Clinical experience with 118 brain tissue oxygen partial pressure catheter probes

Assessing the physiological concentration and targets of nitric oxide in brain tissue

Low nanomolar concentrations of nitric oxide activate guanylyl cyclase to produce cGMP, which has diverse physiological effects. Higher concentrations inhibit mitochondrial respiration at cytochrome c oxidase and this has been proposed to be important physiologically, increasing oxygen permeation into tissue (by reducing the oxygen use of cells near blood vessels), activating AMP kinase, and regulating the relationship between cerebral blood flow and oxygen use. It is unclear, however, whether nitric oxide can accumulate physiologically to concentrations at which inhibition of respiration occurs. In rat cerebellar slices, we activated nitric oxide production from each isoform of nitric oxide synthase. Only activation of inducible nitric oxide synthase, which is expressed pathologically, caused any significant inhibition of respiration. Modelling oxygen and nitric oxide concentrations predicted that, in vivo, physiological nitric oxide levels are too low to affect respiration. Even pathologically, the nitric oxide concentration may only rise to 2.5 nm, producing a 1.5% inhibition of respiration. Thus, under physiological conditions, nitric oxide signals do not inhibit respiration but are well-tuned to the dynamic range of guanylyl cyclase activation.

Monitoring and intraoperative management of elevated intracranial pressure and decompressive craniectomy

There are numerous clinical scenarios wherein a critically ill patient may present with neurologic dysfunction. In a general sense these scenarios often involve ischemia, trauma, or neuroexcitation. Each of these may include a period of decreased cerebral perfusion pressure, usually due to elevated intracranial pressure (ICP), eventually compromising cerebral blood flow sufficiently to produce permanent neuronal loss, infarction, and possibly brain death. Elevated ICP is thus a common pathway for neural demise and it may arise from a variety of causes, many of which may result in a neurosurgical procedure intended to ameliorate the impact or etiology of elevated ICP.

In vitro comparison of two generations of Licox and Neurotrend catheters

BACKGROUND

Clinical reports on brain tissue oxygen tension differ in suggested threshold values for defining cerebral ischemia using the Licox and Neurotrend/Paratrend system. We evaluated in vitro performance of both first and second generation devices.

MATERIALS AND METHODS

Response rate and accuracy in solutions with oxygen tensions from 0 to 150 mm Hg were measured.

FINDINGS

Ninety-five percent Response times were 102 +/- 13 seconds for first generation Licox probes and 135 +/- 24 s for Paratrend (n = 6, each probe), with second generation probes at 134 +/- 4 and 116 +/- 16 s respectively. At pO2 150 mmHg Licox and Paratrend probes were accurate with 2.2% and 2.1% error, respectively and 2.6% and 4.1% for later generation. At pO2 18 mmHg, Paratrend overestimated by 16.5% (absolute error range 2.18 to 4.18 mmHg), 7.4% for Neurotrend, Licox underestimated by 1.8% (absolute error range 0.08 to 0.52 mmHg) with 3.6% for the second generation probe.

CONCLUSIONS

Differences between the first generation probe types, while statistically significant (p < 0.001), may not be clinically relevant. Overestimation of pO2 by Neurotrend and small underestimation by Licox partially explain differences in published thresholds for cerebral ischemia. The Neurotrend was slightly more accurate and faster than the Paratrend system.

The role of lung function in brain tissue oxygenation following traumatic brain injury

Object

Previous studies have demonstrated that periods of low brain tissue oxygen tension (PbtO2) are associated with poor outcome after head trauma but have primarily focused on cerebral and hemodynamic factors as causes of low PbtO2. The purpose of this study was to investigate the influence of lung function on PbtO2 with an oxygen challenge (increase in fraction of inspired oxygen [FiO2] concentration to 1.0).

Methods

This prospective observational cohort study was performed in the neurointensive care unit of the Level 1 trauma center at San Francisco General Hospital. Thirty-seven patients with severe traumatic brain injury (TBI) undergoing brain tissue oxygen monitoring as part of regular care underwent an oxygen challenge, consisting of an increase in FiO2 concentration from baseline to 1.0 for 20 minutes. Partial pressure of arterial oxygen (PaO2), PbtO2, and the ratio of PaO2 to FiO2 (the PF ratio) were determined before and after oxygen challenge.

Results

Patients with higher PF ratios achieved greater PbtO2 during oxygen challenge than those with a low PF ratio because they achieved a higher PaO2 after an oxygen challenge. Lung function, specifically the PF ratio, is a major determinant of the maximal PbtO2 attained during an oxygen challenge.

Conclusions

Given that patients with TBI are at risk for pulmonary complications such as pneumonia, severe atelectasis, and adult respiratory distress syndrome, lung function must be considered when interpreting brain tissue oxygenation.

Monitoring brain tissue oxymetry: Will it change management of critically ill neurologic patients?

Based on the assumption that brain ischemia and hypoxia are central causes of brain damage, the maintenance of an adequate tissue oxygenation is a primary objective in the field of neurocritical care. Thus, monitoring brain tissue oxymetry, allowing the possibility to discriminate between normal and critically impaired tissue oxygenation, is recognized as an essential part of the management of the neurological critically ill patient. The clinical usefulness of this neuromonitoring tool in the area of neurosciences (traumatic brain injury, aneurysm surgery, arteriovenous malformation resection, brain tumors) is discussed. Monitoring brain tissue oxymetry not only allows the detection of impending cerebral ischemia, thus providing the clinician with essential information for the management and correction of harmful intracerebral events, but it also helps in understanding the pathophysiology of neuro-injury. It can also be used as a "surrogate end point" to evaluate putative therapies, targeting therapy towards improved cerebral oxygenation. As brain tissue oxygenation correlates closely with outcome, several outcome categories have been differentiated, aiding in predicting prognosis after injury. The rationale for monitoring brain tissue oxygenation is to provide essential information about oxygen supply and utilization in this specific tissue bed, thus reducing secondary brain damage and improving neurological outcome.

Oxygen tension controls the expansion of human CNS precursors and the generation of astrocytes and oligodendrocytes

Human neural precursor proliferation and potency is limited by senescence and loss of oligodendrocyte potential. We found that in vitro expansion of human postnatal brain CD133(+) nestin(+) precursors is enhanced at 5% oxygen, while raising oxygen tension to 20% depletes precursors and promotes astrocyte differentiation even in the presence of mitogens. Higher cell densities yielded more astrocytes regardless of oxygen tension. This was reversed by noggin at 5%, but not 20%, oxygen due to a novel repressive effect of low oxygen on bone morphogenetic protein (BMP) signaling. When induced to differentiate by mitogen withdrawal, 5% oxygen-expanded precursors generated 17-fold more oligodendrocytes than cells expanded in 20% oxygen. When precursors were expanded at 5% oxygen and then differentiated at 20% oxygen, oligodendrocyte maturation was further enhanced 2.5-fold. These results indicate that dynamic control of oxygen tension regulates different steps in fate and maturation and may be crucial for treating neurodegenerative diseases.

The endovascular operating room as an extension of the intensive care unit: changing strategies in the management of neurovascular disease

Technological advances within the field of endovascular neurosurgery have influenced the management of the neurovascular patient within the intensive care unit (ICU). The endovascular operating room has, in fact, become an extension of the ICU in certain cases. Given the rapid development of new endovascular technologies, it is more important than ever for neurosurgeons to remain intimately involved with the care of their patients within the ICU. This article offers an overview of the evolution in ICU management of neurovascular disease and provides a framework for the incorporation of the endovascular operating room in the intensive care management of patients with this disease.

