Cerebral oxygen and microdialysis monitoring during aneurysm surgery: effects of blood pressure, cerebrospinal fluid drainage, and temporary clipping on infarction

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.

Usefulness of Intraoperative Neurophysiological Monitoring During the Clipping of Unruptured Intracranial Aneurysm: Diagnostic Efficacy and Detailed Protocol

Background: Intraoperative neurophysiological monitoring (IONM) has been widely applied in brain vascular surgeries to reduce postoperative neurologic deficit (PND). This study aimed to investigate the effect of IONM during clipping of unruptured intracranial aneurysms (UIAs).

Methods: Between January 2013 and August 2020, we enrolled 193 patients with 202 UIAs in the N group (clipping without IONM) and 319 patients with 343 UIAs in the M group (clipping with IONM). Patients in the M group were intraoperatively monitored for motor evoked potentials (MEPs) and somatosensory evoked potentials (SSEPs). Irreversible evoked potential (EP) change was defined as EP deterioration that did not recover until surgery completion. Sustained PND was defined as neurological symptoms lasting for more than one postoperative month.

Results: Ten (3.1%) and 13 (6.7%) in the M and N groups, respectively, presented with PND. Compared with the N group, the M group had significantly lower occurrence rates of sustained PND [odds ratio (OR) = 0.36, 95% confidence interval (CI) = 0.13–0.98, p = 0.04], ischemic complications (OR = 0.39, 95% CI = 0.15–0.98, p = 0.04), and radiologic complications (OR = 0.40, 95% CI = 0.19–0.82, p = 0.01). Temporary clipping was an independent risk factor for ischemic complications (ICs) in the total patient group (OR = 6.18, 95% CI = 1.75–21.83, p = 0.005), but not in the M group (OR = 5.53, 95% CI = 0.76–41.92, p = 0.09). Regarding PND prediction, considering any EP changes (MEP and/or SSEP) showed the best diagnostic efficiency with a sensitivity of 0.900, specificity of 0.940, positive predictive value of 0.321, negative predictive value (NPV) of 0.997, and a negative likelihood ratio (LR) of 0.11.

Conclusion: IONM application during UIA clipping can reduce PND and radiological complications. The diagnostic effectiveness of IONM, specifically the NPV and LR negative values, was optimal upon consideration of changes in any EP modality.

Perioperative Management of Aneurysmal Subarachnoid Hemorrhage

Aneurysmal subarachnoid hemorrhage is an acute neurologic emergency. Prompt definitive treatment of the aneurysm by craniotomy and clipping or endovascular intervention with coils and/or stents is needed to prevent rebleeding. Extracranial manifestations of aneurysmal subarachnoid hemorrhage include cardiac dysfunction, neurogenic pulmonary edema, fluid and electrolyte imbalances, and hyperglycemia. Data on the impact of anesthesia on long-term neurologic outcomes of aneurysmal subarachnoid hemorrhage do not exist. Perioperative management should therefore focus on optimizing systemic physiology, facilitating timely definitive treatment, and selecting an anesthetic technique based on patient characteristics, severity of aneurysmal subarachnoid hemorrhage, and the planned intervention and monitoring. Anesthesiologists should be familiar with evoked potential monitoring, electroencephalographic burst suppression, temporary clipping, management of external ventricular drains, adenosine-induced cardiac standstill, and rapid ventricular pacing to effectively care for these patients.

Adverse intraoperative events during surgical repair of ruptured cerebral aneurysms: a systematic review

Compared with endovascular techniques, clipping of ruptured cerebral aneurysms has been shown to associate with increased morbidity in several studies. Despite this, clipping remains the preferred option for many aneurysms. The objective of this study is to describe the reported adverse events of open repair of ruptured cerebral aneurysms and their impact on patient outcome. The PubMed, Embase and Cochrane databases were searched between June 1999 and June 2019 to identify original studies of at least 100 patients undergoing surgical repair of ruptured cerebral aneurysms and in which adverse event rates were reported. Thirty-six studies reporting adverse events in a total of 12,410 operations for repair of ruptured cerebral aneurysms were included. Surgical adverse events were common with 36 event types reported including intraoperative rupture (median rate of 16.6%), arterial injury (median rate of 3.8%) and brain swelling (median rate 5.6%). Only 6 surgical events were statistically shown to associate with poor outcomes by any author and for intraoperative rupture (the most frequently analysed), there was an even split between authors finding a statistical association with poor outcome and those finding no association. Even with modern surgical techniques, the technical demands of surgical aneurysm repair continue to lead to a high rate of intraoperative adverse events. Despite this, it is not known which of these intraoperative events are the most important contributors to the poor outcomes often seen in these patients. More research directed towards identifying the events that most drive operative morbidity has the potential to improve outcomes for these patients.

Effect of neuromonitor-guided titrated care on brain tissue hypoxia after opioid overdose cardiac arrest

INTRODUCTION

Brain tissue hypoxia may contribute to preventable secondary brain injury after cardiac arrest. We developed a porcine model of opioid overdose cardiac arrest and post-arrest care including invasive, multimodal neurological monitoring of regional brain physiology. We hypothesized brain tissue hypoxia is common with usual post-arrest care and can be prevented by modifying mean arterial pressure (MAP) and arterial oxygen concentration (PaO2).

METHODS

We induced opioid overdose and cardiac arrest in sixteen swine, attempted resuscitation after 9 min of apnea, and randomized resuscitated animals to three alternating 6-h blocks of standard or titrated care. We invasively monitored physiological parameters including brain tissue oxygen (PbtO2). During standard care blocks, we maintained MAP > 65 mmHg and oxygen saturation 94-98%. During titrated care, we targeted PbtO2 > 20 mmHg.

RESULTS

Overall, 10 animals (63%) achieved ROSC after a median of 12.4 min (range 10.8-21.5 min). PbtO2 was higher during titrated care than standard care blocks (unadjusted β = 0.60, 95% confidence interval (CI) 0.42-0.78, P < 0.001). In an adjusted model controlling for MAP, vasopressors, sedation, and block sequence, PbtO2 remained higher during titrated care (adjusted β = 0.75, 95%CI 0.43-1.06, P < 0.001). At three predetermined thresholds, brain tissue hypoxia was significantly less common during titrated care blocks (44 vs 2% of the block duration spent below 20 mmHg, P < 0.001; 21 vs 0% below 15 mmHg, P < 0.001; and, 7 vs 0% below 10 mmHg, P = .01).

CONCLUSIONS

In this model of opioid overdose cardiac arrest, brain tissue hypoxia is common and treatable. Further work will elucidate best strategies and impact of titrated care on functional outcomes.

Clinical Use of Cerebral Microdialysis in Patients with Aneurysmal Subarachnoid Hemorrhage-State of the Art

Objective

To review the published literature on the clinical application of cerebral microdialysis (CMD) in aneurysmal subarachnoid hemorrhage (SAH) patients and to summarize the evidence relating cerebral metabolism to pathophysiology, secondary brain injury, and outcome.

