Endothelium-dependent responses after experimental brain injury

Prognosis of Six-Month Glasgow Outcome Scale in Severe Traumatic Brain Injury Using Hospital Admission Characteristics, Injury Severity Characteristics, and Physiological Monitoring during the First Day Post-Injury

Gold standard prognostic models for long-term outcome in patients with severe traumatic brain injury (TBI) use admission characteristics and are considered useful in some areas but not for clinical practice. In this study, we aimed to build prognostic models for 6-month Glasgow Outcome Score (GOS) in patients with severe TBI, combining baseline characteristics with physiological, treatment, and injury severity data collected during the first 24 h after injury. We used a training dataset of 472 TBI subjects and several data mining algorithms to predict the long-term neurological outcome. Performance of these algorithms was assessed in an independent (test) sample of 158 subjects. The least absolute shrinkage and selection operator (LASSO) led to the highest prediction accuracy (area under the receiving operating characteristic curve = 0.86) in the test set. The most important post-baseline predictor of GOS was the best motor Glasgow Coma Scale (GCS) recorded in the first day post-injury. The LASSO model containing the best motor GCS and baseline variables as predictors outperformed a model with baseline data only. TBI patient physiology of the first day-post-injury did not have a major contribution to patient prognosis six months after injury. In conclusion, 6-month GOS in patients with TBI can be predicted with good accuracy by the end of the first day post-injury, using hospital admission data and information on the best motor GCS achieved during those first 24 h post-injury. Passed the first day after injury, important physiological predictors could emerge from landmark analyses, leading to prediction models of higher accuracy than the one proposed in the current research.

Cerebral blood vessel damage in traumatic brain injury

Traumatic brain injury is a devastating cause of death and disability. Although injury of brain tissue is of primary interest in head trauma, nearly all significant cases include damage of the cerebral blood vessels. Because vessels are critical to the maintenance of the healthy brain, any injury or dysfunction of the vasculature puts neural tissue at risk. It is well known that these vessels commonly tear and bleed as an immediate consequence of traumatic brain injury. It follows that other vessels experience deformations that are significant though not severe enough to produce bleeding. Recent data show that such subfailure deformations damage the microstructure of the cerebral vessels, altering both their structure and function. Little is known about the prognosis of these injured vessels and their potential contribution to disease development. The objective of this review is to describe the current state of knowledge on the mechanics of cerebral vessels during head trauma and how they respond to the applied loads. Further research on these topics will clarify the role of blood vessels in the progression of traumatic brain injury and is expected to provide insight into improved strategies for treatment of the disease.

Cerebrovascular dysfunction following subfailure axial stretch

Cerebral blood vessels are vital to maintaining the health of the brain. Traumatic brain injury (TBI) commonly results in autoregulatory dysfunction and associated failure of cerebral vessels to maintain homeostasis in the brain. While post-injury changes to brain biochemistry are known to contribute to this dysfunction, tissue deformation may also directly alter vascular smooth muscle cell (SMC) function. As a first step toward understanding stretch-induced dysfunction, this study investigates the effect of overstretch on the contractile behavior of SMCs in middle cerebral arteries (MCAs). We hypothesized that vessel function is altered above a threshold of stretch and strain rate. Twenty-four MCAs from Sprague Dawley rats were tested. Following development of basal SMC tone, vessels were subjected to increasing levels of isosmotic extracellular potassium (K⁺). Samples were then subjected to an axial overstretch of either 1.2*λ_IV or 1.3*λ_IV at strain rates of 0.2 or 20s^-1. Following overstretch, SMC contractile behavior was measured again, both immediately and 60min after overstretch. Control vessels were subjected to the same protocol but without overstretch. SMC contractile behavior was characterized using both percent contraction (%C) relative to the fully dilated inner diameter and the K⁺ dose required to evoke the half maximal contractile response (EC50). Control vessels exhibited increased sensitivity to K⁺ in successive characterization tests, so all effects were quantified relative to the time-matched control response. Samples exhibited the typical biphasic response to extracellular K⁺, dilating and contracting in response to small and large K⁺ concentrations, respectively. As hypothesized, axial overstretch altered SMC contractile behavior, as seen in a decrease in %C for sub-maximal contractile K⁺ doses (p<0.05) and an increase in EC50 (p<0.01), but only for the test group stretched rapidly to 1.3*λ_IV. While the change in %C was only significantly different immediately after overstretch, the change to EC50 persisted for 60min. These results indicate that deformation can alter SMC contractile behavior and thus potentially play a role in cerebrovascular autoregulatory dysfunction independent of the pathological chemical environment in the brain post-TBI.

Microvascular Dysfunction and Cognitive Impairment

The impact of vascular risk factors on cognitive function has garnered much interest in recent years. The appropriate distribution of oxygen, glucose, and other nutrients by the cerebral vasculature is critical for proper cognitive performance. The cerebral microvasculature is a key site of vascular resistance and a preferential target for small vessel disease. While deleterious effects of vascular risk factors on microvascular function are known, the contribution of this dysfunction to cognitive deficits is less clear. In this review, we summarize current evidence for microvascular dysfunction in brain. We highlight effects of select vascular risk factors (hypertension, diabetes, and hyperhomocysteinemia) on the pial and parenchymal circulation. Lastly, we discuss potential links between microvascular disease and cognitive function, highlighting current gaps in our understanding.

Cellular and molecular mechanisms of injury and spontaneous recovery

Until recently, most have assumed that traumatic brain injury (TBI) was singularly associated with the overt destruction of brain tissue resulting in subsequent morbidity or death. More recently, experimental and clinical studies have shown that the pathobiology of TBI is more complex, involving a host of cellular and subcellular changes that impact on neuronal function and viability while also affecting vascular reactivity and the activation of multiple biological response pathways. Here we review the brain's response to injury, examining both focal and diffuse changes and their implications for post-traumatic brain dysfunction and recovery. TBI-induced neuronal dysfunction and death as well as the diffuse involvement of multiple fiber projections are discussed together with considerations of how local axonal membrane changes or channelopathy translate into local ionic dysregulation and axonal disconnection. Concomitant changes in the cerebral microcirculation are also discussed and their relationship with the parallel changes in the brain's metabolism is considered. These cellular and subcellular events occurring within neurons and their blood supply are correlated with multiple biological response modifiers evoked by generalized post-traumatic inflammation and the parallel activation of oxidative stress processes. The chapter closes with considerations of recovery following focal or diffuse injury. Evidence for dynamic brain reorganization/repair is presented, with considerations of traumatically induced circuit disruption and their progression to either adaptive or in some cases, maladaptive reorganization.

Capillary transit time heterogeneity and flow-metabolism coupling after traumatic brain injury

Most patients who die after traumatic brain injury (TBI) show evidence of ischemic brain damage. Nevertheless, it has proven difficult to demonstrate cerebral ischemia in TBI patients. After TBI, both global and localized changes in cerebral blood flow (CBF) are observed, depending on the extent of diffuse brain swelling and the size and location of contusions and hematoma. These changes vary considerably over time, with most TBI patients showing reduced CBF during the first 12 hours after injury, then hyperperfusion, and in some patients vasospasms before CBF eventually normalizes. This apparent neurovascular uncoupling has been ascribed to mitochondrial dysfunction, hindered oxygen diffusion into tissue, or microthrombosis. Capillary compression by astrocytic endfeet swelling is observed in biopsies acquired from TBI patients. In animal models, elevated intracranial pressure compresses capillaries, causing redistribution of capillary flows into patterns argued to cause functional shunting of oxygenated blood through the capillary bed. We used a biophysical model of oxygen transport in tissue to examine how capillary flow disturbances may contribute to the profound changes in CBF after TBI. The analysis suggests that elevated capillary transit time heterogeneity can cause critical reductions in oxygen availability in the absence of 'classic' ischemia. We discuss diagnostic and therapeutic consequences of these predictions.