A new technique for the mapping of oxygen tension on the brain surface

Most measurements of oxygen tension (PO(2)) in the brain have been performed using oxygen microelectrodes. However, the insertion of microelectrodes into the brain per se causes cortical injury and hence could lead to erroneous PO(2) measurements. The recently developed "quenching lifetime method" requires the injection of fluorescent chemicals into the blood circulation. To address this issue, we tested the feasibility of our O(2)-sensitive fluorescent membrane technique in the rat brain, and visualized the spatial distribution of PO(2) on the brain surface as epifluorescent microscopic patterns. An O(2)-quenching fluorescence dye, tris (1,10-phenanthroline) Ru(2+), was immobilized in a highly gas-permeable, thin silicone-rubber film formed on a microscope coverslip. Unlike the original method, which was intended for transparent rat mesenteric tissue, any change in the redox state in the brain tissue will influence the optical measurement of PO(2). Thus, in the present study, the O(2)-sensing membrane was further coated with a thin opaque silicone-rubber to minimize this type of influence. This new method enabled us to visualize the PO(2) gradient on the rat brain without causing cortical injuries. In an ischemia/reperfusion model using Pulsinelli's four-vessel occlusion rats, the changes in the PO(2) were highly heterogeneous during the ischemic period and this heterogeneity, both temporal and spatial, was higher in the off-arteriolar area than in the peri-arteriolar area.

Imaging of brain hypoxia in permanent and temporary middle cerebral artery occlusion in the rat using 18F-fluoromisonidazole and positron emission tomography: a pilot study

In acute stroke, the target of therapy is the severely hypoxic but salvageable tissue. Previous human studies using 18F-fluoromisonidazole and positron emission tomography (18F-FMISO PET) have shown high tracer retention indicative of tissue hypoxia, which had normalized at repeat scan >48 h later. In the only validation study of 18F-FMISO, using ex vivo autoradiography in thread middle cerebral artery occluded (MCAo) rats, there was unexpected high uptake as late as 22 h after reperfusion, raising questions about the use of 18F-FMISO as a hypoxia tracer. Here we report a pilot study of 18F-FMISO PET in experimental stroke. Spontaneous hypertensive rats were subjected to distal clip MCAo. Three-hour dynamic PET was performed in 7 rats: 3 normals, 1 with permanent MCAo (two sessions: 30 mins and 48 h after clip), and 3 with temporary MCAo (45 mins, n=1; 120 mins, n=2; scanning started 30 mins after clip removal). Experiments were terminated by perfusion-fixation for standard histopathology. Late tracer retention was assessed by both compartmental modelling and simple side-to-side ratios. In the initial PET session of the permanent MCAo rat, striking trapping of 18F-FMISO was observed in the affected cortex, which had normalized 48 h later; histopathology revealed pannecrosis. In contrast, there was no demonstrable tracer retention in either temporary MCAo models, and histopathology showed ischemic changes only. These results document elevated 18F-FMISO uptake in the stroke area only in the early phase of MCAo, but not after early reperfusion nor when tissue necrosis has developed. These findings strongly support the validity of 18F-FMISO as a marker of viable hypoxic tissue/penumbra after stroke.

Special Cases: Mechanical Ventilation of Neurosurgical Patients

Mechanical ventilation has evolved greatly over the last half century, guided primarily by improved comprehension of the relevant pathology/physiology. Neurosurgical patients are a unique subgroup of patients who heavily use this technology for both support, and less commonly, as a therapy. Such patients demand special consideration with regard to mode of ventilation, use of positive end-expiratory pressure, and monitoring. In addition, meeting the ventilatory needs of neurosurgical patients while minimizing ventilatory-induced lung damage can be a challenging aspect of care.

Brain tissue oxygen monitoring in pediatric patients with severe traumatic brain injury

OBJECT

Intracranial pressure (ICP) and cerebral perfusion pressure (CPP) monitoring are fundamental to the management of severe traumatic brain injury (TBI). In adults, brain tissue oxygen monitoring (specifically PO2) and treatment have been shown to be safe additions to conventional neurocritical care and are associated with improved outcome. Brain tissue oxygen monitoring, however, has not been described in pediatric patients with TBI. In this report, the authors present preliminary experience with the use of ICP and PO2 monitoring in this population.

METHODS

Pediatric patients (age <18 years) with severe TBI (Glasgow Coma Scale score <8) admitted to a Level 1 trauma center who underwent ICP and PO2 monitoring were evaluated. Therapy was directed at maintaining ICP below 20 mm Hg and age-appropriate CPP (> or =40 mm Hg). Data obtained in six patients (two girls and four boys ranging in age from 6-16 years) were analyzed. Brain tissue oxygen levels were significantly higher (p < 0.01) at an ICP of less than 20 mm Hg (PO2 29.29 +/- 7.17 mm Hg) than at an ICP of greater than or equal to 20 mm Hg (PO2 22.83 +/- 13.85 mm Hg). Significant differences (p < 0.01) were also measured when CPP was less than 40 mm Hg (PO2 2.53 +/- 7.98 mm Hg) and greater than or equal to 40 mm Hg (PO2 28.97 +/- 7.85 mm Hg).

CONCLUSIONS

Brain tissue oxygen monitoring may be a safe and useful addition to ICP monitoring in the treatment of pediatric patients with severe TBI.

Direct cerebral oxygenation monitoring--a systematic review of recent publications

This review has been compiled to assess publications related to the clinical application of direct cerebral tissue oxygenation (PtiO2) monitoring published in international, peer-reviewed scientific journals. Its goal was to extract relevant, i.e. positive and negative information on indications, clinical application, safety issues and impact on clinical situations as well as treatment strategies in neurosurgery, neurosurgical anaesthesiology, neurosurgical intensive care, neurology and related specialties. For completeness' sake it also presents some related basic science research. PtiO2 monitoring technology is a safe and valuable cerebral monitoring device in neurocritical care. Although a randomized outcome study is not available its clinical utility has repeatedly been clearly confirmed because it adds a monitoring parameter, independent from established cerebral monitoring devices. It offers new insights into cerebral physiology and pathophysiology. Pathologic values have been established in peer-reviewed research, which are not only relevant to outcome but are treatable. The benefits clearly outweigh the risks, which remains unchallenged in all publications retrieved. It is particularly attractive because it offers continuous, real-time data and is available at the bedside.

Monitoring cerebral oxygenation in traumatic brain injury

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

Transfusion of erythrocyte concentrates produces a variable increment on cerebral oxygenation in patients with severe traumatic brain injury: a preliminary study

OBJECTIVE

To investigate the long-term influence of erythrocyte transfusion on cerebral oxygenation in patients with severe traumatic brain injury.

DESIGN

Prospective and observational study.

SETTING

Neurotrauma intensive care unit of trauma center level I.

PATIENTS

Sixty consecutive, hemodynamically stable patients with severe traumatic brain injury, pretransfusion hemoglobin<100g/l, non-bleeding and monitored through intracranial pressure and brain tissue partial pressure of oxygen (PtiO(2)) catheters were included.

INTERVENTIONS

Transfusion of 1-2 units of red blood cells.

MEASUREMENTS AND RESULTS

Ten sets of variables (pretransfusion, end of transfusion, and 1, 2, 3, 4, 5, 6, 12 and 24h after transfusion) were recorded, including: PtiO(2), cerebral perfusion pressure (CPP), end-tidal CO(2), peripheral saturation of oxygen, temperature, hemoglobin, lactate and PaO(2)/FiO(2) ratio. Transfusion was associated with an increase in PtiO(2) during a 6-h period, with a peak at 3h (26.2%; p=0.0001) in 78.3% of the patients. No relationship was observed between PtiO(2), CPP and hemoglobin increments. The relative increment in PtiO(2) at hour 3 was only correlated with baseline PtiO(2) (r(2) 0.166; p=0.001). All of the patients with basal PtiO(2)<15mmHg showed an increment in PtiO(2) versus 74.5% of patients with basal PtiO(2)>or=15mmHg (p<0.01, hour 3).

