Segmental vascular resistance after mild controlled cortical impact injury in the rat

In an effort to localize the site at which increased resistance occurs after brain trauma, pial arteriole diameter and pressure were assessed after mild controlled cortical impact (CCI) injury in rats using an open cranial window technique. The authors tested the hypothesis that an increase in resistance accompanied by vasoconstriction occurs at the level of the pial arterioles within the injured cortex of the brain. At 1 hour after mild CCI injury, ipsilateral cerebral blood flow was significantly reduced by 42% compared with sham injury (n = 4; < 0.05). Pial arteriole diameter and pressure remained unchanged. Resistance in the larger arteries (proximal resistance), however, was significantly greater after CCI injury (1.87 +/- 0.26 mm Hg/[mL. 100 g. min]) compared with sham injury (0.91 +/- 0.21 mm Hg/[mL. 100 g. min]; < 0.0001). Resistance in small vessels, arterioles, and venules (distal resistance) was also significantly greater after CCI injury (1.13 +/- 0.05 mm Hg/[mL. 100 g. min]) compared with sham injury (0.74 +/- 0.13 mm Hg/[mL. 100 g. min]; = 0.0001). The authors conclude that, at 1 hour after mild CCI injury, changes in vascular resistance comprise a 53% increase in the resistance distal to the area of injury and, surprisingly, a 105% increase in resistance in the arteries proximal to the injury site.

Neuroprotective effects of L-arginine administration after cortical impact injury in rats: dose response and time window

Administration of L-arginine has been shown to increase cerebral blood flow and reduce neurological damage after experimental traumatic brain injury. The purpose of this study was to examine the optimal dose and time window for these neuroprotective effects. In a dose response experiment, doses of L-arginine ranging from 37.5 to 600 mg/kg were administered 5 min after a 5-m/s, 3-mm, controlled cortical impact in rats. The amount of brain injury found at 2 weeks after injury, both at the contusion site and in the ipsilateral hippocampus, were inversely related to the dose of L-arginine administered. Both 300- and 600-mg/kg doses of L-arginine significantly reduced contusion volume. The 300-mg/kg dose significantly increased the neuron density in the CA1 region of the hippocampus. Physiological effects of L-arginine were also dose-related. The greatest reduction in intracranial pressure occurred with the 300-mg/kg dose of L-arginine. Doses up to 300 mg/kg were well tolerated, but the 600-mg/kg dose resulted in transient hypotension. In another experiment, 300 mg/kg L-arginine was administered at times varying from 5 min to 48 h after injury. Contusion volume was significantly reduced when the L-arginine was given at 5 min and 1 h after injury. The protective effect was less when the same dose was given at the later times, but there was no evidence of an adverse effect even when the L-arginine was administered 48 h after injury.

Brain tissue PO2: correlation with cerebral blood flow

This investigation analyzed 22 xenon CT cerebral blood flow (CBF) studies from 18 severely head-injured patients (Glasgow motor score < 6) who underwent xenon CT scanning while brain tissue oxygen tension (PbtO2) was being monitored. CBF was determined both in a localized region of interest around the actual or estimated location of the tip of the PbtO2 probe and in the entire corresponding CT slice. Linear regression analysis was used to examine the relationship between these CBF measurements and PbtO2 values recorded immediately prior to the xenon CT CBF study. PbtO2 varied linearly with both regional CBF (rCBF) and global CBF measurements, but the average global CBF value was significantly higher than the average rCBF value. Very low values were significantly less common for global CBF than for rCBF. Further investigation is necessary to determine how probe placement near contused areas vs. in normal tissue affects our understanding of the relationship between rCBF, global CBF, PbtO2, and cerebral oxygen consumption.