The endothelium, a protagonist in the pathophysiology of critical illness: focus on cellular markers

The endotheliumis key in the pathophysiology of numerous diseases as a result of its precarious function in the regulation of tissue homeostasis. Therefore, its clinical evaluation providing diagnostic and prognostic markers, as well as its role as a therapeutic target, is the focus of intense research in patientswith severe illnesses. In the critically ill with sepsis and acute brain injury, the endothelium has a cardinal function in the development of organ failure and secondary ischemia, respectively. Cellular markers of endothelial function such as endothelial progenitor cells (EPC) and endothelialmicroparticles (EMP) are gaining interest as biomarkers due to their accessibility, although the lack of standardization of EPC and EMP detection remains a drawback for their routine clinical use. In this paper we will review data available on EPC, as a general marker of endothelial repair, and EMP as an equivalent of damage in critical illnesses, in particular sepsis and acute brain injury. Their determination has resulted in new insights into endothelial dysfunction in the critically ill. It remains speculative whether their determination might guide therapy in these devastating acute disorders in the near future.

Vascular neural network phenotypic transformation after traumatic injury: potential role in long-term sequelae

The classical neurovascular unit (NVU), composed primarily of endothelium, astrocytes, and neurons, could be expanded to include smooth muscle and perivascular nerves present in both the up- and downstream feeding blood vessels (arteries and veins). The extended NVU, which can be defined as the vascular neural network (VNN), may represent a new physiological unit to consider for therapeutic development in stroke, traumatic brain injury, and other brain disorders (Zhang et al., Nat Rev Neurol 8(12):711-716, 2012). This review is focused on traumatic brain injury and resultant post-traumatic changes in cerebral blood flow, smooth muscle cells, matrix, blood-brain barrier structures and function, and the association of these changes with cognitive outcomes as described in clinical and experimental reports. We suggest that studies characterizing TBI outcomes should increase their focus on changes to the VNN, as this may yield meaningful therapeutic targets to resolve posttraumatic dysfunction.

Protecting against vascular disease in brain

Endothelial cells exert an enormous influence on blood vessels throughout the circulation, but their impact is particularly pronounced in the brain. New concepts have emerged recently regarding the role of this cell type and mechanisms that contribute to endothelial dysfunction and vascular disease. Activation of the renin-angiotensin system plays a prominent role in producing these abnormalities. Both oxidative stress and local inflammation are key mechanisms that underlie vascular disease of diverse etiology. Endogenous mechanisms of vascular protection are also present, including antioxidants, anti-inflammatory molecules, and peroxisome proliferator-activated receptor-γ. Despite their clear importance, studies of mechanisms that underlie cerebrovascular disease continue to lag behind studies of vascular biology in general. Identification of endogenous molecules and pathways that protect the vasculature may result in targeted approaches to prevent or slow the progression of vascular disease that causes stroke and contributes to the vascular component of dementia and Alzheimer's disease.

The adverse pial arteriolar and axonal consequences of traumatic brain injury complicated by hypoxia and their therapeutic modulation with hypothermia in rat

This study examined the effect of posttraumatic hypoxia on cerebral vascular responsivity and axonal damage, while also exploring hypothermia's potential to attenuate these responses. Rats were subjected to impact acceleration injury (IAI) and equipped with cranial windows to assess vascular reactivity to topical acetylcholine, with postmortem analyses using antibodies to amyloid precursor protein to assess axonal damage. Animals were subjected to hypoxia alone, IAI and hypoxia, IAI and hypoxia before induction of moderate hypothermia (33 degrees C), IAI and hypoxia induced during hypothermic intervention, and IAI and hypoxia initiated after hypothermia. Hypoxia alone had no impact on vascular reactivity or axonal damage. Acceleration injury and posttraumatic hypoxia resulted in dramatic axonal damage and altered vascular reactivity. When IAI and hypoxia were followed by hypothermic intervention, no axonal or vascular protection ensued. However, when IAI was followed by hypoxia induced during hypothermia, axonal and vascular protection followed. When this same hypoxic insult followed the use of hypothermia, no benefit ensued. These studies show that early hypoxia and delayed hypoxia exert damaging axonal and vascular consequences. Although this damage is attenuated by hypothermia, this follows only when hypoxia occurs during hypothermia, with no benefit found if the hypoxic insult proceeds or follows hypothermia.

Polyamine catabolism is enhanced after traumatic brain injury

Polyamines spermine and spermidine are highly regulated, ubiquitous aliphatic cations that maintain DNA structure and function as immunomodulators and as antioxidants. Polyamine homeostasis is disrupted after brain injuries, with concomitant generation of toxic metabolites that may contribute to secondary injuries. To test the hypothesis of increased brain polyamine catabolism after traumatic brain injury (TBI), we determined changes in catabolic enzymes and polyamine levels in the rat brain after lateral controlled cortical impact TBI. Spermine oxidase (SMO) catalyzes the degradation of spermine to spermidine, generating H2O2 and aminoaldehydes. Spermidine/spermine-N(1)-acetyltransferase (SSAT) catalyzes acetylation of these polyamines, and both are further oxidized in a reaction that generates putrescine, H2O2, and aminoaldehydes. In a rat cortical impact model of TBI, SSAT mRNA increased subacutely (6-24 h) after TBI in ipsilateral cortex and hippocampus. SMO mRNA levels were elevated late, from 3 to 7 days post-injury. Polyamine catabolism increased as well. Spermine levels were normal at 6 h and decreased slightly at 24 h, but were normal again by 72 h post-injury. Spermidine levels also decreased slightly (6-24 h), then increased by approximately 50% at 72 h post-injury. By contrast, normally low putrescine levels increased up to sixfold (6-72 h) after TBI. Moreover, N-acetylspermidine (but not N-acetylspermine) was detectable (24-72 h) near the site of injury, consistent with increased SSAT activity. None of these changes were seen in the contralateral hemisphere. Immunohistochemical confirmation indicated that SSAT and SMO were expressed throughout the brain. SSAT-immunoreactivity (SSAT-ir) increased in both neuronal and nonneuronal (likely glial) populations ipsilateral to injury. Interestingly, bilateral increases in cortical SSAT-ir neurons occurred at 72 h post-injury, whereas hippocampal changes occurred only ipsilaterally. Prolonged increases in brain polyamine catabolism are the likely cause of loss of homeostasis in this pathway. The potential for simple therapeutic interventions (e.g., polyamine supplementation or inhibition of polyamine oxidation) is an exciting implication of these studies.

The long-term microvascular and behavioral consequences of experimental traumatic brain injury after hypothermic intervention

Traumatic brain injury (TBI) has been demonstrated to induce cerebral vascular dysfunction that is reflected in altered responses to various vasodilators. While previous reports have focused primarily on the short-term vascular alterations, few have examined these vascular changes for more than 7 days, or have attempted to correlate these alterations with any persisting behavioral changes or potential therapeutic modulation. Accordingly, we evaluated the long-term microvascular and behavioral consequences of experimental TBI and their therapeutic modulation via hypothermia. In this study, one group was injured with no treatment, another group was injured and 1 h later was treated with 120 min of hypothermia followed by slow rewarming, and a third group was non-injured. Animals equipped with cranial windows for visualization of the pial microvasculature were challenged with various vasodilators, including acetylcholine, hypercapnia, adenosine, pinacidil, and sodium nitroprusside, at either 1 or 3 weeks post-TBI. In addition, all animals were tested for vestibulomotor tasks at 1 week post-TBI, and animals surviving for 3 weeks post-TBI were tested in a Morris water maze (MWM). The results of this investigation demonstrated that TBI resulted in long-term vascular dysfunction in terms of altered vascular reactivity to various vasodilators, which was significantly improved with the use of a delayed 120-min hypothermic treatment. In contrast, data from the MWM task indicated that injured animals revealed persistent deficits in the spatial memory test performance, with hypothermia exerting no protective effects. Collectively, these data illustrate that TBI can evoke long-standing brain vascular and spatial memory dysfunction that manifest different responses to hypothermic intervention. These findings further illustrate the complexity of TBI and highlight the fact that the chosen hypothermic intervention may not necessarily exert a global protective response.