CONCLUSIONS

Erythrocyte transfusion is associated with a variable and prolonged increment of cerebral tissue oxygenation in anemic patients with severe traumatic brain injury. Low baseline PtiO(2) levels (<15mmHg) could define those patients who benefit the most from erythrocyte transfusion.

Pediatric traumatic brain injury: past, present and future

Traumatic brain injury (TBI) is the leading cause of death and disability in children. Evidence-based guidelines for the management of this population are available; however, the data highlight significant deficiencies with few treatment standards or guidelines. Considering the limited availability of resources, it is necessary to define realistic goals. Attention should be given to injury prevention, developing standardized pediatric admission and outcome evaluations, increasing the utility and spectrum of physiological and biochemical testing, and defining the evolving role of imaging in TBI.

Continuous monitoring of the microcirculation in neurocritical care: an update on brain tissue oxygenation

PURPOSE OF REVIEW

This article summarizes recent clinical and experimental studies of parenchymal brain tissue oxygen monitoring and considers future directions for its use in neurocritical care.

RECENT FINDINGS

Recent reports have focused on the relationship between brain tissue oxygen tension (PbrO2) and other physiologic parameters such as mean arterial pressure, cerebral perfusion pressure, cerebral blood flow, and fraction of inspired oxygen. PbrO2 appears to reflect both regional and systemic oxygen concentrations as well as microvascular perfusion through natural tissue gradients. Defining an absolute critically low PbrO2 threshold has been challenging, but levels below 14 mmHg may have a pathophysiologic basis. Newer studies have examined dynamic changes in PbrO2 during oxygen reactivity testing and during augmentation of cerebral perfusion pressure. PbrO2 monitoring has now been described in a wide range of neurocritical care conditions including head trauma, subarachnoid hemorrhage, nontraumatic intracerebral hemorrhage, brain death, and brain tumor resection.

SUMMARY

The use of brain tissue oxygen monitoring is maturing as a tool to detect and treat secondary brain injury. PbrO2 measurements can provide continuous quantitative data about injury pathophysiology and severity that may help optimize neurointensive care management. Prospective trials of PbrO2 guided treatment protocols are now needed to demonstrate impact on clinical outcomes.

Recognizing and treating ischemic insults to the brain: the role of brain tissue oxygen monitoring

This article describes the potential application of brain tissue oxygen monitoring technology in the care of patients who have sustained traumatic brain injury (TBI) or subarachnoid hemorrhage (SAH). To accomplish this objective, a review of the intracranial dynamics that are created by primary and secondary brain injury, and the challenges of optimizing oxygen delivery to the injured brain are presented. Furthermore, interventions that facilitate cerebral oxygen supply and reduce oxygen consumption are identified. Finally, application of this technology is highlighted by using case vignettes of patients who have TBI or SAH.

[Cerebral ischemic threshold in clinical practice]

The ischemic threshold is reached when the availability of oxygen in the cerebral tissue does not cover oxygen requirement. For a patient sedated, with constant PaO(2) and haemoglobin, the cerebral blood flow (CBF) global and local is the essential factor to maintain such a balance. At a cellular level, ischemia occurs when the CBF is below 20-25 ml/min. However, this threshold probably varies with the patient and also within the normal or perilesional tissue. A cerebral perfusion pressure (CPP) of 60 mmHg, recommended for a cerebral perfusion allowing a sufficient CBF for normal brain, does not prevent ischemia. Monitoring aimed to control parameters of the aerobic metabolism (PtiO(2), SjO(2) and microdialysis) and to detect the ischemic threshold allows to adapt the CPP to each patient and continuously.

Continuous assessment of cerebrovascular autoregulation after traumatic brain injury using brain tissue oxygen pressure reactivity

OBJECTIVE

To evaluate whether two newly developed indexes of brain tissue oxygen pressure reactivity (ORx and bPtio2) provide information on the status of cerebrovascular autoregulation after traumatic brain injury. This was accomplished by analyzing the relationship between these indexes and an index of cerebrovascular pressure reactivity (PRx). PRx is an established parameter for estimation of cerebrovascular autoregulation.

DESIGN

Retrospective analysis of prospectively collected data.

SETTING

Neurosurgical intensive care unit of a university hospital.

PATIENTS

Twenty-seven patients suffering from severe traumatic brain injury.

INTERVENTIONS

Continuous monitoring of mean arterial blood pressure, intracranial pressure, cerebral perfusion pressure, and partial pressure of brain tissue oxygen (Ptio2) was performed for an average of 6.5 days. ORx was calculated as a moving correlation coefficient between values of cerebral perfusion pressure and Ptio2. The bPtio2 was calculated as a moving value of the slope of the linear regression function between cerebral perfusion pressure and Ptio2. PRx was calculated as a moving correlation coefficient between values for intracranial pressure and mean arterial blood pressure. Outcome was assessed at 6 months after traumatic brain injury (Glasgow Outcome Scale).

MEASUREMENTS AND MAIN RESULTS

Both ORx and bPtio2 correlated significantly with PRx (r=.55 for ORx, r=.52 for bPtio2, p<.01). PRx and ORx showed a significantly negative correlation to the monitored Ptio2 values (r=-.42 for PRx, r=-.41 for ORx, p<.05) and outcome (r=-.52 for PRx, r=-.62 for ORx, p<.01), whereas bPtio2 did not.

CONCLUSIONS

ORx and, to a lesser extent, bPtio2 correlated with the autoregulatory marker PRx and provide additional information about the status of cerebrovascular autoregulation after traumatic brain injury. The data also suggested that patients with impaired autoregulation are at increased risk for secondary cerebral hypoxia.

The role of tissue oxygen monitoring in patients with acute brain injury

Cerebral ischaemia is implicated in poor outcome after brain injury, and is a very common post-mortem finding. The inability of the brain to store metabolic substrates, in the face of high oxygen and glucose requirements, makes it very susceptible to ischaemic damage. The clinical challenge, however, remains the reliable antemortem detection and treatment of ischaemic episodes in the intensive care unit. Outcomes have improved in the traumatic brain injury setting after the introduction of progressive protocol-driven therapy, based, primarily, on the monitoring and control of intracranial pressure, and the maintenance of an adequate cerebral perfusion pressure through manipulation of the mean arterial pressure. With the increasing use of multi-modal monitoring, the complex pathophysiology of the injured brain is slowly being unravelled, emphasizing the heterogeneity of the condition, and the requirement for individualization of therapy to prevent secondary adverse hypoxic cerebral events. Brain tissue oxygen partial pressure (Pb(O2) monitoring is emerging as a clinically useful modality, and this review examines its role in the management of brain injury.

Cerebral oxygenation in major pediatric trauma: its relevance to trauma severity and outcome

INTRODUCTION

Trauma is the commonest cause of death in the pediatric population, which is prone to diffuse primary brain injury aggravated by secondary insults (eg, hypoxia, hypotension). Standard monitoring involves intracranial pressure (ICP) and cerebral perfusion pressure, which do not reflect true cerebral oxygenation (oxygen delivery [Do(2)]). We explore the merits of a brain tissue oxygen-directed critical care guide.