Microdialysate nitrate/nitrite levels following severe head injury

Nitric oxide (NO) has important regulatory functions within the central nervous system. The purpose of this study was to measure the concentration of nitric oxide in the brain after severe traumatic brain injury. NO is oxidized in vivo to nitrate and nitrite. Measurement of these products gives an index of NO production. Laboratory studies have shown a good correlation between NO measured directly with an electrode, and indirectly by microdialysis nitrate/nitrite. Using chemiluminescence method we measured nitrate/nitrite levels in 2024 microdialysate samples obtained from 24 patients during the first five days following severe head injury. We used CMA 70 probe (AB Microdialysis, Sweden) perfused by normal saline at a rate of 2 microliters/min. The median values of nitrate/nitrite for the whole group were highest on day 1 and gradually decreased over the 5 day monitoring period (day 1-19.2 mumol/l, day 5-12.7 mumol/l). Average values were lowest in the patients that died of their injury (14.3 mumol/l), and highest in patients who recovered by 3 months after injury with a moderate or severe disability (25.8 mumol/l or 31.9 mumol/l). In addition, there was a strong interaction between the severity of neurological injury and the change in dialysate nitrate/nitrite over time. The results suggest that nitric oxide may have a role in secondary injury mechanisms, but that this role is complex and varies as the injury evolves over time.

Measurement of the nitric oxide metabolites nitrate and nitrite in the human brain by microdialysis

To examine the feasibility of measuring the nitric oxide (NO) metabolites nitrate and nitrite in microdialysate samples from the human brain, microdialysis probes were placed in normal appearing cerebral cortex of severely head injured patients in the Neurosurgical Intensive Care Unit at Ben Taub General Hospital. Nitrate/nitrite analysis was performed using NO chemiluminescence. Low micromolar levels of NO metabolites were consistently and easily detected. These levels seen are comparable to levels reported in CSF but tissue tortuosity and probe recovery considerations suggest that the absolute concentrations at the probe site are probably ten fold higher. Microdialysis with measurement of nitric oxide metabolites is technically feasible and may provide valuable insights into both normal neurochemistry and neurochemical derangements in disease.

Comparison of microdialysate arginine and glutamate levels in severely head-injured patient

L-arginine concentrations in the brain are of interest following TBI because L-arginine is the immediate precursor of nitric oxide (NO). In addition, in vitro studies suggest that glutamate, which is a mediator of secondary injury after TBI, may stimulate release of arginine from glial cells. This study examines arginine concentrations in brain tissue using the microdialysis technique after human TBI. From 78 TBI patients, a total of 1739 microdialysate samples were collected using a CMA-70 probe perfused with normal saline at 2 microliters/min and concentrations of amino acids in microdialysate were determined. Amino acid concentrations for each patient were averaged for 8-hour periods during the first 3 days after injury, and daily for postinjury days 4 and 5. Following an initial rapid decrease in arginine, the dialysate arginine concentrations were low on days 1-3 and then increased over the days 4-5 after injury. In contrast, the microdialysate glutamate levels decreased slowly over the first 48 hours after TBI and thereafter remained low. Thirty-five episodes of jugular venous desaturation (SjvO2 < 50%) occurred during monitoring. Arginine and glutamate levels simultaneously doubled during desaturation and decreased as the clinical episode resolved. The low concentrations of arginine during the first 3 days after TBI may indicate that substrate unavailability could contribute to the decreased NO concentrations that have been observed after TBI. The simultaneous increase in glutamate and arginine during ischemic events is consistent with experimental data which has observed that glutamate induces release of arginine.

L-arginine and free radical scavengers increase cerebral blood flow and brain tissue nitric oxide concentrations after controlled cortical impact injury in rats

To examine the mechanism of the increase in cerebral blood flow induced by L-arginine administration after traumatic brain injury, the cerebral hemodynamic effects of L-arginine, D-arginine, and the free radical scavengers superoxide dismutase (SOD) and catalase were compared in the controlled cortical impact injury model in rats. Animals were anesthetized with isoflurane. Measured parameters included mean blood pressure, intracranial pressure, cerebral blood flow using laser Doppler flowmetry (LDF) and brain tissue nitric oxide (NO) concentrations using an NO electrode. L-arginine, but not D-arginine, administration resulted in a significant increase in tissue NO concentrations and an improvement in LDF at the impact site, compared to control animals given saline. Administration of SOD alone and in combination with catalase resulted in a significant increase in brain tissue NO concentrations. However, LDF was consistently improved only when both SOD and catalase were given. These studies support the theory that L-arginine administration improves post-traumatic cerebral blood flow by increasing NO production. Free radical production after trauma may also contribute to the reduction in CBF by inactivating NO.