The role of oxidative stress and NADPH oxidase in cerebrovascular disease

The study of reactive oxygen species (ROS) and oxidative stress remains a very active area of biological research, particularly in relation to cellular signaling and the role of ROS in disease. In the cerebral circulation, oxidative stress occurs in diverse forms of disease and with aging. Within the vessel wall, ROS produce complex structural and functional changes that have broad implications for regulation of cerebral perfusion and permeability of the blood-brain barrier. These oxidative-stress-induced changes are thought to contribute to the progression of cerebrovascular disease. Here, we highlight recent findings in relation to oxidative stress in the cerebral vasculature, with an emphasis on the emerging role for NADPH oxidases as a source of ROS and the role of ROS in models of disease.

Cerebral vascular responsiveness after experimental traumatic brain injury: the beneficial effects of delayed hypothermia combined with superoxide dismutase administration

OBJECT

Traumatic brain injury (TBI) induces cerebral vascular dysfunction reflected in altered responses to vasodilators such as acetylcholine and hypercapnia. It has been demonstrated that the use of either posttraumatic hypothermia or free radical scavengers offered vascular protection when those treatments were delivered early after the injury, losing efficacy when the initiation of either treatment was delayed. Because immediate posttraumatic treatment is not realistic in the clinical setting, the authors undertook this study to investigate whether the combination of delayed hypothermia and the delayed administration of the free radical scavenger superoxide dismutase (SOD) could result in improved vascular protection.

METHODS

Male Sprague-Dawley rats were anesthetized and subjected to either an impact-acceleration or sham injury. Animals were treated either with hypothermia (32 degrees C) initiated 60 minutes after TBI, delayed SOD (60 U/ml) applied 90 minutes after TBI, or a combination of delayed hypothermia (32 degrees C) and delayed SOD (60 U/ml) applied 15 minutes prior to the cessation of hypothermia. In this investigation, the diameter of cerebral pial arterioles was measured at rest and then challenged with vasodilator acetylcholine and hypercapnia. Four vessels were assessed per animal prior to injury and then again up to 6 hours after injury.

RESULTS

Delayed SOD treatment did not enhance vascular function, while delayed hypothermia treatment only partially preserved pial vascular function. However, the combination of delayed hypothermia and delayed SOD significantly preserved vascular function after the injury.

CONCLUSIONS

The results of these studies demonstrate that delayed hypothermia partially preserves vascular function after TBI, while expanding the therapeutic window over which agents such as SOD can now provide enhanced protection.

Intensive care unit management of the trauma patient

OBJECTIVE

The goal of this concise review is to provide an overview of some of the most important intensive care unit issues and approaches that are unique to trauma patients as compared with the general intensive care unit population.

STUDY SELECTION

Clinical trials in trauma patients focusing on hemorrhage control, issues in resuscitation, staged operative repair of multiple injuries, the diagnosis and therapy of the abdominal compartment syndrome, and the treatment of traumatic brain injury were identified on PubMed.

CONCLUSIONS

The intensive care unit care of the trauma patient differs from that of other intensive care unit patients in many ways, one of the most important being the need to continuously integrate operative and nonoperative therapy. Although progress in the care of the injured has been made, death due to uncontrolled bleeding, severe head injury, or the development of multiple organ dysfunction syndrome remains all too common in this patient population. Furthermore, due to the potential nature of the injuries, the conundrum not infrequently arises that the optimal treatment for one injury or organ system, such as preoperative permissive hypotension in actively bleeding patients, may result in suboptimal or even deleterious therapy in the presence of another injury, such as traumatic brain injury.

LEARNING OBJECTIVES

On completion of this article, the reader should be able to:Dr. Deitch has disclosed that he is/was the recipient of grant/research funds from Celgene. Dr. Dayal has disclosed that she has no financial relationships with or interests in any commercial companies pertaining to this educational activity. Lippincott CME Institute, Inc., has identified and resolved all faculty conflicts of interest regarding this educational activity. Visit the Critical Care Medicine Web site (www.ccmjournal.org) for information on obtaining continuing medical education credit.

Perivascular nerve damage in the cerebral circulation following traumatic brain injury

Traumatic brain injury (TBI) causes cerebral vascular dysfunction. Most have assumed that it was the result of endothelial and/or smooth muscle alteration. No consideration, however, has been given to the possibility that the forces of injury may also damage the perivascular nerve network, thereby contributing to the observed abnormalities. To test this premise, we subjected rats to impact acceleration. At 6 h, 24 h and 7 days post-TBI, cerebral basal arteries were removed and processed with antibody targeting protein gene product 9.5 (PGP-9.5), with parallel assessments of 5-hydroxytryptamine (5-HT) accumulation in the perivascular nerves. Additionally, Fluoro-Jade was also used as a marker of axonal degeneration. The perivascular nerve network revealed no abnormality in sham animals. However, by 6 h post injury, Fluoro-Jade reactivity appeared in the perivascular regions, with the number of fibers increasing with time. By 24 h post injury, a significant reduction in the perivascular 5-HT accumulation occurred, together with a reduction in PGP-9.5 fiber staining. At 7 days, a recovery of the PGP-9.5 immunoreactivity occurred, however, it did not reach a control-like distribution. These studies suggest that neurogenic damage occurs following TBI and may be a contributor to some of the associated vascular abnormalities.

Effects of delayed, prolonged hypothermia on the pial vascular response after traumatic brain injury in rats

Object. In the experimental setting, hypothermia has been demonstrated to attenuate the damaging consequences of stroke and traumatic brain injury (TBI). Laboratory studies of TBI have focused primarily on the use of early hypothermic intervention, with little consideration of the potential efficacy of more delayed but prolonged hypothermia, which would constitute a more clinically relevant approach. In this investigation, the authors evaluated whether delayed, prolonged hypothermia after TBI protected the cerebral microcirculation.

Methods. Male Sprague—Dawley rats were equipped with cranial windows for direct visualization of the pial arterial circulation and then subjected to impact-acceleration brain injury. The rats were randomly divided into four experimental groups: Group 1 consisted of normothermic animals; in Group 2 the rats received a 1-hour period of hypothermia (32°C) 30 minutes posttrauma, followed by slow rewarming (32–37°C/90 minutes); and in Groups 3 and 4 the rats received a more delayed induction (at 1 hour postinjury) of either 1 hour (Group 3) or 2 hours (Group 4) of hypothermia, followed by the slow rewarming. The pial arteriolar responses to acetylcholine (ACh) or hypercapnia were measured until up to 6 hours postinjury. With this approach the authors found that the normothermic group demonstrated severely impaired vasoreactivity in terms of ACh-dependent dilation and CO2 reactivity in comparison to baseline values (p < 0.001). In contrast, hypothermia of short duration that was initiated early (30 minutes postinjury) conferred significant cerebrovascular protection (p < 0.001), yet this protection was reduced when the onset of this 1-hour hypothermic period was postponed to 1 hour postinjury. Nevertheless, reduced protection could be significantly improved (p < 0.001) with prolongation of the hypothermic period to 2 hours.

Conclusions. The results of this study show that early as well as delayed but prolonged hypothermia attenuate the impaired vascular responsiveness seen after TBI, indicating the potential clinical usefulness of this treatment.

Traumatic Cerebral Vascular Injury: The Effects of Concussive Brain Injury on the Cerebral Vasculature

In terms of human suffering, medical expenses, and lost productivity, head injury is one of the major health care problems in the United States, and inadequate cerebral blood flow is an important contributor to mortality and morbidity after traumatic brain injury. Despite the importance of cerebral vascular dysfunction in the pathophysiology of traumatic brain injury, the effects of trauma on the cerebral circulation have been less well studied than the effects of trauma on the brain. Recent research has led to a better understanding of the physiologic, cellular, and molecular components and causes of traumatic cerebral vascular injury. A more thorough understanding of the direct and indirect effects of trauma on the cerebral vasculature will lead to improvements in current treatments of brain trauma as well as to the development of novel and, hopefully, more effective therapeutic strategies.