METHODS

Sixteen patients with major trauma (Injury Severity Score, >16/Pediatric Trauma Score [PTS], <7) had partial pressure of brain tissue oxygen (Pbto(2)) monitor (Licox; Integra Neurosciences, Plainsboro, NJ) placed under local anesthesia using twist-drill craniostomy and definitive management of associated injuries. Pbto(2) levels directed therapy intensity level (ventilator management, inotrops, blood transfusion, and others). Patient demographics, short-term physiological parameters, Pbto(2), ICP, Glasgow Coma Score, trauma scores, and outcomes were analyzed to identify the patients at risk for low Do(2).

RESULTS

There were 10 males and 6 females (mean age, 14 years) sustaining motor vehicle accident (14), falls (1), and assault (1), with a mean Injury Severity Score of 36 (16-59); PTS, 3 (0-7); and Revised Trauma Score, 5.5 (4-11). Eleven patients (70%) had low Do(2) (Pbto(2), <20 mm Hg) on admission despite undergoing standard resuscitation affected by fraction of inspired oxygen, Pao(2), and cerebral perfusion pressure (P = .001). Eubaric hyperoxia improved cerebral oxygenation in the low-Do(2) group (P = .044). The Revised Trauma Score (r = 0.65) showed moderate correlation with Pbto(2) and was a significant predictor for low Do(2) (P = .001). In patients with Pbto(2) of less than 20 mm Hg, PTS correlated with cerebral oxygenation (r = 0.671, P = .033). The mean 2-hour Pbto(2) and the final Pbto(2) in survivors were significantly higher than deaths (21.6 vs 7.2 mm Hg [P = .009] and 25 vs 11 mm Hg [P = .01]). Although 4 of 6 deaths were from uncontrolled high ICP, PTS and 2-hour low Do(2) were significant for roots for mortality.

CONCLUSIONS

Pbto(2) monitoring allows for early recognition of low-Do(2) situations, enabling appropriate therapeutic intervention.

Brain tissue oxygen monitoring in intracerebral hemorrhage

INTRODUCTION

Brain tissue oxygen (PbrO2) monitoring is an emerging technique for detection of secondary brain injury in neurocritical care. Although it has been extensively reported in traumatic brain injury and aneurysmal subarachnoid hemorrhage, its use in nontraumatic intracerebral hemorrhage (ICH) has not been well described. We report complementary preliminary studies in a large animal model and in patients that demonstrate the feasibility of PbrO2 monitoring after ICH.

METHODS

To assess early events after ICH, Licox Clark-type oxygen probes were inserted in the bilateral frontal white matter of four anesthetized swine that subsequently underwent right parietal hematoma formation in an experimental model of ICH. Intracranial pressure (ICP) was monitored as well. Seven patients with acute ICH, who were undergoing ICP monitoring as part of standard neurocritical care, had placement of a frontal oxygen probe, with subsequent monitoring for up to 7 days.

RESULTS

In the swine ICH model, a rise in ICP early after hematoma formation was accompanied by a decrease in ipsilateral and contralateral PbrO2. Secondary increases in hematoma volume resulted in further decreases in PbrO2 over the first hour after ICH. In patients undergoing oxygen monitoring, low PbrO2 (<15 mmHg) was common. In these patients, changes in FiO2, mean arterial pressure, and cerebral perfusion pressure (but not ICP) predicted subsequent change in PbrO2.

CONCLUSION

Brain tissue oxygen monitoring is feasible in ICH patients, as well as in a swine model of ICH. Translational research that emphasizes complementary information derived from human and animal studies may yield additional insights not available from either alone.

Relative importance of hypertension compared with hypervolemia for increasing cerebral oxygenation in patients with cerebral vasospasm after subarachnoid hemorrhage

Object. Hypervolemia and hypertension therapy is routinely used for prophylaxis and treatment of symptomatic cerebral vasospasm at many institutions. Nevertheless, there is an ongoing debate about the preferred modality (hypervolemia, hypertension, or both), the degree of therapy (moderate or aggressive), and the risk or benefit of hypervolemia, moderate hypertension, and aggressive hypertension in patients following subarachnoid hemorrhage.

Methods. Monitoring data and patient charts for 45 patients were retrospectively searched to identify periods of hypervolemia, moderate hypertension, or aggressive hypertension. Measurements of central venous pressure, fluid input, urine output, arterial blood pressure, intracranial pressure, and oxygen partial pressure (PO2) in the brain tissue were extracted from periods ranging from 1 hour to 24 hours. For these periods, the change in brain tissue PO2 and the incidence of complications were analyzed.

During the 55 periods of moderate hypertension, an increase in brain tissue PO2 was found in 50 cases (90%), with complications occurring in three patients (8%). During the 25 periods of hypervolemia, an increase in brain oxygenation was found during three intervals (12%), with complications occurring in nine patients (53%). During the 10 periods of aggressive hypervolemic hypertension, an increase in brain oxygenation was found during six of the intervals (60%), with complications in five patients (50%).

Conclusions. When hypervolemia treatment is applied as in this study, it may be associated with increased risks. Note, however, that further studies are needed to determine the role of this therapeutic modality in the care of patients with cerebral vasospasm. In poor-grade patients, moderate hypertension (cerebral perfusion pressure 80–120 mm Hg) in a normovolemic, hemodiluted patient is an effective method of improving cerebral oxygenation and is associated with a lower complication rate compared with hypervolemia or aggressive hypertension therapy.

Reduced mortality rate in patients with severe traumatic brain injury treated with brain tissue oxygen monitoring

Object. An intracranial pressure (ICP) monitor, from which cerebral perfusion pressure (CPP) is estimated, is recommended in the care of severe traumatic brain injury (TBI). Nevertheless, optimal ICP and CPP management may not always prevent cerebral ischemia, which adversely influences patient outcome. The authors therefore determined whether the addition of a brain tissue oxygen tension (PO2) monitor in the treatment of TBI was associated with an improved patient outcome.

Methods. Patients with severe TBI (Glasgow Coma Scale [GCS] score < 8) who had been admitted to a Level I trauma center were evaluated as part of a prospective observational database. Patients treated with ICP and brain tissue PO2 monitoring were compared with historical controls matched for age, pathological features, admission GCS score, and Injury Severity Score who had undergone ICP monitoring alone. Therapy in both patient groups was aimed at maintaining an ICP less than 20 mm Hg and a CPP greater than 60 mm Hg. Among patients whose brain tissue PO2 was monitored, oxygenation was maintained at levels greater than 25 mm Hg. Twenty-five patients with a mean age of 44 ± 14 years were treated using an ICP monitor alone. Twenty-eight patients with a mean age of 38 ± 18 years underwent brain tissue PO2-directed care. The mean daily ICP and CPP levels were similar in each group. The mortality rate in patients treated using conventional ICP and CPP management was 44%. Patients who also underwent brain tissue PO2 monitoring had a significantly reduced mortality rate of 25% (p < 0.05).

Conclusions. The use of both ICP and brain tissue PO2 monitors and therapy directed at brain tissue PO2 is associated with reduced patient death following severe TBI.

Muscle tissue oxygen tension and oxidative metabolism during ischemia and reperfusion

BACKGROUND

Recent studies have shown a relationship between alterations in tissue oxygen metabolism and cellular changes following ischemia and reperfusion, such as energy store depletion and intracellular acidosis. The aim of this study was to evaluate the relationship between tissue energy metabolism and intramuscular tissue oxygen tension in the mobilized latissimus dorsi muscle.

MATERIAL AND METHODS

The latissimus dorsi muscle was raised in New Zealand white rabbits (n = 10, 2.5 +/- 0.5 kg). During 4 h of ischemia and 2 h of reperfusion, the intramuscular tissue oxygen tension (Licox PO2-microcatheter probe) and the status of phosphorylated muscle energy metabolites were measured using a high-field 31P-NMR spectrometer. Linear correlation was performed between 31P-NMR data and tissue oxygen tension readings.

