Cerebral hemodynamic effects of phenylephrine and L-arginine after cortical impact injury

Targeting hydrogen sulfide and nitric oxide to repair cardiovascular injury after trauma

The systemic cardiovascular effects of major trauma, especially neurotrauma, contribute to death and permanent disability in trauma patients and treatments are needed to improve outcomes. In some trauma patients, dysfunction of the autonomic nervous system produces a state of adrenergic overstimulation, causing either a sustained elevation in catecholamines (sympathetic storm) or oscillating bursts of paroxysmal sympathetic hyperactivity. Trauma can also activate innate immune responses that release cytokines and damage-associated molecular patterns into the circulation. This combination of altered autonomic nervous system function and widespread systemic inflammation produces secondary cardiovascular injury, including hypertension, damage to cardiac tissue, vascular endothelial dysfunction, coagulopathy and multiorgan failure. The gasotransmitters nitric oxide (NO) and hydrogen sulfide (H2S) are small gaseous molecules with potent effects on vascular tone regulation. Exogenous NO (inhaled) has potential therapeutic benefit in cardio-cerebrovascular diseases, but limited data suggests potential efficacy in traumatic brain injury (TBI). H2S is a modulator of NO signaling and autonomic nervous system function that has also been used as a drug for cardio-cerebrovascular diseases. The inhaled gases NO and H2S are potential treatments to restore cardio-cerebrovascular function in the post-trauma period.

Revisiting Traumatic Brain Injury: From Molecular Mechanisms to Therapeutic Interventions

Studying the complex molecular mechanisms involved in traumatic brain injury (TBI) is crucial for developing new therapies for TBI. Current treatments for TBI are primarily focused on patient stabilization and symptom mitigation. However, the field lacks defined therapies to prevent cell death, oxidative stress, and inflammatory cascades which lead to chronic pathology. Little can be done to treat the mechanical damage that occurs during the primary insult of a TBI; however, secondary injury mechanisms, such as inflammation, blood-brain barrier (BBB) breakdown, edema formation, excitotoxicity, oxidative stress, and cell death, can be targeted by therapeutic interventions. Elucidating the many mechanisms underlying secondary injury and studying targets of neuroprotective therapeutic agents is critical for developing new treatments. Therefore, we present a review on the molecular events following TBI from inflammation to programmed cell death and discuss current research and the latest therapeutic strategies to help understand TBI-mediated secondary injury.

Cerebrovascular Response to Phenylephrine in Traumatic Brain Injury: A Scoping Systematic Review of the Human and Animal Literature

Intravenous phenylephrine (PE) is utilized commonly in critical care for cardiovascular support. Its impact on the cerebrovasculature is unclear and its use may have important implications during states of critical neurological illness. The aim of this study was to perform a scoping review of the literature on the cerebrovascular/cerebral blood flow (CBF) effects of PE in traumatic brain injury (TBI), evaluating both animal models and human studies. We searched MEDLINE, BIOSIS, EMBASE, Global Health, SCOPUS, and the Cochrane Library from inception to January 2020. We identified 12 studies with various animal models and 4 studies in humans with varying TBI pathology. There was a trend toward a consistent increase in mean arterial pressure (MAP) by the injection of PE systemically, and by proxy, an increase of the cerebral perfusion pressure (CPP). There was a consistent constriction of cerebral vessels by PE reported in the small number of studies documenting such a response. However, the heterogeneity of the literature on the CBF/cerebral blood volume (CBV) response makes the strength of the conclusions on PE limited. Studies were heterogeneous in design and had significant limitations, with most failing to adjust for confounding factors in cerebrovascular/CBF response. This review highlights the significant knowledge gap on the cerebrovascular/CBF effects of PE administration in TBI, calling for further study on the impact of PE on the cerebrovasculature both in vivo and in experimental settings.

Intracranial Pressure Monitoring in Experimental Traumatic Brain Injury: Implications for Clinical Management

Traumatic brain injury (TBI) is often associated with long-term disability and chronic neurological sequelae. One common contributor to unfavorable outcomes is secondary brain injury, which is potentially treatable and preventable through appropriate management of patients in the neurosurgical intensive care unit. Intracranial pressure (ICP) is currently the predominant neurological-specific physiological parameter used to direct the care of severe TBI (sTBI) patients. However, recent clinical evidence has called into question the association of ICP monitoring with improved clinical outcome. The detailed cellular and molecular derangements associated with intracranial hypertension (IC-HTN) and their relationship to injury phenotype and neurological outcomes are not completely understood. Various animal models of TBI have been developed, but the clinical applicability of ICP monitoring in the pre-clinical setting has not been well-characterized. Linking basic mechanistic studies in translational TBI models with investigation of ICP monitoring that more faithfully replicates the clinical setting will provide clinical investigators with a more informed understanding of the pathophysiology of IC-HTN, thus facilitating development of improved therapies for sTBI patients.

Arginase overexpression in neurons and its effect on traumatic brain injury

Arginine is a semi-essential amino acid which serves as a substrate for nitric oxide (NO) production by nitric oxide synthase (NOS) and a precursor for various metabolites including ornithine, creatine, polyamines, and agmatine. Arginase competes with nitric oxide synthase for substrate arginine to produce orthinine and urea. There is contradictory evidence in the literature on the role of nitric oxide in the pathophysiology of traumatic brain injury (TBI). These contradictory perspectives are likely due to different NOS isoforms - endothelial (eNOS), inducible (iNOS) and neuronal (nNOS) which are expressed in the central nervous system. Of these, the role of nNOS in acute injury remains less clear. This study aimed to employ a genetic approach by overexpressing arginase isoforms specifically in neurons using a Thy-1 promoter to manipulate cell autonomous NO production in the context of TBI. The hypothesis was that increased arginase would divert arginine from pathological NO production. We generated 2 mouse lines that overexpress arginase I (a cytoplasmic enzyme) or arginase II (a mitochondrial enzyme) in neurons of FVB mice. We found that two-weeks after induction of controlled cortical injury, overexpressing arginase I but not arginase II in neurons significantly reduced contusion size and contusion index compared to wild-type (WT) mice. This study establishes enhanced neuronal arginase levels as a strategy to affect the course of TBI and provides support for the potential role of neuronal NO production in this condition.

Acute Physiology and Neurologic Outcomes after Brain Injury in SCOP/PHLPP1 KO Mice

Suprachiasmatic nucleus circadian oscillatory protein (SCOP) (a.k.a. PHLPP1) regulates long-term memory consolidation in the brain. Using a mouse model of controlled cortical impact (CCI) we tested if (1) brain tissue levels of SCOP/PHLPP1 increase after a traumatic brain injury (TBI), and (2) if SCOP/PHLPP1 gene knockout (KO) mice have improved (or worse) neurologic outcomes. Blood chemistry (pH, pCO₂, pO₂, pSO₂, base excess, sodium bicarbonate, and osmolarity) and arterial pressure (MAP) differed in isoflurane anesthetized WT vs. KOs at baseline and up to 1 h post-injury. CCI injury increased cortical/hippocampal SCOP/PHLPP1 levels in WTs 7d and 14d post-injury. Injured KOs had higher brain tissue levels of phosphorylated AKT (pAKT) in cortex (14d post-injury), and higher levels of phosphorylated MEK (pMEK) in hippocampus (7d and 14d post-injury) and in cortex (7d post-injury). Consistent with an important role of SCOP/PHLPP1 on memory function, injured-KOs had near normal performance on the probe trial of the Morris water maze, whereas injured-WTs were impaired. CA1/CA3 hippocampal survival was lower in KOs vs. WTs 24 h post-injury but equivalent by 7d. No difference in 21d cortical lesion volume was detected. SCOP/PHLPP1 overexpression in cultured rat cortical neurons had no effect on 24 h cell death after a mechanical stretch-injury.