Angiotensin II produces superoxide-mediated impairment of endothelial function in cerebral arterioles

BACKGROUND AND PURPOSE

Angiotensin II (Ang II) produces oxidative stress in vascular cells in culture and in extracranial conduit arteries. The goal of this study was to examine the hypothesis that Ang II produces superoxide-mediated impairment of endothelial function in cerebral microvessels.

METHODS

Diameter of cerebral arterioles (baseline diameter=104+/-3 microm) was measured with the use of a closed cranial window in anesthetized rabbits. Topical application of Ang II was used to avoid effects on arterial pressure.

RESULTS

Ang II (0.1 to 1 micromol/L for 2 hours) had no effect on baseline diameter (change in diameter of -3+/-2% in response to 1 micromol/L Ang II) but produced concentration-dependent inhibition of vasodilatation to the endothelium-dependent agonist bradykinin. For example, 1 micromol/L Ang II inhibited responses to 1 nmol/L bradykinin by almost 80%. These inhibitory effects of Ang II were prevented by the superoxide scavenger 4,5-dihydroxy-1,3-benzene-disulfonic acid (Tiron; 10 mmol/L) or diphenylene iodonium (DPI; 3 micromol/L), an inhibitor of NAD(P)H oxidase. Ang II did not inhibit vasodilatation in response to nitroprusside, an endothelium-independent vasodilator.

CONCLUSIONS

These findings are the first evidence that local Ang II produces superoxide-mediated vascular dysfunction in cerebral microvessels. The results with DPI suggest that the source of superoxide may be an NAD(P)H oxidase.

Posttraumatic hypothermia followed by slow rewarming protects the cerebral microcirculation

In the clinical and laboratory setting, multiple reports have suggested the efficacy of hypothermia in blunting the damaging consequences of traumatic brain injury (TBI). With the use of posttraumatic hypothermia, it has been recognized that the time of initiation and duration of hypothermia are important variables in determining the degree of neuroprotection provided. Further, it has been recently recognized that the rate of posttraumatic rewarming is an important variable, with rapid rewarming exacerbating neuronal/axonal damage in contrast to slow rewarming which appears to provide enhanced neuroprotection. Although these findings have been confirmed in the brain parenchyma, no information exists for the cerebral microcirculation on the potential benefits of posttraumatic hypothermia followed by either slow or rapid rewarming. In the current communication we assess these issues in the pial circulation using a well-characterized model of TBI. Rats were prepared for the placement of cranial widows for direct assessment of the pial microcirculation prior to and after the induction of impact acceleration injury followed by moderate hypothermia with either subsequent slow or rapid rewarming strategies. The cranial windows allowed for the measurement of pial vessel diameter to assess ACh-dependent and CO2 reactivity in the chosen paradigms. ACh was applied topically to assess ACh-dependent dilation, while CO2 reactivity was assessed by changing the concentration of the inspired gas. Through this approach, it was found that posttraumatic hypothermia followed by slow rewarming maintained normal arteriolar vascular responses in terms of ACh-dependent dilation and CO2 reactivity. In contrast, arterioles subjected to TBI followed by normothermia or hypothermia and rapid rewarming showed impaired vasoreactivity in terms of their ACh-dependent and CO2 responses. This study provides additional evidence of the benefits of posttraumatic hypothermia followed by slow rewarming, demonstrating for the first time that the previously described neuroprotective effects extend to the cerebral microcirculation.

Jugular venous oxygen saturation or arteriovenous difference of lactate content and outcome in children with severe traumatic brain injury

OBJECTIVE

To assess the association between neurologic out-come and the alterations of jugular venous oxygen saturation (SjvO2) or the increase in arteriovenous difference of lactate content (AVDL) in children with severe traumatic brain injury.

DESIGN

Observational prospective cohort study.

SETTING

Multidisciplinary pediatric intensive care unit of a university hospital.

PATIENTS

A total of 27 pediatric patients with severe traumatic brain injury, with a Glasgow Coma Scale after resuscitation of <9, who were admitted to the pediatric intensive care unit within 36 hrs after injury.

INTERVENTIONS

Intermittent measurement of SjvO2 and AVDL.

MEASUREMENTS AND MAIN RESULTS

SjvO2 and AVDL were assessed simultaneously every 6 hrs. The primary dependent variable measured was assessed independently 3 months after trauma according to the Pediatric Cerebral Performance Category. Patients were classified into two groups: group 1 (favorable outcome, Pediatric Cerebral Performance Category 1-3) and group 2 (unfavorable outcome, Pediatric Cerebral Performance Category 4-6); 81% were included in group 1 and 19% in group 2. A total of 354 measurements of SjvO2 and AVDL were made, with a mean of 13.1 +/- 7.9 per patient. The number of abnormal measurements of SjvO2 and increased AVDL used to predict the neurologic outcome was selected according to the area under the receiver operating characteristic curve. Mortality was 15% (four patients). The strongest association was found between a poor neurologic outcome and two or more pathologic AVDL measurements (higher than -0.37 mmol/L; relative risk, 17.6; 95% confidence interval, 2.5-112.5; p = .001). The presence of two or more measurements of SjvO2 of < or = 55% was significantly associated with a poor neurologic outcome (relative risk, 6.6; 95% confidence interval, 1.5-29.7; p = .003). The frequency of measurements of SjvO2 of > or = 75% was not different between groups 1 and 2.

CONCLUSION

In children with severe traumatic brain injury, two or more measurements of SjvO2 of < or = 55% or two or more pathologic AVDL measurements were associated with a poor neurologic outcome. Further studies are needed to recommend the use of these variables as a guideline to optimize treatment.

Brain tissue oxygen monitoring for assessment of autoregulation: preliminary results suggest a new hypothesis

Brain tissue oxygen monitoring (P(ti)O2 (Neurotrend, Codman, Germany) was employed in addition to standard intracranial pressure (ICP) and cerebral perfusion pressure (CPP) monitoring in seven patients with severe neuronal damage of heterogeneous etiology. The correlation between P(ti)O2 changes and CPP fluctuations during periods of 30 minutes were analyzed, when CPP was above 70 mmHg and lower than 100 mmHg. A new ratio, the CPP-oxygen-reactivity (COR) index was calculated as COR=delta p(ti)O2 %/delta CPP%. The patient COR values were compared to those found in the brain of six noninjured anesthetized piglets. The analysis was performed to determine the significance of synchronous fluctuations of CPP and P(ti)O2, when CPP is above the lower threshold of autoregulation. The correlation between CPP variations and p(ti)O2 variations was found to be strong (R(mean)) = 0.74 +/- 0.17) in the patients and was weak in the uninjured animals (R(mean)) =0.38 +/- 0.43). The COR (mean) was 2.05 +/- 0.57 in patients and 0.78 +/- 0.6 in the animals. In the injured brain of our patients, we observed an unexpectedly close correlation between P(ti)O2 and CPP variations when CPP levels were within a therapeutically targeted range (70 to 100 mmHg). In a porcine model, we could not find this relationship in the noninjured brain. We speculate that an increased COR might be indicative for an impaired local pressure autoregulation. The preliminary data suggest that COR values above "1" might be pathologic. However, the reported sample sizes are too small to provide sufficient statistical power to justify inferential statistical analyses. As such, results are presented with descriptive statistics only, and should be regarded as a hypothesis.

Management of severe head injury: institutional variations in care and effect on outcome

OBJECTIVE

The purpose of this study was three-fold: a) to examine variations in care of patients with severe head injury in academic trauma centers across the United States; b) to determine the proportion of patients who received care according to the Brain Trauma Foundation guidelines; and c) to correlate the outcome from severe traumatic brain injury with the care received.

DESIGN

Retrospective data collection for consecutive patients with closed head injury and long bone fracture admitted over an 8-month period.

SETTING

Thirty-four academic trauma centers in the United States

PATIENTS

All patients admitted with a presenting Glasgow Coma Scale score < or = 8.