RESULTS

The tissue oxygen tension (PO2) values correlated significantly with phosphocreatine (PCr) (r = 0.96, P < 0.001), beta-adenosin triphosphate (beta-ATP) (r = 0.64, P <0.01), and intracellular pH (r = 0.82, P <0.001).

CONCLUSIONS

On the basis of these findings, we conclude that the data provided by tissue oxygen tension measurement offer a real time minimally invasive estimate of muscle oxidative metabolism during ischemia and reperfusion.

Effect of perfluorocarbons on brain oxygenation and ischemic damage in an acute subdural hematoma model in rats

Object. This study was conducted to determine whether perfluorocarbons (PFCs) improve brain oxygenation and reduce ischemic brain damage in an acute subdural hematoma (SDH) model in rats.

Methods. Forty adult male Sprague—Dawley rats were allocated to four groups: 1) controls, acute SDH treated with saline and 30% O2; 2) 30-PFC group, acute SDH treated with PFC infusion in 30% O2; 3) 100-O2 group, acute SDH treated with 100% O2; and 4) 100-PFC group, acute SDH treated with PFC plus 100% O2. Ten minutes after the induction of acute SDH, a single dose of PFC was infused and 30% or 100% O2 was administered simultaneously. Four hours later, half of the rats were killed by perfusion for histological study to assess the extent of ischemic brain damage. The other half were used to measure brain tissue oxygen tension (PO2). The volume of ischemic brain damage was 162.4 ± 7.6 mm3 in controls, 165.3 ± 11.3 mm3 in the 30-PFC group, 153.4 ± 17.3 mm3 in the 100-O2 group, and 95.9 ± 12.8 mm3 in the 100-PFC group (41% reduction compared with controls, p = 0.002). Baseline brain tissue PO2 values were approximately 20 mm Hg, and after induction of acute SDH, PO2 rapidly decreased and remained at 1 to 2 mm Hg. Treatment with either PFC or 100% O2 improved brain tissue PO2, with final values of 5.14 and 7.02 mm Hg, respectively. Infusion of PFC with 100% O2 improved brain tissue PO2 the most, with a final value of 15.16 mm Hg.

Conclusions. Data from the current study demonstrated that PFC infusion along with 100% O2 can significantly improve brain oxygenation and reduce ischemic brain damage in acute SDH.

Brain energy metabolism and intracranial pressure in idiopathic adult hydrocephalus syndrome

BACKGROUND

The symptoms in idiopathic adult hydrocephalus syndrome (IAHS) are consistent with pathology involving the periventricular white matter, presumably reflecting ischaemia and CSF hydrodynamic disturbance.

OBJECTIVE

To investigate whether a change in intracranial pressure (ICP) can affect energy metabolism in deep white matter.

METHODS

A microdialysis catheter, a brain tissue oxygen tension probe, and an ICP transducer were inserted into the periventricular white matter 0-7 mm from the right frontal horn in 10 patients with IAHS. ICP and intracerebral Ptio2 were recorded continuously during lumbar CSF constant pressure infusion test. ICP was raised to pressure levels of 35 and 45 mm Hg for 10 minutes each, after which CSF drainage was undertaken. Microdialysis samples were collected every three minutes and analysed for glucose, lactate, pyruvate, and glutamate.

RESULTS

When raising the ICP, a reversible drop in the extracellular concentrations of glucose, lactate, and pyruvate was found. Comparing the values during baseline to values at the highest pressure level, the fall in glucose, lactate, and pyruvate was significant (p < 0.05, Wilcoxon sign rank). There was no change in glutamate or the lactate to pyruvate ratio during ICP elevation. Ptio2 did not decrease during ICP elevation, but was significantly increased following CSF drainage.

CONCLUSIONS

Raising intracranial pressure induces an immediate and reversible change in energy metabolism in periventricular white matter, without any sign of ischaemia. Theoretically, frequent ICP peaks (B waves) over a long period could eventually cause persisting axonal disturbance and subsequently the symptoms noted in IAHS.

Impact of pyrexia on neurochemistry and cerebral oxygenation after acute brain injury

BACKGROUND

Postischaemic pyrexia exacerbates neuronal damage. Hyperthermia related cerebral changes have still not been well investigated in humans.

OBJECTIVE

To study how pyrexia affects neurochemistry and cerebral oxygenation after acute brain injury.

METHODS

18 acutely brain injured patients were studied at the onset and resolution of febrile episodes (brain temperature > or = 38.7 degrees C). Intracranial pressure (ICP), brain tissue oxygen tension (PbrO2), and brain tissue temperature (Tbr) were recorded continuously; jugular venous blood was sampled intermittently. Microdialysis probes were inserted in the cerebral cortex and in subcutaneous tissue. Glucose, lactate, pyruvate, and glutamate were measured hourly. The lactate to pyruvate ratio was calculated.

RESULTS

Mean (SD) Tbr rose from 38 (0.5) to 39.3 (0.3) degrees C. Arteriojugular oxygen content difference (AJD(O2)) fell from 4.2 (0.7) to 3.8 (0.5) vol% (p < 0.05) and PbrO2 rose from 32 (21) to 37 (22) mm Hg (p < 0.05). ICP increased slightly and no significant neurochemical alterations occurred. Opposite changes were recorded when brain temperature returned towards baseline.

CONCLUSIONS

As long as substrate and oxygen delivery remain adequate, hyperthermia on its own does not seem to induce any further significant neurochemical alterations. Changes in cerebral blood volume may, however, affect intracranial pressure.

Trends in monitoring patients with aneurysmal subarachnoid haemorrhage

After aneurysmal subarachnoid haemorrhage (SAH), the clinical outcome depends upon the primary haemorrhage and a number of secondary insults in the acute post-haemorrhagic period. Some secondary insults are potentially preventable but prevention requires prompt recognition of cerebral or systemic complications. Currently, several neuro-monitoring techniques are available; this review describes the most frequently used techniques and discusses indications for their use, and their value in diagnosis and prognosis. None of the techniques, when considered in isolation, has proved sufficient after SAH. Furthermore, the use of multi-modality monitoring is hampered by a lack of clinical studies that identify combinations of specific techniques in terms of clinical information and reliability. However, ischaemia at the tissue level can be detected by intracerebral microdialysis technique. Used together with the conventional monitoring systems, for example intracranial pressure measurements, transcranial Doppler ultrasound and modern neuro-imaging, direct assessment of biochemical markers by intracerebral microdialysis is promising in the advancement of neurointensive care of patients with SAH. A successfully implemented monitoring system provides answers but it also raises valuable new questions challenging our current understanding of the brain injury after SAH.

Brain tissue oxygen (PtiO2): a clinical comparison of two monitoring devices

BACKGROUND

We investigated the difference between two commercially available sensors for continuous monitoring of brain tissue oxygen (PtiO2). One is a single parameter probe for PtiO2 monitoring (Licox), the other is a multiparamter sensor (Neurotrend) further including measurement of brain temperature, pH, and partial pressure of tissue carbon dioxide.

METHODS

In seven patients after subarachnoid hemorrhage or traumatic brain injury continuous monitoring of PtiO2 was performed simultaneously using Licox and Neurotrend.