Dynamic autoregulatory response after severe head injury

OBJECT

The purpose of this study was to evaluate the extent and timing of impairment of cerebral pressure autoregulation after severe head injury.

METHODS

In a prospective study of 122 patients with severe head trauma (median Glasgow Coma Scale Score 6), dynamic tests of pressure autoregulation were performed every 12 hours during the first 5 days postinjury and daily during the next 5 days. The autoregulatory index ([ARI] normal value 5 +/- 1.1) was calculated for each test. The changes in the ARI over time were examined and compared with other physiological variables. The ARI averaged 2.8 +/- 1.9 during the first 12 hours postinjury, and continued to decrease to a nadir of 1.7 +/- 1.1 at 36 to 48 hours postinjury. At this nadir, in 87% of the patients the value was less than 2.8. This continued deterioration in the ARI during the first 36 to 48 hours postinjury occurred despite an increase in cerebral blood flow ([CBF], p < 0.05) and in middle cerebral artery blood flow velocity ([BFV], p < 0.001), and could not be explained by changes in cerebral perfusion pressure, end-tidal CO2, or cerebral metabolic rate of O2. A marked decrease in cerebrovascular resistance ([CVR], p < 0.001) accompanied this deterioration in the ARI. Patients with a relatively higher BFV on Day 1 had a lower CVR (p < 0.05) and more impaired pressure autoregulation than those with a lower BFV.

CONCLUSIONS

The inability of cerebral vessels to regulate CBF normally may play a role in the vulnerability of the injured brain to secondary ischemic insults. These studies indicate that this vulnerability continues and even increases beyond the first 24 hours postinjury. Local factors affecting cerebrovascular tone may be responsible for these findings.

Organ donation rates in a neurosurgical intensive care unit

OBJECT

The number of patients waiting for organ transplantation continues to grow, while organs are donated by very few of the thousands of potential donors who die every year. The authors' neurosurgical intensive care unit (NICU) has worked closely with coordinators from the local organ procurement organization (OPO) for many years. In this study, the authors analyze donation rates in the NICU and discuss factors that may be important in maximizing these rates.

METHODS

All referrals from the NICU to the OPO from 1996 to 1999 were analyzed. Of the 180 referrals, 98 patients were found to be medically suitable as potential donors. Another 15 patients died of hemodynamic collapse shortly after admission to the NICU. If one assumes that all 15 patients would have been suitable donors, the unsuccessful resuscitation rate becomes 15 (13.3%) of 113. Of the 98 eligible donors, consent was obtained and organs or tissue were recovered in 72, yielding a successful organ procurement rate of 73.5%.

CONCLUSIONS

Close working relationships among physicians, nurses, and OPO coordinators can result in higher donation rates than have been reported previously. Aggressive resuscitation and stabilization of all patients, early identification of potential organ donors, prompt declaration of brain death, and attempts by the OPO coordinator to build rapport with families are all important factors that may increase donation rates. Because most organ donors have sustained catastrophic intracranial events, neurosurgeons are uniquely positioned to influence organ donation policies at their hospitals and thus to salvage some benefit from tragic cases of overwhelming brain injury.

Clinical trials in head injury

Traumatic brain injury (TBI) remains a major public health problem globally. In the United States the incidence of closed head injuries admitted to hospitals is conservatively estimated to be 200 per 100,000 population, and the incidence of penetrating head injury is estimated to be 12 per 100,000, the highest of any developed country in the world. This yields an approximate number of 500,000 new cases each year, a sizeable proportion of which demonstrate significant long-term disabilities. Unfortunately, there is a paucity of proven therapies for this disease. For a variety of reasons, clinical trials for this condition have been difficult to design and perform. Despite promising pre-clinical data, most of the trials that have been performed in recent years have failed to demonstrate any significant improvement in outcomes. The reasons for these failures have not always been apparent and any insights gained were not always shared. It was therefore feared that we were running the risk of repeating our mistakes. Recognizing the importance of TBI, the National Institute of Neurological Disorders and Stroke (NINDS) sponsored a workshop that brought together experts from clinical, research, and pharmaceutical backgrounds. This workshop proved to be very informative and yielded many insights into previous and future TBI trials. This paper is an attempt to summarize the key points made at the workshop. It is hoped that these lessons will enhance the planning and design of future efforts in this important field of research.