MEASUREMENTS AND MAIN RESULTS

Variations in care were assessed, including prehospital intubation, intracranial pressure monitoring, use of osmotic agents, hyperventilation, and computed tomography scan utilization. Aggressive centers were defined as those placing intracranial pressure monitors in >50% of patients meeting the Brain Trauma Foundation criteria for intracranial pressure monitoring. The primary outcome variables were mortality, functional status at discharge, and length of stay. Kaplan-Meier survival analysis was performed for aggressive vs. nonaggressive centers. A Cox proportional hazard model was used to evaluate the association between type of center and mortality rate. Length of stay was evaluated by using linear regression.

RESULTS

There was considerable variation in the rates of prehospital intubation, intracranial pressure monitoring, intracranial pressure-directed therapy, and head computed tomography scan utilization across centers. Management at an aggressive center was associated with a significant reduction in the risk of mortality (hazard ratio, 0.43; 95% confidence interval, 0.27-0.66). There was no statistically significant difference in functional status at the time of discharge for survivors. Adjusted length of stay for survivors at aggressive centers was shorter, compared with the length of stay at nonaggressive centers: -6 days (95% confidence interval, -14 to 2 days).

CONCLUSION

Considerable national variation in the care of severely head-injured patients persists. An "aggressive" management strategy is associated with decreased mortality rate for patients with severe head injury, with no significant difference in functional status at discharge among survivors.

Influence of early posttraumatic hypothermia therapy on local cerebral blood flow and glucose metabolism after fluid-percussion brain injury

Object

Using autoradiographic image averaging, the authors recently described prominent foci of marked glucose metabolism-greater-than-blood-flow uncoupling in the acutely traumatized rat brain. Because hypothermia is known to ameliorate injury in this and other injury models, the authors designed the present study to assess the effects of post-traumatic therapeutic hypothermia on the local cerebral metabolic rate of glucose (LCMRglu) and local cerebral blood flow (LCBF) following moderate parasagittal fluid-percussion head injury (FPI) in rats.

Methods

Either cranial hypothermia (30°C) or normothermia (37°C) was induced for 3 hours in matched groups of rats immediately after FPI; LCMRglu and LCBF were assessed 3 hours after concluding these temperature manipulations.

In rats subjected to FPI, regardless of whether normothermia or hypothermia ensued, LCBF was reduced relative to the sham-injury groups. In addition, when FPI was followed by hypothermia (FPI–30°C group), the subsequent LCBF was significantly lower (35–38% on average) than in FPI–37°C rats. Statistical mapping of LCBF difference imaging data revealed confluent cortical and subcortical zones of significantly reduced LCBF (largely ipsilateral to the prior injury) in FPI–30°C rats relative to the FPI–37°C group. Local glucose utilization was reduced in both hemispheres of FPI–37°C rats relative to the sham-injury group and was lower in the right (traumatized) hemisphere than in the left. However, LCMRglu values were largely unaffected by temperature manipulation in either the FPI or sham-injury groups. The LCMRglu/LCBF ratio was nearly doubled in FPI–30°C rats relative to the FPI–37°C group, in a diffuse and bihemispheric fashion. Linear regression analysis comparing LCMRglu and LCBF revealed that the FPI–37°C and FPI–30°C data sets were completely nonoverlapping, whereas the two sham-injury data sets were intermixed.

Conclusions

Despite its proven neuroprotective efficacy, early posttraumatic hypothermia (30°C for 3 hours) nonetheless induces a moderate decline in cerebral perfusion without the (anticipated) improvement in cerebral glucose utilization, so that a state of mild metabolism-greater-than-blood-flow dissociation is perpetuated.

Superoxide levels and function of cerebral blood vessels after inhibition of CuZn-SOD

The goal of this study was to examine the role of endogenous copper/zinc (CuZn)-superoxide dismutase (SOD) on superoxide levels and on responses of cerebral blood vessels to stimuli that are mediated by nitric oxide (acetylcholine) and cyclooxygenase-dependent mechanisms (bradykinin and arachidonic acid). Levels of superoxide in the rabbit basilar artery were measured using lucigenin-enhanced chemiluminescence (5 microM lucigenin). Diethyldithiocarbamate (DDC; 10 mM), an inhibitor of CuZn-SOD, increased superoxide levels by approximately 2.4-fold (P < 0.05) from a baseline value of 1.0 +/- 0.2 relative light units x min(-1) x mm(-2) (means +/- SE). The diameter of cerebral arterioles (baseline diameter, 99 +/- 3 microm) was also measured using a closed cranial window in anesthetized rabbits. Topical application of DDC attenuated responses to acetylcholine, bradykinin, and arachidonate, but not nitroprusside. For example, 10 microM arachidonic acid dilated cerebral arterioles by 40 +/- 5 and 2 +/- 2 microm under control conditions and after DDC, respectively (P < 0.05). These inhibitory effects of DDC were reversed by the superoxide scavenger 4,5-dihydroxy-1,3-benzenedisulfonic acid (10 mM). Arachidonate increased superoxide levels in the basilar artery moderately under normal conditions and this increase was greatly augmented in the presence of DDC. These findings suggest that endogenous CuZn-SOD limits superoxide levels under basal conditions and has a marked influence on increases in superoxide in vessels exposed to arachidonic acid. The results also suggest that nitric oxide- and cyclooxygenase-mediated responses in the cerebral microcirculation are dependent on normal activity of CuZn-SOD.

Elevated jugular venous oxygen saturation after severe head injury

Object

The aim of this study was to investigate the incidence of elevated (≥ 75%) jugular venous oxygen saturation (SjvO2) and its relationship to cerebral hemodynamic and metabolic parameters and to outcome after severe head injury.

Methods

Data from 450 severely head injured patients admitted to the Neurosurgical Intensive Care Unit of Ben Taub General Hospital were analyzed retrospectively. The SjvO2 was measured in blood obtained from indwelling jugular bulb catheters. Patients were classified into the following categories: high (Group I), normal (Group II), or low SjvO2 (Group III) if their mean SjvO2 over the duration of monitoring was 75% or higher, 74 to 56%, or 55% or lower, respectively.

A high SjvO2 occurred in 19.1% of patients. There was no consistent relationship between SjvO2 and simultaneous cerebral blood flow (CBF) or cerebral perfusion pressure measurements. Compared with Groups II and III, the patients in Group I had a significantly higher CBF and lower cerebral metabolic rate of oxygen (CMRO2). In Group I, the out- comes were death or persistent vegetative state in 48.8% of patients and severe disability in 25.6%. These outcomes were significantly worse than for patients in Group II. Within Group I, the patients with a poor neurological outcome were older and more likely to have suffered a focal head injury; they demonstrated a lower CMRO2 and a greater rate of cerebral lactate production than the patients who attained a favorable outcome.

Conclusions

Posttraumatic elevation of SjvO2 is common but cannot be automatically equated with hyperemia. Instead, elevated SjvO2 is a heterogeneous condition that is associated with poor outcome after head injury and may carry important implications for the management of comatose patients.

Effects of hyperbaric oxygenation

OBJECT

Hyperbaric oxygenation (HBO) therapy has been shown to reduce mortality by 50% in a prospective randomized trial of severely brain injured patients conducted at the authors' institution. The purpose of the present study was to determine the effects of HBO on cerebral blood flow (CBF), cerebral metabolism, and intracranial pressure (ICP), and to determine the optimal HBO treatment paradigm.

METHODS

Oxygen (100% O2, 1.5 atm absolute) was delivered to 37 patients in a hyperbaric chamber for 60 minutes every 24 hours (maximum of seven treatments/patient). Cerebral blood flow, arteriovenous oxygen difference (AVDO2), cerebral metabolic rate of oxygen (CMRO2), ventricular cerebrospinal fluid (CSF) lactate, and ICP values were obtained 1 hour before and 1 hour and 6 hours after a session in an HBO chamber. Patients were assigned to one of three categories according to whether they had reduced, normal, or raised CBF before HBO. In patients in whom CBF levels were reduced before HBO sessions, both CBF and CMRO2 levels were raised 1 hour and 6 hours after HBO (p < 0.05). In patients in whom CBF levels were normal before HBO sessions, both CBF and CMRO2 levels were increased at 1 hour (p < 0.05), but were decreased by 6 hours after HBO. Cerebral blood flow was reduced 1 hour and 6 hours after HBO (p < 0.05), but CMRO2 was unchanged in patients who had exhibited a raised CBF before an HBO session. In all patients AVDO2 remained constant both before and after HBO. Levels of CSF lactate were consistently decreased 1 hour and 6 hours after HBO, regardless of the patient's CBF category before undergoing HBO (p < 0.05). Intracranial pressure values higher than 15 mm Hg before HBO were decreased 1 hour and 6 hours after HBO (p < 0.05). The effects of each HBO treatment did not last until the next session in the hyperbaric chamber.

