The physiology behind direct brain oxygen monitors and practical aspects of their use

Microcirculatory Impairment and Cerebral Injury in Hydrocephalus and the Effects of Cerebrospinal Fluid Diversion

BACKGROUND AND OBJECTIVES

Hydrocephalus is characterized by progressive enlargement of cerebral ventricles, resulting in impaired microvasculature and cerebral hypoperfusion. This study aimed to demonstrate the microvascular changes in hydrocephalic rats and the effects of cerebrospinal fluid (CSF) release on cerebral blood flow (CBF).

METHODS

On postnatal day 21 (P21), male Wistar rats were intracisternally injected with either a kaolin suspension or saline. On P47, Evan's ratio (ER) was measured using MRI. On P49, the arteriolar diameter and vascular density of the pia were quantified using a capillary video microscope. The CBF was measured using laser Doppler flowmetry. The expressions of NeuN and glial fibrillary acidic protein determined by immunochemical staining were correlated with the ER. The CBF and rotarod test performance were recorded before and after CSF release. The expressions of 4-hydroxynonenal (4-HNE) and c-caspase-3 were studied on P56.

RESULTS

Ventriculomegaly was induced to varying degrees, resulting in the stretching and abnormal narrowing of pial arterioles, which regressed with increasing ER. Quantitative analysis revealed significant decreases in the arteriolar diameter and vascular density in the hydrocephalic group compared with those in the control group. In addition, the CBF in the hydrocephalic group decreased to 30%-50% of that in the control group. In hydrocephalus, the neurons appear distorted, and the expression of 4-HNE and reactive astrogliosis increase in the cortex. After CSF was released, improvements in the CBF and rotarod test performance were inversely associated with the ER. In addition, the levels of 4-HNE and c-caspase-3 were further elevated.

CONCLUSION

Rapid ventricular dilatation is associated with severe microvascular distortion, vascular regression, cortical hypoperfusion, and cellular changes that impair the recovery of CBF and motor function after CSF release. Moreover, CSF release may induce reperfusion injury. This pathophysiology should be taken into account when treating hydrocephalus.

Monitoring of Brain Tissue Oxygen Tension in Cardiac Arrest: a Translational Systematic Review from Experimental to Clinical Evidence

BACKGROUND

Cardiac arrest (CA) is a sudden event that is often characterized by hypoxic-ischemic brain injury (HIBI), leading to significant mortality and long-term disability. Brain tissue oxygenation (PbtO2) is an invasive tool for monitoring brain oxygen tension, but it is not routinely used in patients with CA because of the invasiveness and the absence of high-quality data on its effect on outcome. We conducted a systematic review of experimental and clinical evidence to understand the role of PbtO2 in monitoring brain oxygenation in HIBI after CA and the effect of targeted PbtO2 therapy on outcomes.

METHODS

The search was conducted using four search engines (PubMed, Scopus, Embase, and Cochrane), using the Boolean operator to combine mesh terms such as PbtO2, CA, and HIBI.

RESULTS

Among 1,077 records, 22 studies were included (16 experimental studies and six clinical studies). In experimental studies, PbtO2 was mainly adopted to assess the impact of gas exchanges, drugs, or systemic maneuvers on brain oxygenation. In human studies, PbtO2 was rarely used to monitor the brain oxygen tension in patients with CA and HIBI. PbtO2 values had no clear association with patients' outcomes, but in the experimental studies, brain tissue hypoxia was associated with increased inflammation and neuronal damage.

CONCLUSIONS

Further studies are needed to validate the effect and the threshold of PbtO2 associated with outcome in patients with CA, as well as to understand the physiological mechanisms influencing PbtO2 induced by gas exchanges, drug administration, and changes in body positioning after CA.

The Role of Brain Tissue Oxygenation Monitoring in the Management of Subarachnoid Hemorrhage: A Scoping Review

Monitoring of brain tissue oxygenation (PbtO2) is an important component of multimodal monitoring in traumatic brain injury. Over recent years, use of PbtO2 monitoring has also increased in patients with poor-grade subarachnoid hemorrhage (SAH), particularly in those with delayed cerebral ischemia. The aim of this scoping review was to summarize the current state of the art regarding the use of this invasive neuromonitoring tool in patients with SAH. Our results showed that PbtO2 monitoring is a safe and reliable method to assess regional cerebral tissue oxygenation and that PbtO2 represents the oxygen available in the brain interstitial space for aerobic energy production (i.e., the product of cerebral blood flow and the arterio-venous oxygen tension difference). The PbtO2 probe should be placed in the area at risk of ischemia (i.e., in the vascular territory in which cerebral vasospasm is expected to occur). The most widely used PbtO2 threshold to define brain tissue hypoxia and initiate specific treatment is between 15 and 20 mm Hg. PbtO2 values can help identify the need for or the effects of various therapies, such as hyperventilation, hyperoxia, induced hypothermia, induced hypertension, red blood cell transfusion, osmotic therapy, and decompressive craniectomy. Finally, a low PbtO2 value is associated with a worse prognosis, and an increase of the PbtO2 value in response to treatment is a marker of good outcome.