Methods

Study selection: Two reviewers identified all manuscripts reporting on the clinical use of CMD in aneurysmal SAH patients from MEDLINE. All identified studies were grouped according to their focus on brain metabolic changes during the early and subacute phase after SAH, their association with mechanisms of secondary brain injury and outcome.

Results

The review demonstrated: (1) limited literature is available in the very early phase before the aneurysm is secured. (2) Brain metabolic changes related to early and delayed secondary injury mechanisms may be used in addition to other neuromonitoring parameters in the critical care management of SAH patients. (3) CMD markers of ischemia may detect delayed cerebral ischemia early (up to 16 h before onset), underlining the importance of trend analysis. (4) Various CMD-derived parameters may be associated with patient outcome at 3–12 months, including CMD-lactate-to-pyruvate-ratio, CMD-glucose, and CMD-glutamate.

Conclusion

The clinical use of CMD is an emerging area in the literature of aneurysmal SAH patients. Larger prospective multi-center studies on interventions based on CMD findings are needed.

HIF1A gene polymorphisms and human diseases: Graphical review of 97 association studies

Hypoxia-inducible factors (HIFs) belong to a family of transcription factors (TF) responsive to a low O2 availability, which is often a characteristic feature of solid tumors. The alpha subunit of the HIF heterodimer is O2 -sensitive, and once stabilized in hypoxia, it functions as a master regulator of various genes involved in hypoxia pathway. Changes in the HIF1A (hypoxia inducible factor 1, alpha subunit) nucleotide sequence or expression has been shown to be associated with the development of several diseases. Because of increasing research interest in HIF1A gene a review of association studies was needed. We here reviewed published data on single nucleotide polymorphisms (SNPs) in HIF1A in various diseases; in total, 34 SNPs were tested for an association with 49 phenotypes, and the results were visualized using the Cytoscape software. Among all collected polymorphisms 16 SNPs showed significant associations with 40 different phenotypes, including six SNPs associated with 14 cancer types. Missense SNPs (rs11549465 and rs11549467) within the oxygen-dependent degradation domain were most frequently studied. The study provides a comprehensive tool for researchers working in this area and may contribute to more accurate disease diagnosis and identification of therapeutic targets.

Consensus statement from the 2014 International Microdialysis Forum

Microdialysis enables the chemistry of the extracellular interstitial space to be monitored. Use of this technique in patients with acute brain injury has increased our understanding of the pathophysiology of several acute neurological disorders. In 2004, a consensus document on the clinical application of cerebral microdialysis was published. Since then, there have been significant advances in the clinical use of microdialysis in neurocritical care. The objective of this review is to report on the International Microdialysis Forum held in Cambridge, UK, in April 2014 and to produce a revised and updated consensus statement about its clinical use including technique, data interpretation, relationship with outcome, role in guiding therapy in neurocritical care and research applications.

Monitoring of brain and systemic oxygenation in neurocritical care patients

Maintenance of adequate oxygenation is a mainstay of intensive care, however, recommendations on the safety, accuracy, and the potential clinical utility of invasive and non-invasive tools to monitor brain and systemic oxygenation in neurocritical care are lacking. A literature search was conducted for English language articles describing bedside brain and systemic oxygen monitoring in neurocritical care patients from 1980 to August 2013. Imaging techniques e.g., PET are not considered. A total of 281 studies were included, the majority described patients with traumatic brain injury (TBI). All tools for oxygen monitoring are safe. Parenchymal brain oxygen (PbtO2) monitoring is accurate to detect brain hypoxia, and it is recommended to titrate individual targets of cerebral perfusion pressure (CPP), ventilator parameters (PaCO2, PaO2), and transfusion, and to manage intracranial hypertension, in combination with ICP monitoring. SjvO2 is less accurate than PbtO2. Given limited data, NIRS is not recommended at present for adult patients who require neurocritical care. Systemic monitoring of oxygen (PaO2, SaO2, SpO2) and CO2 (PaCO2, end-tidal CO2) is recommended in patients who require neurocritical care.

Current advances in the diagnosis of vasospasm

Symptomatic vasospasm leading to delayed ischemia and neurological deficits is one of the most serious complications after aneurysmal subarachnoid hemorrhage (SAH). Reliable and early detection of symptomatic vasospasm is one of the major goals in the management of patients with SAH. In awake patients, the close clinical neurological examination still remains the most important diagnostic measure. In comatous or sedated patients, cerebral angiography remains the mainstay of the diagnostic workup for vasospasm. However, angiography does not allow assessing the hemodynamic relevance of vasospasm and is not suited for early identification of cerebral hypoperfusion and ischemia. Therefore, a large panel of new monitoring techniques for the assessment of regional cerebral perfusion has been recently introduced into the clinical management of SAH patients. This article briefly reviews the most relevant methods for monitoring cerebral perfusion and discusses their clinical predictive value for the diagnosis of vasospasm. On the basis of the currently available monitoring technologies, an algorithm for the diagnosis of vasospasm is presented.

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.

[Detection of a cerebral ischaemia episode during surgery by monitoring the brain tissue oxygen pressure]

The detection and treatment of cerebral ischaemia and tissue hypoxia for the prevention of secondary injury are the basic objectives during anaesthesia for neurosurgical procedures. The monitoring of the tissue oxygen pressure is direct and can enable potentially harmful situations to be detected in real time. Although it was initially used in neurocritical patients, its use has extended to surgical patients. We present the case of a patient subjected to surgical resection of a dural arteriovenous fistula in which the brain tissue oxygen pressure around the area of the lesion was monitored. The finding of an episode of cerebral tissue hypoxia during closure of the craniotomy determined the treatment of the patient. We highlight the possible use of this neuromonitoring for the rapid detection of regional cerebral hypoxia events in the peri-operative period of vascular neurosurgery, procedures that have a significant risk of, mainly ischaemic, hypoxia episodes.

Monitoring techniques for prevention of procedure-related ischemic damage in aneurysm surgery

OBJECTIVE

To describe the application of intraoperative monitoring techniques during aneurysm surgery and to discuss the advantages and limitations of these techniques in prevention of postoperative neurologic deficits.

METHODS

Articles found in the literature through PubMed for the time frame 1980-2011 and the authors' personal files were reviewed.

RESULTS

Various techniques for detection of vascular insufficiency are available, including direct methods to measure cerebral blood flow and indirect methods to evaluate the integrity of neurologic pathways.

CONCLUSIONS

The choice of monitoring modality should be governed by the vessel and by the vascular territory most at risk during the planned procedure with proper awareness of the potential limits related to each technique. Aneurysm surgery monitoring should help to address issues of continuity and provide a morphologic and functional assessment. Although the use of monitoring devices is still not routine in aneurysm surgery and no standards have been established, combining different monitoring techniques is crucial to optimize aneurysm surgery and avoid or minimize complications.