A Direct Comparison between Norepinephrine and Phenylephrine for Augmenting Spinal Cord Perfusion in a Porcine Model of Spinal Cord Injury

Current clinical guidelines recommend elevating the mean arterial blood pressure (MAP) to increase spinal cord perfusion in patients with acute spinal cord injury (SCI). This is typically achieved with vasopressors such as norepinephrine (NE) and phenylephrine (PE). These drugs differ in their pharmacological properties and potentially have different effects on spinal cord blood flow (SCBF), oxygenation (PO₂), and downstream metabolism after injury. Using a porcine model of thoracic SCI, we evaluated how these vasopressors influenced intraparenchymal SCBF, PO₂, hydrostatic pressure, and metabolism within the spinal cord adjacent to the injury site. Yorkshire pigs underwent a contusion/compression SCI at T10 and were randomized to receive either NE or PE for MAP elevation of 20 mm Hg, or no MAP augmentation. Prior to injury, a combined SCBF/PO₂ sensor, a pressure sensor, and a microdialysis probe were inserted into the spinal cord adjacent to T10 at two locations: a "proximal" site and a "distal" site, 2 mm and 22 mm from the SCI, respectively. At the proximal site, NE and PE resulted in little improvement in SCBF during cord compression. Following decompression, NE resulted in increased SCBF and PO₂, whereas decreased levels were observed for PE. However, both NE and PE were associated with a gradual decrease in the lactate to pyruvate (L/P) ratio after decompression. PE was associated with greater hemorrhage through the injury site than that in control animals. Combined, our results suggest that NE promotes better restoration of blood flow and oxygenation than PE in the traumatically injured spinal cord, thus providing a physiological rationale for selecting NE over PE in the hemodynamic management of acute SCI.

A Period of Controlled Elevation of IOP (CEI) Produces the Specific Gene Expression Responses and Focal Injury Pattern of Experimental Rat Glaucoma

Purpose

We determine if several hours of controlled elevation of IOP (CEI) will produce the optic nerve head (ONH) gene expression changes and optic nerve (ON) damage pattern associated with early experimental glaucoma in rats.

Methods

The anterior chambers of anesthetized rats were cannulated and connected to a reservoir to elevate IOP. Physiologic parameters were monitored. Following CEI at various recovery times, ON cross-sections were graded for axonal injury. Anterior ONHs were collected at 0 hours to 10 days following CEI and RNA extracted for quantitative PCR measurement of selected messages. The functional impact of CEI was assessed by electroretinography (ERG).

Results

During CEI, mean arterial pressure (99 ± 6 mm Hg) and other physiologic parameters remained stable. An 8-hour CEI at 60 mm Hg produced significant focal axonal degeneration 10 days after exposure, with superior lesions in 83% of ON. Message analysis in CEI ONH demonstrated expression responses previously identified in minimally injured ONH following chronic IOP elevation, as well as their sequential patterns. Anesthesia with cannulation at 20 mm Hg did not alter these message levels. Electroretinographic A- and B-waves, following a significant reduction at 2 days after CEI, were fully recovered at 2 weeks, while peak scotopic threshold response (pSTR) remained mildly but significantly depressed.

Conclusions

A single CEI reproduces ONH message changes and patterns of ON injury previously observed with chronic IOP elevation. Controlled elevation of IOP can allow detailed determination of ONH cellular and functional responses to an injurious IOP insult and provide a platform for developing future therapeutic interventions.

Beneficial effects of L-arginine on 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced neuronal degeneration in substantia nigra of Balb/c mice

BACKGROUND

L-arginine has been recently investigated and proposed to reduce neurological damage after various experimental models of neuronal cellular damage. In this study, we aim to evaluate the beneficial effects of L-arginine administration on the numerical density of dark neurons (DNs) in the substantia nigra pars compacta (SNc) of Balb/c mice subjected to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administration.

MATERIALS AND METHODS

Male Balb/c mice were randomly divided into 4 groups (n = 7 each): MPTP only; saline only (control); MPTP + L-arginine; and L-arginine only. The animals were infused intranasally with a single intranasal administration of the proneurotoxin MPTP (1 mg/nostril). L-arginine (300 mg/kg) was administrated intraperitoneally once daily for 1-week starting from 3 days after MPTP administration. Cavalieri principle method was used to estimate the numerical density of DNs in the SNc of different studied groups.

RESULTS

Twenty days following MPTP administration, the number of DNs was significantly increased when compared to sham-control and L-arginine-control groups (P < 0.05). Nevertheless, our results showed that L-arginine administration significantly decreased the numerical density of DNs in SNc of mice.

CONCLUSION

This investigation provides new insights in experimental models of Parkinson's disease, indicating that L-arginine represents a potential treatment agent for dopaminergic neuron degeneration in SNc observed in Parkinson's disease patients.

Comparison of false-negative/positive results of intraoperative evoked potential monitoring between no and partial neuromuscular blockade in patients receiving propofol/remifentanil-based anesthesia during cerebral aneurysm clipping surgery: A retrospective analysis of 685 patients

Although the elicited responses of motor evoked potential (MEP) monitoring are very sensitive to suppression by anesthetic agents and muscle relaxants, the use of neuromuscular blockade (NMB) during MEP monitoring is still controversial because of serious safety concerns and diagnostic accuracy. Here, we evaluated the incidence of unacceptable movement and compared false-negative MEP results between no and partial NMB during cerebral aneurysm clipping surgery. We reviewed patient medical records for demographic data, anesthesia regimen, neurophysiology event logs, MEP results, and clinical outcomes. Patients were divided into 2 groups according to the intraoperative use of NMB: no NMB group (n = 276) and partial NMB group (n = 409). We compared the diagnostic accuracy of MEP results to predict postoperative outcomes between both groups. Additionally, we evaluated unwanted patient movement during MEP monitoring in both groups. Of the 685 patients, 622 (90.8%) manifested no intraoperative changes in MEP and no postoperative motor deficits. Twenty patients showed postoperative neurologic deficits despite preserved intraoperative MEP. False-positive MEP results were 3.6% in the no NMB group and 3.9% in the partial NMB group (P = 1.00). False-negative MEP results were 1.1% in the no NMB group and 4.2% in the partial NMB group (P = 0.02). No spontaneous movement or spontaneous respiration was observed in either group. Propofol/remifentanil-based anesthesia without NMB decreases the stimulation intensity of MEPs, which may reduce the false-negative ratio of MEP monitoring during cerebral aneurysm surgery. Our anesthetic protocol enabled reliable intraoperative MEP recording and patient immobilization during cerebral aneurysm clipping surgery.

Erythropoietin for the Treatment of Subarachnoid Hemorrhage: A Feasible Ingredient for a Successful Medical Recipe

Subarachnoid hemorrhage (SAH) following aneurysm bleeding accounts for 6% to 8% of all cerebrovascular accidents. Although an aneurysm can be effectively managed by surgery or endovascular therapy, delayed cerebral ischemia is diagnosed in a high percentage of patients resulting in significant morbidity and mortality. Cerebral vasospasm occurs in more than half of all patients after aneurysm rupture and is recognized as the leading cause of delayed cerebral ischemia after SAH. Hemodynamic strategies and endovascular procedures may be considered for the treatment of cerebral vasospasm. In recent years, the mechanisms contributing to the development of vasospasm, abnormal reactivity of cerebral arteries and cerebral ischemia following SAH, have been investigated intensively. A number of pathological processes have been identified in the pathogenesis of vasospasm, including endothelial injury, smooth muscle cell contraction from spasmogenic substances produced by the subarachnoid blood clots, changes in vascular responsiveness and inflammatory response of the vascular endothelium. To date, the current therapeutic interventions remain ineffective as they are limited to the manipulation of systemic blood pressure, variation of blood volume and viscosity and control of arterial carbon dioxide tension. In this scenario, the hormone erythropoietin (EPO) has been found to exert neuroprotective action during experimental SAH when its recombinant form (rHuEPO) is administered systemically. However, recent translation of experimental data into clinical trials has suggested an unclear role of recombinant human EPO in the setting of SAH. In this context, the aim of the current review is to present current evidence on the potential role of EPO in cerebrovascular dysfunction following aneurysmal subarachnoid hemorrhage.

Endothelial nitric oxide synthase mediates the cerebrovascular effects of erythropoietin in traumatic brain injury

Background: Erythropoietin (Epo) improves post-traumatic cerebral blood flow (CBF), pressure autoregulation, and vascular reactivity to l-arginine. This study examines the dependence of these cerebral hemodynamic effects of Epo on nitric oxide generated by endothelial nitric oxide synthase (eNOS).

Methods: Using laser Doppler flow imaging, CBF was monitored in wild-type (WT) and eNOS-deficient mice undergoing controlled cortical impact followed by administration of Epo (5000 U/kg) or normal saline.