The intracranial compartmental syndrome: a proposed model for acute brain injury monitoring and management

For decades, one of the main targets in the management of severe acute brain injury (ABI) has been intracranial hypertension (IH) control. However, the determination of IH has suffered variations in its thresholds over time without clear evidence for it. Meanwhile, progress in the understanding of intracranial content (brain, blood and cerebrospinal fluid) dynamics and recent development in monitoring techniques suggest that targeting intracranial compliance (ICC) could be a more reliable approach rather than guiding actions by predetermined intracranial pressure values. It is known that ICC impairment forecasts IH, as intracranial volume may rapidly increase inside the skull, a closed bony box with derisory expansibility. Therefore, an intracranial compartmental syndrome (ICCS) can occur with deleterious brain effects, precipitating a reduction in brain perfusion, thereby inducing brain ischemia. The present perspective review aims to discuss the ICCS concept and suggest an integrative model for the combination of modern invasive and noninvasive techniques for IH and ICC assessment. The theory and logic suggest that the combination of multiple ancillary methods may enhance ICC impairment prediction, pointing proactive actions and improving patient outcomes.

The use of 100% compared to 50% oxygen during ineffective experimental cardiopulmonary resuscitation improves brain oxygenation

INTRODUCTION

Perfusion pressure and chest compression quality are generally considered key determinants of brain oxygenation during cardiopulmonary resuscitation (CPR) and the impact of oxygen administration is less clear. We compared ventilation with 100% and 50% oxygen during ineffective manual chest compressions and hypothesized that 100% oxygen would improve brain oxygenation.

METHODS

Ventricular fibrillation (VF) was induced electrically in anaesthetized pigs and left untreated for 5 minutes, followed by randomization to ineffective manual CPR with ventilation of 50% or 100% oxygen. The first defibrillation was performed 10 minutes after induction of VF, and CPR continued with mechanical chest compressions (LUCAS2™) and defibrillation every 2 minutes until 36 minutes or return of spontaneous circulation (ROSC). Brain oxygenation was measured with near-infrared spectroscopy (rSO₂) and invasive brain tissue oxygen (PbtO₂) with a probe (NEUROVENT-PTO, RAUMEDIC) inserted into frontal brain tissue. Cerebral oxygenation was compared between groups with Mann-Whitney U tests and linear mixed models.

RESULTS

Twenty-eight pigs were included in the study: 14 subjects in each group. During ineffective chest compressions relative PbtO₂ was higher in the group ventilated with 100% compared to 50% oxygen (5.2 mmHg [1.4-20.5] vs 2.2 [0.8-6.8], p = 0.001), but there was no difference in rSO₂ (22% [16-28] vs 18 [15-25], p = 0.090). The use of 50% or 100% oxygen showed no difference in relative PbtO₂ (p = 1.00) and rSO₂ (p = 0.206) during mechanical CPR.

CONCLUSIONS

The use of 100% compared to 50% oxygen during ineffective manual CPR improved brain oxygenation measured invasively in brain tissue, but there was no difference in rSO₂.

Multimodal monitoring: practical recommendations (dos and don'ts) in challenging situations and uncertainty

With the advancements in modern medicine, new methods are being developed to monitor patients in the intensive care unit. Different modalities evaluate different aspects of the patient's physiology and clinical status. The complexity of these modalities often restricts their use to the realm of clinical research, thereby limiting their use in the real world. Understanding their salient features and their limitations can aid physicians in interpreting the concomitant information provided by multiple modalities to make informed decisions that may affect clinical care and outcomes. Here, we present a review of the commonly used methods in the neurological intensive care unit with practical recommendations for their use.

Effects of hypothermia and hypothermia combined with hypocapnia on cerebral tissue oxygenation in piglets

BACKGROUND

Hypothermia and its combination with hypocapnia are frequently associated with anesthesia.

AIMS

The goal was to investigate the effects of hypothermia and hypothermia combined with hypocapnia (hypothermia-hypocapnia) on cerebral tissue oxygenation in anesthetized piglets.

METHODS

Twenty anesthetized piglets were randomly allocated to hypothermia (n = 10) or hypothermia-hypocapnia (n = 10). Cerebral monitoring comprised a tissue oxygen partial pressure (PtO₂ ), a laser Doppler probe, and a near-infrared spectroscopy sensor, measuring regional oxygen saturation (rSO₂ ). After baseline recordings, hypothermia (35.5-36.0°C) with or without hypocapnia (target PaCO₂ : 28-30 mm Hg) was induced. Once treatment goals were achieved (Tr0), they were maintained for 30 minutes (Tr30).

