Hyperbaric oxygen reduces infarct volume in rats by increasing oxygen supply to the ischemic periphery

Secondary deterioration in a patient with cerebral and coronary arterial gas embolism after brief symptom resolution: a case report

Introduction

Hyperbaric oxygen treatment (HBOT) is recommended for arterial gas embolism (AGE) with severe symptoms. However, once symptoms subside, there may be a dilemma to treat or not.

Case presentation

A 71-year-old man was noted to have a mass shadow in his left lung, and a transbronchial biopsy was performed with sedation. Flumazenil was intravenously administered at the end of the procedure. However, the patient remained comatose and developed bradycardia, hypotension, and ST-segment elevation in lead II. Although the ST changes spontaneously resolved, the patient had prolonged disorientation. Whole- body computed tomography revealed several black rounded lucencies in the left ventricle and brain, confirming AGE. The patient received oxygen and remained supine. His neurological symptoms gradually improved but worsened again, necessitating HBOT. HBOT was performed seven times, after which neurological symptoms resolved almost completely.

Conclusions

AGE can secondarily deteriorate after symptoms have subsided. We recommend that HBOT be performed promptly once severe symptoms appear, even if they resolve spontaneously.

Synergistic neuroprotective effects of hyperbaric oxygen and methylprednisolone following contusive spinal cord injury in rat

OBJECTIVE

Recent studies revealed the neuroprotective effects of hyperbaric oxygen (HBO) on spinal cord injury (SCI). Meanwhile, the use of methylprednisolone (MP) is one of the current protocols with limited effects in SCI patients. Accordingly, the aim of the present study was to investigate the effect of combined HBO and MP treatment on SCI.

DESIGN

The present study was conducted on five groups of rats each as follows: Sham group (underwent laminectomy alone at T9 level vertebra); SCI group (underwent moderate contusive SCI); MP group (underwent SCI and received MP); HBO group (underwent SCI and received HBO); HBO + MP group (underwent SCI and simultaneously received MP and HBO). Blood serum and Spinal cord tissue samples were taken 48 h after SCI for analysis of serum ferric reducing antioxidant power (FRAP) and tissue malodialdehyde (MDA) levels as well as immunohistochemistry of caspase-3 and tumor necrosis factor-alpha (TNF-α). Neurological function was evaluated by the Basso-Beattie-Bresnehan (BBB) locomotion scores until the end of experiments. Additionally, histopathology was assessed at the end of the study.

SETTING

Mazandaran University of Medical Sciences, Sari, Iran.

RESULTS

Combination therapy with HBO and MP in the HBO + MP group significantly decreased MDA as well as increased FRAP levels compared to other treatment groups. Meanwhile, attenuated TNF-α and Caspase-3 expression could be significantly detected in the HBO + MP group. At the end of treatment, the neurological outcome was significantly improved and the extent of injured spinal tissue was also significantly reduced in the HBO + MP compared to other treatment groups.

CONCLUSION

The results suggest that combined therapy with MP and HBO has synergistic effects on SCI treatment.

A Case Series of 39 United States Veterans with Mild Traumatic Brain Injury Treated with Hyperbaric Oxygen Therapy

Importance: The Defense and Veterans Brain Injury Center reported 358,088 mild traumatic brain injury (mTBI) among U.S. service members worldwide between the years 2000 and 2020. Veterans with mTBI have higher rates of Post-Traumatic Stress Disorder (PTSD), depressive disorder, substance use disorder, anxiety disorder, and suicide than their healthy counterparts. Currently, there is no effective treatment for mTBI. Objective: To assess the efficacy of hyperbaric oxygen therapy (HBOT) as a treatment option for mTBI. Design, Setting, Participants: This is a case series of 39 U.S. Veterans diagnosed with mTBI and treated with HBOT. Of these participants, 36 were men and 3 women, and their ages ranged between 28 and 69. The treatment was administered by The 22 Project (a veteran-centered nonprofit organization) using monoplace hyperbaric chambers located in Delray Beach, Florida. Neuroimaging using Single Photon Emission Computer Tomography (SPECT) brain scans performed pre- and post-HBOT were made available for secondary analysis. Nilearn Python Library was utilized to visualize the corresponding neuroimaging data. A two-sided paired t-test in R was used to compare the pre- and post-treatment results. Intervention: A full treatment of HBOT involved 40 sessions. Each session consisted of the administration of 100% oxygen at 1.5 atmospheres for 90 min, twice a day, for 20 days, Mondays to Fridays only. Main Outcome and Measure: Perfusion in the brain’s Brodmann Areas (BA) comparing pre- and post-HBOT using NeuroGam software analysis from brain SPECT scan neuroimaging and multi-symptom self-reports. Results: A comparison between the pre- and post-HBOT brain scans showed significant improvement in the brain perfusion, and the difference was statistically significant (p < 0.001). Separately, participants reported reduced pain, improved mood, and better sleep, an outcome that translated into an average of about 46.6% improvement in the measured symptoms. Conclusions and Relevance: This series demonstrated that HBOT could be a useful treatment for mTBI in U.S. veterans. The participants in the study showed marked improvement in both brain perfusion measured on SPECT scan imaging and measured mTBI symptoms. This is the first study to use brain SPECT scans with quantitative numerical measurements to demonstrate improvement in brain perfusion in veterans with mild TBI treated with HBOT and measured mTBI symptoms. Future research studies are currently being done to validate these important findings.

A review on the neuroprotective effects of hyperbaric oxygen therapy

Hyperbaric oxygen therapy, intermittent breathing of 100% oxygen at a pressure upper than sea level, has been shown to be some of the neuroprotective effects and used therapeutically in a wide range of neurological disorders. This review summarizes current knowledge about the neuroprotective effects of hyperbaric oxygen therapy with their molecular mechanisms in different models of neurological disorders.

Neuroprotective effects of hyperbaric oxygen (HBO) therapy on neuronal death induced by sciatic nerve transection in rat

Background

Recent studies shows that hyperbaric oxygen (HBO) therapy exerts some protective effects against neural injuries. The purpose of this study was to determine the neuroprotective effects of HBO following sciatic nerve transection (SNT).

Methods

Rats were randomly divided into five groups (n = 14 per group): Sham-operated (SH) group, SH + HBO group, SNT group, and SNT + pre- and SNT + post-HBO groups (100% oxygen at 2.0 atm absolute, 60 min/day for five consecutive days beginning on 1 day before and immediately after nerve transaction, respectively). Spinal cord segments of the sciatic nerve and related dorsal root ganglions (DRGs) were removed 4 weeks after nerve transection for biochemical assessment of malodialdehyde (MDA) levels in spinal cord, biochemical assessment of superoxide dismutase (SOD) and catalse (CAT) activities in spinal cord, immunohistochemistry of caspase-3, cyclooxigenase-2 (COX-2), S100beta (S100ß), and terminal deoxynucleotidyl transferase dUTP nick end labeling (TUNEL) in spinal cord and DRG.

Results

The results revealed that MDA levels were significantly decreased in the SNT + pre-HBO group, while SOD and CAT activities were significantly increased in SNT + pre- and SNT + post-HBO treated rats. Attenuated caspase-3 and COX-2 expression, and TUNEL reaction could be significantly detected in the HBO-treated rats after nerve transection. Also, HBO significantly increased S100ß expression.

Conclusions

Based on these results, we can conclude that pre- and post-HBO therapy had neuroprotective effects against sciatic nerve transection-induced degeneration.