Results: Cerebral blood flow decreased in all groups post-injury with the greatest reductions occurring at the impact site. Epo administration resulted in significantly higher CBF in the peri-contusional sites in the WT mice [70.2 ± 3.35% in Epo-treated compared to 53 ± 3.3% of baseline in saline-treated mice (p < 0.0001)], but no effect was seen in the eNOS-deficient mice. No CBF differences were found at the core impact site where CBF dropped to 20–25% of baseline in all groups.

Conclusion: These differences between eNOS-deficient and WT mice indicate that the Epo mediated improvement in CBF in traumatic brain injury is eNOS dependent.

Endogenous nitric-oxide synthase inhibitor ADMA after acute brain injury

Previous results on nitric oxide (NO) metabolism after traumatic brain injury (TBI) show variations in NO availability and controversial effects of exogenous nitric oxide synthase (NOS)-inhibitors. Furthermore, elevated levels of the endogenous NOS inhibitor asymmetric dimethylarginine (ADMA) were reported in cerebro-spinal fluid (CSF) after traumatic subarachnoid hemorrhage (SAH). Therefore, we examined whether ADMA and the enzymes involved in NO- and ADMA-metabolism are expressed in brain tissue after TBI and if time-dependent changes occur. TBI was induced by controlled cortical impact injury (CCII) and neurological performance was monitored. Expression of NOS, ADMA, dimethylarginine dimethylaminohydrolases (DDAH) and protein-arginine methyltransferase 1 (PRMT1) was determined by immunostaining in different brain regions and at various time-points after CCII. ADMA and PRMT1 expression decreased in all animals after TBI compared to the control group, while DDAH1 and DDAH2 expression increased in comparison to controls. Furthermore, perilesionally ADMA is positively correlated with neuroscore performance, while DDAH1 and DDAH2 are negatively correlated. ADMA and its metabolizing enzymes show significant temporal changes after TBI and may be new targets in TBI treatment.

Traumatic brain injury: oxidative stress and neuroprotection

SIGNIFICANCE

A vast amount of circumstantial evidence implicates high energy oxidants and oxidative stress as mediators of secondary damage associated with traumatic brain injury. The excessive production of reactive oxygen species due to excitotoxicity and exhaustion of the endogenous antioxidant system induces peroxidation of cellular and vascular structures, protein oxidation, cleavage of DNA, and inhibition of the mitochondrial electron transport chain.

RECENT ADVANCES

Different integrated responses exist in the brain to detect oxidative stress, which is controlled by several genes termed vitagens. Vitagens encode for cytoprotective heat shock proteins, and thioredoxin and sirtuins.

CRITICAL ISSUES AND FUTURE DIRECTIONS

This article discusses selected aspects of secondary brain injury after trauma and outlines key mechanisms associated with toxicity, oxidative stress, inflammation, and necrosis. Finally, this review discusses the role of different oxidants and presents potential clinically relevant molecular targets that could be harnessed to treat secondary injury associated with brain trauma.

Hypothermia decreases cerebrospinal fluid asymmetric dimethylarginine levels in children with traumatic brain injury

OBJECTIVES

Pathological increases in asymmetric dimethylarginine, an endogenous nitric oxide synthase inhibitor, have been implicated in endothelial dysfunction and vascular diseases. Reduced nitric oxide early after traumatic brain injury may contribute to hypoperfusion. Currently, methods to quantify asymmetric dimethylarginine in the cerebrospinal fluid have not been fully explored. We aimed to develop and validate a method to determine asymmetric dimethylarginine in the cerebrospinal fluid of a pediatric traumatic brain injury population and to use this method to assess the effects of 1) traumatic brain injury and 2) therapeutic hypothermia on this mediator.

DESIGN, SETTING, AND PATIENTS

An ancillary study to a prospective, phase II randomized clinical trial of early hypothermia in a tertiary care pediatric intensive care unit for children with Traumatic brain injury admitted to Children's Hospital of Pittsburgh.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

A UPLC-MS/MS method was developed and validated to quantitate asymmetric dimethylarginine. A total of 56 samples collected over 3 days with injury onset were analyzed from the cerebrospinal fluid of consented therapeutic hypothermia (n = 9) and normothermia (n = 10) children. Children undergoing diagnostic lumbar puncture (n = 5) were enrolled as controls. Asymmetric dimethylarginine was present at a quantifiable level in all samples. Mean asymmetric dimethylarginine levels were significantly increased in normothermic Traumatic brain injury children compared with that in control (0.19 ± 0.08 µmol/L and 0.11 ± 0.02 µmol/L, respectively, p = 0.01), and hypothermic children had significantly reduced mean asymmetric dimethylarginine levels (0.11 ± 0.05 µmol/L) vs. normothermic (p = 0.03) measured on day 3. Patient demographics including age, gender, and nitric oxide levels (measured as nitrite and nitrate using liquid chromatography coupled with Griess reaction) did not significantly differ between normothermia and hypothermia groups. Also, nitric oxide levels did not correlate with asymmetric dimethylarginine concentrations.

CONCLUSIONS

Asymmetric dimethylarginine levels were significantly increased in the cerebrospinal fluid of traumatic brain injury children. Early hypothermia attenuated this increase. The implications of attenuated asymmetric dimethylarginine on nitric oxide synthases activity and regional cerebral blood flow after traumatic brain injury by therapeutic hypothermia deserve future study.

The anesthetic effects on vasopressor modulation of cerebral blood flow in an immature swine model

BACKGROUND

The effect of various sedatives and anesthetics on vasopressor modulation of cerebral blood flow (CBF) in children is unclear. In adults, isoflurane has been described to decrease CBF to a lesser extent than fentanyl and midazolam. Most large-animal models of neurocritical care use inhaled anesthetics for anesthesia. Investigations involving modulations of CBF would have improved translatability within a model that more closely approximates the current practice in the pediatric intensive care unit.

METHODS

Fifteen 4-week-old piglets were given 1 of 2 anesthetic protocols: total IV anesthesia (TIVA) (midazolam 1 mg/kg/h and fentanyl 100 μg/kg/h, n = 8) or ISO (isoflurane 1.5%-2% and fentanyl 100 μg/kg/h, n = 7). Mean arterial blood pressure, intracranial pressure (ICP), CBF, and brain tissue oxygen tension were measured continuously as piglets were exposed to escalating doses of arginine vasopressin, norepinephrine (NE), and phenylephrine (PE).

RESULTS

Baseline CBF was similar in the 2 groups (ISO 38 ± 10 vs TIVA 35 ± 26 mL/100 g/min) despite lower baseline cerebral perfusion pressure in the ISO group (45 ± 11 vs 71 ± 11 mm Hg; P < 0.0005). Piglets in the ISO group displayed increases in ICP with PE and NE (11 ± 4 vs 16 ± 4 mm Hg and 11 ± 8 vs 18 ± 5 mm Hg; P < 0.05), but in the TIVA group, only exposure to PE resulted in increases in ICP when comparing maximal dose values with baseline data (11 ± 4 vs 15 ± 5 mm Hg; P < 0.05). Normalized CBF displayed statistically significant increases regarding anesthetic group and vasopressor dose when piglets were exposed to NE and PE (P < 0.05), suggesting an impairment of autoregulation within ISO, but not TIVA.

CONCLUSION

The vasopressor effect on CBF was limited when using a narcotic-benzodiazepine-based anesthetic protocol compared with volatile anesthetics, consistent with a preservation of autoregulation. Selection of anesthetic drugs is critical to investigate mechanisms of cerebrovascular hemodynamics, and in translating critical care investigations between the laboratory and bedside.

Inhaled nitric oxide reduces secondary brain damage after traumatic brain injury in mice

Ischemia, especially pericontusional ischemia, is one of the leading causes of secondary brain damage after traumatic brain injury (TBI). So far efforts to improve cerebral blood flow (CBF) after TBI were not successful because of various reasons. We previously showed that nitric oxide (NO) applied by inhalation after experimental ischemic stroke is transported to the brain and induces vasodilatation in hypoxic brain regions, thus improving regional ischemia, thereby improving brain damage and neurological outcome. As regional ischemia in the traumatic penumbra is a key mechanism determining secondary posttraumatic brain damage, the aim of the current study was to evaluate the effect of NO inhalation after experimental TBI. NO inhalation significantly improved CBF and reduced intracranial pressure after TBI in male C57 Bl/6 mice. Long-term application (24 hours NO inhalation) resulted in reduced lesion volume, reduced brain edema formation and less blood-brain barrier disruption, as well as improved neurological function. No adverse effects, e.g., on cerebral auto-regulation, systemic blood pressure, or oxidative damage were observed. NO inhalation might therefore be a safe and effective treatment option for TBI patients.