CONCLUSIONS

The increased CMRO2 and decreased CSF lactate levels after treatment indicate that HBO may improve aerobic metabolism in severely brain injured patients. This is the first study to demonstrate a prolonged effect of HBO treatment on CBF and cerebral metabolism. On the basis of their data the authors assert that shorter, more frequent exposure to HBO may optimize treatment.

Potentiated endothelium-derived hyperpolarizing factor-mediated dilations in cerebral arteries following mild head injury

Evidence in the literature suggests that endothelium-derived hyperpolarizing factor (EDHF) may act in a compensatory manner such that during conditions of compromised nitric oxide (NO), EDHF serves as a back-up mechanism. Given that constitutive NO synthase is chronically downregulated after head trauma, we tested the hypothesis that EDHF is potentiated following injury. Male adult rats were subjected to either sham injury (n = 27) or mild controlled cortical impact (CCI) injury (n = 26). Branches of the middle cerebral artery (MCA) directly within the contusion site were harvested either 1 or 24 h later, pressurized to 60 mm Hg in a vessel chamber and allowed to develop spontaneous tone. Relaxation to luminal application of adenosine triphosphate (ATP) was similar in all groups. Relaxation to ATP in the presence of L-NAME (N(G)-nitro-L-arginine methyl ester) and indomethacin was similar in all groups except for vessels isolated at 24 h following mild CCI injury. In this case, L-NAME and indomethacin had no effect on the ATP-mediated dilation. The ATP-mediated dilation in L-NAME and indomethacin-treated MCA branches was inhibited by charybdotoxin, an inhibitor of large conductance Ca2+-sensitive K+ channels. These findings suggest that there is a significant potentiation of the EDHF-mediated dilation to ATP in cerebral arteries isolated at 24 h following mild CCI injury.

Delayed hemorrhagic hypotension exacerbates the hemodynamic and histopathologic consequences of traumatic brain injury in rats

Alterations in cerebral autoregulation and cerebrovascular reactivity after traumatic brain injury (TBI) may increase the susceptibility of the brain to secondary insults, including arterial hypotension. The purpose of this study was to evaluate the consequences of mild hemorrhagic hypotension on hemodynamic and histopathologic outcome after TBI. Intubated, anesthetized male rats were subjected to moderate (1.94 to 2.18 atm) parasagittal fluid-percussion (FP) brain injury. After TBI, animals were exposed to either normotension (group 1: TBI alone, n = 6) or hypotension (group 2: TBI + hypotension, n = 6). Moderate hypotension (60 mm Hg/30 min) was induced 5 minutes after TBI or sham procedures by hemorrhage. Sham-operated controls (group 3, n = 7) underwent an induced hypotensive period, whereas normotensive controls (group 4, n = 4) did not. For measuring regional cerebral blood flow (rCBF), radiolabeled microspheres were injected before, 20 minutes after, and 60 minutes after TBI (n = 23). For quantitative histopathologic evaluation, separate groups of animals were perfusion-fixed 3 days after TBI (n = 22). At 20 minutes after TBI, rCBF was bilaterally reduced by 57% +/- 6% and 48% +/- 11% in cortical and subcortical brain regions, respectively, under normotensive conditions. Compared with normotensive TBI rats, hemodynamic depression was significantly greater with induced hypotension in the histopathologically vulnerable (P1) posterior parietal cortex (P < 0.01). Secondary hypotension also increased contusion area at specific bregma levels compared with normotensive TBI rats (P < 0.05), as well as overall contusion volume (0.96 +/- 0.46 mm(3) vs. 2.02 +/- 0.51 mm(3), mean +/- SD, P < 0.05). These findings demonstrate that mild hemorrhagic hypotension after FP injury worsens local histopathologic outcome, possibly through vascular mechanisms.

Endothelial cell activation following moderate traumatic brain injury

Traumatic brain injury (TBI) initiates a cascade of acute and chronic injury responses which include disturbances in the cerebrovasculature that may result in the activation of the microvascular endothelial development of a dysfunction endothelium. The present study examines endothelial cell (EC) activation in a percussion model of moderate TBI. The criteria for endothelial activation used in these studies was surface expression of a number of markers collectively termed endothelial activation antigens. Temporal induction of the major histocompatibility (MHC) class II molecules, E-selectin (CD62E), vascular cell adhesion molecule (VACM-1) (CD106) as well as altered expression of constitutively expressed intercellular adhesion molecule-1 (ICAM-1) (CD54), the glucose transporter protein (glut-1), the transferrin receptor (tfR) (CD71), and MHC class I molecules was examined at various times following impact. Induction of E-selectin and increased expression of ICAM-1 was seen by 2 h post-impact (PI) and was sustained through 24 h PI. Decreased expression of immunologically reactive glut-1 and tfR was observed by 2-4 h PI and remained low up to 24 h PI. No induction of VCAM-1, MHC class II molecules or altered constitutive expression or MHC class I molecules was seen. Changes in EC activation were observed predominantly at the site of impact and were diminished temporarily. These results indicate that mild concussive injury to the brain results in activation of the endothelium.

The relation between acute physiological variables and outcome on the Glasgow Outcome Scale and Disability Rating Scale following severe traumatic brain injury

The relation between outcome and duration of adverse physiological events was studied, using suggested critical physiological values. Subjects were 184 patients with severe traumatic brain injury who received continuous monitoring of intracranial pressure (ICP), mean arterial pressure (MAP), cerebral perfusion pressure (CPP), and jugular venous oxygen saturation. Longer durations of adverse physiological events were significantly related to Glasgow Outcome Scale (GOS) scores and Disability Rating Scale (DRS) scores for all variables at all timepoints postinjury. When analyses excluded patients who died, the relation between adverse physiological events and GOS was nonsignificant; however, duration of ICP, MAP, and CPP still accounted for a significant portion of the variance in DRS scalres. The relative sensitivity of the GOS and DRS is discussed.

The Brain Trauma Foundation. The American Association of Neurological Surgeons. The Joint Section on Neurotrauma and Critical Care. Guidelines for cerebral perfusion pressure

Maintenance of a CPP above 70 mm Hg is a therapeutic option that may be associated with a substantial reduction in mortality and improvement in quality of survival and is likely to enhance perfusion to ischemic regions of the brain following severe TBI. No study has demonstrated that the incidence of intracranial hypertension, morbidity, or mortality is increased by the active maintenance of CPP above 70 mm Hg, even if this means normalizing the intravascular volume or inducing systemic hypertension.

L-arginine partially restores the diminished CO2 reactivity after mild controlled cortical impact injury in the adult rat

Using an open cranial window technique, the authors investigated the mechanisms associated with the suppressed CO2 reactivity after mild controlled cortical impact (CCI) injury in rats. The dilation of arterioles (n = 7) to hypercapnia before injury was 38 +/- 12%, which was significantly reduced both at 1 hour (23 +/- 15% dilation) and at 2 hours after injury (11 +/- 19% dilation). In the presence of L-arginine (10 mmol/L topical suffusion, 300 mg/kg intravenous infusion), the dilation of pial arterioles (n = 6) to hypercapnia was partially restored to 30 +/- 6% at 2 hours after injury. In the presence of the nitric oxide (NO) donor, S-nitroso-N-acetylpenicillamine (SNAP) (10(-8) mol/L topical suffusion), the dilation of pial arterioles (n = 5) to hypercapnia remained diminished (5 +/- 7%) at 2 hours after injury. The dilation of pial arterioles (n = 4) to hypercapnia also remained suppressed (5 +/- 6%) with topical suffusion of the free radical scavengers, polyethylene glycol-superoxide dismutase (60 units/mL) and polyethylene glycol-catalase (40 units/mL). The authors have shown that L-arginine at least partially restores the diminished response to hypercapnia after mild CCI injury. Furthermore, these data suggest that the beneficial effects of L-arginine are mediated by a combination of providing substrate for NO synthase and scavenging free radicals.