RESULTS

No changes in PtO₂ but a significant increase in rSO₂ (Tr0 (mean difference 8.9[95% CI for difference3.99 to 13.81], P < .001); Tr30 (10.8[6.20 to 15.40], P < .001)) were detected during hypothermia. With hypothermia-hypocapnia, a decrease in PtO₂ (Tr0 (-3.2[-6.01 to -0.39], P = .021; Tr30 (-3.3[-5.8 to -0.80], P = .006)) and no significant changes in rSO₂ occurred. Cerebral blood flow decreased significantly from baseline to Tr0 independently of treatment (-0.89[-0.18 to -0.002], P = .042), but this was more consistently observed with hypothermia-hypocapnia.

CONCLUSIONS

The hypothermia-induced reduction in oxygen delivery was compensated by lowered metabolic demand. However, hypothermia was not able to compensate for an additional reduction in oxygen delivery caused by simultaneous hypocapnia. This resulted in a PtO₂ drop, which was not reflected by a downshift in rSO₂ .

Dental Pulp Stem Cell-Derived Factors Alleviate Subarachnoid Hemorrhage-Induced Neuroinflammation and Ischemic Neurological Deficits

Aneurysmal subarachnoid hemorrhage (aSAH), characterized by the extravasation of blood into the subarachnoid space caused by an intracranial aneurysm rupture, may lead to neurocognitive impairments and permanent disability and usually carries poor outcome. Dental or gingiva-derived stem cells have been shown to contribute to immune modulation and neuroregeneration, but the underlying mechanisms are unclear. In the present study, we sought to investigate whether dental pulp stem cells (DPSCs) secrete certain factor(s) that can ameliorate the neural damage and other manifestations in a rat aSAH model. Twenty-four hours after the induction of aSAH, microthrombosis, cortical vasoconstriction, and the decrease in microcirculation and tissue oxygen pressure were detected. Intrathecal administration of DPSC-derived conditioned media (DPSC-CM) ameliorated aSAH-induced vasoconstriction, neuroinflammation, and improved the oxygenation in the injured brain. Rotarod test revealed that the aSAH-induced cognitive and motor impairments were significantly improved by this DPSC-CM administration. Cytokine array indicated the major constituent of DPSC-CM was predominantly insulin growth factor-1 (IGF-1). Immunohistochemistry staining of injured brain tissue revealed the robust increase in Iba1-positive cells that were also ameliorated by DPSC-CM administration. Antibody-mediated neutralization of IGF-1 moderately deteriorated the rescuing effect of DPSC-CM on microcirculation, Iba1-positive cells in the injured brain area, and the cognitive/motor impairments. Taken together, the DPSC-derived secretory factors showed prominent therapeutic potential for aSAH. This therapeutic efficacy may include improvement of microcirculation, alleviation of neuroinflammation, and microglial activation; partially through IGF-1-dependent mechanisms.

The physiological determinants of near-infrared spectroscopy-derived regional cerebral oxygenation in critically ill adults

Background

To maintain adequate oxygen delivery to tissue, resuscitation of critically ill patients is guided by assessing surrogate markers of perfusion. As there is no direct indicator of cerebral perfusion used in routine critical care, identifying an accurate strategy to monitor brain perfusion is paramount. Near-infrared spectroscopy (NIRS) is a non-invasive technique to quantify regional cerebral oxygenation (rSO₂) that has been used for decades during cardiac surgery which has led to targeted algorithms to optimize rSO₂ being developed. However, these targeted algorithms do not exist during critical care, as the physiological determinants of rSO₂ during critical illness remain poorly understood.

Materials and methods

This prospective observational study was an exploratory analysis of a nested cohort of patients within the CONFOCAL study (NCT02344043) who received high-fidelity vital sign monitoring. Adult patients (≥ 18 years) admitted < 24 h to a medical/surgical intensive care unit were eligible if they had shock and/or required mechanical ventilation. Patients underwent rSO₂ monitoring with the FORESIGHT oximeter for 24 h, vital signs were concurrently recorded, and clinically ordered arterial blood gas samples and hemoglobin concentration were also documented. Simultaneous multiple linear regression was performed using all available predictors, followed by model selection using the corrected Akaike information criterion (AICc).

Results

Our simultaneous multivariate model included age, heart rate, arterial oxygen saturation, mean arterial pressure, pH, partial pressure of oxygen, partial pressure of carbon dioxide (PaCO₂), and hemoglobin concentration. This model accounted for a significant proportion of variance in rSO₂ (R² = 0.58, p < 0.01) and was significantly associated with PaCO₂ (p < 0.05) and hemoglobin concentration (p < 0.01). Our selected regression model using AICc accounted for a significant proportion of variance in rSO₂ (R² = 0.54, p < 0.01) and was significantly related to age (p < 0.05), PaCO₂ (p < 0.01), hemoglobin (p < 0.01), and heart rate (p < 0.05).