Early cerebral perfusion pressure augmentation with phenylephrine after traumatic brain injury may be neuroprotective in a pediatric swine model

OBJECTIVE

Cerebral perfusion pressure<40 mm Hg following pediatric traumatic brain injury has been associated with increased mortality independent of age, and current guidelines recommend maintaining cerebral perfusion pressure between 40 mm Hg-60 mm Hg. Although adult traumatic brain injury studies have observed an increased risk of complications associated with targeting a cerebral perfusion pressure>70, we hypothesize that targeting a cerebral perfusion pressure of 70 mm Hg with the use of phenylephrine early after injury in the immature brain will be neuroprotective.

DESIGN

Animals were randomly assigned to injury with a cerebral perfusion pressure of 70 mm Hg or 40 mm Hg. Diffuse traumatic brain injury was produced by a single rapid rotation of the head in the axial plane. Cerebral microdialysis, brain tissue oxygen, intracranial pressure, and cerebral blood flow were measured 30 min-6 hrs postinjury. One hour after injury, cerebral perfusion pressure was manipulated with the vasoconstrictor phenylephrine. Animals were euthanized 6 hrs posttraumatic brain injury, brains fixed, and stained to assess regions of cell injury and axonal dysfunction.

SETTING

University center.

SUBJECT

Twenty-one 4-wk-old female swine.

MEASUREMENTS AND MAIN RESULTS

Augmentation of cerebral perfusion pressure to 70 mm Hg resulted in no change in axonal dysfunction, but significantly smaller cell injury volumes at 6 hrs postinjury compared to cerebral perfusion pressure 40 (1.1% vs. 7.4%, p<.05). Microdialysis lactate/pyruvate ratios were improved at cerebral perfusion pressure 70 compared to cerebral perfusion pressure 40. Cerebral blood flow was higher in the cerebral perfusion pressure 70 group but did not reach statistical significance. Phenylephrine was well tolerated and there were no observed increases in serum lactate or intracranial pressure in either group.

CONCLUSIONS

Targeting a cerebral perfusion pressure of 70 mm Hg resulted in a greater reduction in metabolic crisis and cell injury volumes compared to a cerebral perfusion pressure of 40 mm Hg in an immature swine model. Early aggressive cerebral perfusion pressure augmentation to a cerebral perfusion pressure of 70 mm Hg in pediatric traumatic brain injury before severe intracranial hypertension has the potential to be neuroprotective, and further investigations are needed.

Preventing flow-metabolism uncoupling acutely reduces axonal injury after traumatic brain injury

We have previously presented evidence that the development of secondary traumatic axonal injury is related to the degree of local cerebral blood flow (LCBF) and flow-metabolism uncoupling. We have now tested the hypothesis that augmenting LCBF in the acute stages after brain injury prevents further axonal injury. Data were acquired from rats with or without acetazolamide (ACZ) that was administered immediately following controlled cortical impact injury to increase cortical LCBF. Local cerebral metabolic rate for glucose (LCMRglc) and LCBF measurements were obtained 3 h post-trauma in the same rat via ¹⁸F-fluorodeoxyglucose and ¹⁴C-iodoantipyrine co-registered autoradiographic images, and compared to the density of damaged axonal profiles in adjacent sections, and in additional groups at 24 h used to assess different populations of injured axons stereologically. ACZ treatment significantly and globally elevated LCBF twofold above untreated-injured rats at 3 h (p<0.05), but did not significantly affect LCMRglc. As a result, ipsilateral LCMRglc:LCBF ratios were reduced by twofold to sham-control levels, and the density of β-APP-stained axons at 24 h was significantly reduced in most brain regions compared to the untreated-injured group (p<0.01). Furthermore, early LCBF augmentation prevented the injury-associated increase in the number of stained axons from 3-24 h. Additional robust stereological analysis of impaired axonal transport and neurofilament compaction in the corpus callosum and cingulum underlying the injury core confirmed the amelioration of β-APP axon density, and showed a trend, but no significant effect, on RMO14-positive axons. These data underline the importance of maintaining flow-metabolism coupling immediately after injury in order to prevent further axonal injury, in at least one population of injured axons.

Effects of hemodilution after traumatic brain injury in juvenile rats

BACKGROUND

Normovolemic hemodilution (HD) in adult animal studies has shown exacerbation of traumatic brain injury (TBI) lesion volumes. Similar studies in juvenile rats have not been reported and outcomes are likely to be different. This study investigated the effects of normovolemic hemodilution (21% hematocrit) in a juvenile TBI (jTBI) model.

METHODS

Twenty 17-day-old rats underwent moderate cortical contusion impact injury (CCI) and were divided into four groups: CCI/hemodilution (HD) (group HD), CCI/no HD (group C), Sham/HD (group SHD), and Sham/no HD (group S). Regional laser Doppler flowmetry (LDF), edema formation (MRI-T2WI), water mobility assessed using diffusion weighted imaging (MRI-DWI), open field activity tests, and histological analyses were evaluated for lesion characteristics.

RESULTS

Hemodilution significantly increased blood flow in the HD compared to the C group after TBI. T2WI revealed a significantly increased extravascular blood volume in HD at 1, 7, and 14 days post-CCI. Edematous tissue and total contusional lesion volume were higher in HD-treated animals at 1 and 14 days. DWI revealed that HD, SHD, and C groups had elevated water mobility compared to S groups in the ipsilateral cortex and striatum. Histology showed a larger cortical lesion in the C than HD group. Open field activity was increased in HD, C, and SHD groups compared to the S group.

CONCLUSIONS

Hemodilution results in significant brain hyperemia with increased edema formation, extravascular blood volume, and water mobility after jTBI. Hemodilution results in less cortical damage but did not alter behavior. Hemodilution is likely not to be clinically beneficial following jTBI.

Vasopressor use and effect on blood pressure after severe adult traumatic brain injury

BACKGROUND

We describe institutional vasopressor usage, and examine the effect of vasopressors on hemodynamics: heart rate (HR), mean arterial blood pressure (MAP), intracranial pressure (ICP), cerebral perfusion pressure (CPP), brain tissue oxygenation (PbtO(2)), and jugular venous oximetry (SjVO(2)) in adults with severe traumatic brain injury (TBI).

METHODS

We performed a retrospective analysis of 114 severely head injured patients who were admitted to the neurocritical care unit of Level 1 trauma center and who received vasopressors (phenylephrine, norepinephrine, dopamine, vasopressin or epinephrine) to increase blood pressure

RESULTS

Phenylephrine was the most commonly used vasopressor (43%), followed by norepinephrine (30%), dopamine (22%), and vasopressin (5%). Adjusted for age, gender, injury severity score, vasopressor dose, baseline blood pressure, fluid administration, propofol sedation, and hypertonic saline infusion, phenylephrine use was associated with 8 mmHg higher mean arterial pressure (MAP) than dopamine (P = 0.03), and 12 mmHg higher cerebral perfusion pressure (CPP) than norepinephrine (P = 0.02) during the 3 h after vasopressor start. There was no difference in ICP between the drug groups, either at baseline or after vasopressor treatment.

CONCLUSIONS

Most severe TBI patients received phenylephrine. Patients who received phenylephrine had higher MAP and CPP than patients who received dopamine and norepinephrine, respectively.

