Extracellular levels of glucose and lactate measured by quantitative microdialysis in the human brain.

The aim of this study was to use quantitative microdialysis to estimate the true extracellular concentrations of glucose and lactate in minimally disturbed human brain. These values are important as criteria for microdialytical monitoring in critical care patients and for determining therapy. Microdialysis procedures were carried out during tumor operations, the probe being inserted distant from the site of manipulation in minimally disturbed tissue. Two methods were used: 1. The zero net flux method of Lönnroth. 2. The low flow method (10 mm membrane length, flow rate 0.3 microliter min-1, high in vivo recovery). Both methods gave similar values of about 2000 microM for lactate and slightly less for glucose (1700 microM). Glucose levels correspond with those measured by other methods in humans, allowing for the fact that our patients were anesthetised. Extracellular glucose levels were positively correlated with blood glucose values measured before the operation, and with extracellular lactate. Results confirm that extracellular glucose is zero when blood glucose is about 2 mM.

Microdialytical monitoring of uric and ascorbic acids in the brains of patients after severe brain injury and during neurovascular surgery.

OBJECTIVES: Microdialysis has been extensively used to monitor brain metabolism in the extracellular fluid of patients with severe head injury, to detect the onset of secondary ischaemic damage. The aim was to investigate whether concentrations of uric and ascorbic acids were altered in such patients. Both these compounds play a part in free radical metabolism, which is accelerated after ischaemia and brain injury. METHODS: Patients with aneurysm or bypass operations were monitored intraoperatively to assess concentrations in minimally disturbed tissue. Afterwards, 13 patients with severe head trauma were monitored for up to 13 days in the intensive care unit. RESULTS: Intraoperatively, concentrations of both ascorbic and uric acids were significantly higher in the bypass group than in patients with aneurysm, which might be attributed to chronic ischaemic conditions caused by the unilateral occlusion of the carotid artery. In the patients with trauma, mean values of uric acid, varying between 6 microM and 180 microM, did not correlate with type of injury (contusion or diffuse) or duration of monitoring time. Patients who died had significantly higher concentrations of uric acid than those with a good outcome. Ascorbic acid could be detected only intermittently, probably due to technical problems. Concentrations of these two compounds could not be correlated with clinical findings during the course of monitoring. CONCLUSIONS: Although uric and ascorbic acids are influenced by ischaemic conditions-for example, in bypass patients, neither compound is suitable for monitoring for free radical activity after severe head injury. Patients with a bad outcome tended to have higher concentrations of uric acid.

17beta-Estradiol reduces cortical lesion size in the glutamate excitotoxicity model by enhancing extracellular lactate: a new neuroprotective pathway.

Estrogens play an important role in neuronal function and in protecting neurones in the cerebral cortex against pathological conditions. An in vivo model of glutamate excitotoxicity in which glutamate is applied to the cortex of rats through a microdialysis probe has been used to investigate the neuroprotective processes initiated by 17beta-estradiol. Rats were pre-treated with 17beta-estradiol (i.v.) before local application of 100 mM glutamate into the cortex through a microdialysis probe. Pre-treatment with 17beta-estradiol significantly reduced the size of the glutamate-induced cortical lesion. In the cortical microdialysates collected from the probe, a peak of lactate was observed immediately after glutamate application. After 17beta-estradiol pre-treatment this peak of lactate was significantly higher with estradiol than without 120 min after glutamate application, reaching 700% basal level at the end of measurement. The level of extracellular glucose was markedly decreased with and without 17beta-estradiol pre-treatment. Local blockage of neuronal lactate transporters with alpha-cyano-4-hydroxycinnamate (4-CIN) completely abolished the neuroprotective effect of 17beta-estradiol and induced a larger cortical lesion. An accumulation of extracellular lactate was observed after inhibition of the lactate transporters suggesting that transport of lactate into neurones is necessary for the neuroprotective effect of 17beta-estradiol. The anti-estrogen tamoxifen also abolished the neuroprotective effect of 17beta-estradiol on the lesion size and inhibited the production of lactate. These results suggest a new neuroprotective mechanism of 17beta-estradiol by activating glutamate-stimulated lactate production, which is estrogen receptor-dependent.

A four lumen screwing device for multiparametric brain monitoring.

We describe multiparametric monitoring in severe head trauma using a new screwing device. Our aim was to create a screw which would make the implantation of the probes and thus multiparametric monitoring easier. The new screw allows us to implant 3 probes (microdialysis, Paratrend and an intracranial pressure device) through one burr hole. The screw has four channels, the fourth being for ventricular drainage. We monitored 13 patients with severe head trauma (GCS = 3-8) for up to 7 days. Brain tissue pO2, pCO2, pH, and temperature were measured on-line with the Paratrend 7 machine. The microdialytic parameters glucose, lactate, pyruvate and glutamate were determined semi on-line with a CMA 600 enzymatic analyser. There were no complications in any of the patients that could be ascribed to the screw.