Conclusions

Known and established physiological determinants of oxygen delivery accounted for a significant proportion of the rSO₂ signal, which provides evidence that NIRS is a viable modality to assess cerebral oxygenation in critically ill adults. Further elucidation of the determinants of rSO₂ has the potential to develop a NIRS-guided resuscitation algorithm during critical illness.

Trial registration

This trial is registered on clinicaltrials.gov (Identifier: NCT02344043), retrospectively registered January 8, 2015.

Electronic supplementary material

The online version of this article (10.1186/s40635-019-0247-0) contains supplementary material, which is available to authorized users.

Impaired microcirculation after subarachnoid hemorrhage in an in vivo animal model

The influence of aneurysmal subarachnoid hemorrhage (SAH) on brain microcirculation has not yet been systematically investigated. We established an animal model to examine (1) the brain surface microcirculation (2) the influences of cerebrospinal fluid (CSF) from aneurysmal SAH on the brain surface microcirculation. A rat SAH model was induced by injection of autologous arterial blood into the cisterna magnum, and the brain surface microcirculation was evaluated by a capillary videoscope with craniotomy at the fronto-parietal region. CSF from SAH rats and SAH patients was applied on the brain surface of naïve rats to assess the resulting microcirculatory changes. In the SAH rats, diffuse constriction of cortical arterioles within 24 hours of SAH was observed. Similar patterns of microcirculation impairment were induced on normal rat brain surfaces via application of CSF from SAH rats and SAH patients. Furthermore, the proportion of subjects with arteriolar vasoconstriction was significantly higher in the group of SAH patients with delayed ischemic neurological deficits (DIND) than in those without DIND (p < 0.001). This study demonstrated impaired microcirculation on brain surface arterioles in a rat model of SAH. CSF from SAH rats and patients was responsible for impairment of brain surface microcirculation.

The influence of norepinephrine and phenylephrine on cerebral perfusion and oxygenation during propofol-remifentanil and propofol-remifentanil-dexmedetomidine anaesthesia in piglets

Background

Vasopressors are frequently used to increase blood pressure in order to ensure sufficient cerebral perfusion and oxygenation (CPO) during hypotensive periods in anaesthetized patients. Efficacy depends both on the vasopressor and anaesthetic protocol used. Propofol–remifentanil total intravenous anaesthesia (TIVA) is common in human anaesthesia, and dexmedetomidine is increasingly used as adjuvant to facilitate better haemodynamic stability and analgesia. Little is known of its interaction with vasopressors and subsequent effects on CPO. This study investigates the CPO response to infusions of norepinephrine and phenylephrine in piglets during propofol–remifentanil and propofol–remifentanil–dexmedetomidine anaesthesia. Sixteen healthy female piglets (25–34 kg) were randomly allocated into a two-arm parallel group design with either normal blood pressure (NBP) or induced low blood pressure (LBP). Anaesthesia was induced with propofol without premedication and maintained with propofol–remifentanil TIVA, and finally supplemented with continuous infusion of dexmedetomidine. Norepinephrine and phenylephrine were infused in consecutive intervention periods before and after addition of dexmedetomidine. Cerebral perfusion measured by laser speckle contrast imaging was related to cerebral oxygenation as measured by an intracerebral Licox probe (partial pressure of oxygen) and transcranial near infrared spectroscopy technology (NIRS) (cerebral oxygen saturation).

Results

During propofol–remifentanil anaesthesia, increases in blood pressure by norepinephrine and phenylephrine did not change cerebral perfusion significantly, but cerebral partial pressure of oxygen (Licox) increased following vasopressors in both groups and increases following norepinephrine were significant (NBP: P = 0.04, LBP: P = 0.02). In contrast, cerebral oxygen saturation (NIRS) fell significantly in NBP following phenylephrine (P = 0.003), and following both norepinephrine (P = 0.02) and phenylephrine (P = 0.002) in LBP. Blood pressure increase by both norepinephrine and phenylephrine during propofol–remifentanil–dexmedetomidine anaesthesia was not followed by significant changes in cerebral perfusion. Licox measures increased significantly following both vasopressors in both groups, whereas the decreases in NIRS measures were only significant in the NBP group.

Conclusions

Cerebral partial pressure of oxygen measured by Licox increased significantly in concert with the vasopressor induced increases in blood pressure in healthy piglets with both normal and low blood pressure. Cerebral oxygenation assessed by intracerebral Licox and transcranial NIRS showed opposing results to vasopressor infusions.

Electronic supplementary material

The online version of this article (10.1186/s13028-018-0362-z) contains supplementary material, which is available to authorized users.