The novel nitric oxide synthase inhibitor 4-amino-tetrahydro-L-biopterine prevents brain edema formation and intracranial hypertension following traumatic brain injury in mice

Brain edema formation, resulting in increased intracranial pressure (ICP), is one of the most deleterious consequences of traumatic brain injury (TBI). Nitric oxide (NO) has previously been shown to be involved in the damage of the blood-brain barrier (BBB) and, thus, in the formation of post-traumatic brain edema; however, this knowledge never resulted in a clinically relevant therapeutic option because available NO synthase inhibitors have serious side effects in man. The aim of the current study was to investigate the therapeutic efficacy of VAS203, a novel tetrahydrobiopterine (BH3)-based NOS inhibitor, in experimental TBI. When added to isolated vessels rings obtained from rat basilar and middle cerebral arteries (n = 32-35) VAS203 showed the same vasoconstrictive effect as the classical NO synthase inhibitor L-(G)-nitro-arginine-methylester (L-NAME). VAS203 passed the BBB both in healthy and traumatized mouse brain (C57/BL6, n = 5 per group) and did not show any systemic side effects at therapeutic concentrations. When administered 30 min after experimental TBI (controlled cortical impact, 2.2 mg/kg/min i.v., n = 7 per group), VAS203 prevented any further increase in ICP or deterioration of cerebral blood flow. This effect was dose-dependent and long-lasting (i.e., 24 h after trauma, brain edema formation was still significantly reduced [-40%, p < 0.008; n = 7 per group] and functional improvements were present up to 7 days after TBI [p < 0.02 on post-trauma day 6; n = 8 per group]). Therefore, VAS203 may represent a promising candidate for the treatment of acute intracranial hypertension following TBI.

Pressure autoregulation, intracranial pressure, and brain tissue oxygenation in children with severe traumatic brain injury

OBJECT

Cerebral pressure autoregulation is an important neuroprotective mechanism that stabilizes cerebral blood flow when blood pressure (BP) changes. In this study the authors examined the association between autoregulation and clinical factors, BP, intracranial pressure (ICP), brain tissue oxygen tension (PbtO(2)), and outcome after pediatric severe traumatic brain injury (TBI). In particular we examined how the status of autoregulation influenced the effect of BP changes on ICP and PbtO(2).

METHODS

In this prospective observational study, 52 autoregulation tests were performed in 24 patients with severe TBI. The patients had a mean age of 6.3 +/- 3.2 years, and a postresuscitation Glasgow Coma Scale score of 6 (range 3-8). All patients underwent continuous ICP and PbtO(2) monitoring, and transcranial Doppler ultrasonography was used to examine the autoregulatory index (ARI) based on blood flow velocity of the middle cerebral artery after increasing mean arterial pressure by 20% of the baseline value. Impaired autoregulation was defined as an ARI < 0.4 and intact autoregulation as an ARI >or= 0.4. The relationships between autoregulation (measured as both a continuous and dichotomous variable), outcome, and clinical and physiological variables were examined using multiple logistic regression analysis.

RESULTS

Autoregulation was impaired (ARI < 0.4) in 29% of patients (7 patients). The initial Glasgow Coma Scale score was significantly associated with the ARI (p = 0.02, r = 0.32) but no other clinical factors were associated with autoregulation status. Baseline values at the time of testing for ICP, PbtO(2), the ratio of PbtO(2)/PaO(2), mean arterial pressure, and middle cerebral artery blood flow velocity were similar in the patients with impaired or intact autoregulation. There was an inverse relationship between ARI (continuous and dichotomous) with a change in ICP (continuous ARI, p = 0.005; dichotomous ARI, p = 0.02); that is, ICP increased with the BP increase when ARI was low (weak autoregulation). The ARI (continuous and dichotomous) was also inversely associated with a change in PbtO(2) (continuous ARI, p = 0.002; dichotomous ARI, p = 0.02). The PbtO(2) increased when BP was increased in most patients, even when the ARI was relatively high (stronger autoregulation), but the magnitude of this response was still associated with the ARI. There was no relationship between the ARI and outcome.

CONCLUSIONS

These data demonstrate the influence of the strength of autoregulation on the response of ICP and PbtO(2) to BP changes and the variability of this response between individuals. The findings suggest that autoregulation testing may assist clinical decision-making in pediatric severe TBI and help better define optimal BP or cerebral perfusion pressure targets for individual patients.

Cerebral pressure autoregulation in traumatic brain injury

An understanding of normal cerebral autoregulation and its response to pathological derangements is helpful in the diagnosis, monitoring, management, and prognosis of severe traumatic brain injury (TBI). Pressure autoregulation is the most common approach in testing the effects of mean arterial blood pressure on cerebral blood flow. A gold standard for measuring cerebral pressure autoregulation is not available, and the literature shows considerable disparity in methods. This fact is not surprising given that cerebral autoregulation is more a concept than a physically measurable entity. Alterations in cerebral autoregulation can vary from patient to patient and over time and are critical during the first 4-5 days after injury. An assessment of cerebral autoregulation as part of bedside neuromonitoring in the neurointensive care unit can allow the individualized treatment of secondary injury in a patient with severe TBI. The assessment of cerebral autoregulation is best achieved with dynamic autoregulation methods. Hyperventilation, hyperoxia, nitric oxide and its derivates, and erythropoietin are some of the therapies that can be helpful in managing cerebral autoregulation. In this review the authors summarize the most important points related to cerebral pressure autoregulation in TBI as applied in clinical practice, based on the literature as well as their own experience.

Role of vasopressor administration in patients with acute neurologic injury

INTRODUCTION

Pharmacologic blood pressure elevation is often utilized to prevent or treat ischemia in patients with acute neurologic injury, and routinely requires administration of vasopressor agents. Depending on the indication, vasopressor agents may be administered to treat hypotension or to induce hypertension.

METHODS

Although numerous guideline statements exist regarding the management of blood pressure in these patients, most recommendations are based largely on Class III evidence. Further, there are few randomized controlled trials comparing vasopressor agents in these patients and selection is often guided by expert consensus.

RESULTS

We discuss the clinical evidence regarding vasopressor administration for blood pressure management in patients with acute neurologic injury. The effect of various vasopressors on cerebral hemodynamics is also discussed.

CONCLUSION

Although high-quality clinical data are scarce, the available evidence suggests that norepinephrine should be considered as the vasopressor of choice when blood pressure elevation is indicated in patients with acute neurologic injury.

Drug targets for traumatic brain injury from poly(ADP-ribose)polymerase pathway modulation

The deleterious pathophysiological cascade induced after traumatic brain injury (TBI) is initiated by an excitotoxic process triggered by excessive glutamate release. Activation of the glutamatergic N-methyl-D-aspartate receptor, by increasing calcium influx, activates nitric oxide (NO) synthases leading to a toxic production of NO. Moreover, after TBI, free radicals are highly produced and participate to a deleterious oxidative stress. Evidence has showed that the major toxic effect of NO comes from its combination with superoxide anion leading to peroxynitrite formation, a highly reactive and oxidant compound. Indeed, peroxynitrite mediates nitrosative stress and is a potent inducer of cell death through its reaction with lipids, proteins and DNA. Particularly DNA damage, caused by both oxidative and nitrosative stresses, results in activation of poly(ADP-ribose) polymerase (PARP), a nuclear enzyme implicated in DNA repair. In response to excessive DNA damage, massive PARP activation leads to energetic depletion and finally to cell death. Since 10 years, accumulating data have showed that inactivation of PARP, either pharmacologically or using PARP null mice, induces neuroprotection in experimental models of TBI. Thus TBI generating NO, oxidative and nitrosative stresses promotes PARP activation contributing in post-traumatic motor, cognitive and histological sequelae. The mechanisms by which PARP inhibitors provide protection might not entirely be related to the preservation of cellular energy stores, but might also include other PARP-mediated mechanisms that needed to be explored in a TBI context. Ten years of experimental research provided rational basis for the development of PARP inhibitors as treatment for 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.

Effects of l-arginine on cerebral blood flow, microvascular permeability, number of perfused capillaries, and brain water content in the traumatized mouse brain

It is has been suggested that decreased production of the vasodilatory and anti-aggregative substance NO (nitric oxide) may result in lower cerebral blood flow (CBF) in injured areas of the traumatized brain. The NO-precursor L-arginine has been shown to counteract CBF decreases early after trauma, but microcirculatory and more long-term effects on CBF of L-arginine have not been investigated. In an attempt to analyze effects of L-arginine on the microcirculation in the traumatized brain, the present study was designed to evaluate the effects of L-arginine compared to vehicle (0.9% saline) following a standardized controlled cortical-impact brain trauma in mice. Cerebral blood flow (autoradiography [(14)C]-iodoantipyrine), number of perfused capillaries (FITC-dextran fluorescence technique), brain water content (wet vs. dry weight) and the blood to brain transfer constant K(i) for [(51)Cr]-EDTA were analyzed in the injured and the contralateral cortex. Cortical blood flow in the injured cortex was 0.43+/-0.3 mL/g/min and 0.81+/-0.3 mL/g/min 3 h after trauma in the vehicle and L-arginine groups, respectively (p<0.05), and no treatment effect was seen 24 h after trauma. The number of perfused capillaries decreased following trauma and was unaffected by L-arginine. K(i) increased following trauma and was unaffected by L-arginine. Brain water content was lower in the L-arginine group than in the vehicle group 3 h after trauma and there was no difference between the groups 24 h after trauma. We conclude that L-arginine reduces brain edema formation and improves cortical blood flow in the early phase after a brain trauma, whereas no circulatory effects can be seen after prolonged treatment.