Hyperbaric oxygenation and 20-hydroxyeicosatetreanoic acid inhibition reduce stroke volume in female diabetic Sprague-Dawley rats

NEW FINDINGS

What is the central question of this study? Is there a beneficial effect and what are the mechanisms of acute and multiple hyperbaric oxygenation (HBO₂ ) exposures on the outcome of cerebral tissue injury induced by a transient middle cerebral artery occlusion model in diabetic female rats? Are 20-hydroxyeicosatetreanoic acid and epoxyeicosatrienoic acids involved? What is the main finding and its importance? Equal reduction of cortical and total infarct size in rats treated with HBO₂ and HET0016 (20-hydroxyeicosatetreanoic acid production inhibitor) and significant mRNA upregulation of epoxyeicosatrienoic acid-producing enzymes (Cyp2J3 and Cyp2C11) in treated groups suggest that HBO₂ and HET0016 are highly effective stroke treatments and that cytochrome P450 metabolites are involved in this therapeutic effect. We evaluated the effects of acute and repetitive hyperbaric oxygenation (HBO₂ ), 20-hydroxyeicosatetreanoic acid (20-HETE) inhibition by N-hydroxy-N'-(4-butyl-2methylphenyl)-formamidine (HET0016) and their combination on experimental stroke outcomes. Streptozotocin-induced type 1 diabetic Sprague-Dawley female rats (n = 42; n = 7 per group), were subjected to 30 min of transient middle cerebral artery occlusion (t-MCAO)-reperfusion and divided into the following groups: (1) control group, without treatment; and groups exposed to: (2) HBO₂ ; (3) multiple HBO₂ (HBO₂ immediately and second exposure 12 h after t-MCAO); (4) HET0016 pretreatment (1 mg kg^-1 , 3 days before t-MCAO) combined with HBO₂ after t-MCAO; (5) HET0016 treatment (1 h before, during and for 6 h after t-MCAO); and (6) HET0016 treatment followed by HBO₂ after t-MCAO. Messenger RNA expression of CYP2J3, CYP2C11, CYP4A1, endothelial nitric oxide synthase and epoxide hydrolase 2 was determined by real-time qPCR. Cortical infarct size and total infarct size were equally and significantly reduced in HBO₂ - and HET0016-treated rats. Combined treatment with HET0016 and HBO₂ provided no significant additive effect compared with HET0016 treatment only. Messenger RNA of Cyp2J3 was significantly increased in all study groups, and mRNA of Cyp2C11 was significantly increased in the multiple HBO₂ group and the HET0016 treatment followed by HBO₂ group, compared with the control group. Expression of endothelial nitric oxide synthase was significantly increased after HBO₂ treatments, and expression of epoxide hydrolase 2 was increased in all groups compared with the control group. In diabetic female Sprague-Dawley rats, HBO₂ and HET0016 are highly effective stroke treatments, suggesting the involvement of cytochrome P450 metabolites and the NO pathway in this therapeutic effect.

The efficacy of hyperbaric oxygen in hemorrhagic stroke: experimental and clinical implications

Hemorrhagic stroke, accounting for 10-30% of stroke cases, carries high rates of morbidity and mortality. This review presents the current knowledge on the efficacy of hyperbaric oxygen (HBO)-based modalities in the preclinical research on hemorrhagic stroke. Both preconditioning and post-treatment with HBO are considered as prospective therapeutic options. High efficacy of HBO therapy (HBOT) for brain hemorrhage has been noted. We found that moderate hyperbaric pressures appear optimal for therapeutic effect, while the therapeutic window of opportunity is short. HBO preconditioning offers more modest neuroprotective benefit as compared to HBO post-treatment for experimental intracerebral hemorrhage. We advocate for mandatory calculations of percent changes in the experimentally investigated indexes of HBO effectiveness and stress the need to design new clinical trials on HBO for hemorrhagic stroke.

Hyperbaric Oxygen Reduces Infarction Volume and Hemorrhagic Transformation Through ATP/NAD+/Sirt1 Pathway in Hyperglycemic Middle Cerebral Artery Occlusion Rats

BACKGROUND AND PURPOSE

Energy depletion is a critical factor leading to cell death and brain dysfunction after ischemic stroke. In this study, we investigated whether energy depletion is involved in hyperglycemia-induced hemorrhagic transformation after ischemic stroke and determined the pathway underlying the beneficial effects of hyperbaric oxygen (HBO).

METHODS

After 2-hour middle cerebral artery occlusion, hyperglycemia was induced by injecting 50% dextrose (6 mL/kg) intraperitoneally at the onset of reperfusion. Immediately after it, rats were exposed to HBO at 2 atmospheres absolutes for 1 hour. ATP synthase inhibitor oligomycin A, nicotinamide phosphoribosyl transferase inhibitor FK866, or silent mating type information regulation 2 homolog 1 siRNA was administrated for interventions. Infarct volume, hemorrhagic volume, and neurobehavioral deficits were recorded; the level of blood glucose, ATP, and nicotinamide adenine dinucleotide and the activity of nicotinamide phosphoribosyl transferase were monitored; the expression of silent mating type information regulation 2 homolog 1, acetylated p53, acetylated nuclear factor-κB, and cleaved caspase 3 were detected by Western blots; and the activity of matrix metalloproteinase-9 was assayed by zymography.

RESULTS

Hyperglycemia deteriorated energy metabolism and reduced the level of ATP and nicotinamide adenine dinucleotide and exaggerated hemorrhagic transformation, blood-brain barrier disruption, and neurological deficits after middle cerebral artery occlusion. HBO treatment increased the levels of the ATP and nicotinamide adenine dinucleotide and consequently increased silent mating type information regulation 2 homolog 1, resulting in attenuation of hemorrhagic transformation, brain infarction, as well as improvement of neurological function in hyperglycemic middle cerebral artery occlusion rats.

CONCLUSIONS

HBO induced activation of ATP/nicotinamide adenine dinucleotide/silent mating type information regulation 2 homolog 1 pathway and protected blood-brain barrier in hyperglycemic middle cerebral artery occlusion rats. HBO might be promising approach for treatment of acute ischemic stroke patients, especially patients with diabetes mellitus or treated with r-tPA (recombinant tissue-type plasminogen activator).

Hyperbaric oxygen therapy and preconditioning for ischemic and hemorrhagic stroke

To date, the therapeutic methods for ischemic and hemorrhagic stroke are still limited. The lack of oxygen supply is critical for brain injury following stroke. Hyperbaric oxygen (HBO), an approach through a process in which patients breathe in 100% pure oxygen at over 101 kPa, has been shown to facilitate oxygen delivery and increase oxygen supply. Hence, HBO possesses the potentials to produce beneficial effects on stroke. Actually, accumulated basic and clinical evidences have demonstrated that HBO therapy and preconditioning could induce neuroprotective functions via different mechanisms. Nevertheless, the lack of clinical translational study limits the application of HBO. More translational studies and clinical trials are needed in the future to develop effective HBO protocols.

Normobaric oxygen treatment in acute ischemic stroke: a clinical perspective

Acute ischemic stroke is a common and serious neurological disease. Oxygen therapy has been shown to increase oxygen supply to ischemic tissues and improve outcomes after cerebral ischemia/reperfusion. Normobaric hyperoxia (NBO), an easily applicable and non-invasive method, shows protective effects on acute ischemic stroke animals and patients in pilot studies. However, many critical scientific questions are still unclear, such as the therapeutic time window of NBO, the long-term effects and the benefits of NBO in large clinic trials. In this article, we review the current literatures on NBO treatment of acute ischemic stroke in preclinical and clinical studies and try to analyze and identify the key gaps or unknowns in our understanding about NBO. Based on these analyses, we provide suggestions for future studies.