Monitoring of brain oxygenation in surgery of ruptured middle cerebral artery aneurysms

Background:

The occurrence of brain ischemic lesions, due to temporary arterial occlusion or incorrect placement of the definitive clip, is a major complication of aneurysm surgery. Temporary clipping is a current technique during surgery and there is no reliable method of predicting the possibility of ischemia due to extended regional circulatory interruption. Even with careful inspection, misplacement of the definitive clip can be difficult to detect. Brain tissue oxygen concentration (PtiO₂) was monitored during surgery of middle cerebral artery (MCA) aneurysm presenting with subarachnoid hemorrhage (SAH), for detection of changes in brain oxygenation due to reduced blood flow, as a predictor of ischemic events, during temporary clipping and after definitive clipping.

Methods:

PtiO₂ was monitored during surgery of 13 patients harboring MCA aneurysms presenting with SAH, using a polarographic microcatheter (Licox, GMS, Kiel, Germany) placed in the territory of MCA.

Results

A decrease in PtiO₂ values was verified in every period of temporary clipping. Brain infarction occurred in 2 patients; in both cases, there was a decrease in PtiO₂ greater than 80% from basal value, a minimum value of less than 2 mmHg persisting for 2 or more minutes during temporary clipping, and an incomplete recovery of PtiO₂ after definitive clipping. In 2 patients, incomplete recovery of values after definitive clipping led to verification of inappropriate placement and repositioning of the clip.

Conclusion:

The results suggest that intraoperative monitoring of PtiO₂ may be a useful method of detection of changes in brain tissue oxygenation during MCA aneurysm surgery. Postoperative infarction in the territory of MCA developed in cases with an abrupt decrease of PtiO₂ and a very low and persistent minimum value, during temporary clipping, and an incomplete recovery after definitive clipping. Verification of clip position should be considered when there is an incomplete recovery or a persistent fall in PtiO₂ after definitive clipping.

Monitoring of brain tissue oxygenation in surgery of middle cerebral artery incidental aneurysms

Introduction:

The management of incidental unruptured aneurysms remains a matter of controversy; middle-sized or large anterior circulation incidental aneurysms, in young or middle age patients, should be considered for treatment. Surgical clipping is an accepted treatment for middle cerebral artery unruptured aneurysms. Ischemic events can occur even in cases of incidental aneurysm surgery. Since regional cerebral blood flow can be compromised due to temporary arterial clipping or to incorrect placement of defi nitive clip, we performed intra-operative monitoring of brain tissue oxygen concentration (PtiO₂), to detect changes in brain oxygenation due to reduced blood fl ow, eventually leading to ischemia, during surgery of middle cerebral artery incidental aneurysms.

Methods:

PtiO₂ monitoring was performed during surgery of eight patients harboring incidental MCA aneurysms, using a polarographic microcatheter (Licox, GMS – Kiel, Germany), placed in the temporal lobe on the side of the lesion, from dural opening to dural closure.

Results:

Basal values varied between 2.3 and 27.3 mmHg; these values are lower than those previously described in the literature as “normal” for uninjured brain or in cases of subarachnoid hemorrhage. In all patients, a significant decrease in PtiO2 was found in every period of temporary clipping of MCA. Post-operative infarction in the territory of middle cerebral artery occurred in one patient and, in that case, there was a persistent minimum value of 0.6 mmHg, without recovery after the placement of the definitive clip. In another patient, an incorrect placement of the definitive clip could be predicted by a decrease in PtiO₂ value.

Conclusions:

PtiO₂ monitoring during aneurysm surgery shows brain tissue perfusion in real time and there is a correlation between any episode of reduced blood flow to the affected vascular territory during surgery and a decrease of PtiO2 values. Unexpected low basal values were obtained in “uninjured” brain, with no influence from subarachnoid hemorrhage. The values of risk for brain infarction during temporary arterial occlusion still need further studies, but an incomplete recovery or a persistent fall in PtiO₂ values after definitive clipping should be considered as an indication for verification of the position of the clip.

Measurement of Cortical Microcirculation During Intracranial Aneurysm Surgery by Combined Laser-Doppler Flowmetry and Photospectrometry

BACKGROUND:

Accidental vessel occlusion is one major risk of intracranial aneurysm surgery potentially causing cerebral ischemia. The intraoperative assessment of cerebral ischemia remains a technological challenge.

OBJECTIVE:

As a novel approach, cortical tissue integrity was monitored using simultaneous measurements of regional capillary-venous cerebral blood flow (rvCBF), oxygen saturation (Srvo2), and hemoglobin amount (rvHb) during aneurysm surgery.

METHODS:

Fifteen patients scheduled for aneurysm surgery of the anterior and posterior circulation were included. A fiber optic probe was placed on the cortex associated with the distal branch of the aneurysmatic vessel. Blinded measurements by combined laser-Doppler flowmetry (rvCBF) and photospectrometry (Srvo2, rvHb) were performed before and after surgical clipping or trapping of the aneurysm. Data were correlated with postoperative imaging and neurological outcome.

RESULTS:

Cortical measurements could be successfully performed in all patients. Significant increase (&gt;25% change from baseline) or decrease (&lt;25% change from baseline) of rvCBF, Srvo2, and rvHb was detectable in 33 to 46% of patients after surgical intervention. Severe decrease (&gt;50% change from baseline) of all parameters or solitary of rvCBF was correlated to reduced cerebral perfusion and neurological deficits in 2 patients.

CONCLUSION:

Combined laser-Doppler flowmetry and photospectrometry provides real-time information on cortical microcirculation. Intraoperative alterations of parameters (rvCBF, Srvo2, rvHb) might reflect changes of cerebral tissue integrity during intracranial aneurysm surgery.

Prospective Comparison of Intraoperative Vascular Monitoring Technologies During Cerebral Aneurysm Surgery

BACKGROUND:

Microscope integrated intraoperative near-infrared indocyanine green angiography (ICGA) provides assessment of the cerebral vasculature in the operating field.

OBJECTIVE:

To prospectively compare the value of ICGA-derived information during cerebral aneurysm surgery with data simultaneously generated from other intraoperative monitoring and vascular imaging techniques.

METHODS:

Data from 104 patients with 123 cerebral aneurysms who were operated on were prospectively recorded. Results of intraoperative vascular monitoring and descriptions of how this information influenced intraoperative decision making were analyzed.