Hemodynamic manipulation in the neuro-intensive care unit: cerebral perfusion pressure therapy in head injury and hemodynamic augmentation for cerebral vasospasm

PURPOSE OF REVIEW

The intent of this manuscript is to summarize the pathophysiologic basis for hemodynamic manipulation in subarachnoid hemorrhage and traumatic brain injury, highlight the most recent literature and present expert opinion on indications and use.

RECENT FINDINGS

Hemodynamic augmentation with vasopressors and inotropes along with hypervolemia are the mainstay of treatment of vasospasm due to subarachnoid hemorrhage. Considerable variation continues to exist regarding fluid management and the use of vasopressors and inotropes. Blood pressure augmentation, volume expansion and cardiac contractility enhancement improve cerebral blood flow in ischemic areas, ameliorate vasospasm and improve clinical condition. In patients suffering from severe traumatic brain injury, while every attempt is made to control intracranial hypertension, cerebral perfusion-directed therapy with fluids and vasopressors is also used to keep cerebral perfusion pressure above 60-70 mmHg. Yet, recent observations suggest that posttraumatic mitochondrial dysfunction has been proposed as an alternative explanation for lower cerebral blood flow after acute trauma.

SUMMARY

Hemodynamic manipulation is routinely used in the management of patients with acute vasospasm following subarachnoid hemorrhage and severe head injury. The rationale is to improve blood flow to the injured brain and prevent secondary ischemia.

Cerebral blood flow in traumatic contusions is predominantly reduced after an induced acute elevation of cerebral perfusion pressure

OBJECTIVE

To evaluate the response to an acute elevation of cerebral perfusion pressure (CPP) of the regional cerebral blood flow (rCBF) measured in the edematous area of traumatic contusions.

METHODS

rCBF was measured in the intracontusional low-density area, in the pericontusional healthy-appearing brain tissue surrounding the contusion, in a healthy-appearing area in the contralateral hemisphere, in 16 head-injured patients with 16 traumatic contusions larger than 2 cm at baseline, and after 20 minutes of norepinephrine-induced 20-mmHg elevation of CPP levels.

RESULTS

After an induced acute elevation of CPP from baseline values of 65.8 ml/100 g/min (standard deviation, 8.6) to final values of 88.7 ml/100 g/min (standard deviation, 8.9; P < or = 0.0001), we found that rCBF mean levels decreased in the intracontusional low-density area (P = 0.0278), and change in rCBF was inversely associated to the baseline values. After grouping contusions according to the rCBF response to induced acute CPP elevation, rCBF mean values recorded at baseline were significantly lower in lesions with "rCBF improvement" than in those with "rCBF reduction" in the intracontusional low-density area (P = 0.0435).

CONCLUSION

Our findings suggest that CPP elevation induced by norepinephrine is effective in improving contusional rCBF only in selected cases, which are represented by a subset of contusions with critical perfusion, which can be identified by rCBF measurements. Conversely, in contusions with rCBF higher than critical low values, the CPP elevation could probably induce a temporary breakdown of the blood brain barrier, and the norepinephrine leads to a vasoconstriction with a worsening of regional perfusion.

EFFECTS OF HYPERTONIC ARGININE ON CEREBRAL BLOOD FLOW AND INTRACRANIAL PRESSURE AFTER TRAUMATIC BRAIN INJURY COMBINED WITH HEMORRHAGIC HYPOTENSION

Hypertonic saline solutions improve cerebral blood flow (CBF) when used for acute resuscitation from hemorrhagic hypotension accompanying some models of traumatic brain injury (TBI); however, the duration of increased CBF is brief. Because the nitric oxide synthase substrate l-arginine provides prolonged improvement in CBF after TBI, we investigated whether a hypertonic resuscitation fluid containing l-arginine would improve CBF in comparison to hypertonic saline without l-arginine in a model of moderate, paramedian, fluid-percussion TBI followed immediately by hemorrhagic hypotension (mean arterial pressure [MAP] = 60 mm Hg for 45 min). Sprague-Dawley rats were anesthetized with 4.0% isoflurane, intubated and ventilated with 1.5%-2.0% isoflurane in oxygen/air (50:50). After preparation for TBI and measurement of CBF using laser Doppler flowmetry and measurement of intracranial pressure (ICP) using an implanted transducer, rats were subjected to moderate (2.0 atm) TBI, hemorrhaged for 45 min, and randomly assigned to receive an infusion of hypertonic saline (7.5%, 2,400 mOsm total; 6 mL/kg; n = 6) or hypertonic saline with 50, 100, or 300 mg/kg L-arginine (2,400 mOsm; 6 mL/kg; n = 6 in each of the three dose groups) and then monitored for 120 min after the end of infusion. CBF was measured continuously and calculated as a percent of the pre-TBI baseline during the hemorrhage period, after reinfusion of one of the hypertonic arginine solutions, and 30, 60, and 120 min after reinfusion. All four hypertonic solutions initially improved MAP, which, by 120 min after infusion, had decreased nearly to the levels observed during hemorrhage. ICP remained below baseline levels during resuscitation in all groups, although ICP was slightly greater (P = NS) than baseline in the hypertonic saline group. CBF increased similarly in all groups during infusion and then decreased similarly in all groups. At 120 min after infusion, CBF was highest in the group infused with hypertonic saline, but the difference was not significant. We conclude that the improvement of MAP, ICP, and CBF produced by hypertonic saline alone after TBI and hemorrhagic hypotension is not significantly enhanced by the addition of L-arginine at these doses.

Blood pressure management during acute ischaemic stroke

Control of hypertension is a well-established goal of primary stroke prevention. Management of blood pressure in patients during acute ischaemic stroke, however, is complicated by the need to maintain brain perfusion. Lowering blood pressure in the acute setting may avoid the deleterious effects of high blood pressure but may also lead to cerebral hypoperfusion and worsening of the ischaemic stroke. Little information is available from clinical trials concerning optimal blood pressure management in acute stroke. Current protocols of thrombolytic therapy require strict blood pressure control below certain prescribed limits; however, in most acute stroke patients not treated with thrombolysis, blood pressure reduction is not routinely recommended and guidelines for target blood pressures are difficult to justify. Preliminary studies, in fact, suggest that there may be a role for blood pressure elevation in the treatment of some patients with acute ischaemic stroke.

Significance of a reduced cerebral blood flow during the first 12 hours after traumatic brain injury

BACKGROUND

It is controversial whether a low cerebral blood flow (CBF) simply reflects the severity of injury or whether ischemia contributes to the brain's injury. It is also not clear whether posttraumatic cerebral hypoperfusion results from intracranial hypertension or from pathologic changes of the cerebral vasculature. The answers to these questions have important implications for whether and how to treat a low CBF.

METHODS

We performed a retrospective analysis of 77 patients with severe traumatic brain injury who had measurement of CBF within 12 hours of injury. CBF was measured using xenon-enhanced computed tomography (XeCT). Global CBF, physiological parameters at the time of XeCT, and outcome measures were analyzed.

RESULTS

Average global CBF for the 77 patients was 36+/-16 mL/100 g/minutes. Nine patients had an average global CBF<18 (average 12+/-5). The remaining 68 patients had a global CBF of 39+/-15. The initial ICP was >20 mmHg in 90% and >30 mmHg in 80% of patients in the group with CBF<18, compared to 33% and 16%, respectively, in the patients with CBF>or=18. Mortality was 90% at 6 months postinjury in patients with CBF<18. Mortality in the patients with CBF>18 was 19% at 6 months after injury.