The consequences of traumatic brain injury on cerebral blood flow and autoregulation: a review

In this decade, the brain argueably stands as one of the most exciting and challenging organs to study. Exciting in as far as that it remains an area of research vastly unknown and challenging due to the very nature of its anatomical design: the skull provides a formidable barrier and direct observations of intraparenchymal function in vivo are impractical. Moreover, traumatic brain injury (TBI) brings with it added complexities and nuances. The development of irreversible damage following TBI involves a plethora of biochemical events, including impairment of the cerebral vasculature, which render the brain at risk to secondary insults such as ischemia and intracranial hypertension. The present review will focus on alterations in the cerebrovasculature following TBI, and more specifically on changes in cerebral blood flow (CBF), mediators of CBF including local chemical mediators such as K+, pH and adenosine, endothelial mediators such as nitric oxide and neurogenic mediators such as catecholamines, as well as pressure autoregulation. It is emphasized that further research into these mechanisms may help attenuate the prevalence of secondary insults and therefore improve outcome following TBI.

Influence of early posttraumatic hypothermia therapy on local cerebral blood flow and glucose metabolism after fluid-percussion brain injury

OBJECT

Using autoradiographic image averaging, the authors recently described prominent foci of marked glucose metabolism-greater-than-blood-flow uncoupling in the acutely traumatized rat brain. Because hypothermia is known to ameliorate injury in this and other injury models, the authors designed the present study to assess the effects of posttraumatic therapeutic hypothermia on the local cerebral metabolic rate of glucose (LCMRglu) and local cerebral blood flow (LCBF) following moderate parasagittal fluid-percussion head injury (FPI) in rats.

METHODS

Either cranial hypothermia (30 degrees C) or normothermia (37 degrees C) was induced for 3 hours in matched groups of rats immediately after FPI; LCMRglu and LCBF were assessed 3 hours after concluding these temperature manipulations. In rats subjected to FPI, regardless of whether normothermia or hypothermia ensued, LCBF was reduced relative to the sham-injury groups. In addition, when FPI was followed by hypothermia (FPI-30 degrees C group), the subsequent LCBF was significantly lower (35-38% on average) than in FPI-37 degrees C rats. Statistical mapping of LCBF difference imaging data revealed confluent cortical and subcortical zones of significantly reduced LCBF (largely ipsilateral to the prior injury) in FPI-30 degrees C rats relative to the FPI-37 degrees C group. Local glucose utilization was reduced in both hemispheres of FPI-37 degrees C rats relative to the sham-injury group and was lower in the right (traumatized) hemisphere than in the left. However, LCMRglu values were largely unaffected by temperature manipulation in either the FPI or sham-injury groups. The LCMRglu/LCBF ratio was nearly doubled in FPI-30 degrees C rats relative to the FPI-37 degrees C group, in a diffuse and bihemispheric fashion. Linear regression analysis comparing LCMRglu and LCBF revealed that the FPI-37 degrees C and FPI-30 degrees C data sets were completely nonoverlapping, whereas the two sham-injury data sets were intermixed.

CONCLUSIONS

Despite its proven neuroprotective efficacy, early posttraumatic hypothermia (30 degrees C for 3 hours) nonetheless induces a moderate decline in cerebral perfusion without the (anticipated) improvement in cerebral glucose utilization, so that a state of mild metabolism-greater-than-blood-flow dissociation is perpetuated.

Effects of L-NAME and 7-NI on NOS catalytic activity and behavioral outcome after traumatic brain injury in the rat

Traumatic brain injury (TBI) produces transient increases in constitutive nitric oxide synthase (cNOS) activity and prolonged behavioral abnormalities. This study investigated the effects of nitro-L-arginine-methyl ester (L-NAME) and 3-bromo-7-nitroindazole (7-NI) treatment on cNOS catalytic activity and sensorimotor behavioral outcome after TBI. Rats underwent moderate (1.8-2.2 atm) parasagittal fluid percussion brain injury (FPI). At 5 min after FPI, cNOS activity was significantly increased within the damaged cerebral cortex of vehicle-treated rats compared to the noninjured contralateral cortex (206.7 +/- 150.5 % of contralateral, p < 0.01). Pretreatment with L-NAME and 7-NI significantly reduced injury-induced cNOS activation (47.7 +/- 42.6 %, p < 0.05, and 96.16 +/- 12.76, p < 0.05, respectively). Pretreatment with L-NAME and 7-NI also inhibited cNOS activity within the contralateral noninjured cerebral cortex compared to vehicle-treated rats (L-NAME 43.7 +/- 12.47%, p < 0.05; 7-NI 36.8 +/- 7.47%, p < 0.05). Furthermore, pretreatment with 7-NI, but not L-NAME, significantly reduced forelimb placing sensorimotor deficits (3.14 +/- 1.07, p < 0.05) at 1 day after TBI compared to vehicle-treated rats (5.38 +/- 0.42). These data indicate that inhibition of injury-induced elevations in neuronal NOS activity has a beneficial effect on neurological outcome after parasagittal FPI brain injury.

Endogenous glutathione protects cerebral endothelial cells from traumatic injury

Blood-brain barrier breakdown and edema, indicative of cerebrovascular injury, are characteristic pathophysiologic outcomes following head trauma. These injuries result from both primary mechanical damage and from secondary events initiated by the traumatic insult. Free radicals are recognized as mediators of secondary injury in a number of trauma models. In this study, we used a novel in vitro model of traumatic microvascular injury to test the hypothesis that endogenous glutathione protects cerebral endothelial cells from secondary autooxidative injury following mechanical trauma. Porcine brain cerebral endothelial cells were grown in tissue culture wells with Silastic membrane bottoms, and cellular injury was induced by displacing the membrane different distances with user-defined pressure pulses from a customized device. The resultant endothelial cell injury 2 h following stretch was determined by measuring lactate dehydrogenase in the culture media. Significant stretch-dependent increases in endothelial injury were elicited that depended in a nonlinear fashion on the degree of membrane displacement. Depletion of intracellular glutathione with buthionine sulfoximine (1 mM) increased the extent of traumatic endothelial cell injury by 17-56%, particularly at low to moderate levels of traumatic injury (30-40% of total endothelial cell LDH release). Conversely, traumatic injury was reduced by 22-45% when endothelial cell glutathione levels were augmented threefold (to 140+/-8 nmol/mg protein) by preincubating cells with 2 mM glutathione; the extent of protection was inversely proportional to the extent of the traumatic stretch. Traumatic endothelial cell injury was also significantly and dose-dependently attenuated (up to 40%) by treatment with the xanthine oxidase inhibitor oxypurinol (50 and 100 microM). These results demonstrate that cerebral endothelial cells are the targets of hydrogen peroxide-mediated injury secondary to trauma-induced superoxide radical formation via the xanthine oxidase pathway. The neutralization of peroxides by the endogenous glutathione redox cycle provides endothelial cells a finite capacity to reduce free radical-mediated traumatic injury; this cycle may be amenable to therapeutic manipulation to mitigate posttraumatic edema and other manifestations of vascular dysfunction.

Elevated jugular venous oxygen saturation after severe head injury

OBJECT

The aim of this study was to investigate the incidence of elevated (> or = 75%) jugular venous oxygen saturation (SjvO2) and its relationship to cerebral hemodynamic and metabolic parameters and to outcome after severe head injury.