RESULTS:

Clip repositioning was necessary in 30 of 123 aneurysms (24.4%) treated. Parent artery occlusion was documented by microvascular Doppler ultrasound in 4 aneurysms. ICGA disclosed parent artery stenoses not detected by sonography in 7 cases. Neuroendoscopy was used in 13 cases of midline aneurysms to confirm perforator patency after clipping, and disclosed aneurysm misclipping undetected by ICGA and digital subtraction angiography in 1 aneurysm. The information from DSA and ICGA corresponded in 120 of 123 aneurysms operated on (97.5 %). In 1 patient, ICGA underestimated a relevant parent artery stenosis detected by digital subtraction angiography. In 2 patients with relevant aneurysmal misclipping, digital subtraction angiography and ICGA led to conflicting results that could be clarified only when both methods were used and interpreted together.

CONCLUSION:

The intraoperative monitoring and vascular imaging methods compared were complementary rather than competitive in nature. None of the devices used were absolutely reliable when used as a stand-alone method. Correct intraoperative assessment of aneurysm occlusion, perforating artery patency, and parent artery reconstruction was possible in all patients when these techniques were used in combination.

Brain tissue oxygenation during dexmedetomidine administration in surgical patients with neurovascular injuries

Investigations in dogs have shown substantial dexmedetomidine (Dex)-induced reductions in cerebral blood flow (CBF) unaccompanied by reductions in cerebral metabolic rate (CMR). If this effect were to occur in humans in areas of injured brain in which CBF is already low, oxygen delivery might be critically impaired. The institutional use of brain PO2 monitoring during neurovascular surgery and the use of Dex as a component of the anesthetic allowed insight into this issue. Data from 5 neurovascular surgery patients, 2 for excision of arteriovenous malformations (AVMs), and 3 for intracranial aneurysm clipping were reviewed retrospectively. All had acute, lesion-related neurologic deficits. During general anesthesia with sufentanil and sevoflurane, with or without N2O, a parenchymal brain tissue PO2 (PbrO2) electrode was placed directly in the territory at risk from the pending neurosurgical intervention. After a stable PbrO2 value was achieved, Dex was administered by bolus (1 μg/kg over 10 min) and infusion (0.5 to 0.7 μg/kg/min). Mean arterial pressure (MAP), heart rate (HR), and PbrO2 were observed continuously for at least 25 minutes. Baseline PbrO2 values were relatively low (≤16 mm Hg) in 4 of the 5 patients, a pattern consistent with antecedent neurologic insult. In the 15 minutes after initiation of Dex administration, the pattern was one of a modest increase in Pbr02 (maximum 11.1%; P=0.0147) occurring roughly in parallel with a modest increase in MAP [maximum 3.5 mm Hg (4.5%); P=0.041]. HR did not change. Clinically significant reduction of PbrO2 did not occur before neurosurgical interventions. These observations provide no support for a direct cerebral vasoconstrictive effect of Dex in humans that is independent of any vasoconstriction that may occur as a consequence of Dex-induced reduction in CMR. At a minimum, any such effect was insufficient to have an adverse effect on oxygen delivery to brain parenchyma.

Relationship between brain interstitial fluid tumor necrosis factor-α and cerebral vasospasm after aneurysmal subarachnoid hemorrhage

Tumor necrosis factor-alpha (TNF-alpha) has a crucial role in the onset of hemolysis-induced vascular injury and cerebral vasoconstriction. We hypothesized that TNF-alpha measured from brain interstitial fluid would correlate with the severity of vasospasm following aneurysmal subarachnoid hemorrhage (aSAH). From a consecutive series of 10 aSAH patients who underwent cerebral microdialysis (MD) and evaluation of vasospasm by CT angiogram (CTA) or digital subtraction angiography (DSA), TNF-alpha levels from MD were measured at 8-hour intervals from aSAH days 4-6 using enzyme-linked immunosorbent assay. An attending neuroradiologist blinded to the study independently evaluated each CTA and DSA and assigned a vasospasm index (VI). Five patients had a VI<2 and 5 patients had a VI>2, where the median VI was 2 (range 0-13). The median log TNF-alpha area under the curve (AUC) was 1.64pg/mL *day (interquartile range 1.48-1.71) for the VI<2 group, and 2.11pg/mL *day (interquartile range 1.95-2.47) for the VI>2 group (p<0.01). Thus, in this small series of poor-grade aSAH patients, the AUC of TNF-alpha levels from aSAH days 4-6 correlates with the severity of radiographic vasospasm. Further analysis in a larger population is warranted based on our preliminary findings.

Methods of monitoring brain oxygenation

INTRODUCTION

Posttraumatic brain ischemia or hypoxia is a major potential cause of secondary injury that may lead to poor outcome. Avoidance, or amelioration, of this secondary injury depends on early diagnosis and intervention before permanent injury occurs. However, tools to monitor brain oxygenation continuously in the neuro-intensive care unit have been lacking.

DISCUSSION

In recent times, methods of monitoring aspects of brain oxygenation continuously by the bedside have been evaluated in several experimental and clinical series and are potentially changing the way we manage head-injured patients. These monitors have the potential to alert the clinician to possible secondary injury and enable intervention, help interpret pathophysiological changes (e.g., hyperemia causing raised intracranial pressure), monitor interventions (e.g., hyperventilation for increased intracranial pressure), and prognosticate. This review focuses on jugular venous saturation, brain tissue oxygen tension, and near-infrared spectroscopy as practical methods that may have an important role in managing patients with brain injury, with a particular focus on the available evidence in children. However, to use these monitors effectively and to understand the studies in which these monitors are employed, it is important for the clinician to appreciate the technical characteristics of each monitor, as well as respective strengths and limitations of each. It is equally important that the clinician understands relevant aspects of brain oxygen physiology and head trauma pathophysiology to enable correct interpretation of the monitored data and therefore to direct an appropriate therapeutic response that is likely to benefit, not harm, the patient.

Expression and functional activity of nucleoside transporters in human choroid plexus

Background

Human equilibrative nucleoside transporters (hENTs) 1-3 and human concentrative nucleoside transporters (hCNTs) 1-3 in the human choroid plexus (hCP) play a role in the homeostasis of adenosine and other naturally occurring nucleosides in the brain; in addition, hENT1, hENT2 and hCNT3 mediate membrane transport of nucleoside reverse transcriptase inhibitors that could be used to treat HIV infection, 3'-azido-3'-deoxythymidine, 2'3'-dideoxycytidine and 2'3'-dideoxyinosine. This study aimed to explore the expression levels and functional activities of hENTs 1-3 and hCNTs 1-3 in human choroid plexus.

Methods

Freshly-isolated pieces of lateral ventricle hCP, removed for various clinical reasons during neurosurgery, were obtained under Local Ethics Committee approval. Quantification of mRNAs that encoded hENTs and hCNTs was performed by the hydrolysis probes-based reverse transcription real time-polymerase chain reaction (RT-qPCR); for each gene of interest and for 18 S ribosomal RNA, which was an endogenous control, the efficiency of PCR reaction (E) and the quantification cycle (Cq) were calculated. The uptake of [³H]inosine by the choroid plexus pieces was investigated to explore the functional activity of hENTs and hCNTs in the hCP.