Progress in AQP Research and New Developments in Therapeutic Approaches to Ischemic and Hemorrhagic Stroke

Cerebral edema often manifests after the development of cerebrovascular disease, particularly in the case of stroke, both ischemic and hemorrhagic. Without clinical intervention, the influx of water into brain tissues leads to increased intracranial pressure, cerebral herniation, and ultimately death. Strategies to manage the development of edema constitute a major unmet therapeutic need. However, despite its major clinical significance, the mechanisms underlying cerebral water transport and edema formation remain elusive. Aquaporins (AQPs) are a class of water channel proteins which have been implicated in the regulation of water homeostasis and cerebral edema formation, and thus represent a promising target for alleviating stroke-induced cerebral edema. This review examines the significance of relevant AQPs in stroke injury and subsequently explores neuroprotective strategies aimed at modulating AQP expression, with a particular focus on AQP4, the most abundant AQP in the central nervous system.

Hyperbaric oxygen modalities are differentially effective in distinct brain ischemia models

The effectiveness and efficacy of hyperbaric oxygen (HBO) preconditioning and post-treatment modalities have been demonstrated in experimental models of ischemic cerebrovascular diseases, including global brain ischemia, transient focal and permanent focal cerebral ischemia, and experimental neonatal hypoxia-ischemia encephalopathy. In general, early and repetitive post-treatment of HBO appears to create enhanced protection against brain ischemia whereas delayed HBO treatment after transient focal ischemia may even aggravate brain injury. This review advocates the level of injury reduction upon HBO as an important component for translational evaluation of HBO based treatment modalities. The combined preconditioning and HBO post-treatment that would provide synergistic effects is also worth considering.

The Efficacy of Hyperbaric Oxygen Therapy on Middle Cerebral Artery Occlusion in Animal Studies: A Meta-Analysis

Background

Inconsistent results have been reported for hyperbaric oxygen therapy (HBO) for acute stroke. We conducted a systematic review and meta-analysis to evaluate the benefit of HBO in animal studies of middle cerebral artery occlusion (MCAO).

Methods

A systematic search of the literature published prior to September 2015 was performed using Embase, Medline (OvidSP), Web of Science and PubMed. Keywords included “hyperoxia” OR “hyperbaric oxygen” OR “HBO” AND “isch(a)emia” OR “focal cerebral ischemia” OR “stroke” OR “infarct” OR “middle cerebral artery occlusion (MCAO).” The primary endpoints were the infarct size and/or neurological outcome score evaluated after HBO treatment in MCAO. Heterogeneity was analyzed using Cochrane Library’s RevMan 5.3.5.

Results

Fifty-one studies that met the inclusion criteria were identified among the 1198 studies examined. When compared with control group data, HBO therapy resulted in infarct size reduction or improved neurological function (32% decrease in infarct size; 95% confidence interval (CI), range 28%–37%; p < 0.00001). Mortality was 18.4% in the HBO group and 26.7% in the control group (RR 0.72, 95% CI, 0.54–0.98; p = 0.03). Subgroup analysis showed that a maximal neuro-protective effect was reached when HBO was administered immediately after MCAO with an absolute atmospheric pressure (ATA) of 2.0 (50% decrease; 95% CI, 43% -57% decrease; p < 0.0001) and more than 6 hours HBO treatment (53% decrease; 95% CI, 41% -64% decrease; p = 0.0005).

Conclusions

HBO had a neuro-protective effect and improved survival in animal models of MCAO, especially in animals given more than 6 hours of HBO and when given immediately after MCAO with 2.0 ATA.

Protection mechanism of early hyperbaric oxygen therapy in rats with permanent cerebral ischemia

[Purpose] The purpose of this study was to investigate whether early hyperbaric oxygen is useful in rats with permanent cerebral ischemia, and whether its mechanism relates to the inhibition of the tumor necrosis factor-alpha-protein kinase C-alpha pathway. [Subjects] Healthy, male Sprague-Dawley rats (N = 108) were the subjects. [Methods] After middle cerebral artery occlusion models were successfully made, rats were randomly divided into sham-operated, cerebral ischemia, and hyperbaric oxygen groups. At 4 and 12 hours after modeling, the volume of cerebral infarction was determined by triphenyltetrazolium chloride staining, and brain water content was measured using the dry and wet method. The expression of tumor necrosis factor-alpha and protein kinase C-alpha in the ischemic penumbra tissue was measured using Western blot analysis. [Results] The data showed that at 4 and 12 hours after modeling, cerebral infarct volume and brain water content decreased in the hyperbaric oxygen group, and expression of tumor necrosis factor-alpha and phospho-protein kinase C-alpha in the ischemic penumbra tissue also decreased. [Conclusion] Our study demonstrates that early hyperbaric oxygen therapy has protective effects on brain tissue after cerebral ischemia, possibly via inhibition of tumor necrosis factor-alpha and phospho-protein kinase C-alpha.

Effect of hyperbaric oxygen preconditioning on peri-hemorrhagic focal edema and aquaporin-4 expression

The aim of the present study was to investigate the effect of hyperbaric oxygen preconditioning (HBO-PC) on peri-hemorrhagic focal edema and aquaporin-4 (AQP-4) expression in an experimental intracerebral hemorrhage (ICH) rat model. Sixty-six Sprague Dawley® rats were divided into three groups: The sham-surgery group (SHG; n=6); the control group (A-ICH; n=30), in which the rats were injected with autologous blood; and the experimental HBO-PC group (P-HBO; n=30). The rats underwent brain edema and AQP-4 detection at 5 postoperative time-points (24, 48 and 72 h and 5 and 7 days). The water content in the brain tissues of the A-ICH animals was higher than that in the brain tissues of the SHG rats at each time-point (P<0.05), and the edema in the P-HBO was significantly more severe 24 and 48 h postoperatively than that at 7 days postoperatively (P<0.05). The difference between the P-HBO and A-ICH was significant at 48 and 72 h postoperatively (P<0.05). AQP-4 was expressed in the post-hemorrhagic rat brains of all groups; the SHG animals exhibited low expression, while the A-ICH animals exhibited an increased expression 24 h postoperatively. In the A-ICH, expression peaked at 48 h postoperatively and began to decrease gradually after 72 h. At the 7-day time-point, the expression level in the A-ICH was closer to but still higher than that of the SHG animals (P<0.05). The differences between the P-HBO and A-ICH animals at the postoperative 24-h, 48-h and 7-day time-points were statistically significant (P<0.05). In conclusion, HBO-PC may downregulate AQP-4 expression to reduce the intracerebral edema, thus strengthening tolerance to ICH and protecting the nerves.