CONCLUSION

In patients with CBF<18 mL/100 g/minutes, intracranial hypertension plays a major causative role in the reduction in CBF. Treatment would most likely be directed at controlling intracranial pressure, but the early, severe intracranial hypertension also probably indicates a severe brain injury. For levels of CBF between 18 and 40 mL/100 g/minutes, the presence of regional hypoperfusion was a more important factor in reducing the average CBF.

Characterization of the effects of adenosine receptor agonists on cerebral blood flow in uninjured and traumatically injured rat brain using continuous arterial spin-labeled magnetic resonance imaging

Hypoperfusion after traumatic brain injury may exacerbate damage. Adenosine, a vasodilator, regulates cerebral blood flow (CBF). Treatment with adenosine receptor agonists has shown benefit in experimental CNS trauma; however, their effects on CBF after injury remain undefined. We used magnetic resonance imaging to assess CBF in uninjured rats both early and at 24 h after intrahippocampal administration of either the nonselective adenosine receptor agonist 2-chloroadenosine (2-CA, 12 nmol) or the A(2A)-receptor agonist 2-p-(2-carboxyethyl)-phenethylamino-5'-N-ethylcarbox-amidoadenosine (CGS 21680, 6 nmol). We also assessed the effects of these agents on cerebral metabolic rate for glucose (CMRglu). We then assessed the effect of 2-CA on CBF at 3.5 to 5 h after controlled cortical impact (CCI). Injection of 2-CA into uninjured rat brain produced marked increases in CBF in ipsilateral hippocampus and cortex versus vehicle (P<0.05); CBF increases persisted even at 24 h. Measurement of hippocampal levels of 2-CA showed persistent increases to 24 h. CGS 21680 produced even more marked global increases in CBF than seen with 2-CA (2-6-fold versus vehicle, P<0.05 in 10/12 regions of interest (ROIs)). Neither agonist altered CMRglu versus vehicle. After CCI, 2-CA increased CBF in ipsilateral hippocampal and hemispheric ROIs (P<0.05 versus vehicle), but the response was attenuated at severe injury levels. We report marked increases in CBF after injection of adenosine receptor agonists into uninjured rat brain despite unaltered CMRglu. 2-Chloroadenosine produced enduring increases in CBF in uninjured brain and attenuated posttraumatic hypoperfusion. Future studies of adenosine-related therapies in CNS injury should address the role of CBF.

Resuscitation with Pressors after Traumatic Brain Injury

BACKGROUND

The purpose of the study was to compare initial resuscitation with arginine vasopressin (AVP), phenylephrine (PE), or isotonic crystalloid fluid alone after traumatic brain injury and vasodilatory shock.

STUDY DESIGN

Anesthetized, ventilated swine (n = 39, 30 +/- 2 kg) underwent fluid percussion traumatic brain injury followed by hemorrhage (30 +/- 2mL/kg) to a mean arterial pressure < 30mmHg, then were randomized to 1 of 5 groups to maintain mean arterial pressure > 60mmHg for 30 to 60minutes, then cerebral perfusion pressure > 60mmHg for 60 to 300minutes, either unlimited crystalloid fluid only (n = 9), arginine vasopressin + fluid (n = 9), phenylephrine + fluid (n = 9), arginine vasopressin only (n = 5), or phenylephrine only (n = 5). Heterologous transfusions were administered if hematocrit was < 13, and mannitol was administered if intracranial pressure was > 20 mmHg. Cerebrovascular reactivity was evaluated with serial CO(2) challenges.

RESULTS

In all groups, physiologic variables were similar at baseline and at the end of shock. On resuscitation, all achieved mean arterial pressure and cerebral perfusion pressure goals. Brain tissue PO(2)s were similar. With fluid only, more blood and mannitol were required, intracranial pressure and peak inspiratory pressure were higher, and cerebrovascular reactivity was decreased (all p < 0.05 versus pressor + fluid). With either pressor + fluid, cardiac output, heart rate, lactate, and mixed venous O(2) saturation were similar to fluid only, but total fluid requirements and urine output were both reduced (p < 0.05). With either pressor only, intracranial pressure remained low, but mixed venous O(2) saturation, cardiac output, and urine output were decreased (all p < 0.05 versus other groups).

CONCLUSIONS

To correct vasodilatory shock after traumatic brain injury, a resuscitation strategy that combined either phenylephrine or arginine vasopressin plus crystalloid was superior to either fluid alone or pressor alone.

The effects of traumatic brain injury on cerebral blood flow and brain tissue nitric oxide levels and cytokine expression

Adult, male, Sprague-Dawley rats were anesthetized, intubated, and mechanically ventilated with 1.5-2.0% isoflurane in oxygen (30%) and air. Rats were prepared for fluid percussion traumatic brain injury (TBI), laser Doppler flowmetry, and measurement of brain tissue nitric oxide (NO) levels using an ISO-NO electrode system. After preparation, isoflurane was reduced to 1.5%, and the rats were randomly assigned to receive sham (n = 6), moderate (1.9 atm, n = 6), or severe (2.8 atm, n = 6) parasagittal fluid percussion TBI. CBF and brain tissue NO levels were measured for 4 h, and then isoflurane levels were increased to 4.0% and the rats were decapitated and the brains were removed. Total RNA was isolated from rat brains and cytokine expression was determined. Laser Doppler flow velocity remained constant in the sham-injured rats but decreased significantly in rats subjected to moderate (p < 0.05) or severe (p < 0.05) TBI. Brain tissue NO levels remained constant in the sham-injured rats but decreased significantly (p < 0.01) after moderate TBI. Severe TBI produced slight, insignificant reductions in NO levels. Cytokine expression was very low in the shaminjured rats. TBI-induced expression of mRNAs for interleukin-1 alpha (IL-1alpha), IL-1beta, IL-6, and tumor necrosis factor-alpha (TNFa). IL-1alpha and IL-1beta mRNA expression increased significantly (p < 0.05 vs. sham-injury) after severe TBI and IL-6 and TNFa mRNA expression increased significant (p < 0.05 vs. sham-injury) after both moderate and severe TBI. Other cytokine mRNA expression was unchanged after TBI.

Cerebral pressure autoregulation is intact and is not influenced by hypothermia after traumatic brain injury in rats

In head-injured patients and experimental traumatic brain injury (TBI), important cerebrovascular abnormalities include decreases in cerebral blood flow (CBF) and impairment of cerebral pressure autoregulation. We evaluated CBF and pressure autoregulation after fluid percussion injury (FPI) and hypothermia in rats with the hypothesis that hypothermia would ameliorate changes in posttraumatic CBF. Male Sprague-Dawley rats, intubated and mechanically ventilated, were prepared for parasagittal FPI (1.8 atm) and laser Doppler CBF flow (LDF) measurement. The abdominal aorta was cannulated for rapid removal and reinfusion of blood. Baseline autoregulatory testing in all groups consisted of LDF measurements at normothermia and a mean arterial pressure (MAP) of 100 mm Hg, followed by randomly ordered changes of MAP to 80, 60, and 40 mm Hg. Animals were then randomized to one of five groups: normothermic control without FPI; normothermia with FPI; hypothermic control (32 degrees C) without FPI; hypothermia initiated before FPI; and hypothermia initiated immediately after FPI injury. For each group, a complete, randomly ordered autoregulatory sequence was performed at 30 and 60 min after FPI or sham TBI. In a second study, rats were prepared identically, maintained at normothermic temperatures and autoregulation was tested before and after TBI using a set of randomly ordered levels of hypotension or using progressive reductions in MAP (i.e., 80, 60, 40 mm Hg) with the hypothesis that the technical manner and timing of decreasing of the blood pressure would effect CBF after TBI. Due to high acute mortality, the group in which hypothermia was induced before FPI was excluded from the analysis. At baseline, autoregulation was similar in all groups. There was no change in CBF or autoregulation in the normothermic control group at 30 and 60 min. In the other groups at 30 and 60 min, there was a similar, statistically significant decrease in absolute CBF (i.e., a decrease of 27-57% of baseline values), but pressure autoregulation was intact except at the lowest blood pressure tested at 60 min, where there was a slight improvement in the hypothermic group. Thus, in these experiments, absolute CBF decreased with hypothermia and FPI, while neither hypothermia nor FPI significantly altered autoregulation. In the second study, autoregulatory function was not different before TBI when comparing random and sequential blood pressure changes, but, when comparing the groups after TBI at the 60 mm Hg blood pressure level, CBF was significantly lower in the sequential group than in the random order group. This suggests that the mechanism of creating hypotension, whether random or sequential, significantly affects the measurement of CBF and autoregulation after TBI in rats.