Results

RT-qPCR revealed that the mRNA encoding the intracellularly located transporter hENT3 was the most abundant, with E^-Cq value being only about 40 fold less that the E^-Cq value for 18 S ribosomal RNA; mRNAs encoding hENT1, hENT2 and hCNT3 were much less abundant than mRNA for the hENT3, while mRNAs encoding hCNT1 and hCNT2 were of very low abundance and not detectable. Uptake of [³H]inosine by the CP samples was linear and consisted of an Na⁺-dependent component, which was probably mediated by hCNT3, and Na⁺-independent component, mediated by hENTs. The latter component was not sensitive to inhibition by S-(4-nitrobenzyl)-6-thioinosine (NBMPR), when used at a concentration of 0.5 μM, a finding that excluded the involvement of hENT1, but it was very substantially inhibited by 10 μM NBMPR, a finding that suggested the involvement of hENT2 in uptake.

Conclusion

Transcripts for hENT1-3 and hCNT3 were detected in human CP; mRNA for hENT3, an intracellularly located nucleoside transporter, was the most abundant. Human CP took up radiolabelled inosine by both concentrative and equilibrative processes. Concentrative uptake was probably mediated by hCNT3; the equilibrative uptake was mediated only by hENT2. The hENT1 transport activity was absent, which could suggest either that this protein was absent in the CP cells or that it was confined to the basolateral side of the CP epithelium.

Hypertonic saline in patients with poor-grade subarachnoid hemorrhage improves cerebral blood flow, brain tissue oxygen, and pH

BACKGROUND AND PURPOSE

Delayed cerebral ischemia and infarction due to reduced CBF remains the leading cause of poor outcome after aneurysmal subarachnoid hemorrhage. Hypertonic saline (HS) is associated with an increase in CBF. This study explores whether CBF enhancement with HS in patients with poor-grade subarachnoid hemorrhage is associated with improved cerebral tissue oxygenation.

METHODS

Continuous monitoring of arterial blood pressure, intracranial pressure, cerebral perfusion pressure, brain tissue oxygen, carbon dioxide, pH, and middle cerebral artery flow velocity was performed in 44 patients. Patients were given an infusion (2 mL/kg) of 23.5% HS. In 16 patients, xenon CT scanning was also performed. CBF in a region surrounding the tissue oxygen sensor was calculated. Data are mean+/-SD.

RESULTS

Thirty minutes postinfusion, a significant increase in arterial blood pressure, cerebral perfusion pressure, flow velocity, brain tissue pH, and brain tissue oxygen was seen together with a decrease in intracranial pressure (P<0.05). Intracranial pressure remained reduced for >300 minutes and flow velocity elevated for >240 minutes. A significant increase in brain tissue oxygen persisted for 240 minutes. Average baseline regional CBF was 33.9+/-13.5 mL/100 g/min, rising by 20.3%+/-37.4% (P<0.05) after HS. Patients with favorable outcome responded better to HS in terms of increased CBF, brain tissue oxygen, and pH and reduced intracranial pressure compared with those with an unfavorable outcome. A sustained increase in brain tissue oxygen (beyond 210 minutes) was associated with favorable outcome (P<0.023).

CONCLUSIONS

HS augments CBF in patients with poor-grade subarachnoid hemorrhage and significantly improves cerebral oxygenation for 4 hours postinfusion. Favorable outcome is associated with an improvement in brain tissue oxygen beyond 210 minutes.

Effect of nitrous oxide use on long-term neurologic and neuropsychological outcome in patients who received temporary proximal artery occlusion during cerebral aneurysm clipping surgery

BACKGROUND

The authors explored the relationship between nitrous oxide use and neurologic and neuropsychological outcome in a population of patients likely to experience intraoperative cerebral ischemia: those who had temporary cerebral arterial occlusion during aneurysm clipping surgery.

METHODS

A post hoc analysis of a subset of the data from the Intraoperative Hypothermia for Aneurysm Surgery Trial was conducted. Only subjects who had temporary arterial occlusion during surgery were included in the analysis. Metrics of short-term and long-term (i.e., 3 months after surgery) outcome were evaluated via both univariate and multivariate logistic regression analysis. An odds ratio (OR) greater than 1.0 denotes a worse outcome in patients receiving nitrous oxide.

RESULTS

The authors evaluated 441 patients, of which 199 received nitrous oxide. Patients receiving nitrous oxide had a greater risk of delayed ischemic neurologic deficits (i.e., the clinical manifestation of vasospasm) (OR, 1.78, 95% confidence interval [CI], 1.08-2.95; P = 0.025). However, at 3 months after surgery, there was no difference in any metric of gross neurologic outcome: Glasgow Outcome Score (OR, 0.67; CI, 0.44-1.03; P = 0.065), Rankin Score (OR, 0.74; CI, 0.47-1.16; P = 0.192), National Institutes of Health Stroke Scale (OR, 1.02; CI, 0.66-1.56; P = 0.937), or Barthel Index (OR, 0.69; CI, 0.38-1.25; P = 0.22). The risk of impairment on at least one test of neuropsychological function was reduced in those who received nitrous oxide (OR, 0.56; CI, 0.36-0.89; P = 0.013).

CONCLUSION

In this patient population, use of nitrous oxide was associated with an increased risk for the development of delayed ischemic neurologic deficits; however, there was no evidence of detriment to long-term gross neurologic or neuropsychological outcome.

NICEM consensus on neurological monitoring in acute neurological disease

This manuscript summarises the consensus on neuromonitoring in neuro-intensive care promoted and organised by the Neuro-Intensive Care and Emergency Medicine (NICEM) Section of the European Society of Intensive Care Medicine (ESICM). It is expected that continuous monitoring using multi-modal techniques will help to overcome the limitations of each individual method and will provide a better diagnosis. More specific treatment can then be applied; however, it remains to be determined which combination of parameters is optimal. The questions discussed and addressed in this manuscript are: (1) Who should have ICP monitoring and for how long? (2) What ICP technologies are available and what are their relative advantages/disadvantages? (3) Should CPP monitoring and autoregulation testing be used? (4) When should brain tissue oxygen tension (PbrO(2)) be monitored? (5) Should structurally normal or abnormal tissue be monitored with PbrO(2)? (6) Should microdialysis be considered in complex cases? It is hoped that this document will prove useful to clinicians working in NICU and also to those developing specialist NICU services within their hospital practice.