The effect of concentration and duration of normobaric oxygen in reducing caspase-3 and -9 expression in a rat-model of focal cerebral ischaemia

The aim of this study was to determine the effect of different concentrations of normobaric oxygen (NBO) on neurological function and the expression of caspase-3 and -9 in a rat model of acute cerebral ischaemia. Sprague-Dawley rats (n=120) were randomly divided into four groups (n=30 per group), including 3 groups given NBO at concentrations of 33%, 45% or 61% and one control group given air (21% oxygen). After 2h of ischaemic occlusion, each group was further subdivided into six subgroups (n=5) during reperfusion according to the duration (3, 6, 12, 24, 48 or 72h) and concentration of NBO (33%, 45% or 61%) or air treatment. The Fluorescence Quantitative polymerase chain reaction (PCR) and immunohistochemistry were used to detect caspase-3 and -9 mRNA and protein relative expression respectively. The Neurologic Impairment Score (NIS) was significantly lower in rats given 61% NBO ≥3h after reperfusion when compared to the control group (P<0.05, Mann-Whitney U). NBO significantly reduced caspase-3 and -9 mRNA and protein expression when compared to the control group at all NBO concentrations and time points (P<0.05, ANOVA). The expression of caspase-3 and -9 was lower in the group given 61% NBO compared any other group, and this difference was statistically significant when compared to the group given 33% NBO for ≥48h and the control group (both P<0.05, ANOVA). These findings indicate that NBO may inhibit the apoptotic pathway by reducing caspase-3 and -9 expression, thereby promoting neurological functional recovery after stroke.

Hyperbaric oxygen treatment in the experimental spinal cord injury model

BACKGROUND CONTEXT

Spinal cord trauma is a major cause of mortality and morbidity. Although no known treatment for spinal cord injury exists, a limited number of effective treatment modalities and procedures are available that improve secondary injury. Hyperbaric oxygen (HBO) treatment has been used to assist in neurologic recovery after cranial injury or ischemic stroke.

PURPOSE

To report the findings on the effectiveness of HBO treatment on rats with experimental traumatic spinal cord injury. Improvement was evaluated through motor strength assessment and nitrite level assay testing.

STUDY DESIGN

We randomly distributed 40 rats among 5 groups of 8 rats each: sham incurable trauma, induced trauma, HBO treatment begun at the 1st hour, HBO treatment begun at the 6th hour, and HBO treatment begun at the 24th hour.

METHOD

The HBO treatment was administered to rats in three of the groups and conducted in two 90-minute sessions, under an absolute atmospheric pressure of 2.4 at 100% oxygen for 5 days. In the motor strength evaluations, all the rats were observed during the inclined plane test and clinical motor examination on the first, third, and fifth days. In addition, the nitrite levels of spinal cord tissues on the sixth day were also studied.

RESULTS

Results from the inclined plane levels and motor strength test from all the three groups undergoing HBO treatment were higher than those from Group 2. It was also determined that early HBO treatment resulted in higher recovery rates (groups 3 and 4). The highest levels were seen in the group in which the HBO treatments were started in the first hour (Group 3). It was noted that nitrite levels of rats in the group exposed to trauma increased, compared with the sham group, but increased levels also diminished after HBO treatments. Again, the greatest decrease in nitrite levels was evident in the group where the HBO treatment was started the earliest (Group 3).

CONCLUSIONS

Prompt HBO treatment after trauma significantly contributed to the clinical, histopathologic, and biochemical recovery of the rats.

Administration of a second generation perfluorochemical in combination with hyperbaric oxygenation does not provide additional benefit in a model of permanent middle cerebral artery occlusion in rats

Objective

Both, second generation perfluorochemicals (Oxycyte®) and hyperbaric oxygen (HBO) have been shown to reduce necrotic tissue volume if administered early after experimental cerebral ischemia. With the idea of exponentiation of oxygen delivery to ischemic tissue, this study was conducted to investigate the combined effect of both treatment modalities on the extent of ischemic brain damage.

Methods

Permanent focal cerebral ischemia was induced in rats by middle cerebral artery occlusion (MCAO). Animals were assigned randomly to one of the following treatment groups: Control (0.9% NaCl, 1 ml/100 g i.v.), Oxycyte® (1 ml/100 g i.v.), HBO (1 bar hyperbaric oxygenation for 1 h) and HBO + Oxycyte® (1 ml/100 g i.v. combined with 1 bar hyperbaric oxygenation for 1 h). Injection of NaCl or Oxycyte® was performed following MCAO. After injection, breathing was changed to 100% oxygen in Oxycyte®-, HBO- and HBO + Oxycyte®-groups. After eight hours the necrotic volume was calculated from serial coronal sections stained with silver-nitrate and corrected for the extent of swelling.

Results

Hemodynamic and metabolic parameters were not affected by infusion of Oxycyte®. Total necrosis volume was significantly reduced in HBO-treated animals (223 ± 70 mm³), when compared to control animals (335 ± 36 mm³). In animals after Oxycyte®-treatment alone (299 ± 33 mm³) or combined HBO + Oxycyte®-treatment (364 ± 50 mm³) did not show a significantly smaller necrosis volume compared to control animals (necrosis volumes are given as mean ± SD).

Discussion

These results suggest that combination of hyperbaric oxygenation and Oxycyte® administered immediately after onset of vascular occlusion does not provide an additional neuroprotective effect in the early phase of brain ischemia.

Collateral therapeutics for cerebral ischemia

Current therapeutic strategies for acute ischemic stroke focus on vessel recanalization or penumbral neuroprotection without consideration of collaterals. Collateral circulation defines the extent of the ischemic penumbra, providing blood flow to tissues at risk of infarction downstream from an occluded artery. Therefore, leptomeningeal collaterals are a principal delivery route for oxygen, nutrients and potential therapeutic agents. Understanding of collateral anatomy and physiology is essential for the development of effective stroke treatments. Diagnostic imaging modalities may illustrate the penumbra from the collateral perspective, defining regions of relative ischemic vulnerability. Although specific collateral therapeutics are unrealized, insight may be gleaned from subtle details of prior stroke studies. Future advances will result from nascent research in therapeutic arteriogenesis and gene therapy adapted to the specific features of the cerebral circulation.

The effects of hyperbaric air and hyperbaric oxygen on blood-brain barrier integrity in rats

Hyperbaric oxygen (HBO) treatment yields conflicting results on blood-brain barrier (BBB) integrity under various pathological conditions and the effects of HBO on healthy brain is poorly understood. In this experimental study, the effects of HBO on BBB integrity were investigated in comparison with hyperbaric air (HBA) in intact rats. Four sessions of HBA or HBO were applied to intact rats in 24h. BBB integrity was functionally and structurally evaluated by determining extravasation of Evans blue (EB) dye and horseradish peroxidase (HRP) tracers. In immunohistochemical evaluation, relative staining intensity for occludin, a tight junction (TJ) protein, and aquaporin 4 (AQP4), a water-channel protein, was detected in the barrier type of microvessels of brain by image analysis. BBB permeability to EB dye significantly increased in animals in HBO treatment group compared to those in HBA and control groups (p<0.05). The immunoreactivity of occludin, a tight junction protein, remained essentially unaltered in capillaries of hippocampus in all groups. In animals exposed to HBO, AQP4 immunoreactivity significantly increased in parietal cortex compared to those in HBA and control groups (p<0.01). Ultrastructurally, frequent vesicles containing HRP reaction products were observed in capillary endothelial cells in cerebral cortex and hippocampus of rats subjected to both HBA and HBO. Our results indicate that the HBO administration to intact rats increased BBB permeability to both EB and HRP while HBA increased only HRP extravasation in these animals. The results of this study suggest that HBA also impairs the BBB integrity in intact rats as well as HBO.

