVDpCO2 is not specific for global cerebral ischemia, but its observation over time could be potentially more useful.

[Intracerebral monitoring of a patient with vasopasm]

Delayed neurological deficit occurs among 30% of patients after aneurysmal subarachnoid haemorrhage, mainly related to cerebral vasospasm. The early detection of cerebral ischemia remains problematic. Conventional cerebral monitoring (as intracranial pressure and cerebral perfusion pressure) appears to be insufficient, because cerebral ischemia may occur without elevated intracranial pressure. Global cerebral monitoring as venous jugular oxygen saturation are useful for regional monitoring. Local monitoring as oxygen tissue partial pressure (PtiO2) and microdialysis are sensible for brain ischemia detection, but may also ignore episodes occurring in non-monitored brain area. For the detection of most episodes of brain ischemia, several monitoring system should be use performing a multimodal intracerebral monitoring. Brain microdialysis and oxygen tissue partial pressure are promising monitoring system.

Structural and functional neuroimaging in mild-to-moderate head injury

Head injury is a major cause of disability and death in adults. Significant developments in imaging techniques have contributed to the knowledge of the pathophysiology of head injury. Although extensive research is available on severe head injury, less is known about mild-to-moderate head injury despite the fact that most patients sustain this type of injury. In this review, we focus on structural and functional imaging techniques in patients with mild-to-moderate head injury. We discuss CT and MRI, including different MRI sequences, single photon emission computed tomography, perfusion-weighted MRI, perfusion CT, PET, magnetic resonance spectroscopy, functional MRI and magnetic encephalography. We outline the advantages and limitations of these various techniques in the contexts of the initial assessment and identification of brain abnormalities and the prediction of 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.

Pathophysiology of traumatic brain injury

The knowledge of the pathophysiology after traumatic head injury is necessary for adequate and patient-oriented treatment. As the primary insult, which represents the direct mechanical damage, cannot be therapeutically influenced, target of the treatment is the limitation of the secondary damage (delayed non-mechanical damage). It is influenced by changes in cerebral blood flow (hypo- and hyperperfusion), impairment of cerebrovascular autoregulation, cerebral metabolic dysfunction and inadequate cerebral oxygenation. Furthermore, excitotoxic cell damage and inflammation may lead to apoptotic and necrotic cell death. Understanding the multidimensional cascade of secondary brain injury offers differentiated therapeutic options.

Imaging after brain injury

Head injury remains an important cause of death and disability in young adults. This review will discuss the role of structural imaging using computed tomography (CT) and magnetic resonance imaging (MRI) and physiological imaging using CT perfusion, 131Xe CT, MRI and spectroscopy (MRS), single photon emission computed tomography, and positron emission tomography (PET) in the assessment, management, and prediction of outcome after head injury. CT allows rapid assessment of brain pathology which ensures patients who require urgent surgical intervention receive appropriate care. Although MRI provides greater spatial resolution, particularly within the posterior fossa and deep white matter, a complete assessment of the burden of injury requires imaging of cerebral physiology. Physiological imaging techniques can only provide 'snap shots' of physiology within the injured brain, but they can be repeated, and such data can be used to assess the impact of therapeutic interventions. Perfusion imaging based on CT techniques (xenon CT and CT perfusion) can be implemented easily in most hospital centres, and provide quantitative perfusion data in addition to structural images. PET imaging provides unparalleled insights into cerebral physiology and pathophysiology, but is not widely available and is primarily a research tool. MR technology continues to develop and is becoming generally available. Using a complex variety of sequences, MR can provide data concerning both structural and physiological derangements. Future developments with such imaging techniques should improve understanding of the pathophysiology of brain injury and provide data that should improve management and prediction of functional outcome.

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.

Global cerebral blood flow and CPP after severe head injury: a xenon-CT study

OBJECTIVE

To evaluate the association between global cerebral blood flow and different cerebral perfusion pressure ranges in severe head injury.

DESIGN

A retrospective study

SETTING

Neurosurgical and trauma patients in an intensive care unit in a regional hospital.

PATIENTS AND PARTICIPANTS

Out of a series of 237 consecutive patients with severe head injuries (GCS<or=8), 162 were submitted to ICP monitoring and 89 of them underwent 180 xenon-CT studies and cerebral perfusion pressure (CPP) measurement. The xenon-CT studies did not include any unsalvageable patients nor any mistakenly diagnosed as severe on entry.

INTERVENTIONS

None.

MEASUREMENTS AND RESULTS

Most (95.6%) of xenon-CT studies were obtained with CPP values within the 50-90 mmHg range. Perfusion data were grouped according to CPP values: (1) below 50 mmHg, (2) 50-60 mmHg, (3) 60-70 mmHg, (4) above 70 mmHg. Global cerebral blood flow did not differ among the groups (p=0.49). No differences in physiological variables were found among the CPP groups, except for intracranial pressure, higher in the group with CPP below 50 mmHg, and mean arterial pressure, higher in the CPP above 70 mmHg group (p<0.0001). No differences were found for cerebral metabolic rate of oxygen and lactate.