Multimodality monitoring in neurocritical care

Multimodality monitoring of cerebral physiology encompasses the application of different monitoring techniques and integration of several measured physiologic and biochemical variables into assessment of brain metabolism, structure, perfusion, and oxygenation status. Novel monitoring techniques include transcranial Doppler ultrasonography, neuroimaging, intracranial pressure, cerebral perfusion, and cerebral blood flow monitors, brain tissue oxygen tension monitoring, microdialysis, evoked potentials, and continuous electroencephalogram. Multimodality monitoring enables immediate detection and prevention of acute neurologic injury as well as appropriate intervention based on patients' individual disease states in the neurocritical care unit. Real-time analysis of cerebral physiologic, metabolic, and cardiovascular parameters simultaneously has broadened knowledge about complex brain pathophysiology and cerebral hemodynamics. Integration of this information allows for more precise diagnosis and optimization of management of patients with brain injury.

Aneurysmal subarachnoid haemorrhage and the anaesthetist

The anaesthetist may be involved at various stages in the management of subarachnoid haemorrhage (SAH). Thus, familiarity with epidemiological, pathophysiological, diagnostic, and therapeutic issues is as important as detailed knowledge of the optimal intraoperative anaesthetic management. As the prognosis of SAH remains poor, prompt diagnosis and appropriate treatment are essential, because early treatment may improve outcome. It is, therefore, important to rule out SAH as soon as possible in all patients complaining of sudden onset of severe headache lasting for longer than an hour with no alternative explanation. The three main predictors of mortality and dependence are impaired level of consciousness on admission, advanced age, and a large volume of blood on initial cranial computed tomography. The major complications of SAH include re-bleeding, cerebral vasospasm leading to immediate and delayed cerebral ischaemia, hydrocephalus, cardiopulmonary dysfunction, and electrolyte disturbances. Prophylaxis and therapy of cerebral vasospasm include maintenance of cerebral perfusion pressure (CPP) and normovolaemia, administration of nimodipine, triple-H therapy, balloon angioplasty, and intra-arterial papaverine. Occlusion of the aneurysm after SAH is usually attempted surgically ('clipping') or endovascularly by detachable coils ('coiling'). The need for an adequate CPP (for the prevention of cerebral ischaemia and cerebral vasospasm) must be balanced against the need for a low transmural pressure gradient of the aneurysm (for the prevention of rupture of the aneurysm). Effective measures to prevent or attenuate increases in intracranial pressure, brain swelling, and cerebral vasospasm throughout all phases of anaesthesia are prerequisite for optimal outcome.

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.

[Monitoring of cerebral oxygenation with SvjO(2) or PtiO(2)]

Jugular venous oxygen saturation (SvjO(2)) monitoring has been developed in order to detect cerebral ischaemia. The interpretation of SvjO(2) values remains nevertheless complex, and should be associated with cerebral haemodynamic multimonitoring with ICP and transcranial Doppler. With the hypothesis of a constant cerebral oxygen consummation, and also with a constant haematocrit, SvjO(2) variations correlates with cerebral blood flow variations. After a brain trauma, an SvjO(2)<50% or>75% is associated with a bad prognosis. To maintain SvjO(2)>50% constitutes a reasonable therapeutic objective, but the benefice associated with such a strategy has not been validated. Oxygen partial pressure (PtiO(2)) in the brain parenchyma may be monitored in the non-lesioned area (usually frontal) in order to detect a global cerebral ischaemia, or in the penumbra of a cerebral lesion in order to detect a local ischaemia. The values associated with an ischemic risk are not fully defined and may be under 10-15 mmHg. A concomitant metabolic monitoring by cerebral microdialysis is of importance to fully address the real cerebral local ischaemic burden. Scientific studies are mainly focused on patients with a brain traumatism. Nor SvjO(2), nor PtiO(2) monitoring have at present been demonstrated to be associated with a clinical benefit, and their use should be restricted to scientific research.

Future of brain support: multimodality monitoring

Multimodality monitoring of cerebral physiology encompasses the application of different monitoring techniques and integration of several measured physiological and biochemical variables into the assessment of brain metabolism, structure, perfusion and oxygenation status, in addition to clinical evaluation. Novel monitoring techniques include transcranial Doppler ultrasonography, neuroimaging, intracranial pressure, cerebral perfusion and cerebral blood flow monitors, brain tissue oxygen tension monitoring, microdialysis, evoked potentials and continuous electroencephalography. Multimodality monitoring enables the immediate detection and prevention of acute neurological events, as well as appropriate intervention based on a patient’s individual disease state in the neurocritical care unit. Simultaneous real-time analysis of cerebral physiological, metabolic and cardiovascular parameters has broadened knowledge regarding complex brain pathophysiology and cerebral hemodynamics. Integration of this information allows for a more precise diagnosis and optimization of management of patients with brain injury.

Application of rapid-sampling, online microdialysis to the monitoring of brain metabolism during aneurysm surgery

OBJECTIVE

To introduce rapid-sampling microdialysis for the early detection of adverse metabolic changes in tissue at risk during aneurysm surgery.

METHODS

A microdialysis catheter was inserted under direct vision into at-risk cortex at the start of surgery. This monitoring was sustained throughout the course of the operation, during which intraoperative events, for example, temporary arterial occlusion or lobe retraction, were precisely documented. A continuous online flow of dialysate was fed into a mobile bedside glucose and lactate analyser. This comprises flow-injection dual-assay enzyme-based biosensors capable of determining values of metabolites every 30 seconds.

RESULTS

Eight patients underwent clipping or wrapping of intracranial aneurysms and were monitored. Time between events and detection: 9 minutes. Mean change in metabolite value +/- standard deviation: temporal lobe retraction lactate, +656 +/- 562 micromol/L (n = 7, P < 0.05); glucose, -123 +/- 138 micromol/L (n = 6, P = 0.08). Glucose intravenous bolus infusion glucose, +512 +/- 244 micromol/L (n = 5, P < 0.01); peak at mean time after bolus, 16 minutes. Temporary proximal clip lactate, +731 +/- 346 micromol/L (n = 6, P < 0.01); glucose, -139 +/- 96 micromol/L (n = 5, P < 0.05); mean clip time, 8.6 minutes.

CONCLUSION

The technique detects changes 9 minutes after intraoperative events occur (limited only by probe-to-sensor tubing length and dialysate flow rate). This provides reliable information to the surgeon and anesthetist promptly. It is a useful method for monitoring glucose and lactate in dialysate, particularly when rapid, transient changes in brain analyte levels need to be determined and the alternative offline methodology would be inadequate.

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.

