On-line monitoring of cerebral pH by microdialysis

Lamellar perfusion and energy metabolism in a preferential weight bearing model

BACKGROUND

Supporting limb laminitis (SLL) is suspected to be caused by lamellar ischaemia as a consequence of increased mechanical load.

OBJECTIVES

Examine the effects of prolonged preferential weight bearing (PWB) on lamellar perfusion and metabolism.

STUDY DESIGN

In vivo experiment.

METHODS

Microdialysis probes were inserted in the lamellar and sublamellar dermis of one forelimb in 13 Standardbred horses. In six horses, a platform shoe (contralateral forelimb) was used to induce increased load on the microdialysis-instrumented forelimb (PWB). The remaining seven horses were controls (CON). All horses were housed in stocks with limb weight distribution logged continuously for 92 hours. Microdialysate was collected and analysed every 4 hours for glucose, lactate, pyruvate, and lactate to pyruvate ratio (L:P). Microdialysis urea clearance was used to estimate lamellar perfusion. Data were analysed using a mixed-effects linear regression model.

RESULTS

Median [IQR] load on the microdialysis-instrumented limb was equivalent to 38.7% bwt. [37.3-40.3] in PWB and 27.3% bwt. [26.6-28] in CON. Limb offloading frequency increased in CON (P < .001) but not PWB (P = .2). Lamellar microdialysate glucose decreased in PWB (P < .001) and CON (P = .004), however, the rate of decrease was higher in PWB (P = .007). Lamellar L:P increased in PWB (P < .001) and peaked at 196 [79-656], whereas L:P did not change over time in CON (P = .6) and peaked at 42 [41-49]. Lamellar urea clearance decreased in PWB (P < .001) but not in CON (P = .3). Sublamellar L:P and urea clearance did not change over time in either group (P > .05).

MAIN LIMITATIONS

The PWB model may not be representative of naturally occurring SLL.

CONCLUSIONS

Evidence of lamellar ischaemia (increased L:P and decreased urea clearance) was detected exclusively in the lamellar dermis of PWB feet subjected to persistently increased load. Lamellar ischaemia is a consequence of increased mechanical load and likely contributes to the development of SLL.

Low-dose Carbicarb improves cerebral outcome after asphyxial cardiac arrest in rats

STUDY OBJECTIVE

Controversy surrounds the use of buffers during cardiac arrest to correct acidosis. The objective of this study was to determine whether attenuation or neutralization of cerebral acidosis by Carbicarb alters hippocampal glutamate levels, neuronal cell death, and neurologic deficits after reperfusion from asphyxial cardiac arrest in rats.

METHODS

Rats were prospectively randomized to either a control (n=45), low-dose Carbicarb (LDC; 3 mL/kg, n=45), or high-dose Carbicarb (HDC; 6 mL/kg, n=45) group in a blinded fashion during resuscitation after 8 minutes of asphyxial cardiac arrest. Microdialysis was used to assess brain pH and glutamate. A neurologic deficit score and neuronal cell death in the hippocampus were determined at day 7.

RESULTS

Resuscitation was greatest in LDC rats (42/45) and least in HDC rats (28/45) versus that in control rats (34/45). Brain pH was higher in the LDC and HDC rats 10 minutes after resuscitation and remained higher than that of control rats for 120 minutes after resuscitation. Glutamate levels at 10 to 120 minutes after reperfusion were lowest in the LDC rats. LDC rats had the lowest neurologic deficit score (1+/-2) versus that of control rats (13+/-8) and HDC rats (19+/-6). Hippocampal neuronal cell death was lowest in LDC rats (30+/-20) versus that in control rats (86+/-47) and HDC rats (233+/-85).

CONCLUSION

LDC administered during resuscitation from asphyxial cardiac arrest attenuated acidosis, improved resuscitation, and reduced neurologic deficits and the number of dead hippocampal neurons. Neutralization of cerebral acidosis with HDC increased the number of dead hippocampal neurons and neurologic deficits after resuscitation from cardiac arrest in rats.

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.