The association between neuronal nitric oxide synthase and neuronal sensitivity in the brain after brain injury

Injury to the central nervous system is the leading cause of disability in the United States. Neuronal death is one of the causes of disability. Among patients who survive this type of injury, various degrees of recovery in brain function are observed. The molecular basis of functional recovery is poorly understood. Clinical observations and research using experimental injury models have implicated several metabolites in the cascade of events that lead to neuronal degeneration. The levels of intracellular ATP (energy source) and pH are decreased, whereas levels of extracellular glutamate, intracellular calcium ions, and oxidative damage to RNA/DNA, protein, and lipid are increased. These initiating events can be associated with energy failure and mitochondrial dysfunction, resulting in functional or structural brain damage. The injured brain is known to express immediate early genes. Recent studies show that reactive oxygen species (ROS) cause lesions in genes from which mRNA is transcribed as part of the endogenous neuroprotective response. Although degenerating proteins and lipids may contribute to necrosis significantly after severe injury, abnormalities in genetic material, if not repaired, disturb cellular function at every level by affecting replication, transcription, and translation. These lesions include abnormal nucleic acids, known as oxidative lesions of DNA (ODLs) or of RNA (ORLs). In this review, we focus on our current understanding of the various effects of neuronal nitric oxide synthase on the formation of modified bases in DNA and RNA that are induced in the brain after injury, and how ODLs and ORLs affect cell function.

The effects of L-arginine on cerebral hemodynamics after controlled cortical impact injury in the mouse

Traumatic brain injury (TBI) induces vascular changes that may influence neurological outcome by causing the brain to be more susceptible to secondary ischemic insults. In rat models of TBI, L-arginine administration has been shown to restore cerebral blood flow and improve neurological outcome. The purpose of this study was to determine if hypoperfusion occurs in a mouse model of TBI and if L-arginine administration has the same beneficial effects after injury in the mouse. C57BL6 mice were anesthetized with isoflurane, intubated and mechanically ventilated, and underwent a 3-m/sec, 1.5-mm deformation cortical impact injury. Five minutes after injury, L-arginine, 300 mg/kg, or saline were administered. Arterial blood pressure, intracranial pressure, and laser Doppler flow at the impact site were monitored for 3 h after the injury. The cerebral hemodynamic effects of the TBI induced by cortical impact injury were similar to that previously observed in rats. Intracranial hypertension, with ICP peaking at 46+/-2 mm Hg, and systemic hypotension both contributed to a reduction in CPP. In addition, LDF decreased significantly at the impact site. L-Arginine administration restored LDF to near baseline levels without increasing ICP. These studies demonstrate that cerebral hemodynamics can be measured in mouse models of TBI. The changes in cerebral hemodynamics are relatively simlar to those see in the rat model of cortical impact injury and suggest an important role for nitric oxide metabolism in the maintenance of cerebral blood flow following TBI.

Elevated jugular venous oxygen saturation after severe head injury

Object

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

Methods

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

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

Conclusions

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

Adult respiratory distress syndrome: a complication of induced hypertension after severe head injury

OBJECT

The factors involved in the development of adult respiratory distress syndrome (ARDS) after severe head injury were studied. The presence of ARDS complicates the treatment of patients with severe head injury, both because hypoxia causes additional injury to the brain and because therapies that are used to protect the lungs and improve oxygenation in patients with ARDS can reduce cerebral blood flow (CBF) and increase intracranial pressure (ICP). In a recent randomized trial of two head-injury management strategies (ICP-targeted and CBF-targeted), a fivefold increase in the incidence of ARDS was observed in the CBF-targeted group.

METHODS

Injury severity, physiological data, and treatment data in 18 patients in whom ARDS had developed were compared with the remaining 171 patients in the randomized trial in whom it had not developed. Logistic regression analysis was used to study the interaction of the factors that were related to the development of ARDS. In the final exact logistic regression model, several factors were found to be significantly associated with an increased risk of ARDS: administration of epinephrine (5.7-fold increased risk), administration of dopamine in a larger than median dose (10.8-fold increased risk), and a history of drug abuse (3.1-fold increased risk).