Aspects on the Physiological and Biochemical Foundations of Neurocritical Care

Neurocritical care (NCC) is a branch of intensive care medicine characterized by specific physiological and biochemical monitoring techniques necessary for identifying cerebral adverse events and for evaluating specific therapies. Information is primarily obtained from physiological variables related to intracranial pressure (ICP) and cerebral blood flow (CBF) and from physiological and biochemical variables related to cerebral energy metabolism. Non-surgical therapies developed for treating increased ICP are based on knowledge regarding transport of water across the intact and injured blood-brain barrier (BBB) and the regulation of CBF. Brain volume is strictly controlled as the BBB permeability to crystalloids is very low restricting net transport of water across the capillary wall. Cerebral pressure autoregulation prevents changes in intracranial blood volume and intracapillary hydrostatic pressure at variations in arterial blood pressure. Information regarding cerebral oxidative metabolism is obtained from measurements of brain tissue oxygen tension (PbtO2) and biochemical data obtained from intracerebral microdialysis. As interstitial lactate/pyruvate (LP) ratio instantaneously reflects shifts in intracellular cytoplasmatic redox state, it is an important indicator of compromised cerebral oxidative metabolism. The combined information obtained from PbtO2, LP ratio, and the pattern of biochemical variables reveals whether impaired oxidative metabolism is due to insufficient perfusion (ischemia) or mitochondrial dysfunction. Intracerebral microdialysis and PbtO2 give information from a very small volume of tissue. Accordingly, clinical interpretation of the data must be based on information of the probe location in relation to focal brain damage. Attempts to evaluate global cerebral energy state from microdialysis of intraventricular fluid and from the LP ratio of the draining venous blood have recently been presented. To be of clinical relevance, the information from all monitoring techniques should be presented bedside online. Accordingly, in the future, the chemical variables obtained from microdialysis will probably be analyzed by biochemical sensors.

Pathophysiology and Treatment of Severe Traumatic Brain Injuries in Children

Traumatic brain injuries (TBIs) in children are a major cause of morbidity and mortality worldwide. Severe TBIs account for 15,000 admissions annually and a mortality rate of 24% in children in the United States. The purpose of this article is to explore pathophysiologic events, examine monitoring techniques, and explain current treatment modalities and nursing care related to caring for children with severe TBI. The primary injury of a TBI is because of direct trauma from an external force, a penetrating object, blast waves, or a jolt to the head. Secondary injury occurs because of alterations in cerebral blood flow, and the development of cerebral edema leads to necrotic and apoptotic cellular death after TBI. Monitoring focuses on intracranial pressure, cerebral oxygenation, cerebral edema, and cerebrovascular injuries. If abnormalities are identified, treatments are available to manage the negative effects caused to the cerebral tissue. The mainstay treatments are hyperosmolar therapy; temperature control; cerebrospinal fluid drainage; barbiturate therapy; decompressive craniectomy; analgesia, sedation, and neuromuscular blockade; and antiseizure prophylaxis.

Can Therapeutic Hypothermia Be Guided by Advanced Neuromonitoring in Neurocritical Care Patients? A Review

The impact of therapeutic hypothermia (TH) on long-term neurological outcome is still controversial. Data on the effects of TH on brain homeostasis are mostly derived from experimental research. Invasive multimodal neuromonitoring techniques may provide additional insight into pathophysiological changes associated with primary or secondary brain injury in humans. In this study we describe the principles of multimodal neuromonitoring and its potential in the clinical setting of TH. We call for more research using multimodal neuromonitoring techniques in patients undergoing TH to optimize cooling and rewarming strategies.

Standards of scoring, monitoring, and parameter targeting in German neurocritical care units: a national survey

BACKGROUND

Optimal management of physiological parameters in neurological/neurosurgical intensive care units (NICUs) is largely unclear as high-quality evidence is lacking. The aim of this survey was to investigate if standards exist in the use of clinical scores, systemic and cerebral monitoring and the targeting of physiology values and in what way this affects clinical management in German NICUs.

METHODS

National survey, on-line anonymized questionnaire. German departments stating to run a neurological, neurosurgical or interdisciplinary neurological/neurosurgical intensive care unit were identified by a web-based search of all German hospitals and contacted via email.

RESULTS

Responses from 78 German NICUs were obtained. Of 19 proposed clinical/laboratory/radiological scores only 5 were used regularly by >60 %. Bedside neuromonitoring (NM) predominantly consisted of transcranial Doppler sonography (94 %), electroencephalography (92 %) and measurement of intracranial pressure (ICP) (90 %), and was installed if patients had or were threatened by elevated ICP (86 %), had specific diseases like subarachnoid hemorrhage (51 %) or were comatose (35 %). Although mean trigger values for interventions complied with guidelines or wide-spread customs, individual trigger values varied widely, e.g., for hyperglycemia (maximum blood glucose between 120 and 250 mg/dl) or for anemia (minimum hemoglobin values between 5 and 10 g/dl).

CONCLUSIONS

Although apparently aiming for standardization in neurocritical care, German NICUs show substantial differences in NM and monitoring-associated interventions. In terms of scoring and monitoring methods, German NICUs seem to be quite conservative. These survey results suggest a need of prospective and randomized interventional trials in neurocritical care to help define standards and target values.