Translational neurochemical research in acute human brain injury: the current status and potential future for cerebral microdialysis

Microdialysis (MD) was introduced as an intracerebral sampling method for clinical neurosurgery by Hillered et al. and Meyerson et al. in 1990. Since then MD has been embraced as a research tool to measure the neurochemistry of acute human brain injury and epilepsy. In general investigators have focused their attention to relative chemical changes during neurointensive care, operative procedures, and epileptic seizure activity. This initial excitement surrounding this technology has subsided over the years due to concerns about the amount of tissue sampled and the complicated issues related to quantification. The interpretation of mild to moderate MD fluctuations in general remains an issue relating to dynamic changes of the architecture and size of the interstitial space, blood-brain barrier (BBB) function, and analytical imprecision, calling for additional validation studies and new methods to control for in vivo recovery variations. Consequently, the use of this methodology to influence clinical decisions regarding the care of patients has been restricted to a few institutions. Clinical studies have provided ample evidence that intracerebral MD monitoring is useful for the detection of overt adverse neurochemical conditions involving hypoxia/ischemia and seizure activity in subarachnoid hemorrhage (SAH), traumatic brain injury (TBI), thromboembolic stroke, and epilepsy. There is some data strongly suggesting that MD changes precede the onset of secondary neurological deterioration following SAH, hemispheric stroke, and surges of increased ICP in fulminant hepatic failure. These promising investigations have relied on MD-markers for disturbed glucose metabolism (glucose, lactate, and pyruvate) and amino acids. Others have focused on trying to capture other important neurochemical events, such as excitotoxicity, cell membrane degradation, reactive oxygen species (ROS) and nitric oxide (NO) formation, cellular edema, and BBB dysfunction. However, these other applications need additional validation. Although these cerebral events and their corresponding changes in neurochemistry are important, other promising MD applications, as yet less explored, comprise local neurochemical provocations, drug penetration to the human brain, MD as a tool in clinical drug trials, and for studying the proteomics of acute human brain injury. Nevertheless, MD has provided new important insights into the neurochemistry of acute human brain injury. It remains one of very few methods for neurochemical measurements in the interstitial compartment of the human brain and will continue to be a valuable translational research tool for the future. Therefore, this technology has the potential of becoming an established part of multimodality neuro-ICU monitoring, contributing unique information about the acute brain injury process. However, in order to reach this stage, several issues related to quantification and bedside presentation of MD data, implantation strategies, and quality assurance need to be resolved. The future success of MD as a diagnostic tool in clinical neurosurgery depends heavily on the choice of biomarkers, their sensitivity, specificity, and predictive value for secondary neurochemical events, and the availability of practical bedside methods for chemical analysis of the individual markers. The purpose of this review was to summarize the results of clinical studies using cerebral MD in neurosurgical patients and to discuss the current status of MD as a potential method for use in clinical decision-making. The approach was to focus on adverse neurochemical conditions in the injured human brain and the MD biomarkers used to study those events. Methodological issues that appeared critical for the future success of MD as a routine intracerebral sampling method were addressed.

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.

Clampage vasculaire temporaire

In many situations, temporary artery occlusion is an integral component of aneurysm surgery. The use of temporary clip may allow safer and easier aneurysmal dissection and clipping. Several points, concerning the duration and overall risks of temporary occlusion and the method of choice for cerebral function monitoring have to be discussed.

MATERIAL AND METHODS

Non exhaustive review of neurosurgical literature.

DISCUSSION

Temporary clip application decreases the risk of intraoperative aneurysmal rupture. The analysis of data published in the literature showed that several questions remain open concerning the optimal method of neuroprotection and cerebral function monitoring, as well as the limit of occlusion duration. Other clinical trials are needed to assess the efficacy and safety of this technique.

Cerebral microdialysis of patients with severe traumatic brain injury exhibits highly individualistic patterns as visualized by cluster analysis with self-organizing maps

OBJECTIVE

To analyze patterns of cerebral microdialysis in patients with traumatic brain injury and, with a neural network methodology, investigate pattern relationships to intracranial pressure and cerebral perfusion pressure.

DESIGN

Retrospective.

SETTING

University hospital, adult neurosurgical intensive care unit.

PATIENTS

Twenty-six patients with severe traumatic brain injury. All consecutive traumatic brain injured patients (Glasgow Coma Scale < or =8) with microdialysis monitoring, analyzing glutamate, lactate, pyruvate, and glucose in both penumbral and nonpenumbral tissue.

INTERVENTIONS

None; patients received the unit's standard neurointensive care procedure.

MEASUREMENTS AND MAIN RESULTS

We used 2084 hrs of complete microdialysis data sets (eight markers) to train Kohonen self-organizing maps. The self-organizing map algorithm is a data-clustering method that reduces high-dimensional information to a two-dimensional representation on a grid (map), retaining local relationships in the data. Maps were colored (overlaid) for intracranial pressure, cerebral perfusion pressure, and outcome, to explore relationships with underlying microdialysis patterns. The maps exhibited a striking clustering of patients, with unique microdialysis patterns that were recognizable throughout the analysis period. This also held true for most microdialysis patterns characteristic of ischemia. These patients with ischemic patterns can have good outcomes, suggesting a disparity between microdialysis values and severity of traumatic brain injury.

CONCLUSION

Using an artificial neural network-like clustering technique, Kohonen self-organizing maps, we have shown that cerebral microdialysis, in traumatic brain injury, exhibits strikingly individualistic patterns that are identifiable throughout the analysis period. Because patients form their own clusters, microdialysis patterns, during periods of increased intracranial pressure or decreased cerebral perfusion pressure, will be found within these clusters. Consequently, no common pattern of microdialysis can be seen among patients within the range of our data. We suggest that these individualistic patterns reflect not only metabolic states of traumatic brain injury but also local gradients seen with small volume sampling. Future investigation should focus on relating these patterns, and movement within and from clusters, to metabolic states of the complex pathophysiology of traumatic brain injury.

Effect of cerebral perfusion pressure augmentation on regional oxygenation and metabolism after head injury*

OBJECTIVE

In this study we have used O positron emission tomography, brain tissue oxygen monitoring, and cerebral microdialysis to assess the effects of cerebral perfusion pressure augmentation on regional physiology and metabolism in the setting of traumatic brain injury.

DESIGN

Prospective interventional study.

SETTING

Neurosciences critical care unit of a university hospital.

PATIENTS

Eleven acutely head-injured patients requiring norepinephrine to maintain cerebral perfusion pressure.

INTERVENTIONS

Using positron emission tomography, we have quantified the response to an increase in cerebral perfusion pressure in a region of interest around a brain tissue oxygen sensor (Neurotrend) and microdialysis catheter. Oxygen extraction fraction and cerebral blood flow were measured with positron emission tomography at a cerebral perfusion pressure of approximately 70 mm Hg and approximately 90 mm Hg using norepinephrine to control cerebral perfusion pressure. All other aspects of physiology were kept stable.