Intracerebral microdialysis and its clinical application: a review

In vivo intracerebral microdialysis is an important neurochemical technique that has been used extensively in the experimental setting. Relatively recently, techniques have been developed to utilize this method in human subjects. The past decade has seen the advent of clinical investigations utilizing in vivo microdialysis in a number of neuropathological states. This review summarizes the principles of in vivo microdialysis techniques, as applied to humans, while discussing the significance of recent investigations for future clinical development.

Theory and practice of microdialysis--prospect for future clinical use

The application of microdialysis for neurochemical monitoring in neurosurgery and neurointensive care is rapidly expanding in a number of clinical centers around the world. In order for microdialysis to become a future routine method in these clinical settings a number of problems, outlined in this communication, must be solved by the clinical researchers and the commercial companies. Regardless of the future success as a routine method, it is already obvious that microdialysis will be an important clinical research tool for years to come, providing new important insights into the pathophysiology of acute human brain injury.

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.

Cerebral microdialysis combined with single-neuron and electroencephalographic recording in neurosurgical patients. Technical note

Monitoring physiological changes in the brain parenchyma has important applications in the care of neurosurgical patients. A technique is described for measuring extracellular neurochemicals by cerebral microdialysis with simultaneous recording of electroencephalographic (EEG) and single-unit (neuron) activity in selected targets in the human brain. Forty-two patients with medically intractable epilepsy underwent stereotactically guided implantation of a total of 423 intracranial depth electrodes to delineate potentially resectable seizure foci. The electrodes had platinum alloy contacts for EEG recordings and four to nine 40-microm microwires for recording single-unit neuron activity. Eighty-six electrodes also included microdialysis probes introduced via the electrode lumens. During monitoring on the neurosurgical ward, electrophysiological recording and cerebral microdialysis sampling were performed during seizures, cognitive tasks, and sleep-waking cycles. The technique described here could be used in developing novel approaches for evaluation and treatment in a variety of neurological conditions such as head injury, subarachnoid hemorrhage, epilepsy, and movement disorders.

Cortical brain microdialysis and temperature monitoring during hypothermic circulatory arrest in humans

OBJECTIVES

Critical vascular surgery of the brain or the heart occasionally requires total cessation of the circulatory system. Profound hypothermia is used to protect the brain from ischaemic injury. This study explores the use of microdialysis to measure metabolic indices of ischaemia: glutamate, lactate, and pH, and cerebral temperature during profound hypothermia and circulatory arrest.

METHODS

Effluent from a microdialysis catheter placed in the cerebral cortex of three patients undergoing complete circulatory arrest was continuously sampled. Samples were pooled over 10 minute periods and glutamate and lactate concentrations were measured postoperatively. Brain temperature and pH were measured on line intraoperatively. Electroencephalography and monitoring of somatosensory evoked potentials and brainstem auditory evoked potentials were simultaneously carried out.

RESULTS

Patient 1 had normal glutamate and lactate. PH was 6.75 to 6.85 and increased to 6.9 after warming ensued. Patient 2 had raised glutamate and lactate during most measurements. The glutamate concentrations peaked at 305 microM/l at the start of the measurements and fell below 20 microM/l after warming. The lactate concentrations peaked at 680 microM/l before cooling, rose to 1040 microM/l during the cooling process, decreased to 212 microM/l during circulatory arrest, and rose again to 620 microM/l after warming. The pH started at 7.06 and continued a downward course until stabilising at a pH of 6.5 after circulatory arrest. Patient 3 had a transient, mild increase in glutamate and lactate during the cooling and warming period. pH was stable throughout.

CONCLUSION

Microdialysis combined with temperature and pH measurements of the cerebral cortex promises to be an important tool in detecting cerebral ischaemia. Further studies are needed to validate our findings and test the feasibility of modifying ischaemic changes.