Basic physiology of hyperbaric oxygen in brain

Oxygen is the proverbial 'double-edged sword' in that it is a necessity for life in moderation and toxic and detrimental to life in excess. This too is the dilemma in hyperbaric oxygen (HBO) treatment in cerebral ischemic-anoxic insults such as stroke, head injury, near drowning, asphyxia, cardiac arrest, etc., i.e. the brain at risk, where regions of ischemia are beside regions of marked hyperemia. The natural heterogeneity of normal brain tissue oxygenation compounds the problem with different microvascular brain regions living at various levels of oxygenation from 0 to arterial PO(2) as an added complication. The application of HBO, whether normobaric or hyperbaric, will result in brain tissue oxygenation ranging from normoxic to highly hyperoxic with the latter possibly exacerbating the injury sustained. On this basis, the application of multiple therapeutic interventions may be considered, for example, HBO in combination with free radical scavengers or inhibitors of free radical generating enzymes. Despite these difficulties in moderating oxygen delivery to treat cerebral ischemic-anoxic insults, overwhelming preclinical evidence indicates that HBO administered during or within 2 hours post-insult effectively attenuates the severity of brain damage sustained. The primary disconnection between pre-clinical and clinical efficacy of HBO then appears to be the time of application. Clinically, HBO therapy is applied at the earliest 6 hours post-insult but usually between 12 hours or longer post-insult. Pre-hospital application of HBO may be required for clear-cut demonstration of clinical efficacy.

Pathophysiology of an hypoxic–ischemic insult during the perinatal period

Hypoxia-ischemia is a leading cause of morbidity and mortality in the perinatal period with an incidence of 1/4000 live births. Biochemical events such as energy failure, membrane depolarization, brain edema, an increase of neurotransmitter release and inhibition of uptake, an increase of intracellular Ca(2+), production of oxygen-free radicals, lipid peroxidation, and a decrease of blood flow are triggered by hypoxia-ischemia and may lead to brain dysfunction and neuronal death. These abnormalities can result in mental impairments, seizures, and permanent motor deficits, such as cerebral palsy. The physical and emotional strain that is placed on the children affected and their families is enormous. The care that these individuals need is not only confined to childhood, but rather extends throughout their entire life span, so it is very important to understand the pathophysiology that follows a hypoxic-ischemic insult. This review will highlight many of the mechanisms that lead to neuronal death and include the emerging area of white matter injury as well as the role of inflammation and will provide a summary of therapeutic strategies. Hypothermia and oxygen will also be discussed as treatments that currently lack a specific target in the hypoxic/ischemic cascade.

Early administration of a second-generation perfluorochemical decreases ischemic brain damage in a model of permanent middle cerebral artery occlusion in the rat

OBJECTIVES

Perfluorochemicals (PFCs) may exert a neuroprotective function in the early phase of ischemia by improving the oxygen supply to the endangered tissue. We have, therefore, investigated the effect of Oxycyte, a second-generation perfluorocarbon solution, on the extent of early ischemic brain damage in a model of permanent focal cerebral ischemia.

METHODS

Eight hours of permanent focal cerebral ischemia was induced in isoflurane anesthetized male Sprague-Dawley rats by unilateral middle cerebral artery (MCA) thread occlusion under the control of laser Doppler flowmetry (LDF). Animals were assigned to one of the following treatment groups: nO2-NaCl and hO2-NaCl-NaCl (0.9%, 1 ml/100 g i.v.) and nO2-Oxycyte and hO2-Oxycyte-Oxycyte (1 ml/100 g i.v.). The injection of NaCl or Oxycyte was performed immediately after MCA occlusion. After injection, breathing was changed to pure oxygen in groups hO2-NaCl and hO2-Oxycyte while animals in groups nO2-NaCl and nO2-Oxycyte were allowed to breathe air. The necrotic volume was calculated from serial coronal sections stained with silver-nitrate. In addition, nitrotyrosine production was studied by immunohistochemistry.

RESULTS

Upon MCA occlusion, animals showed a reduction of cerebral blood flow of approximately 80% of the LDF signal in all groups. Hemodynamic and metabolic parameters were not affected by the infusion of Oxycyte. The total infarct volume was reduced in hO2-Oxycyte animals [group nO2-NaCl: 341+/-31 mm3 (mean+/-SD), group hO2-NaCl: 351+/-33 mm3, group nO2-Oxycyte: 354+/-24 mm3, and group hO2-Oxycyte: 300+/-29 mm3, p < 0.05 versus all other groups]. Moreover, hO2-Oxycyte animals showed lesser intensity of nitrotyrosine staining when compared with hO2-NaCl animals.

DISCUSSION

These results suggest that Oxycyte administered immediately after the onset of vascular occlusion may exert neuroprotective effects in the early phase of brain ischemia.

A review of oxygen therapy in ischemic stroke

Neuroprotective drugs have so far failed clinical trials, at high cost, and intravenous tissue plasminogen activator (i.v. tPA) remains the only FDA-approved acute stroke therapy. Hyperoxia, acting via multiple direct and indirect mechanisms, may be a powerful neuroprotective strategy to salvage acutely ischemic brain tissue and extend the time window for acute stroke treatment. Of the available oxygen delivery methods, hyperbaric oxygen therapy (HBO) appears to be the most potent, while even normobaric oxygen therapy (NBO) may be effective if started promptly after stroke onset. HBO has so far failed to show efficacy in three clinical trials. The failure of these trials is probably attributable to factors such as delayed time to therapy, inadequate sample size and use of excessive chamber pressures. Previous trials did not assess long-term benefit in patients with tissue reperfusion. In this modern era of stroke thrombolysis and advanced neuroimaging, oxygen therapy may have renewed significance. If applied within the first few hours after stroke onset or in patients with imaging evidence of salvageable brain tissue, oxygen therapy could be used to 'buy time' for the administration of thrombolytic or neuroprotective drugs. This article reviews the history and current rationale for using oxygen therapy in stroke, the mechanisms of action of HBO and the results of animal and human studies of hyperoxia in cerebrovascular diseases.

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

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

Normobaric oxygen therapy in acute ischemic stroke: A pilot study in Indian patients

Purpose:

Clinical and radiological assessment of effects of normobaric high-flow oxygen therapy in patients with acute ischemic stroke (AIS).

Materials and Methods:

Patients with anterior circulation ischemic strokes presenting within 12 h of onset, ineligible for intravenous thrombolysis, an National Institute of Health Stroke Scale (NIHSS) score of >4, a mean transit time (MTT) lesion larger than diffusion-weighted image (DWI) (perfusiondiffusion mismatch), and an evidence of cortical hypoperfusion on magnetic resonance imaging (MRI) were included into the trial. Active chronic obstructive pulmonary disease (COPD), requirement of >3/L min oxygen delivery to maintain SaO2 > 95%, rapidly improving neurological deficits, pregnancy, contraindications to MRI, or unstable medical conditions were excluded. The experimental group received humidified oxygen at flow rates of 10 L/min for 12 h. The NIHSS, modified Rankin Score (mRS), Barthel Index (BI) were measured at 0, 1, 7 day of admission and at 3 months follow-up. MRI with DWI/PWI was performed at admission, 24 h later and at 3 months follow-up.

Results:

Of 40 patients (mean age = 55.8 years ± 13.2) (range, 26–82), 20 patients were randomized to normobaric oxygen (NBO). The mean NIHSS in NBO and control groups were 14.25 and 12.7 at admission which decreased to 11.6 and 9.5 on the seventh day, and 9.4 and 9.05 at 3 months, respectively. The mean mRS (3.7/3.7) and BI (58.2/53.9) in NBO and control groups improved to 2/2.2 and 73.05/73.8 at the end of 3 months, respectively.