CONCLUSIONS

There was little correlation between CPP values and global cerebral blood flow levels in our selected patients, probably because pressure autoregulation was preserved. Global metabolism measurements were constant within the groups, suggesting that in patients with controlled physiological variables an interplay between cerebral blood flow and metabolism might be more relevant than the relationship between CPP and cerebral blood flow.

Hyperventilation following head injury: effect on ischemic burden and cerebral oxidative metabolism

OBJECTIVE

To determine whether hyperventilation exacerbates cerebral ischemia and compromises oxygen metabolism (CMRO2) following closed head injury.

DESIGN

A prospective interventional study.

SETTING

A specialist neurocritical care unit.

PATIENTS

Ten healthy volunteers and 30 patients within 10 days of closed head injury.

INTERVENTIONS

Subjects underwent oxygen-15 positron emission tomography imaging of cerebral blood flow, cerebral blood volume, CMRO2, and oxygen extraction fraction. In patients, positron emission tomography studies, somatosensory evoked potentials, and jugular venous saturation (SjO2) measurements were obtained at Paco2 levels of 36+/-3 and 29+/-2 torr.

MEASUREMENTS AND MAIN RESULTS

We estimated the volume of ischemic brain and examined the efficiency of coupling between oxygen delivery and utilization using the sd of the oxygen extraction fraction distribution. We correlated CMRO2 to cerebral electrophysiology and examined the effects of hyperventilation on the amplitude of the cortical somatosensory evoked potential response. Patients showed higher ischemic brain volume than controls (17+/-22 vs. 2+/-3 mL; p<or=.05), with worse matching of oxygen delivery to demand (p<.001). Hyperventilation consistently reduced cerebral blood flow (p<.001) and resulted in increases in oxygen extraction fraction and ischemic brain volume (17+/-22 vs. 88+/-66 mL; p<.0001), which were undetected by SjO2 monitoring. Mean CMRO2 was slightly increased following hyperventilation, but responses were extremely variable, with 28% of patients demonstrating a decrease in CMRO2 that exceeded 95% prediction intervals for zero change in one or more regions. CMRO2 correlated with cerebral electrophysiology, and cortical somatosensory evoked potential amplitudes were significantly increased by hyperventilation.

CONCLUSIONS

The acute cerebral blood flow reduction and increase in CMRO2 secondary to hyperventilation represent physiologic challenges to the traumatized brain. These challenges exhaust physiologic reserves in a proportion of brain regions in many subjects and compromise oxidative metabolism. Such ischemia is underestimated by common bedside monitoring tools and may represent a significant mechanism of avoidable neuronal injury following head trauma.

Transcranial Doppler ultrasound goal-directed therapy for the early management of severe traumatic brain injury

OBJECTIVE

To evaluate the usefulness of early transcranial Doppler ultrasound (TCD) goal-directed therapy after severe traumatic brain injury initiated before invasive cerebral monitoring is available.

DESIGN

Prospective, observational clinical study.

SETTING

Surgical intensive care unit, university hospital.

PATIENTS AND PARTICIPANTS

Twenty-four severely brain-injured patients.

INTERVENTIONS

All patients had TCD measurements immediately on admission (T0) and when invasive cerebral monitoring was available (T1). TCD was considered abnormal when two out of three measured values were outside the following limits: Vm<30 cm/s, Vd<20 cm/s, PI > 1.4. When admission TCD was abnormal, attending physicians modified treatment to increase cerebral perfusion pressure.

MEASUREMENTS AND RESULTS

Admission TCD was performed 18+/-11 min (T0) after admission, whereas cerebral invasive monitoring was available 242+/-116 min (T1) after admission. At T0, 11 (46%) patients had abnormal TCD values (group 1) and 13 had normal TCD values (group 2); mean arterial pressure was comparable between groups. All group 1 patients received mannitol and/or norepinephrine. At T1, mean arterial pressure was increased compared to admission in group 1 (105+/-17 mmHg vs. 89+/-15 mmHg, p<0.05) and only two patients had still an abnormal TCD. Although group 1 patients had higher intracranial pressure than those of group 2 (32+/-13 mmHg vs. 22+/-10 mmHg, p<0.01), both cerebral perfusion pressure and jugular venous oxygen saturation were comparable between the groups.

CONCLUSIONS

The use of TCD at hospital admission allows identification of severely brain-injured patients with brain hypoperfusion. In such high-risk patients, early TCD goal-directed therapy can restore normal cerebral perfusion and might then potentially help in reducing the extent of secondary brain injury.

Monitoring cerebral oxygenation in traumatic brain injury

Ischemia is a common problem after traumatic brain injury (TBI) that eludes detection with standard monitoring. In this review we will discuss four available techniques (SjVO2, PET, NIRS and PbrO2) to monitor cerebral oxygenation. We present technical data including strengths and weaknesses of these systems, information from clinical studies and formulate a vision for the future.