Brain tissue oxygen monitoring identifies cortical hypoxia and thalamic hyperoxia after experimental cardiac arrest in rats

BACKGROUND

Optimization of cerebral oxygenation after pediatric cardiac arrest (CA) may reduce neurological damage associated with the post-CA syndrome. We hypothesized that important alterations in regional partial pressure of brain tissue oxygen (PbO2) occur after resuscitation from CA and that clinically relevant interventions such as hyperoxia and blood pressure augmentation would influence PbO2.

METHODS

Cortical and thalamic PbO2 were monitored in immature rats subjected to asphyxial CA (9 or 12 min asphyxia) and sham-operated rats using oxygen sensors.

RESULTS

Thalamus and cortex showed similar baseline PbO2. Postresuscitation, there was early and sustained cortical hypoxia in an insult-duration dependent fashion. In contrast, thalamic PbO2 initially increased fourfold and afterwards returned to baseline values. PbO2 level was dependent on the fraction of inspired O2, and the response to oxygen was more pronounced after a 9 vs. 12 min CA. After a 12 min CA, PbO2 was modestly affected by blood pressure augmentation using epinephrine in the thalamus but not in the cortex.

CONCLUSION

After asphyxial pediatric CA, there is marked regional variability of cerebral oxygenation. Cortical hypoxia is pronounced and appears early, whereas thalamic hyperoxia is followed by normoxia. Compromised PbO2 in the cortex may represent a relevant and clinically measurable therapeutic target aimed at improving neurological outcome after pediatric CA.

Physiological monitoring of the severe traumatic brain injury patient in the intensive care unit

Traumatic brain injury (TBI) is a major cause of morbidity and mortality worldwide. Despite encouraging animal research, pharmacological agents and neuroprotectants have disappointed in the clinical environment. Current TBI management therefore is directed towards identification, prevention, and treatment of secondary cerebral insults that are known to exacerbate outcome after injury. This strategy is based on a variety of monitoring techniques that include the neurological examination, imaging, laboratory analysis, and physiological monitoring of the brain and other organ systems used to guide therapeutic interventions. Recent clinical series suggest that TBI management informed by multimodality monitoring is associated with improved patient outcome, in part because care is provided in a patient-specific manner. In this review we discuss physiological monitoring of the brain after TBI and the emerging field of neurocritical care bioinformatics.

Parenchymal brain oxygen monitoring in the neurocritical care unit

Patients admitted to the neurocritical care unit (NCCU) often have serious conditions that can be associated with high morbidity and mortality. Pharmacologic agents or neuroprotectants have disappointed in the clinical environment. Current NCCU management therefore is directed toward identification, prevention, and treatment of secondary cerebral insults that evolve over time and are known to aggravate outcome. This strategy is based on a variety of monitoring techniques including use of intraparenchymal monitors. This article reviews parenchymal brain oxygen monitors, including the available technologies, practical aspects of use, the physiologic rationale behind their use, and patient management based on brain oxygen.

Invasive and noninvasive multimodal bedside monitoring in subarachnoid hemorrhage: a review of techniques and available data

Delayed-cerebral ischemia is a major cause of morbidity and mortality in the setting of aneurysmal subarachnoid hemorrhage. Despite extensive research efforts and a breadth of collective clinical experience, accurate diagnosis of vasospasm remains difficult, and effective treatment options are limited. Classically, diagnosis has focused on imaging assessment of the cerebral vasculature. Recently, invasive and noninvasive bedside techniques designed to characterize relevant hemodynamic and metabolic alterations have gained substantial attention. Such modalities include microdialysis, brain tissue oxygenation, jugular bulb oximetry, thermal diffusion cerebral blood flow, and near-infrared spectroscopy. This paper reviews these modalities and examines data pertinent to the diagnosis and management of cerebral vasospasm.

Are initial radiographic and clinical scales associated with subsequent intracranial pressure and brain oxygen levels after severe traumatic brain injury?

BACKGROUND

Prediction of clinical course and outcome after severe traumatic brain injury (TBI) is important.

OBJECTIVE

To examine whether clinical scales (Glasgow Coma Scale [GCS], Injury Severity Score [ISS], and Acute Physiology and Chronic Health Evaluation II [APACHE II]) or radiographic scales based on admission computed tomography (Marshall and Rotterdam) were associated with intensive care unit (ICU) physiology (intracranial pressure [ICP], brain tissue oxygen tension [PbtO2]), and clinical outcome after severe TBI.

METHODS

One hundred one patients (median age, 41.0 years; interquartile range [26-55]) with severe TBI who had ICP and PbtO2 monitoring were identified. The relationship between admission GCS, ISS, APACHE II, Marshall and Rotterdam scores and ICP, PbtO2, and outcome was examined by using mixed-effects models and logistic regression.