Conclusions:

NBO did not improve the clinical scores of stroke outcome in Indian patients with AIS.

Protecting the future: neuroprotective strategies in the pediatric intensive care unit

PURPOSE OF REVIEW

Brain injury is the leading cause of death in pediatric intensive care units, and improvements in therapy and in understanding the pathogenesis are urgently needed. This review presents recent advances in the understanding of neuroprotective therapy and brain-specific monitoring for critically ill pediatric patients.

RECENT FINDINGS

Two neuroprotective strategies are becoming increasingly accepted as they are applied to different mechanisms of brain injury. The rapid application of hypothermia and avoidance of hyperoxia after cardiac arrest and other brain injuries are each being more commonly used as both human and animal data advocating for these approaches accumulate. In addition, more advanced and noninvasive technologies are emerging that are designed to serve as surrogates for brain function and may be used to help predict outcome. Near-infrared spectroscopy is one such commonly used technique that has prompted many studies to understand how to incorporate it into practice.

SUMMARY

Protection of the pediatric brain from both a primary insult and the common subsequent secondary injury is essential for improving long-term neurologic outcomes. Whereas monitoring technology is being constantly modified, it must be proven efficacious in order to understand the utility of new and presumed neuroprotective therapies like hypothermia and avoidance of hyperoxia.

Use of normobaric and hyperbaric oxygen in acute focal cerebral ischemia - a preclinical and clinical review

High socioeconomic burden is attributed to acute ischemic stroke, but treatment strategies are still limited. Normobaric (NBO) and hyperbaric oxygen therapy (HBO) were frequently investigated in preclinical studies following acute focal cerebral ischemia with predominantly beneficial effects in different outcome measurements. Best results were achieved in transient cerebral ischemia, starting HBO early after artery occlusion, and by using relatively high pressures. On molecular level, oxygen application leads to blood-brain barrier stabilization, reduction of excitotoxic metabolites, and inhibition of inflammatory processes. Therefore, NBO and HBO appear excessively hopeful in salvaging impaired brain cells during ischemic stroke. However, harmful effects have been noted contributing to damaging properties, for example, vasoconstriction and free oxygen radicals. In the clinical setting, NBO provided positive results in a single clinical trial, but HBO failed to show efficacy in three randomized trials. To date, the translation of numerous evidentiary experimental results into clinical implementation remains open. Recently, oxygen became interesting as an additional therapy to neuroprotective or recanalization drugs to combine positive effects. Further preclinical research is needed exploring interactions between NBO, HBO, and key factors with multiphasic roles in acute damaging and delayed inflammatory processes after cerebral ischemia, for example, matrix-metalloproteinases and hypoxia-inducible factor-1α.

Metabolic reflow as a therapy for ischemic brain injury

Ischemic neuronal damage is a common feature of occlusive strokes, hemorrhagic strokes, and traumatic brain injury. In addition, ischemia can be an anticipated or unanticipated complication of a variety of surgical procedures. Most therapeutic strategies for managing ischemic injury seek to re-establish blood flow, suppress neural metabolism, and/or limit specific cellular injury cascades. An alternative therapeutic approach is to enhance the delivery of metabolic substrates to ischemic tissue. This strategy is typified by efforts to increase tissue oxygenation by elevating the levels of circulating oxygen. Our studies are examining a complementary approach in which the delivery of metabolic substrates is enhanced by facilitating the diffusion of oxygen and glucose from the vasculature into neural tissue during ischemia. This is achieved by increasing the diffusivity of small molecules in aqueous solutions, such as plasma and interstitial fluid. The carotenoid compound, trans-sodium crocetinate (TSC) is capable of increasing oxygen and glucose diffusivity, and our studies demonstrate that TSC increases cerebral tissue oxygenation in the penumbra of a focal ischemic event. In addition, TSC treatment reduces the volume of cerebral infarction in rodent models of both permanent and temporary focal ischemia. This strategy of "metabolic reflow" thus blunts the metabolic challenge in partially-perfused tissue and reduces ischemic neural injury.

Protection against focal ischemic injury to the brain by trans-sodium crocetinate. Laboratory investigation

OBJECT

Ischemic injury is a potential complication in a variety of surgical procedures and is a particular impediment to the success of surgeries involving highly vulnerable neural tissue. One approach to limiting this form of injury is to enhance metabolic supply to the affected tissue. Trans-sodium crocetinate (TSC) is a carotenoid compound that has been shown to increase tissue oxygenation by facilitating the diffusivity of small molecules, such as oxygen and glucose. The present study examined the ability of TSC to modify oxygenation in ischemic neural tissue and tested the potential neuroprotective effects of TSC in permanent and temporary models of focal cerebral ischemia.

METHODS

Adult male rats (330–370 g) were subjected to either permanent or temporary focal ischemia by simultaneous occlusion of both common carotid arteries and the left middle cerebral artery (3-vessel occlusion [3-VO]). Using the permanent ischemia paradigm, TSC was administered intravenously beginning 10 minutes after the onset of ischemia at 1 of 8 dosages, ranging from 0.023 to 4.580 mg/kg. Cerebral infarct volume was measured 24 hours after the onset of ischemia. The effect of TSC on infarct volume was also tested after temporary (2-hour) ischemia using a dosage of 0.092 mg/kg. In other animals undergoing temporary ischemia, tissue oxygenation was monitored in the ischemic penumbra using a Licox probe.

RESULTS

Administration of TSC reduced infarct volume in a dose-dependent manner in the permanent ischemia model, achieving statistical significance at dosages ranging from 0.046 to 0.229 mg/kg. The most effective dosage of TSC in the permanent ischemia experiment (0.092 mg/kg) was further tested using a temporary (2-hour) ischemia paradigm. Infarct volume was reduced significantly by TSC in this ischemia-reperfusion model as well. Recordings of oxygen levels in the ischemic penumbra of the temporary ischemia model showed that TSC increased tissue oxygenation during vascular occlusion, but reduced the oxygen overshoot (hyperoxygenation) that occurs upon reperfusion.

CONCLUSIONS

The novel carotenoid compound TSC exerts a neuroprotective influence against permanent and temporary ischemic injury when administered soon after the onset of ischemia. The protective mechanism of TSC remains to be confirmed; however, the permissive effect of TSC on the diffusivity of small molecules is a plausible mechanism based on the observed increase in tissue oxygenation in the ischemic penumbra. This represents a form of protection based on “metabolic reflow” that can occur under conditions of partial vascular perfusion. It is particularly noteworthy that TSC could conceivably limit the progression of a wide variety of cellular injury mechanisms by blunting the ischemic challenge to the brain.