FINDINGS

Mean PtiO2 was generally lower when assessed by the Neurotrend, as compared with the Licox (Licox 27.7 mmHg vs. Neurotrend 20.9 mmHg; P = 0.028). The amplitude of PtiO2 elevations during ventilation with 100% oxygen was higher with the Licox, but this did not reach statistical significance (Licox 55.2 mmHg vs. Neurotrend 50.2 mmHg, P = 0.082). Regarding clinical stability of the sensors, only one Neurotrend sensor provided valid function over the desired monitoring period. Five Neurotrend sensors dislocated or broke and one sensor did not show any function after insertion. No malfunction occurred with the Licox sensors.

CONCLUSIONS

Our results suggest that PtiO2 might be lower when assessed by the Neurotrend sensor. The clinical stability of the Neurotrend sensor was of concern and allowed monitoring in one of seven patients over the desired monitoring period of several days only.

Linear correlation between stable intracranial pressure decrease and regional cerebral oxygenation improvement following mannitol administration in severe acute head injury patients

OBJECTIVES

We investigated the relationship between stable decrease in intracranial pressure (ICP) following mannitol administration and variations in regional oxygenation (PtiO2) in severe traumatic brain injured (STBI) patients.

METHODS

Fourteen STBI patients (Glasgow Coma Scale score < or = 8) admitted to the neurointensive care unit during a 12-month period were studied. Multiparameter data, including parenchymal tissue oxygen (PtiO2) and carbon dioxide tension, cerebral perfusion pressure, mean arterial pressure; temperature, pH and pressure reactivity index were measured. Point values from 53 mannitol administrations were extracted every five seconds and divided into five consecutive 30-minute epochs. Mean values and trends were identified. Postadministration epoch with maximum decrease in ICP was selected along with the means of the corresponding variables. These data were compared with baseline to derive the delta values. Mean deltaICP was then compared with deltaPtiO2 in each patient using linear correlation.

FINDINGS

In patients with ICP0 > 20 mmHg (group A), PtiO2 increased in 69.6% of samples, whereas in group B (ICP0 < 20 mmHg), PtiO2 increased in only 50% of the samples. There was a significant negative correlation between mean deltaICP and deltaPtiO2 in both groups; Group A: r = -0.79 (P = 0.01); Group B: r = -0.92 (P = 0.01).

CONCLUSIONS

There is a strong negative correlation between stable decrease in ICP following mannitol administration and improvement in regional oxygenation around the peri-contusional area. The data suggest a potentially favourable interaction between mannitol therapy and cerebral internal milieu in STBI patients.

Brain tissue oxygenation monitoring in acute brain injury

Cerebral ischemia is one of the most important causes of secondary insults following acute brain injury. While intracranial pressure monitoring in the intensive care unit constitutes the cornerstone of neurocritical care monitoring, it does not reflect the state of oxygenation of the injured brain. The holy grail of neuromonitoring is a modality that would reflect accurately real time the status of oxygenation in the tissue of interest, is robust, artefact free and that which provides information that can be used for therapeutic interventions and to improve outcome. Such a device could conceivably be used to augment the sensitivity of current multi-modality monitoring systems in the neurocritical management of brain injured patients. This article examines the availability of data in the literature to support clinical use of local tissue oxygen probes in intensive care.

The effects of hypoxia on the modulation of human TREK-1 potassium channels

Two-pore-domain potassium channels are a family of ion channels that are widely believed to play an important role in maintaining and regulating neuronal excitability. It has been shown that they can be modulated by an extraordinarily diverse range of endogenous and exogenous factors. One particular member of the family, TREK-1 (also known as KCNK2), is activated by increasing temperature, membrane stretch and internal acidosis, but is also sensitive to the presence of certain polyunsaturated fatty acids (such as arachidonic acid), neuroprotectants (such as riluzole) and volatile and gaseous general anaesthetics (such as halothane and nitrous oxide). It has recently been reported that TREK-1 channels are also affected by oxygen concentrations, and that at the levels of hypoxia that occur in the normal human brain, the channels greatly change their properties and, for example, lose their ability to be modulated by arachidonic acid and internal acidosis. These reports seriously challenge the idea that TREK-1 is a target for general anaesthetics and neuroprotectants. However, in this report we show that TREK-1 is not oxygen sensitive, and its ability to be activated by anaesthetics, arachidonic acid and internal acidosis remains unaltered under conditions of hypoxia. We further show that the protocol used by previous workers to prepare hypoxic solutions of arachidonic acid results in the removal of the compound from solution.

The effect of nimodipine on cerebral oxygenation in patients with poor-grade subarachnoid hemorrhage

Object. Nimodipine has been shown to improve neurological outcome after subarachnoid hemorrhage (SAH); the mechanism of this improvement, however, is uncertain. In addition, adverse systemic effects such as hypotension have been described. The authors investigated the effect of nimodipine on brain tissue PO2.

Methods. Patients in whom Hunt and Hess Grade IV or V SAH had occurred who underwent aneurysm occlusion and had stable blood pressure were prospectively evaluated using continuous brain tissue PO2 monitoring. Nimodipine (60 mg) was delivered through a nasogastric or Dobhoff tube every 4 hours. Data were obtained from 11 patients and measurements of brain tissue PO2, intracranial pressure (ICP), mean arterial blood pressure (MABP), and cerebral perfusion pressure (CPP) were recorded every 15 minutes.

Nimodipine resulted in a significant reduction in brain tissue PO2 in seven (64%) of 11 patients. The baseline PO2 before nimodipine administration was 38.4 ± 10.9 mm Hg. The baseline MABP and CPP were 90 ± 20 and 84 ± 19 mm Hg, respectively. The greatest reduction in brain tissue PO2 occurred 15 minutes after administration, when the mean pressure was 26.9 ± 7.7 mm Hg (p < 0.05). The PO2 remained suppressed at 30 minutes (27.5 ± 7.7 mm Hg [p < 0.05]) and at 60 minutes (29.7 ± 11.1 mm Hg [p < 0.05]) after nimodipine administration but returned to baseline levels 2 hours later. In the seven patients in whom brain tissue PO2 decreased, other physiological variables such as arterial saturation, end-tidal CO2, heart rate, MABP, ICP, and CPP did not demonstrate any association with the nimodipine-induced reduction in PO2. In four patients PO2 remained stable and none of these patients had a significant increase in brain tissue PO2.

Conclusions. Although nimodipine use is associated with improved outcome following SAH, in some patients it can temporarily reduce brain tissue PO2.

Neurophysiological monitoring, magnetic resonance imaging, and histological assays confirm the beneficial effects of moderate hypothermia after epidural focal mass lesion development in rodents

OBJECTIVE

To assess the effects of moderate intraischemic hypothermia on neurophysiological parameters in an epidural balloon compression model in rats and to correlate the results with magnetic resonance imaging and histological findings.

METHODS

Neurophysiological monitoring included laser Doppler flow, tissue partial oxygen pressure, and intracranial pressure measurements and electroencephalographic assessments during balloon expansion, sustained inflation, and reperfusion. Moderate intraischemic cooling of animals was extended throughout the reperfusion period, and results were compared with those for normothermic animals. Moreover, histological morphometric and magnetic resonance imaging volumetric analyses of the lesions were performed.

RESULTS

Laser Doppler flow decreased slightly during ischemia (P < 0.05) in animals treated with hypothermia, and flow values demonstrated complete reperfusion, compared with incomplete flow restoration in untreated animals (P < 0.05). During ischemia, the tissue partial oxygen pressure was less than 4.3 mm Hg in both groups. After reperfusion, values returned to the normal range in both groups, but the tissue partial oxygen pressure in hypothermic animals was significantly higher (P = 0.042) and demonstrated 19% higher values, compared with normothermic animals, before rewarming. Moderate hypothermia attenuated a secondary increase in intracranial pressure (P < 0.05), and electroencephalographic findings indicated a trend toward faster recovery (P > 0.05) after reperfusion. Lesion size was reduced by 35% in magnetic resonance imaging volumetric evaluations and by 24.5% in histological morphometric analyses.