RESULTS

Median (25%-75% interquartile range) admission GCS and APACHE II without GCS scores were 3.0 (3-7) and 11.0 (8-13), respectively. Marshall and Rotterdam scores were 3.0 (3-5) and 4.0 (4-5). Mean ICP and PbtO2 during the patients' ICU course were 15.5 ± 10.7 mm Hg and 29.9 ± 10.8 mm Hg, respectively. Three-month mortality was 37.6%. Admission GCS was not associated with mortality. APACHE II (P = .003), APACHE-non-GCS (P = .004), Marshall (P < .001), and Rotterdam scores (P < .001) were associated with mortality. No relationship between GCS, ISS, Marshall, or Rotterdam scores and subsequent ICP or PbtO2 was observed. The APACHE II score was inversely associated with median PbtO2 (P = .03) and minimum PbtO2 (P = .008) and had a stronger correlation with amount of time of reduced PbtO2.

CONCLUSION

Following severe TBI, factors associated with outcome may not always predict a patient's ICU course and, in particular, intracranial physiology.

The relationship between intracranial pressure and brain oxygenation in children with severe traumatic brain injury

BACKGROUND

Intracranial pressure (ICP) monitoring is a cornerstone of care for severe traumatic brain injury (TBI). Management of ICP can help ensure adequate cerebral blood flow and oxygenation. However, studies indicate that brain hypoxia may occur despite normal ICP and the relationship between ICP and brain oxygenation is poorly defined. This is particularly important for children in whom less is known about intracranial dynamics.

OBJECTIVE

To examine the relationship between ICP and partial pressure of brain tissue oxygen (PbtO2) in children with severe TBI (Glasgow Coma Scale score ≤ 8) admitted to Red Cross War Memorial Children's Hospital, Cape Town.

METHODS

The relationship between time-linked hourly and high-frequency ICP and PbtO2 data was examined using correlation, regression, and generalized estimating equations. Thresholds for ICP were examined against reduced PbtO2 using age bands and receiver-operating characteristic curves.

RESULTS

Analysis using more than 8300 hourly (n = 75) and 1 million high-frequency data points (n = 30) demonstrated a weak relationship between ICP and PbtO2 (r = 0.05 and r = 0.04, respectively). No critical ICP threshold for low PbtO2 was identified. Individual patients revealed a strong relationship between ICP and PbtO2 at specific times, but different relationships were evident over longer periods.

CONCLUSION

The relationship between ICP and PbtO2 appears complex, and several factors likely influence both variables separately and in combination. Although very high ICP is associated with reduced PbtO2, in general, absolute ICP has a poor relationship with PbtO2. Because reduced PbtO2 is independently associated with poor outcome, a better understanding of ICP and PbtO2 management in pediatric TBI seems to be needed.

Physiologic Effects of Xenon in Xenon-CT Cerebral Blood Flow Studies on Comatose Patients

Despite more than 30 years of clinical use, questions remain about the safety of xenon gas in Xenon-CT cerebral blood flow (XeCTCBF) studies. In particular, xenon's effect on brain oxygen (PbtO2) in comatose patients is not well defined. Our objective was to assess the effect of a 4.5-min inhalation of 28 % stable xenon on several physiologic variables, including intracranial pressure (ICP), cerebral perfusion pressure (CPP), and PbtO2 in comatose patients (Glasgow Coma Scale [GCS] ≤ 8). Thirty-seven comatose patients who underwent 73 XeCTCBF studies were identified retrospectively from a prospective observational database. Changes in MAP, HR, SaO2, EtCO2, ICP, CPP, and PbtO2 measured at the start of xenon administration and every minute for 5 min thereafter were assessed. The maximum change in each variable also was determined for each scan to tabulate clinically relevant changes. Statistically, but not clinically significant changes in MAP, HR, and EtCO2 were seen. Xenon had no effect on ICP, and a small, but clinically insignificant decrease in CPP and PbtO2, was observed. There was a varied response to xenon in most measured variables. Clinically significant changes in each were infrequent, and readily reversed with the cessation of the gas. We conclude that xenon does not appear to have a clinically significant effect on ICP, CPP, and PbtO2 and so appears safe to evaluate cerebral blood flow in comatose patients.

Comparison between cerebral tissue oxygen tension and energy metabolism in experimental subdural hematoma

BACKGROUND

An experimental swine model (n = 7) simulating an acute subdural hematoma (ASDH) was employed (1) to explore the relation between the brain tissue oxygenation (PbtO(2)) and the regional cerebral energy metabolism as obtained by microdialysis, and (2) to define the lowest level of PbtO(2) compatible with intact energy metabolism.

METHODS

ASDH was produced by infusion of 7 ml of autologous blood (infusion rate 0.5 ml/min) by a catheter placed subdurally. PbtO(2) and microdialysis probes were placed symmetrically in the injured ("bad-side") and non-injured ("good-side") hemispheres. Intracranial pressure (ICP) was monitored in the "good-side."