Altitude mountain sickness among tourist populations: a review and pathophysiology supporting management with hyperbaric oxygen

In the mountain climbing community, conventional prevention of altitude mountain sickness (AMS) relies primarily on a formal acclimatization period. AMS symptoms during mountaineering climbs are managed with medication, oxygen and minor recompression (1524-2438 m altitude) using a portable chamber, such as the Gamow Bag. This is not always an acceptable therapy alternative in a predominantly elderly tourist population. The primary problem with reduced pressure at high altitude is hypoxaemia, which causes increased sympathetic activity, induces pulmonary venous constriction, while increasing pulmonary blood flow and regional perfusion. Rapid assents to altitude contribute to an increased incidence of decompression sickness (DCS). The treatment of choice for DCS is hyperbaric oxygenation, thus, treatment of high-altitude induced hypoxaemia using hyperbaric oxygenation (HBO(2)) is logical. Life Support Technologies group and the Center for Investigation of Altitude Medicine (CIMA, in Cusco, Peru) propose a comprehensive and multidisciplinary approach to AMS management. This approach encompasses traditional and advanced medical interventions including the use of a clinical HBO(2) chamber capable of recompression to three times greater than sea level pressure (3 atmosphere absolute (ATA)). The system uses a series of AMS hyperbaric treatment profiles that LST has previously developed to the US military and NASA, and that take greater advantage of vasoconstrictive effects of oxygen under true hyperbaric conditions of 1.25 ATA. These profiles virtually eliminate AMS rebound after the initial treatment often seen in conventional AMS treatment, where the patient is either treated at altitude, or does not recompress back to sea level or greater pressure (1.25 ATA), but returns directly to the same altitude where AMS symptoms first manifested.

Inhibition of gp91(phox) contributes towards normobaric hyperoxia afforded neuroprotection in focal cerebral ischemia

Oxygen therapy is a promising treatment strategy for ischemic stroke. One potential safety concern with oxygen therapy, however, is the possibility of increased generation of reactive oxygen species (ROS), which could exacerbate ischemic brain injury. Our previous study indicated that normobaric hyperoxia (NBO, 95% O(2) with 5% CO(2)) treatment during ischemia salvaged ischemic brain tissue and significantly reduced ROS generation in transient experimental stroke. In this follow-up study, we tested the hypothesis that suppression of NADPH oxidase is an important mechanism for NBO-induced reduction of ROS generation in focal cerebral ischemia. Male Sprague-Dawley rats were given NBO (95% O(2)) or normoxia (21% O(2)) during 90-min filament occlusion of the middle cerebral artery, followed by 22.5-hour reperfusion. NBO treatment increased the tissue oxygen partial pressure (pO(2)) level in the ischemic penumbra close to the pre-ischemic value, as measured by electronic paramagnetic resonance (EPR), and led to a 30.2% reduction in magnetic resonance imaging (MRI) apparent diffusion coefficients (ADC) lesion volume. Real time PCR and western blot analyses showed that the mRNA and protein expression of NADPH oxidase catalytic subunit gp91(phox) were upregulated in the ischemic brain, which was significantly inhibited by NBO. As a consequence of gp91(phox) inhibition, NBO treatment reduced NADPH oxidase activity in the ischemic brain. Our results suggest that NBO treatment given during ischemia reduces ROS generation via inhibiting NADPH oxidase, which may serve as an important mechanism underlying NBO's neuroprotection in acute ischemic stroke.

Hyperbaric oxygenation alleviates MCAO-induced brain injury and reduces hydroxyl radical formation and glutamate release

The present study examined the effect of hyperbaric oxygen (HBO) on the formation of 2,3-dihydroxybenzoic acid (2,3-DHBA) and 2,5-dihydroxybenzoic acid (2,5-DHBA), the products of salicylate trapping of hydroxyl free radicals, and glutamate release in the striatum during acute ischemia and reperfusion. Non-HBO rats (n = 8) were subjected to 1-h ischemia. Study rats (n = 8) were treated with HBO at 2.8 ATA for 1 h during ischemia. Artificial CSF solution containing 5 mM sodium salicylate was perfused at 1 microl/min. Samples were continuously collected at 15 min intervals and the levels of 2,3-DHBA, 2,5-DHBA, and glutamate were analyzed. The lesion volume was determined by TTC stain. Occlusion of the middle cerebral artery induced a significant increase in the levels of 2,3-DHBA and 2,5-DHBA. A peak of approximately two and fourfold of baseline levels was reached at 45 min and was maintained at elevated levels during reperfusion. The level of glutamate increased approximately two times at 30 min during ischemia, continued to increase, and reached approximately three times baseline level during reperfusion. HBO significantly alleviated brain injury associated with decreased levels of 2,3-DHBA, 2,5-DHBA and glutamate. This study suggests that the decreased glutamate release and the reduced formation of hydroxyl free radicals might contribute to the neuroprotective effect of HBO.

Cerebral gas embolism due to upper gastrointestinal endoscopy

Cerebral gas embolism as a result of upper gastrointestinal endoscopy is a rare complication and bares a high morbidity. A patient is presented who underwent an upper endoscopy for evaluation of a gastric-mediastinal fistula after subtotal oesophagectomy and gastric tube reconstruction because of oesophageal cancer. During the procedure, cerebral gas emboli developed resulting in an acute left-sided hemiparesis. After hyperbaric oxygen therapy, the patient recovered almost completely. The aetiology and treatment is discussed based on the reviewed literature. Once cerebral gas emboli are recognized, patient outcome can be improved by hyperbaric oxygen therapy.

Hyperbaric oxygen therapy and cerebral ischemia: neuroprotective mechanisms

INTRODUCTION

Numerous studies have demonstrated a protective effect of hyperbaric oxygen therapy in experimental ischemic brain injury, and many physiological and molecular mechanisms of hyperbaric oxygen therapy-related neuroprotection have been identified.

METHODS

Review of articles pertaining to hyperbaric oxygen therapy and cerebral ischemia in the National Library of Medicine and National Institutes of Health database, emphasizing mechanisms of hyperbaric oxygen therapy-related neuroprotection.

RESULTS

Hyperbaric oxygen therapy has been shown to ameliorate brain injury in a variety of animal models including focal cerebral ischemia, global cerebral ischemia, neonatal hypoxia-ischemia and subarachnoid hemorrhage. Small human trials of hyperbaric oxygen therapy in focal ischemia have not shown benefit, although one trial of hyperbaric oxygen therapy before cardiopulmonary bypass demonstrated improved neuropsychological and inflammatory outcomes with hyperbaric oxygen therapy. Hyperbaric oxygen therapy is associated with improved cerebral oxygenation, reduced blood-brain barrier breakdown, decreased inflammation, reduced cerebral edema, decreased intracranial pressure, reduced oxidative burden, reduced metabolic derangement, decreased apoptotic cell death and increased neural regeneration.

CONCLUSION

On a molecular level, hyperbaric oxygen therapy leads to activation of ion channels, inhibition of hypoxia inducible factor-1alpha, up-regulation of Bcl-2, inhibition of MMP-9, decreased cyclooxygenase-2 activity, decreased myeloperoxidase activity, up-regulation of superoxide dismutase and inhibition of Nogo-A (an endogenous growth-inhibitory factor). Ongoing research will continue to describe the mechanisms of hyperbaric oxygen therapy-related neuroprotection, and possibly expand hyperbaric oxygen therapy use clinically.