CONCLUSION

Intraischemic hypothermia improves cerebral microcirculation, attenuates a secondary increase in intracranial pressure, facilitates electroencephalographic recovery, and reduces the lesion size.

Cerebral oxygenation following decompressive hemicraniectomy for the treatment of refractory intracranial hypertension

Object. Medically intractable intracranial hypertension is a major cause of morbidity and mortality after severe brain injury. One potential treatment for intracranial hypertension is decompressive hemicraniectomy (DCH). Whether and when to use DCH, however, remain unclear. The authors therefore studied the effects of DCH on cerebral O2 to develop a better understanding of the effects of this treatment on the recovery from injury and disease.

Methods. The study focused on seven patients (mean age 30.6 ± 9.7 years) admitted to the hospital after traumatic brain injury (five patients) or subarachnoid hemorrhage (two patients) as part of a prospective observational database at a Level I trauma center. At admission the Glasgow Coma Scale (GCS) score was 6 or less in all patients. Patients received continuous monitoring of intracranial pressure (ICP), cerebral perfusion pressure (CPP), blood pressure, and arterial O2 saturation. Cerebral oxygenation was measured using the commercially available Licox Brain Tissue Oxygen Monitoring System manufactured by Integra NeuroSciences. A DCH was performed when the patient's ICP remained elevated despite maximal medical management.

Conclusions. All patients tolerated DCH without complications. Before the operation, the mean ICP was elevated in all patients (26 ± 4 mm Hg), despite maximal medical management. After surgery, there was an immediate and sustained decrease in ICP (19 ± 11 mm Hg) and an increase in CPP (81 ± 17 mm Hg). Following DCH, cerebral oxygenation improved from a mean of 21.2 ± 13.8 mm Hg to 45.5 ± 25.4 mm Hg, a 114.8% increase. The change in brain tissue O2 and the change in ICP after DCH demonstrated only a modest relationship (r2 = 0.3). These results indicate that the use of DCH in the treatment of severe brain injury is associated with a significant improvement in brain O2.

The Influence of Allogeneic Red Blood Cell Transfusion Compared with 100% Oxygen Ventilation on Systemic Oxygen Transport and Skeletal Muscle Oxygen Tension After Cardiac Surgery: Retracted

In this study we investigated the effects of allogeneic red blood cell (RBC) transfusion on tissue oxygenation compared with those of 100% oxygen ventilation by using systemic oxygen transport variables and skeletal muscle oxygen tension (PtiO2). Fifty-one volume-resuscitated, mechanically ventilated patients with a nadir hemoglobin concentration in the range from 7.5 to 8.5 g/dL after elective coronary artery bypass grafting were allocated randomly to receive 1 unit (transfusion 1; n = 17) or 2 units (transfusion 2; n = 17) of allogeneic RBCs and ventilation with 40% oxygen or pure oxygen ventilation (100% oxygen; n = 17) and no allogeneic blood for 3 hours. Invasive arterial and pulmonary artery pressures and calculations of oxygen delivery (oxygen delivery index) and consumption indices (oxygen consumption index) were documented at 30-min intervals. PtiO2 was measured continuously by using implantable polarographic microprobes. Systemic oxygen transport variables and PtiO2 were similar between groups at baseline. The oxygen delivery index increased significantly with transfusion of allogeneic RBCs and 100% oxygen ventilation, whereas the oxygen consumption index remained unchanged. Oxygen 100% ventilation increased PtiO2 significantly (from 24.0 +/- 5.1 mm Hg to 34.2 +/- 6.2 mm Hg), whereas no change was found after transfusion of allogeneic RBCs. Peak PtiO2 values were 25.2 +/- 5.2 mm Hg and 26.3 +/- 6.5 mm Hg in the transfusion 1 and 2 groups, respectively. Transfusion of stored allogeneic RBCs was effective only in improving systemic oxygen delivery index, whereas 100% oxygen ventilation improved systemic oxygen transport and PtiO2. This improved oxygenation status was most likely due to an increase in convective oxygen transport with a large driving gradient for diffusion of plasma-dissolved oxygen into the tissue.

Diffusion limited oxygen delivery following head injury*

OBJECTIVE

To use a range of techniques to explore diffusion limitation as a mechanism of cellular hypoxia in the setting of head injury.

DESIGN

A prospective interventional study.

SETTING

A specialist neurocritical care unit.

PATIENTS

Thirteen patients within 7 days of closed head injury underwent imaging studies. Tissue for ultrastructural studies was obtained from a cohort of seven patients who required surgery.

INTERVENTIONS

Cerebral tissue PO2 (PtO2) was obtained using a multiple-variable sensor, and images of oxygen extraction fraction (OEF), derived from positron emission tomography, were used to calculate cerebral venous PO2 (PvO2). These data were used to derive the PvO2-PtO2 gradient in a region of interest around the sensor, which provided a measure of the efficiency of microvascular oxygen delivery. Measurements were repeated after PaCO2 was reduced from 37 +/- 3 to 29 +/- 3 torr (4.9 +/- 0.4 to 3.9 +/- 0.4 kPa) to assess the ability of the microvasculature to increase oxygen unloading during hypocapnia-induced hypoperfusion. Pericontusional tissue was submitted to electron microscopy to illustrate the structural correlates of physiologic findings.

MEASUREMENTS AND MAIN RESULTS

Tissue regions with hypoxic levels of PtO2 (<10 torr) had similar levels of PvO2 compared with nonhypoxic areas and hence displayed larger PvO2-PtO2 gradients (27 +/- 2 vs. 9 +/- 8 torr, p <.001). Despite similar cerebral blood flow reductions with hyperventilation, hypoxic regions achieved significantly smaller OEF increases compared with normoxic regions (7 +/- 5 vs. 16 +/- 6 %, p <.05). Pericontusional tissue showed varying degrees of endothelial swelling, microvascular collapse, and perivascular edema.

CONCLUSIONS

Increased diffusion barriers may reduce cellular oxygen delivery following head injury and attenuate the ability of the brain to increase oxygen extraction in response to hypoperfusion. Global or regional OEF underestimates tissue hypoxia due to such mechanisms.

Influence of cerebral oxygenation following severe head injury on neuropsychological testing

Despite recent advances in the management of severe head injury the mortality and morbidity remains high. Intracranial pressure (ICP) and cerebral perfusion pressure (CPP) are crucial parameters for the correct management at the intensive care unit, due to their therapeutic and prognostic importance. In addition, regional brain tissue oxygenation (ptiO2) seems to be of importance. While different studies demonstrated the impact of cerebral hypoxia on outcome (mortality), no data are available focusing on morbidity (neuropsychological deficits). Therefore, our study is carried out to demonstrate a possible relationship between amount of cerebral oxygenation during acute stage after severe head injury and neuropsychological outcome. Besides ICP and CPP, ptiO2 was monitored in 40 severely head injured patients during the ICU stay from the day of admission until day 10. Monitoring data were stored and amount of hypoxic episodes were calculated. Besides outcome using the Glasgow Outcome Scale neuropsychological testing was performed 2-3 years after injury. Analysing the quality of brain tissue oxygenation, a relationship to the performance in neuropsychological tests could be found. Patients with low brain tissue oxygenation had a worse outcome in neuropsychological testing, especially concerning intelligence and memory. Associated with these deficits patients showed a reduced performance in their profession. Our data suggest a possible predictive value of brain tissue oxygen on morbidity analysing neurocognitive function after head injury. This may implicate monitoring and treatment of cerebral hypoxia.