Extracellular glutamate and other metabolites in and around RG2 rat glioma: an intracerebral microdialysis study.

The current study determined the extracellular content of glutamate, 10 additional amino acids, lactate, glucose and some antioxidants in a rodent model of malignant glioma, its peritumoral space and the adjacent cortex. RG2 tumors were induced in the right frontal cortex of Fischer-344 rats (n = 10) by a standardized procedure to obtain a maximum sagittal tumor width of 3-4 mm diameter. After confirmation of tumor growth and localization by contrast enhanced MRI three microdialysis probes were implanted simultaneously in the cortex: at the tumor implantation site (tumor), 2 mm caudally, brain around tumor (BAT) and 4 mm caudally (cortex) to the site of implantation. Dialysate concentrations of glutamate were increased 3.9-fold in tumor and 2-fold in BAT compared with cortex. Glycine was elevated 11.4-fold in tumor and 2.6-fold in BAT. Lactate was increased 1.7-fold in tumor, 1.2-fold in BAT. Levels of glucose, ascorbic acid and uric acid were not significantly different in tumor, BAT and cortex. The increased dialysate levels of glutamate and glycine in the peritumoral space may contribute to impaired neuronal function and epileptiform activity associated with this tumor type in humans.

Chronic effects of an aminosteroid on microdialytically measured parameters after experimental middle cerebral artery occlusion in the rat.

The effects of the neuroprotective aminosteroid U74006F (tirilazad mesylate, Freedox) were monitored microdialytically in rat cortex during three 4h periods beginning immediately, 25h and 49h after permanent middle cerebral artery occlusion. Either U74006F or vehicle only was administered 15 min, 2h, 6h, 12h and 24h after operation. The dialysate was analysed for on-line pH, ascorbic acid, uric acid, glucose and lactate. The efficacy of post-ischaemic treatment was shown by: a) lesion volume 53h after operation was significantly smaller in U74006F-treated animals; b) microdialytic findings were very similar to those found previously with pre-ischaemic drug application (reduction in release of ascorbic acid, uric acid and lactate, increased pH); c) an effect of U74006F on lactate release could still be seen on days 2 and 3; and d) increases in uric acid on days 2 and 3, possibly reflecting delayed cell death, were smaller in aminosteroid treated animals.

Acute effects of an aminosteroid on microdialytically measured parameters after experimental middle cerebral artery occlusion in the rat.

The effects of the neuroprotective aminosteroid U74389G were monitored microdialytically in rat cortex during 4 h after permanent middle cerebral artery occlusion. Either U74389G or vehicle only was administered one hour before and 2 h after operation. The dialysate was analysed for on-line pH, ascorbic acid, uric acid, glucose and lactate. In aminosteroid treated animals the levels of ascorbic and uric acids were lower in dialysates after occlusion, total release of lactate was significantly reduced and on-line pH was significantly higher than in control animals. Lesion volume at 4 h, which was significantly reduced in treated animals, correlated positively with ascorbic acid release and on-line pH. Results suggest that neuroprotective effects of aminosteroids might be explained by: (a) preservation of intracellular levels of the radical scavenger ascorbic acid with possible concomitant reduction of glutamate release; and (b) reduced lactate release and increased pH which might influence oedema positively. Copyright 1999 Harcourt Publishers Ltd.

[Intraoperative and postoperative microdialysis measurement of the human heart--feasibility and initial results]. / Intraoperative und postoperative Mikrodialysemessungen am menschlichen Herzen--Machbarkeit und erste Resultate.