Normobaric hyperoxia and delayed tPA treatment in a rat embolic stroke model

In a rat embolic stroke (eMCAO) model, the effects of 100% normobaric hyperoxia (NBO) with delayed recombinant tissue plasminogen activator (tPA) administration on ischemic lesion size and safety were assessed by diffusion- and perfusion (PWI)-weighted magnetic resonance imaging. NBO or room air (Air) by a face mask was started at 30 mins posteMCAO and continued for 3.5 h. Tissue plasminogen activator or saline was started at 3 h posteMCAO. Types and location of hemorrhagic transformation were assessed at 24 h and a spectrophotometric hemoglobin assay quantified hemorrhage volume at 10 h. In NBO-treated animals the apparent diffusion coefficient/PWI mismatch persisted during NBO treatment. Relative to Air groups, NBO treatment significantly reduced 24 h infarct volumes by approximately 30% and approximately 15% with or without delayed tPA, respectively (P<0.05). There were significantly more hemorrhagic infarction type 2 hemorrhages in Air/tPA versus Air/saline animals (P<0.05). Compared with Air/tPA, the combination of NBO with tPA did not increase hemorrhage volume at 10 h (4.0+/-2.4 versus 6.6+/-2.6 microL, P=0.065) or occurrence of confluent petechial hemorrhages at 24 h (P>0.05), respectively. Our results suggest that early NBO treatment in combination with tPA at a later time point may represent a safe and effective strategy for acute stroke treatment.

Oxygen therapy in stroke: past, present, and future

Oxygen is frequently administered to patients with suspected stroke. However, the role of oxygen therapy in ischemic stroke remains controversial in light of the failure of three clinical trials of hyperbaric oxygen therapy to show efficacy, and the fear of exacerbating oxygen free radical injury. The previous trials had several shortcomings, perhaps because they were designed on basis of anecdotal case reports and little preclinical data. Most animal studies concerning oxygen therapy in stroke have been conducted over the last 6 years. Emerging data suggests that hyperbaric and even normobaric oxygen therapy can be effective if used appropriately, and raises the tantalizing possibility that hyperoxia can be used to extend the narrow therapeutic time window for stroke thrombolysis. This article reviews the history, rationale, mechanisms of action and adverse effects of hyperoxia, the key results of previous hyperoxia studies, and the potential role of oxygen therapy in contemporary stroke treatment.

Brief exposure to hyperoxia depletes the glial progenitor pool and impairs functional recovery after hypoxic-ischemic brain injury

Patterns of hypoxic-ischemic brain injury in infants and children suggest vulnerability in regions of white matter development, and injured patients develop defects in myelination resulting in cerebral palsy and motor deficits. Reperfusion exacerbates the oxidative stress that occurs after such injuries and may impair recovery. Resuscitation after hypoxic-ischemic injury is routinely performed using 100% oxygen, and this practice may increase the oxidative stress that occurs during reperfusion and further damage an already compromised brain. We show that brief exposure (30 mins) to 100% oxygen during reperfusion worsens the histologic injury in young mice after unilateral brain hypoxia-ischemia, causes an accumulation of the oxidative metabolite nitrotyrosine, and depletes preoligodendrocyte glial progenitors present in the cortex. This damage can be reversed with administration of the antioxidant ebselen, a glutathione peroxidase mimetic. Moreover, mice recovered in 100% oxygen have a more disrupted pattern of myelination and develop a static motor deficit that mimics cerebral palsy and manifests itself by significantly worse performance on wire hang and rotorod motor testing. We conclude that exposure to 100% oxygen during reperfusion after hypoxic-ischemic brain injury increases secondary neural injury, depletes developing glial progenitors, interferes with myelination, and ultimately impairs functional recovery.

Neuroprotection by oxygen in acute transient focal cerebral ischemia is dose dependent and shows superiority of hyperbaric oxygenation

The neuroprotective effect of oxygen after acute stroke in rats has been shown previously. However, the question of optimal dosing still remains unanswered. Thus, we investigated the use of oxygen at different concentrations by either normobaric oxygenation (NBO) or hyperbaric oxygenation (HBO) at different pressures in a model of transient ischemia/reperfusion in rats. Animals underwent 90 min of middle cerebral artery occlusion (MCAO) followed by 90 min of reperfusion before oxygen treatment. Oxygen was applied either by NBO (100% O(2); 1.0 absolute atmosphere, ATA) or HBO (100% O(2); 1.5, 2.0, 2.5 or 3.0 ATA) for 1 h. Primary endpoints were infarct volume and clinical outcome measured 24 h and 7 days following the MCAO. A statistically significant and long-lasting reduction in infarct volume was seen in the HBO 2.5 ATA and 3.0 ATA groups over a period of 7 days. The reduced infarct volume was accompanied with a statistically significant improvement in clinical outcome in the high-dose oxygen-treated groups. The presented data indicate that oxygen is a highly neuroprotective molecule in transient focal cerebral ischemia in rats, when applied early and at high doses. The effect is dose dependent and shows a superiority of HBO over NBO, when the primary endpoints infarct volume reduction and clinical outcome are analyzed. These data are important for the development of new acute stroke treatment studies in humans.

Delayed hyperbaric oxygenation is more effective than early prolonged normobaric hyperoxia in experimental focal cerebral ischemia

Hyperbaric (HBO) and normobaric (NBO) oxygen therapy have been shown to be neuroprotective in focal cerebral ischemia. In previous comparative studies, NBO appeared to be less effective than HBO. However, the experimental protocols did not account for important advantages of NBO in the clinical setting such as earlier initiation and prolonged administration. Therefore, we compared the effects of early prolonged NBO to delayed HBO on infarct size and functional outcome. We also examined whether combining NBO and HBO is of additional benefit. Wistar rats underwent filament-induced middle cerebral artery occlusion (MCAO) for 150 min. Animals breathed either air, 100% O(2) at ambient pressure (NBO; initiated 30 min after MCAO) 100% O(2) at 3 atm absolute (HBO; initiated 90 min after MCAO), or a sequence of NBO and HBO. Infarct volumes and neurological outcome (Garcia score) were examined 7d after MCAO. HBO (174+/-65 mm(3)) significantly reduced mean infarct volume by 31% compared to air (251+/-59 mm(3)) and by 23% compared to NBO treated animals (225+/-63 mm(3)). In contrast, NBO failed to decrease infarct volume significantly. Treatment with NBO+HBO (185+/-101 mm(3)) added no additional benefit to HBO alone. Neurological deficit was significantly smaller in HBO treated animals (Garcia score: 13.3+/-1.2) than in animals treated with air (12.1+/-1.4), but did not differ significantly from NBO (12.4+/-0.9) and NBO+HBO (12.8+/-1.1). In conclusion, HBO is a more effective therapy than NBO in transient experimental ischemia even when accounting for delayed treatment-onset of HBO. The combination of NBO and HBO results in no additional benefit.

High spatial resolution multisite EPR oximetry of transient focal cerebral ischemia in the rat

In vivo electron paramagnetic resonance (EPR) spectroscopy can provide direct noninvasive, continuous, and repeatable measurements of oxygen in tissues. High-spatial-resolution multisite (HSRMS) oximetry is an EPR technique that uses applied magnetic field gradients to extend this capability to multiple implanted probes within the sample and accurately to estimate their respective local pO(2) values. These capabilities are crucial in experiments in which pO(2) varies across space and time and in which information about these variations is needed to describe physiologic and pathophysiologic phenomena and evaluate their responses to interventions such as therapy. One important application is the investigation of transient focal ischemia in the rat brain and the effects of treatment with hyperoxygenation. We used HSRMS oximetry with overmodulation to measure brain tissue oxygenation in a rat stroke model using lithium phthalocyanine as the oxygen probe. Oxygen measurements were made in a small cohort of rats at four implant sites during ischemia and reperfusion after transient focal ischemia initiated by occlusion of the middle cerebral artery. These measurements demonstrate the capabilities of the HSRMS oximetry technique and set the stage for more extensive physiologic studies.