MEASUREMENTS AND MAIN RESULTS

Cerebral perfusion pressure augmentation resulted in a significant increase in brain tissue oxygen (17 +/- 8 vs. 22 +/- 8 mm Hg; 2.2 +/- 1.0 vs. 2.9 +/- 1.0 kPa, p < .001) and cerebral blood flow (27.5 +/- 5.1 vs. 29.7 +/- 6.0 mL/100 mL/min, p < .05) and a significant decrease in oxygen extraction fraction (33.4 +/- 5.9 vs. 30.3 +/- 4.6 %, p < .05). There were no significant changes in any of the microdialysis variables (glucose, lactate, pyruvate, lactate/pyruvate ratio, glycerol). There was a significant linear relationship between brain tissue oxygen and oxygen extraction fraction (r = .21, p < .05); the brain tissue oxygen value associated with an oxygen extraction fraction of 40% (the mean value for oxygen extraction fraction in normal controls) was 14 mm Hg (1.8 kPa). The cerebral perfusion pressure intervention resulted in a greater percentage increase in brain tissue oxygen than the percentage decrease in oxygen extraction fraction; this suggests that the oxygen gradients between the vascular and tissue compartments were reduced by the cerebral perfusion pressure intervention.

CONCLUSIONS

Cerebral perfusion pressure augmentation significantly increased levels of brain tissue oxygen and significantly reduced regional oxygen extraction fraction. However, these changes did not translate into predictable changes in regional chemistry. Our results suggest that the ischemic level of brain tissue oxygen may lie at a level below 14 mm Hg (1.8 kPa); however, the data do not allow us to be more specific.

Cerebral blood flow augmentation in patients with severe subarachnoid haemorrhage

Following aneurysmal subarachnoid haemorrhage (SAH), cerebral blood flow (CBF) may be reduced, resulting in poor outcome due to cerebral ischaemia and subsequent stroke. Hypertonic saline (HS) is known to be effective in reducing intracranial pressure (ICP). We have previously shown a 20-50% increase in CBF in ischaemic regions after intravenous infusion of HS. This study aims to determine the effect of HS on CBF augmentation, substrate delivery and metabolism. Continuous monitoring of arterial blood pressure (ABP), ICP, cerebral perfusion pressure (CPP), brain tissue oxygen (PbO2), middle cerebral artery flow velocity (FV), and microdialysis was performed in 14 poor grade SAH patients. Patients were given an infusion of 23.5% HS, and quantified xenon computerised tomography scanning (XeCT) was carried out before and after the infusion in 9 patients. The results showed a significant increase in ABP, CPP, FV and PbO2, and a significant decrease in ICP (p < 0.05). Nine patients showed a decrease in lactate-pyruvate ratio at 60 minutes following HS infusion. These results show that HS safely and effectively augments CBF in patients with poor grade SAH and significantly improves cerebral oxygenation. An improvement in cerebral metabolic status in terms of lactate-pyruvate ratio is also associated with HS infusion.

Extracellular amino acid changes in patients during reversible cerebral ischaemia

This study investigated the changes in extracellular chemistry during reversible human cerebral ischaemia. Delayed analysis was performed on samples taken from a subgroup of patients during aneurysm surgery previously reported. Frozen microdialysis samples from 14 patients who had all undergone temporary clipping of the ipsilateral internal carotid artery (ICA) were analysed for another 15 amino acids with HPLC and for glycerol with CMA-600. Changes were characterised according to whether cerebral tissue oxygen pressure (PBO2) decreases were brief or prolonged. Brief ICA clipping (maximum duration of 16 minutes) in 11 patients was not associated with changes in amino acids or glycerol. Cerebral ischaemia, defined by a PBO2 decrease below 1.1 kPa for at least 30 minutes during ICA occlusion, occurred in 3 patients. None of whom developed an infarct in the monitored region. This prolonged reversible ischaemia was associated with transient delayed increases in gamma-amino butyric acid (GABA) as well as glutamate and glycerol, each by two-to-three folds. This study demonstrates detectable transient increases in human extracellular glutamate, GABA and glycerol during identified periods of reversible cerebral ischaemia, maximal 30-60 minutes after onset of ischaemia, but not in other amino acids detected by HPLC.

Individual value of brain tissue oxygen pressure, microvascular oxygen saturation, cytochrome redox level, and energy metabolites in detecting critically reduced cerebral energy state during acute changes in global cerebral perfusion

The authors assessed the diagnostic value of brain tissue oxygen tension (PbrO2), microvascular oxygen saturation (SmvO2), cytochrome oxidase redox level (Cyt a+a3 oxidation), and cerebral energy metabolite concentrations in detecting acute critical impairment of cerebral energy homeostasis. Each single parameter as well as derived multimodal indices (arteriovenous difference in oxygen content [AVDO2], cerebral metabolic rate for oxygen [CMRO2], fractional microvascular oxygen extraction [OEF]) were investigated during controlled variation of global cerebral perfusion using a cisternal infusion technique in 16 rabbits. The objective of this study was to determine whether acute changes between normal, moderately, and critically reduced cerebral perfusion as well as frank ischemia defined by local cortical blood flow (lcoBF), brain electrical activity (BEA), and brain stem vasomotor control can be reliably identified by SmvO2, PbrO2, Cyt a+a3 oxidation, or energy metabolites (glutamate, lactate/pyruvate ratio). PbrO2, SmvO2, and Cyt a+a3 oxidation, but not cerebral perfusion pressure, were closely linked to lcoBF and BEA and allowed discrimination between normal, moderately reduced, and critically reduced cerebral perfusion (P < 0.01). Glutamate concentrations and the lactate/pyruvate ratio varied significantly only between moderately reduced cerebral perfusion and frank ischemia (complete loss of BEA and brain stem vasomotor control). Therefore, PbrO2, SmvO2, and Cyt a+a3 oxidation, but not glutamate and the lactate/pyruvate ratio, reliably predict the transition from moderately to critically reduced cerebral perfusion with impending energy failure.

[Intraoperative detection of ischemic brain hypoxia using oxygen tissue pressure microprobes]

OBJECTIVE AND IMPORTANCE

Detection of intraoperative ischemic events could lead to the resolution of their cause and to the prevention of the definitive establishment of a postoperative infarct. We want to illustrate the possibilities that intraoperative monitoring of oxygen tissue pressure (PtiO2) in critical areas during a neurosurgical vascular procedure offers, enhancing its reliability and immediacy in obtaining information about tissue oxygenation status as a marker of ischemia in the vascular territory at risk.

CLINICAL PRESENTATION

We report the case of a 32 year-old male with a deep arteriovenous malformation (AVM) localised in the insular region. The patient had been previously treated with radiosurgery without achieving a satisfactory result.

INTERVENTION

AVM removal was performed through a transylvian transinsular approach. PtiO2 was monitorised at the temporal pole (reference area) and at the posterior temporal region (risk area). Both probes maintained close tissue oxygenation levels until the last stage of the AVM resection when, during the coagulation of a supposed afferent vessel, a brisk fall of the oxygen tissue pressure in the posterior temporal region was detected. An ischemic infarct in this area was observed postoperatively.

CONCLUSIONS

PtiO2 monitoring has a high reliability in the detection of intraoperative tissue hypoxia. Data obtained could lead to early identification of these events and, whatever possible, to resolve this situation preventing the definitive establishment of an ischemic infarct.