STUDY OBJECTIVE: Microdialysis measurements of human cardiac metabolism during and after cardiac operations have not been published up to now. The goal of this study was to evaluate feasibility of the method in a clinical setting and to interpret first results. PATIENTS AND METHODS: In 5 patients microdialysis measurements were made in regular intervals during aortocoronary bypass surgery. Analysis of dialysate was done by high performance liquid chromatography or enzymatic fluorometry. In the last 2 patients measurements were also taken during the postoperative course up to the time of extubation. RESULTS: During aortic cross clamping a mean 7-fold rise of the radical scavenger glutathione was observed (range 0.9-15.4; p = 0.06). During reperfusion the glucose/lactate(Glc/Lac)-rate rose from 0.4 to 3.1 (p = 0.02). Concentrations of ascorbic acid, cysteine and uric acid remained neutral or showed no regular changes. In the 2 patients who were also observed postoperatively, lactate rose significantly at 190 min and 340 min postoperatively (decrease in Glc/Lac-ratio from 2.5 to 0.4 and 2.0 to 0.4 respectively). CONCLUSIONS: Microdialytic measurements of metabolic parameters can be performed on the human heart in a clinical setting. So far no complications have been observed and the microdialysis probe can be installed in such a fashion, that it can be easily removed transcutaneously during the postoperative course. Substances that are important in ischemia and reperfusion can be measured and their concentrations show changes that are not just artefacts. Postoperatively, metabolic alterations may be observed in the myocardial septum that are not recordable with conventional techniques (i.e., pressure measurements, cardiac output, ECG).

Microdialytic monitoring during cerebrovascular surgery.

Using microdialysis, levels of metabolites in the extracellular fluid of the cerebral cortex were monitored during neurovascular surgery (9 aneurysm and 5 extra-intracranial bypass operations). Our aim was to use microdialysis to detect any local ischemia which might be caused by brain retraction or temporary clipping. Parameters were therefore quantified whose levels in the dialysate are known to be influenced by ischemia (on-line pH, ascorbic acid, uric acid, glutathione, cysteine, glucose, lactate, glucose:lactate ratio). In the aneurysm series, on-line pH fell after introduction of the retractor, and in the majority of cases the other parameters also showed changes in accordance with ischemic conditions in the region of the probe. These changes disappeared at the end of retraction, or sometimes even before. During the bypass operations, there were no marked changes in on-line pH or in any of the measured parameters. However, in some of these patients values for the glucose:lactate ratio, ascorbic acid and uric acid lay outside the suggested basal levels for minimally disturbed cortex, indicating possible changes in metabolism caused by inadequate perfusion (carotid artery occlusion). We conclude that microdialysis is a sensitive method of detecting intraoperative changes in cerebral metabolism.

Cerebral microdialysis as a diagnostic tool in acute brain injury.

In the past 10 years, the management of brain injury has shown several advances. Besides new diagnostic tools many new tentative approaches have been investigated, such as jugular bulb measurement of oxygen and lactate differences and near-infrared spectroscopy. The latest tool is microdialysis, which uses a probe as an interface to the brain. This method uses internally perfused semi-permeable membrane probes, which allow neurochemical water-soluble substances to be collected outside the brain for further analysis. Since the late 1980s the first results of microdialysis in neurointensive care show that chemical substances such as lactate, glucose, amino acids, metabolites of several biochemical mechanisms and electrolytes are measurable. This heterogeneous approach now waits for validation for clinical use and for the most challenging aspect, the clinical interpretation and use to improve therapy. The aim of this review is to describe the state of the art of this new technique, including our own experiences and concepts.

Application of glutamate in the cortex of rats: a microdialysis study.

Glutamate, a major neurotransmitter in the brain, is also involved in pathophysiological processes resulting in secondary lesions following ischaemia or trauma. In the present study we investigated the relationship between glutamate excitotoxicity free radical induction (indicated by ascorbic acid level) and glucose-lactate metabolism. Monosodium glutamate was applied through microdialysis probes (500 mM in perfusate) into the cortex of rats for 30 minutes and ascorbic acid (ASC), glucose (GLUC) and lactate (LAC) were measured in dialysates. Glutamate produced a cortical lesion with an average volume of 12.7 +/- 1.4 mm3. Analysis of dialysates revealed a significant increase of ASC (325 +/- 52% of baseline) and LAC (677 +/- 86%) in the core lesion. In the lesion periphery a non-significant and short-lasting elevation was measured for both parameters with a second microdialysis probe (about 1.3 mm frontally to the first probe). A concomitant decrease of GLUC was found in both probes, reaching 29 +/- 8% and 60 +/- 7% of basal levels in the core and periphery of the lesion, respectively. In addition, we studied the delivery characteristics of several glutamate concentrations (10, 100 or 1000 mM in perfusate) during a 90-minute application into the cortex. The delivery of glutamate from the perfusate to the brain was about 33-38% in the first 30 min and afterwards 11 25% of the total in the perfusate. The results show that cortical application of glutamate changes the composition of the extracellular fluid, which could contribute to the development of the lesion.

A concept for the introduction of cerebral microdialysis in neurointensive care.