Liver tissue partial pressure of oxygen and carbon dioxide during partial hepatectomy

BACKGROUND

Data on tissue oxygen partial pressure (PtO2) and carbon dioxide partial pressure (PtCO2) in human liver tissue are limited. We set out to measure changes in liver PtO2 and PtCO2 during changes in ventilation and a 10 min period of ischaemia in patients undergoing liver resection using a multiple sensor (Paratrend Diametrics Medical Ltd, High Wycombe, UK).

METHODS

Liver tissue oxygenation was measured in anaesthetized patients undergoing liver resection using a sensor inserted under the liver capsule. PtO2 and PtCO2 were recorded with FIO2 values of 0.3 and 1.0, at end-tidal carbon dioxide partial pressures of 3.5 and 4.5 kPa and 10 min after the onset of liver ischaemia (Pringle manoeuvre).

RESULTS

Data are expressed as median (interquartile range). Increasing the FIO2 from 0.3 to 1.0 resulted in the PtO2 changing from 4.1 (2.6-5.4) to 4.6 (3.8-5.2) kPa, but this was not significant. During the 10 min period of ischaemia PtCO2 increased significantly (P<0.05) from 6.7 (5.8-7.0) to 11.5 (9.7-15.3) kPa and PtO2 decreased, but not significantly, from 4.3 (3.5-12.0) to 3.3 (0.9-4.1) kPa.

CONCLUSION

PtO2 and PtCO2 were measured directly using a Paratrend sensor in human liver tissue. During anaesthesia, changes in ventilation and liver blood flow caused predictable changes in PtCO2.

Cerebral Cortical Oxygenation: A Pilot Study

BACKGROUND

Cerebral hypoxia (cerebral cortical oxygenation [Pbro2] < 20 mm Hg) monitored by direct measurement has been shown in animal and small clinical studies to be associated with poor outcome. We present our preliminary results observing Pbro2 in patients with traumatic brain injury (TBI).

METHODS

A prospective observational cohort study was performed. Institutional review board approval was obtained. All patients with TBI who required measurement of intracranial pressure (ICP), cerebral perfusion pressure (CPP), and Pbro2 because of a Glasgow Coma Scale score < 8 were enrolled. Data sets (ICP, CPP, Pbro2, positive end-expiratory pressure (PEEP), Pao2, and Paco2) were recorded during routine manipulation. Episodes of cerebral hypoxia were compared with episodes without. Results are displayed as mean +/- SEM; t test, chi2, and Fisher's exact test were used to answer questions of interest.

RESULTS

One hundred eighty-one data sets were abstracted from 20 patients. Thirty-five episodes of regional cerebral hypoxia were identified in 14 patients. Compared with episodes of acceptable cerebral oxygenation, episodes of cerebral hypoxia were noted to be associated with a significantly lower mean Pao2 (144 +/- 14 vs. 165 +/- 8; p < 0.01) and higher mean PEEP (8.8 +/- 0.7 vs. 7.1 +/- 0.3; p < 0.01). Mean ICP and CPP measurements were similar between groups. In a univariate analysis, cerebral hypoxic episodes were associated with Pao2 < or = 100 mm Hg (p < 0.01) and PEEP > 5 cm H2O (p < 0.01), but not ICP > 20 mm Hg, CPP < or = 65 mm Hg, or Pac2 < or = 35 mm Hg.

CONCLUSION

Cerebral oxymetry is confirmed safe in the patient with multiple injuries with TBI. Occult cerebral hypoxia is present in the traumatic brain injured patient despite normal traditional measurements of cerebral perfusion. Further research is necessary to determine whether management protocols aimed at the prevention of cerebral cortical hypoxia will affect outcome.

Subcutaneous tissue oxygen tension after coronary revascularisation with and without cardiopulmonary bypass

Directly measured subcutaneous tissue oxygen tension reflects the adequacy of regional tissue oxygenation and influences wound infection and healing. We tested the hypothesis that off-pump coronary artery bypass would increase subcutaneous tissue oxygen tension by minimizing cardiopulmonary bypass-induced systemic inflammation. Ten consecutive patients scheduled for off-pump coronary artery bypass were compared with 10 undergoing conventional cardiopulmonary bypass. All patients had a tissue oxygen sensor implanted longitudinally into the subcutaneous tissue of the leg in the saphenous vein harvest wound. Data were collected from closure of the saphenous vein wound for 20 h postoperatively. Although more off-pump patients had only one coronary artery grafted, postoperative subcutaneous tissue oxygen tension was significantly higher in off-pump patients throughout the 20-h study. Absolute mean (SD) differences ranged from 2.3 kPa in the first 2 h [14.4 (2.3) vs. 12.1 (2.4) kPa in off-pump and cardiopulmonary bypass, respectively, p = 0.04] to 4.6 kPa at 8-10 h [14.0 (3.5) vs. 9.3 (2.7) kPa, p = 0.007]. In contrast, there were no significant differences in arterial oxygen tension values over this period. Mean arterial pressure and haemoglobin were transiently higher in off-pump patients at 8 h only. We conclude that postoperative subcutaneous tissue oxygen tension was higher for 20 h after off-pump compared with conventional cardiopulmonary bypass.

Trans-sodium crocetinate increases oxygen delivery to brain parenchyma in rats on oxygen supplementation

Trans-sodium crocetinate (TSC) is a vitamin A-analog that increases diffusivity of oxygen in aqueous solutions, including plasma. The current study is the initial investigation of the effects of TSC on oxygen delivery to brain. Adult male rats were intubated and ventilated with 21%, 60%, or 100% oxygen. A craniotomy was performed and a Licox rat brain tissue PO(2) probe inserted into parietal cortex. Rats were then administered intravenous infusions of either TSC or saline and brain tissue PO(2) values were recorded. TSC significantly increased brain tissue oxygen delivery. This effect was minimal in rats ventilated with normal air and substantial in rats on oxygen supplementation. Arterial blood gas parameters did not differ within groups. These results provide clear indication to study the utility of TSC in ameliorating hypoxic/ischemic insults in neurological disorders.

Measuring cerebral oxygenation during normobaric hyperoxia: a comparison of tissue microprobes, near-infrared spectroscopy, and jugular venous oximetry in head injury

We measured simultaneous changes in jugular venous oxygen saturation, brain tissue oxygen tension, and cerebral tissue oxygen index by using near-infrared spectroscopy during normobaric hyperoxygenation in eight severely brain-injured patients. Patients were ventilated at their baseline fraction of inspired oxygen (FIO(2)), followed by stepped changes in FIO(2) to 1.0, 0.6, and 0.02-0.05 less than baseline. There was an increase (P < 0.01) in jugular venous saturation (mean +/- SD) from a baseline value of 79% +/- 7% to 89% +/- 6% and 84% +/- 8% at an FIO(2) of 1.0 and 0.6, respectively. The changes in brain tissue oxygen tension were from a baseline of 30 +/- 5 mm Hg to 147 +/- 36 mm Hg and 63 +/- 6 mm Hg at an FIO(2) of 1.0 and 0.6, respectively (P < 0.01). The baseline tissue oxygen index was 78% +/- 3%, and this increased to 83% +/- 5% and 80% +/- 4% at an FIO(2) of 1.0 and 0.6, respectively. There was a reduction (P < 0.05) in tissue oxygen index to 76.2% +/- 3.0% when the FIO(2) was reduced to less than baseline. The changes in the three variables followed similar patterns but varied in their degree and speed of response. During brain injury, FIO(2) affects measured variables of cerebral oxygenation.