Role of nitric oxide in the regulation of neuronal proliferation, survival and differentiation

Nitric oxide (NO), an important cellular messenger, has been linked to both neurodegenerative and neuroprotective actions. In the present review, we focus on recent data establishing a survival and differentiation role for NO in several neural in vitro and in vivo models. Nitric oxide has been found to be essential for survival of neuronal cell lines and primary neurons in culture under various death challenges. Furthermore, its lack may aggravate some neuropathological conditions in experimental animals. Several cellular pathways and signaling systems subserving this neuroprotective role of NO are considered in the review. Survey of recent data related to the developmental role of NO mainly focus on its action as a negative regulator of neuronal precursor cells proliferation and on its role of promotion of neuronal differentiation. Discussion on discrepancies arising from the literature is focused on the Janus-faced properties of the molecule and it is proposed that most controversial results are related to the intrinsic property of NO to compensate among functionally opposed effects. As an example, the increased proliferation of neural cell precursors under conditions of NO shortage may be, later on in the development, compensated by increased elimination through programmed cell death as a consequence of the lack of the survival-promoting action of the molecule. To elucidate these complex, and possibly contrasting, effects of NO is indicated as an important task for future researches.

Does induced hypertension reduce cerebral ischaemia within the traumatized human brain?

Recent changes in published guidelines for the management of patients with severe head injury are based on data showing that aggressive maintenance of cerebral perfusion pressure (CPP) can worsen outcome due to extracranial complications of therapy. However, it remains unclear whether CPP augmentation could reduce cerebral ischaemia, a finding which might prompt the search for CPP augmentation protocols that avoid these extracranial complications. We studied 10 healthy volunteers and 20 patients within 6 days of closed head injury. All subjects underwent imaging of cerebral blood flow (CBF), blood volume (CBV), oxygen metabolism (CMRO2) and oxygen extraction fraction (OEF) using 15O PET. In addition, for patients, the EEG power ratio index (PRI), burst suppression ratio and somatosensory evoked potentials (SEP) were obtained and CPP was increased from 68 +/- 4 to 90 +/- 4 mmHg using an infusion of norepinephrine and measurements were repeated. Following elevation of CPP, CBF and CBV were increased and CMRO2 and OEF were reduced (P < 0.001 for all comparisons). Regions with a reduction in CMRO2 were associated with the greatest reduction in OEF (r2 = 0.3; P < 0.0001). Although CPP elevation produced a significant fall in the ischaemic brain volume (IBV) (from 15 +/- 16 to 5 +/- 4 ml; P < 0.01) and improved flow metabolism coupling, the IBV was small and clinically insignificant in the majority of these patients. However, the reduction in IBV was directly related to the baseline IBV (r2 = 0.97; P < 0.001) and patients with large baseline IBV showed substantial and clinically significant reductions. CPP augmentation increased the EEG PRI (5.0 +/- 1.5 versus 4.3 +/- 1.4, P < 0.01), implying an overall decrease in neural activity, but these changes did not correlate with the reduction in CMRO2 and there was no change in SEP cortical amplitude (N20-P27). These data provide support for recent changes in recommended CPP levels for head injury management across populations of patients with significant head injury. However, they do not provide guidance on whether the intervention may be more appropriate at earlier stages after injury, or in patients selected because of high baseline IBV. It also remains unclear whether CPP values below 65 mmHg can be safely used in this population. Clarification of the significance of a reduction in CMRO2 and neuronal electrical function will require further study.

Comparison of tetrahydrobiopterin and L-arginine on cerebral blood flow after controlled cortical impact injury in rats

The purpose of this study was to compare the effects of L-arginine and tetrahydrobiopterin administration on post-traumatic cerebral blood flow (CBF) and tissue levels of NO in injured brain tissue. Rats were anesthetized with isoflurane. Mean blood pressure, intracranial pressure, cerebral blood flow using laser Doppler flowmetry (LDF) and brain tissue nitric oxide (NO) concentrations were measured prior to, and for 2 h after a controlled cortical impact injury. L-arginine, 300 mg/kg, tetrahydrobiopterin, 10 mg/kg, or equal volume of saline was given at 5 min after injury. In the saline-treated animals, LDF decreased to 34 +/- 4% of baseline values after injury. NO concentration also decreased by approximately 20 pmol/ml from baseline values. L-arginine and tetrahydrobiopterin administration both resulted in a significant preservation of tissue NO concentrations and an improvement in LDF, compared to control animals given saline. These studies demonstrate that tetrahydrobiopterin administration has a beneficial effect on cerebral blood flow that is similar to L-arginine administration, and may suggest that depletion of tetrahydrobiopterin plays a role in the post-traumatic hypoperfusion of the brain.

Laser Doppler Flow and Brain Tissue PO2 after Cortical Impact Injury Complicated by Secondary Ischemia in Rats Treated with Arginine

BACKGROUND

Traumatic brain injury (TBI) makes the brain susceptible to secondary insults such as ischemia. This study tested the hypothesis that L-arginine would increase regional CBF (rCBF) and brain tissue PO2 (PbtO2) at the injury site.

METHODS

A secondary insult model was employed in rodents. rCBF was measured with laser doppler flowmetry (LDF) and PbtO2 with a PO2 catheter at the impact site. Animals were randomized to receive L-arginine, D-arginine or saline intravenously, 5 minutes after impact.

RESULTS

In animals who received L-arginine, the percentage rCBF from baseline (%CBF) was higher at the impact site after impact (p < 0.001), during bilateral carotid occulation (BCO) (p = 0.001) and during reperfusion (p = 0.032). In contrast, PbtO2 was not significantly increased throughout the experiment for the L-arginine group.

CONCLUSIONS

Administration of L-arginine increased rCBF in the injured brain tissue, and resulted in better preservation of CBF during BCO than D-arginine and saline.

Oxidative DNA lesions in a rodent model of traumatic brain injury

BACKGROUND

Oxidative DNA lesions have not been well studied in traumatic brain injury (TBI).

METHODS

TBI was induced with a controlled cortical impact injury in rats. Brain tissue was examined for 8-hydroxy-2'-deoxyguanosine (oh8dG) using mono-clonal antibodies at different time frames; 15 minutes (n = 4), 30 minutes (n = 7), 60 minutes (n = 6), and 240 minutes (n = 5). The control group consisted of sham-operated animals undergoing the same surgery without the controlled cortical impact injury (n = 5).

RESULTS

An elevation of oh8dG was detected in the nuclear and perinuclear (mitochondrial) regions of the ipsilateral cortex, but seldom in those of the contralateral cortex. The amount of oh8dG in those animals with TBI was significant in all time frames when compared with sham-operated controls (p < 0.001). The oh8dG levels were more prominent at 15 minutes (p < 0.0001) when compared with controls.

CONCLUSION

Oxidative DNA lesions occurred in this model of TBI maximally early after TBI. This suggests that oh8dGs may affect genetic material of the brain and that oh8dGs may adversely affect gene expression that occurs early after head injury.

Nitric oxide in traumatic brain injury

Nitric oxide (NO) is a gaseous chemical messenger which has functions in the brain in a variety of broad physiological processes, including control of cerebral blood flow, interneuronal communications, synaptic plasticity, memory formation, receptor functions, intracellular signal transmission, and release of neurotransmitters. As might be expected from the numerous and complex roles that NO normally has, it can have both beneficial and detrimental effects in disease states, including traumatic brain injury. There are two periods of time after injury when NO accumulates in the brain, immediately after injury and then again several hours-days later. The initial immediate peak in NO after injury is probably due to the activity of endothelial NOS and neuronal NOS. Pre-injury treatment with 7-nitroindazole, which probably inhibits this immediate increase in NO by neuronal NOS, is effective in improving neurological outcome in some models of traumatic brain injury (TBI). After the initial peak in NO, there can be a period of relative deficiency in NO. This period of low NO levels is associated with a low cerebral blood flow (CBF). Administration of L-arginine at this early time improves CBF, and outcome in many models. The late peak in NO after traumatic injury is probably due primarily to the activity of inducible NOS. Inhibition of inducible NOS has neuroprotective effects in most models.