Before microdialysis (MD) can be introduced into the clinic as an improved method of cerebral monitoring, certain ethical, methodological and clinical factors must be considered. Access to the brain for probe insertion is offered by craniotomy or by routine intracranial pressure (ICP) monitoring and the additional lesion is minimal. Care must be taken that the two devices do not interfere with each other. In contrast to ICP monitoring, MD provides information about multiple aspects of brain metabolism. We can monitor either still intact tissue to prevent additional damage, or injured brain to decide on and control therapies. The parameters used must reflect pathological changes an early stage, and the analysis should be available on-line or immediately after sample collection. The effects off factors such as tube length and flow rate on the behaviour of the chosen parameters (in our case on-line pH, radical scavengers and uric acid) in the MD set-up must be investigated in vitro and in animal models before use in the clinic. The range of non-pathological values of parameters of interest in human brain should be known For this purpose we took measurements during an extracranial-intracranial bypass operation, and were able to compare values with those in a severely damaged brain. The mutual chronology of parameter changes and clinical events must be clear. Future aspects include the use of low-flow methods offering nearly 100% recovery, improved analytical methods, and combination of MD with other monitoring methods to obtain more exact information.

Microdialytic monitoring of the cortex during neurovascular surgery.

Using microdialysis combined with suitable analytical methods, levels of metabolites in the extracellular fluid of the cerebral cortex were monitored during neurovascular surgery (9 aneurysm and 5 bypass operations). Our aim was to use microdialysis to detect any local ischaemia which could be caused by brain retraction, temporary clipping and dissecting manoevres. For this purpose, parameters were quantified whose levels in the dialysate are known to be influenced by ischaemia (on-line pH, ascorbic acid, uric acid, glutathione, cysteine, glucose, lactate). In the aneurysm series, the on-line pH fell after introduction of the retractor, and rose after removal: also, in many cases, levels of ascorbic acid, glutathione and lactate increased and glucose decreased. These changes are all in accordance with ischaemic conditions in the region of the probe; they disappeared at the end of retraction, or sometimes even before. During the bypass operations, there were no marked changes in on-line pH or in any of the measured parameters. However, in 2 of these patients ascorbic acid, uric acid and glucose levels were very high during the whole measurement, indicating possible changes in metabolism caused by inadequate perfusion (carotid artery stenosis). We conclude that microdialysis is a sensitive method of detecting intraoperative changes in cerebral metabolism.

A new screwing device for fixing a microdialysis probe in critical care patients.

We describe a new, easy method which extends the use of clinical microdialysis to neurotrauma patients who primarily do not need a decompressing surgical intervention. In all head trauma patients in whom a Camino ICP-monitor is indicated a second hole (2 mm in diameter) is made, and the MD probe is fixed using the new screwing device. Before clinical use the system was tested during postmortem, confirming correct cortical placement of the probe in almost all cases. Two case reports are presented including their metabolic values. An extension to patients with non-traumatic brain disorders might be a future aspect.

Microdialytic monitoring during a cardiovascular operation.

In an aorta-coronary bypass operation, the heart is excluded from the circulation for many minutes, leading to ischemia. During this time the heart is cooled in order to mitigate damage. Microdialysis has been shown to be very suitable for detecting ischaemic changes e.g. in brain. We therefore used this method to study the time courses of several neurochemical parameters which have been shown to indicate ischaemia in animal models (ascorbic acid, glutathione, cysteine, uric acid, glucose, lactate and pH), during such a bypass operation. Three patients were investigated, the microdialysis probe being inserted into the interventricular septum of the heart. Our results show that microdialysis is technically feasible in the human heart in a clinical setting, although the operation becomes more demanding for the surgeon. All the above-mentioned parameters could be detected in the heart muscle. Some of them showed changes characteristic of ischaemia, and the effects of cooling on the metabolism could also be noted. Long term measurements are planned to enable delayed damage to be disclosed.

Experimental and clinical monitoring of glucose by microdialysis.

The aim of this investigation was to assess the use of cerebral extracellular glucose as a parameter for microdialytic monitoring in neurosurgical critical care patients. Samples were collected from four patients with severe head injury and one with subarachnoid haemorrhage for periods of 4.5-67 h. Glucose and lactate were analysed in the dialysates. The ratios of glucose to lactate were calculated to partially allow for changes in microdialytic conditions over time. On-line pH was measured for up to 3 days in three patients. In experiments with spontaneous hypertensive rats we found that extracellular glucose became unmeasurable in the ischemic zone after middle cerebral artery occlusion. Similarly, in 3 patients glucose became undetectable for several hours, and glucose/lactate tended to decrease during measurement. This was accompanied by high ICP in one patient, and by a hypoxic episode in another. In the two other patients glucose/lactate ratios showed a rising trend. Findings indicate that glucose, and the glucose/lactate ratio show some correlations with clinical course and are promising parameters for cerebral monitoring and therapeutic decision making.