RESULTS

ICP, cerebral perfusion pressure (CPP), PbtO(2), glucose, lactate, pyruvate, lactate-pyruvate ratio (LP ratio), glutamate, and glycerol were recorded at baseline (60 min) and post trauma (360 min). After the creation of the ASDH, PbtO(2) decreased significantly in both the hemispheres (P < 0.001). No significant difference was found between the sides post trauma. The LP ratio, glutamate, and glycerol in the "bad-side" increased significantly over the "good-side" where the values remained within the normal limits. A PbtO(2) value below approximately 25 mmHg was found to be associated with disturbed energy metabolism in the "bad-side" but not in the "good-side." No correlation was found between the LP ratio and PbtO(2) in either hemisphere.

CONCLUSIONS

PbtO(2) monitoring accurately describes tissue oxygenation but does not disclose whether the oxygen delivery is sufficient for maintaining cerebral energy metabolism. Accordingly, it may not be possible to define a threshold level for PbtO(2) below which energy failure and permanent tissue damage occurs.

Research and technology in neurocritical care

The daily practice of neurointensivists focuses on the recognition of subtle changes in the neurological examination, interactions between the brain and systemic derangements, and brain physiology. Common alterations such as fever, hyperglycemia, and hypotension have different consequences in patients with brain insults compared with patients of general medical illness. Various technologies have become available or are currently being developed. The session on "research and technology" of the first neurocritical care research conference held in Houston in September of 2009 was devoted to the discussion of the current status, and the research role of state-of-the art technologies in neurocritical patients including multi-modality neuromonitoring, biomarkers, neuroimaging, and "omics" research (proteomix, genomics, and metabolomics). We have summarized the topics discussed in this session. We have provided a brief overview of the current status of these technologies, and put forward recommendations for future research applications in the field of neurocritical care.

Monitoring and detection of vasospasm II: EEG and invasive monitoring

Several non-invasive and invasive modalities have been used to monitor patients for cerebral ischemia after subarachnoid hemorrhage. A literature search was performed to identify original research studies testing monitors that may be used in addition to the standard measures of brain function and cerebral blood flow. Fifty observational studies were identified that evaluated the role of electroencephalography, brain tissue oxygenation monitoring, cerebral microdialysis, thermal diffusion flowmetry, or near-infrared spectroscopy in patients after subarachnoid hemorrhage.

Intracranial monitoring in traumatic brain injury

PURPOSE OF REVIEW

This review focuses on the role of various intracranial monitoring technologies in the diagnosis and therapy of traumatic brain injury injury.

RECENT FINDINGS

There exist many controversial points as to the utility of different intracranial monitoring with regard to improvement of outcomes from severe traumatic brain injury. Most recent studies are confirming that the use of multiple modalities in the neurological ICU setting may offer promising results.

SUMMARY

Increased adherence to guideline-based and protocol-driven neurointensive care utilizing multimodality in monitoring technology for patients with severe traumatic brain injury is likely to give clinicians increased insight into the elusive mechanisms underlying the complex pathophysiology of this disease process and may further improve outcomes in this patient population.

Detection of Cerebral Compromise With Multimodality Monitoring in Patients With Subarachnoid Hemorrhage

BACKGROUND:

Studies in traumatic brain injury suggest that monitoring techniques such as brain tissue oxygen (Pbto2) and cerebral microdialysis may complement conventional intracranial pressure (ICP) and cerebral perfusion pressure (CPP) measurements.

OBJECTIVE:

In this study of poor-grade (Hunt and Hess grade IV and V) subarachnoid hemorrhage (SAH) patients, we examined the prevalence of brain hypoxia and brain energy dysfunction in the presence of normal and abnormal ICP and CPP.

METHODS:

SAH patients who underwent multimodal neuromonitoring and cerebral microdialysis were studied. We examined the frequency of brain hypoxia and energy dysfunction in different ICP and CPP ranges and the relationship between Pbto2 and the lactate/pyruvate ratio (LPR).

RESULTS:

A total of 2394 samples from 19 patients were analyzed. There were 149 samples with severe brain hypoxia (Pbto2 ≤10 mm Hg) and 347 samples with brain energy dysfunction (LPR &gt;40). The sensitivities of abnormal ICP or CPP for elevated LPR and reduced Pbto2 were poor (21.2% at best), and the LPR or Pbto2 was abnormal in many instances when ICP or CPP was normal. Severe brain hypoxia was often associated with an LPR greater than 40 (86% of samples). In contrast, mild brain hypoxia (≤20 mm Hg) and severe brain hypoxia were observed in only 53% and 36% of samples with brain energy dysfunction, respectively.

CONCLUSION:

Our data demonstrate that ICP and CPP monitoring may not always detect episodes of cerebral compromise in SAH patients. Our data suggest that several complementary monitors may be needed to optimize the care of poor-grade SAH patients.