High-performance liquid chromatographic analysis of (S)-alpha-amino-5-phosphonomethyl[1,1'-biphenyl]-3-propanoic acid (EAB 515) in brain and blood microdialysate (on-line) and in plasma ultrafiltrate of freely moving rats

(S)-alpha-Amino-5-phosphonomethyl[1,1'-biphenyl]-3-propanoic acid (EAB 515, I), a competitive antagonist of the N-methyl-D-aspartate receptor, has significant pharmacological activity in the central nervous system (CNS). An extremely sensitive and selective analytical method was developed for the simultaneous analysis of I and its hydroxylated analog (RDC, II) in the microdialysate (MD) and plasma ultrafiltrate (UF) of rats. Microdialysis was used for in vivo sampling of unbound drug in the CSF, cortical extracellular fluid and in the blood of freely moving rats. Compound II was used for retrodialysis-based in vivo calibration of microdialysis probes to estimate the recovery of I. Compound I, being extremely hydrophilic with a high degree of ionization at the physiological pH of 7.4, has limited access to the brain regions. This, combined with its low microdialysis recovery, made the estimation of low brain concentrations of I a challenge. The analytes in MD and UF were separated (within 5 min) by reversed-phase HPLC on a 250 x 4.6 mm I.D. Maxsil 5 microns RP-2 column, and fluorescence of the eluent was monitored at 255 nm (lambda ex) and 320 nm (lambda em). A 0.09% (v/v) aqueous solution of trifluoroacetic acid (1 ml/min) was used as the mobile phase. The response for I in MD and UF samples was linear from 5 to 2000 ng/ml and from 20 to 10,000 ng/ml, respectively. The between-run (n = 6) and within-run (n = 3) variability of the assay was < 15%. Plasma-protein binding of I (fu = 0.68) was determined to be linear from 0.1 to 10 micrograms/ml. The analytical sensitivity, precision and accuracy of this method was suitable for the characterization of the pharmacokinetics and the CNS distribution of I, following administration of intravenous (i.v.) infusion, single i.v. bolus and multiple i.v. bolus doses of I to freely moving rats, with continuous microdialysate sampling of multiple tissues and simultaneous on-line HPLC analysis. Pharmacokinetic parameters for I, as determined from concentrations in blood MD samples with on-line analysis, were in good agreement with those estimated from concentrations in the UF of plasma samples obtained by conventional sampling.

Acute acidosis elevates malonaldehyde in rat brain in vivo

Oxidative stress in brain tissue was measured experimentally in situ using microdialysis to sample the extracellular environment for a lipid peroxidation breakdown product and antioxidants. The extracellular concentrations of the lipid peroxidation product malonaldehyde (MDA) and the antioxidants ascorbic acid (AA) and uric acid (UA) were measured in rat cortex and striatum in vivo using microdialysis coupled to HPLC with UV detection. Tissue acidosis following ischaemia and epileptic seizures may contribute to neuronal damage, which may be mediated by reactive oxygen species. Perfusion of microdialysis probes with acidic artificial cerebrospinal fluid (pH 6) led to a significant increase in the sampled concentration of MDA and the antioxidant ascorbic acid. Simultaneous perfusion of ascorbate (5 mM) with acidic ACSF (pH 6) completely attenuated the rise in lipid peroxidation. This study provides in vivo evidence for acidosis induced oxidative stress in brain tissue and an antioxidant action of ascorbate. The methodology described here can provide direct in vivo information in respect of oxidative stress in experimental situations. The method could equally be applied to the assessment of oxidative stress in a number of pathological models not necessarily confined to the CNS.

A novel microdialysis probe designed for clinical use: potential analytical and therapeutic applications

Significant obstacles to the use of microdialysis in the clinic for diagnostic or therapeutic purposes include the production of dedicated entry port through the skull and the formation of a tract by the insertion of a probe into the parenchyma. We have developed a microdialysis probe that is minimally invasive and can be combined with an intracranial pressure probe, recording electrode, or other intracranial probe, that is minimally invasive. Yet the surface area of this probe is very high, permitting high recovery efficiencies even at relatively high flow rates. This probe design makes possible minimally invasive measurement of the peroxidation product, uric acid, and excitatory amino acids, two analytes that increase in experimental traumatic brain injury in animals. Moreover, its large surface area makes therapeutic applications of microdialysis probes in the brain potentially feasible. A pilot evaluation of the ability of microdialysis to have therapeutic benefit in limiting experimental excitotoxin lesions induced in rat striatum by N-methyl-D-aspartate (NMDA) is reported.

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.

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.

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 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.

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.

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.