Neurochemical monitoring and on-line pH measurements using brain microdialysis in patients in intensive care.

We will report on our preliminary findings using microdialysis to monitor three patients in intensive care with either severe head injury (SHI) or severe subarachnoid hemorrhage (SAH) for up to 72 hours. In addition, basal levels in uninjured brain were assessed during an extra-intracranial bypass operation. Samples were collected hourly or half-hourly (flow rate 2 microliters/min, perfusion medium 0.9% saline). Parameters measured were the antioxidants ascorbic acid, uric acid, glutathione and cysteine. In 2 patients, the pH of the dialysate (pHD) was also measured on-line with a specially constructed flow-through meter, and glucose and lactate levels were assessed in the dialysate. In patient 1 (SHI), there was practically no cerebral perfusion pressure because of high ICP; cysteine and lactate levels were very high and glucose not measurable. In patient 2 (SAH) a hypoxic episode was accompanied by increased uric acid and decreased glucose. In patient 3 (SHI), the pHD reflected normalisation of blood gases after hyperventilation. Results indicate that parameters are in the range known from experimental studies, and can be correlated with clinical situations. The pHD as valuable indicator of metabolic changes is also feasible bedside.

Levels of water-soluble antioxidants in astrocytoma and in adjacent tumor-free tissue.

The aim of the present study was to investigate the oxidative status in astrocytoma. Samples of brain tissue from the centre to the periphery of the tumor were obtained from 11 astrocytoma patients undergoing computer tomography-guided stereotaxic operation, who had been previously treated with the corticosteroid dexamethasone. Part of the sample was investigated histologically for clarification of tumor type, and the presence of neoplastic and non-neoplastic tissue and necrosis. The rest was used for the quantification of the antioxidants ascorbic acid, uric acid, glutathione and cysteine by high performance liquid chromatography, and for quantification of DNA. Levels of antioxidants were calculated as micrograms/g fresh tissue and mumol/g DNA, a parameter related to cell content. There was significantly more DNA in neoplastic samples than in non-neoplastic ones, indicating increased cell density. Uric acid (micrograms/g fresh tissue) was significantly increased in neoplastic compared with non-neoplastic tissue, and levels were even higher in necrotic tissue. There were no significant differences between neoplastic and non-neoplastic tissue levels of ascorbic acid, glutathione or cysteine, expressed as micrograms/g fresh tissue. However, when levels of these three compounds were expressed as mumol/g DNA, i.e. taking into account the higher cell density, ascorbic acid, glutathione and cysteine were significantly reduced in neoplastic samples compared with non-neoplastic ones. Results thus show that there are differences between the antioxidant levels in astrocytoma and non-neoplastic tissue, providing additional support for the hypothesis that free radicals play a role in tumor growth.

On-line monitoring of cerebral pH by microdialysis.

An on-line pH meter that can be mounted in microdialysis systems is described. The pH monitoring system was tested in rat cortex before and after middle cerebral artery occlusion (focal ischemia model). After probe implantation, pH values in the dialysate quickly reached a stable level that depended on perfusion medium (6.72, Ringer; 6.47, 0.9% saline) and flow rate (2 microliters/min). During ischemia, pH values sank rapidly and significantly, whereas lactic and ascorbic acid levels in the dialysate increased 9- to 12-fold. The pH of the dialysate is lower than that of the extracellular fluid because the relative recovery of carbon dioxide is about twice that of bicarbonate at the flow rate used, as shown in in vitro experiments. The pH meter would provide useful additional information during monitoring for ischemia, not only in experimental situations but also during neurosurgical intensive care. In the latter case, the on-line pH value would be a bedside parameter enabling fast feedback for setting analytical priorities and making therapeutical decisions.

[Cerebral microdialysis in neurological and neurosurgical research and clinical application in neurology and neurosurgery]. / Zerebrale Mikrodialyse in neurologischer und neurochirurgischer Forschung und klinische Anwendung in Neurologie und Neurochirurgie.

Microdialysis of the cerebral extracellular space (CES) provides information on the cortical metabolic state by measurement of the concentrations of metabolites and transmitters as well as of the extracellular pH. In trauma patients, this kind of monitoring should reveal damage from secondary cortical ischemia in an early, hopefully reversible state. The method further allows in vivo measurement of drug levels in the CES and investigation of their effect on metabolism. Although microdialysis has mainly been used experimentally, it is now beginning to be applied clinically. We report on our experience with both experimental and clinical work.