CONCLUSIONS

Although this clinical trial was not designed to study the association of management strategy and the occurrence of ARDS, the data strongly indicated that induced hypertension in this high-risk group of patients is associated with the development of symptomatic ARDS.

Ischemic injury and faulty gene transcripts in the brain

The brain has the highest metabolic rate of all organs and depends predominantly on oxidative metabolism as a source of energy. Oxidative metabolism generates reactive oxygen species, which can damage all cellular components, including protein, lipids and nucleic acids. The processes of DNA repair normally remove spontaneous gene damage with few errors. However, cerebral ischemia followed by reperfusion leads to elevated oxidative stress and damage to genes in brain tissue despite a functional mechanism of DNA repair. These critical events occur at the same time as the expression of immediate early genes, the products of which trans-activate late effector genes that are important for sustaining neuronal viability. These findings open the possibility of applying genetic tools to identify molecular mechanisms of gene repair and to derive new therapies for stroke and brain injury.

Extracellular glutamate and aspartate in head injured patients

Eighty-six patients in coma from a severe head injury underwent monitoring of extracellular concentrations of glutamate and aspartate by a microdialysis technique during the first few days after injury. The median value for glutamate was 7.4 microM (interquartile range 3.6-18.8 microM). The median value for aspartate was 2.4 microM (interquartile range 1.1-5.0 microM). Average values for the dialysate concentrations of glutamate and aspartate, were closely related to outcome (p < .001 and p = .002, respectively). Patients who died of their head injury had significantly higher dialysate glutamate and aspartate concentrations compared to patients who recovered to a Glasgow Outcome Score of good recovery or moderate disability. Dialysate glutamate and aspartate levels were also significantly related to type of injury (p = .008 and p = .004, respectively). The highest values were found in patients with gunshot wounds, followed by patients with evacuated and unevacuated mass lesions. Patients with diffuse injuries had the lowest values of glutamate and aspartate. These results suggest that excitatory amino acids may play a role in the evolution of injury to the brain after trauma.

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

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

Ultrasound is a reliable method for determining jugular bulb dominance

Despite widespread application of jugular oximetry devices, the optimal side to cannulate for monitoring cerebral oxygenation is controversial. For most monitoring strategies, the dominant or larger internal jugular vein gives the most representative values for venous oxygen saturation. However, there is little information on how to best determine the dominant side. The purpose of this study was to compare the results of an ultrasound examination to two other standard methods for determining the dominant internal jugular vein, the jugular vein compression test and the computed tomographic (CT) approach. Seventeen patients with severe head injury (GCS <8) were studied. The ultrasound examination showed the mean internal diameter of the right and the left internal jugular veins to be 1.27 cm (standard deviation [SD] 0.16 cm) and 1.21 cm (SD 0.36 cm), respectively. The right internal jugular vein was larger than the left in 11 (65%) of the patients. The diameter of the dominant or larger internal jugular veins were 1.44 cm (SD = 0.22), compared with 1.04 cm (SD = 0.18) on the opposite side (P < .05). The results of the ultrasound method were in agreement with the CT scan method in 94% of the comparisons and with the jugular vein compression test in 82% of comparisons. These studies demonstrate that the ultrasound method provides useful information about the side of the dominant cerebral venous drainage, comparable to other standard methods, without the need for a CT scan or manipulation of intracranial pressure.

Subarachnoid-pleural fistula treated with noninvasive positive-pressure ventilation. Case report

The authors describe the case of a 24-year-old man who underwent an L-1 corpectomy for spinal decompression and stabilization following an injury that caused an L-1 burst fracture. Postoperatively, an accumulation of spinal fluid developed in the pleural space, which was refractory to 1 week of thoracostomy tube drainage and lumbar cerebrospinal fluid (CSF) diversion. The authors then initiated a regimen of positive-pressure ventilation in which a bi-level positive airway pressure (PAP) mask was used. After 5 days, the CSF collection in the pleural space resolved. Use of a bi-level PAP mask represents a safe, noninvasive method of reducing the negative intrathoracic pressure that promotes CSF leakage into the pleural cavity and may be a useful adjunct in the treatment of subarachnoid-pleural fistula.