METHODS

Data from 450 severely head injured patients admitted to the Neurosurgical Intensive Care Unit of Ben Taub General Hospital were analyzed retrospectively. The SjvO2 was measured in blood obtained from indwelling jugular bulb catheters. Patients were classified into the following categories: high (Group I), normal (Group II), or low SjvO2 (Group III) if their mean SjvO2 over the duration of monitoring was 75% or higher, 74 to 56%, or 55% or lower, respectively. A high SjvO2 occurred in 19.1% of patients. There was no consistent relationship between SjvO2 and simultaneous cerebral blood flow (CBF) or cerebral perfusion pressure measurements. Compared with Groups II and III, the patients in Group I had a significantly higher CBF and lower cerebral metabolic rate of oxygen (CMRO2). In Group I, the outcomes were death or persistent vegetative state in 48.8% of patients and severe disability in 25.6%. These outcomes were significantly worse than for patients in Group II. Within Group I, the patients with a poor neurological outcome were older and more likely to have suffered a focal head injury; they demonstrated a lower CMRO2 and a greater rate of cerebral lactate production than the patients who attained a favorable outcome.

CONCLUSIONS

Posttraumatic elevation of SjvO2 is common but cannot be automatically equated with hyperemia. Instead, elevated SjvO2 is a heterogeneous condition that is associated with poor outcome after head injury and may carry important implications for the management of comatose patients.

Monitoring brain oxygen tension in severe head injury: the Rotterdam experience

Cerebral ischemia is considered the central mechanism leading to secondary brain damage in patients with severe head injury. We investigated the technique of continuous monitoring of local brain tissue oxygen tension as parameter for cerebral oxygenation. Eighty-two patients with non penetrating severe head injury were studied. No complications of the monitoring technique were seen. Postmeasurement calibration of the catheters showed a very low zero drift and acceptable sensitivity drift. Low PbrO2 values were seen within the first 12 to 24 hours of injury. Early occurrence of values below 10 mm Hg indicated a poor prognosis. Comparative measurements between two catheters performed in six patients showed differences in absolute values measured, but a good correlation of relative changes was observed. We conclude that continuous monitoring of PbrO2 is reliable, clinically applicable and provides the clinician with a better insight in cerebral oxygenation and hopefully should help in targeting therapy towards improved cerebral oxygenation.

-The effect of ondansetron on intracranial pressure and cerebral perfusion pressure in neurosurgical patients-

OBJECTIVE

To determine the effect of ondansetron on intracranial pressure (ICP), mean arterial pressure (MAP) and cerebral perfusion pressure (CPP).

STUDY DESIGN

Prospective, comparative, randomized double-blind study.

PATIENTS

Twenty-six patients undergoing intracranial surgery.

METHOD

Induction was obtained with propofol (1-2.5 mg.kg-1), fentanyl (1.5 micrograms.kg-1) and pancuronium (0.1 mg.kg-1), and maintenance was achieved with propofol and fentanyl. Intermittent positive pressure ventilation was used to ensure mild hypocapnia at 35 +/- 2 mmHg. Positioning of the patient was followed by 15 minutes steady-state. Patient received thereafter either 8 mg ondansetron or a placebo intravenously. The ICP was measured using a lumbar malleable spinal needle. CPP was calculated using the formula CCP = MAP-ICP. All variables were measured every minute for 15 minutes.

RESULTS

The ICP, MAP and CPP did not differ between the two groups. There were no differences in the highest ICP values in patients receiving either ondansetron or placebo (11 +/- 5 versus 9 +/- 5, mean +/- SD), respectively.

CONCLUSION

Intravenous administration of 8 mg ondansetron affects neither cerebral hemodynamics nor ICP.

Endothelial-mediated dilations following severe controlled cortical impact injury in the rat middle cerebral artery

The mechanisms associated with dysfunction of the cerebral vasculature following head trauma have not yet been fully elucidated. In an attempt to shed more light on the matter, we investigated the endothelial-mediated dilations in the rat middle cerebral artery (MCA) following severe traumatic brain injury (TBI). Rats were subjected to severe controlled cortical impact injury (CCI; 5 m/s, 130 ms duration, 3 mm deformation) over the right parietal cortex. At 24 h postinjury, ipsilateral segments of MCA and corresponding contralateral segments were isolated, mounted in a vessel chamber, and pressurized. The responses to 2 methylthio-ATP (2MeSATP), a selective agonist for the P2Y1 purinoceptors, N(omega)-nitro-L-arginine (L-NAME), an NO synthase inhibitor, and S-nitroso-N-acetylpenicillamine (SNAP), an exogenous NO donor, were determined. 2MeSATP elicited concentration dependent dilations in all MCAs studied. Ipsilateral MCAs harvested following TBI or sham-TBI, showed similar maximum dilations to 2MeSATP [70 +/- 4% (n = 17) and 72 +/- 6% (n = 13), respectively]. However, TBI reduced the concentration of 2MeSATP necessary to elicit one-half of the maximum dilation (EC50) from 15 to 9 nM (p < 0.05). Inhibition of NO synthase with 10(-5) M L-NAME abolished the dilation to 2MeSATP in both TBI and sham-TBI MCAs. The constriction to L-NAME was significantly reduced in TBI MCAs compared to sham vessels. Dilations to SNAP, an NO donor, were not altered by TBI indicating that the mechanisms of dilation involving NO in the vascular smooth muscle were not affected. Unlike other pathological conditions which often diminish endothelial-mediated responses, severe TBI enhanced the sensitivity to 2MeSATP without altering the maximum response.

Head injury in children

Trauma in the United States is the leading cause of death and disability in the pediatric population. Differences of age and development affect recovery and outcome following head injury. Mechanisms and pathophysiology of head injury are varied in both the pediatric and adult populations. Assessments of injury are varied and some measurements are more sensitive than others as well as more specific with regard to certain types of injury. Treatment and management should be tailored to each case in order to effect a positive outcome with respect to brain functioning. Aggressive intervention for prevention of primary and secondary injury must be continued and understanding of the impact of these injuries should provide for a brighter future for these patients.

Effects of ozone exposure in rats on memory and levels of brain and pulmonary superoxide dismutase

Exposure to ozone results in increased production of free radicals, which causes oxidative stress. The objective of this study was to determine the effect of different doses of ozone exposure on memory and to correlate this with pulmonary and brain Cu/Zn superoxide dismutase (SOD) levels. Male Wistar rats were exposed for 4 h to one of the following ozone concentrations: 0, 0.1, 0.2, 0.5, or 1 ppm. Subsequently, they were tested in a passive avoidance conditioning protocol to measure short and long-term memory. Motor activity was determined 1 and 24 h after ozone exposure. Cu/Zn SOD levels in the brain and pulmonary tissue were also measured. Rats exposed for 4 h to 0.2, 0.5, and 1 ppm ozone showed long-term memory deterioration and decreased motor activity, which was reversed 24 h later. Brain and pulmonary Cu/Zn SOD levels were increased in animals exposed to 0.1, 0.2, and 0.5 ppm ozone doses, but decreased in animals exposed to 1 ppm ozone. The results suggest that ozone exposure affects long-term memory possibly in association oxidative stress.

Regulation of the cerebral circulation: role of endothelium and potassium channels

Several new concepts have emerged in relation to mechanisms that contribute to regulation of the cerebral circulation. This review focuses on some physiological mechanisms of cerebral vasodilatation and alteration of these mechanisms by disease states. One mechanism involves release of vasoactive factors by the endothelium that affect underlying vascular muscle. These factors include endothelium-derived relaxing factor (nitric oxide), prostacyclin, and endothelium-derived hyperpolarizing factor(s). The normal vasodilator influence of endothelium is impaired by some disease states. Under pathophysiological conditions, endothelium may produce potent contracting factors such as endothelin. Another major mechanism of regulation of cerebral vascular tone relates to potassium channels. Activation of potassium channels appears to mediate relaxation of cerebral vessels to diverse stimuli including receptor-mediated agonists, intracellular second messenger, and hypoxia. Endothelial- and potassium channel-based mechanisms are related because several endothelium-derived factors produce relaxation by activation of potassium channels. The influence of potassium channels may be altered by disease states including chronic hypertension, subarachnoid hemorrhage, and diabetes.