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

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

Interstitial brain adenosine and xanthine increase during jugular venous oxygen desaturations in humans after traumatic brain injury

OBJECTIVE

Adenosine decreases the cerebral metabolic rate for oxygen and increases cerebral blood flow, and it may play an important role in cerebrometabolic and cerebrovascular responses to hypoperfusion after traumatic brain injury. Jugular venous oxygen saturation is monitored after traumatic brain injury to assess brain oxygen extraction, and desaturations may reflect secondary brain insults. We hypothesized that brain interstitial adenosine and related purine metabolites would be increased during jugular venous oxygen saturation desaturations (<50%) and determined associations between the purines, lactate, and glucose to assess the role of adenosine during secondary insults in humans.

DESIGN

Study of critically ill adults with severe traumatic brain injury.

SETTING

Adult neurointensive care unit.

PATIENTS

We prospectively defined periods of normal saturation and desaturation in six patients after severe traumatic brain injury.

INTERVENTIONS

During these periods, cerebral microdialysis samples of brain interstitial fluid were collected, and adenosine and purine metabolites were measured by high-pressure liquid chromatography.

MEASUREMENTS AND MAIN RESULTS

Adenosine increased 3.1-fold and xanthine increased 2.5-fold during desaturation periods (both p <.05 vs. normal saturation period, signed rank). Adenosine, xanthine, hypoxanthine, and cyclic-adenosine monophosphate correlated with lactate over both study periods (r(2) =.32,.14,.31,.07, and.26, respectively, all p <.05, Pearson product moment correlation).

CONCLUSION

The marked increases in interstitial brain adenosine that occur during jugular venous oxygen desaturations suggest that adenosine may play an important role during periods of secondary insults after traumatic brain injury. The correlation of these metabolites with lactate further suggests that adenosine is increased during periods of enhanced glycolytic metabolism.

Global and regional techniques for monitoring cerebral oxidative metabolism after severe traumatic brain injury

The disturbance of normal mechanisms of oxygen delivery and metabolism is a hallmark of severe traumatic brain injury (TBI). In the past, investigations into the status of cerebral oxygen metabolism depended on changes in the differences in oxygen content between arterial and jugular venous blood. The development of jugular venous oximetry permitted continuous monitoring of jugular venous oxygen saturation, thereby overcoming earlier limitations caused by intermittent sampling. Neuromonitoring techniques that utilize only jugular vein sampling provide information only about global cerebral metabolism, but direct measurement of brain tissue oxygen tension via intraparenchymal probes makes possible the assessment of regional cerebral oxygen metabolism. Regional and global neuromonitoring techniques are not competitive or mutually exclusive. Rather, they are best regarded as complementary, with each providing valuable information that has a direct bearing on patient outcomes. The authors review the currently available techniques used in the monitoring of cerebral oxidative metabolism in patients who have sustained severe TBI.

Midline shift after severe head injury: pathophysiologic implications

OBJECTIVE

To investigate the mechanism of the adverse effect of midline shift after severe traumatic brain injury.

METHODS

This study compared averaged cerebral metabolic parameters of patients with midline shift > 5 mm (S) on initial computerized tomography scan to those of patients with shift < or = 5 mm (NS). The effect of an acute subdural hematoma (SDH) was determined by separating patients into those with and those without SDH and then re-examining the effect of shift in these subgroups.

RESULTS

Four hundred fifty-four patients were studied. Cerebral metabolic rate of oxygen (CMRO2, in mL/100 g per min) was always lower with shift: 1.74 for SDH-S versus 2.21 for SDH-NS (p < 0.001), and 1.80 for non-SDH-S versus 2.24 for non-SDH-NS (p < 0.001). No other major effects of shift were seen in SDH patients. Among non-SDH patients, shift was associated with higher intracranial pressure (ICP): 23.1 mm Hg versus 16.3 mm Hg (p < 0.001). Other differences between shift and nonshift patients in the non-SDH group were due at least in part to interventions to treat the elevated ICP.

CONCLUSION

Midline shift after severe traumatic brain injury is associated with reduced CMRo2, regardless of whether or not SDH is present. The deleterious effects of subdural blood may be related more to the mass effect of large SDHs than to the biochemical abnormalities caused by small amounts of blood in the subdural space.