Fetal cerebral energy metabolism and electrocardiogram during experimental umbilical cord occlusion and resuscitation

Bedside interpretation of cerebral energy metabolism utilizing microdialysis in neurosurgical and general intensive care

The microdialysis technique was initially developed for monitoring neurotransmitters in animals. In 1995 the technique was adopted to clinical use and bedside enzymatic analysis of glucose, pyruvate, lactate, glutamate and glycerol. Under clinical conditions microdialysis has also been used for studying cytokines, protein biomarkers, multiplex proteomic and metabolomic analyses as well as for pharmacokinetic studies and evaluation of blood-brain barrier function. This review focuses on the variables directly related to cerebral energy metabolism and the possibilities and limitations of microdialysis during routine neurosurgical and general intensive care. Our knowledge of cerebral energy metabolism is to a large extent based on animal experiments performed more than 40 years ago. However, the different biochemical information obtained from various techniques should be recognized. The basic animal studies analyzed brain tissue homogenates while the microdialysis technique reflects the variables in a narrow zone of interstitial fluid surrounding the probe. Besides the difference of the volume investigated, the levels of the biochemical variables differ in different compartments. During bedside microdialysis cerebral energy metabolism is primarily reflected in measured levels of glucose, lactate and pyruvate and the lactate to pyruvate (LP) ratio. The LP ratio reflects cytoplasmatic redox-state which increases instantaneously during insufficient aerobic energy metabolism. Cerebral ischemia is characterized by a marked increase in intracerebral LP ratio at simultaneous decreases in intracerebral levels of pyruvate and glucose. Mitochondrial dysfunction is characterized by a moderate increase in LP ratio at a very marked increase in cerebral lactate and normal or elevated levels of pyruvate and glucose. The patterns are of importance in particular for interpretations in transient cerebral ischemia. A new technique for evaluating global cerebral energy metabolism by microdialysis of the draining cerebral venous blood is discussed. In experimental studies it has been shown that pronounced global cerebral ischemia is reflected in venous cerebral blood. Jugular bulb microdialysis has been investigated in patients suffering from subarachnoid hemorrhage, during cardiopulmonary bypass and resuscitation after out of hospital cardiac arrest. Preliminary results indicate that the new technique may give valuable information of cerebral energy metabolism in clinical conditions when insertion of an intracerebral catheter is contraindicated.

Bedside microdialysis for detection of early brain injury after out-of-hospital cardiac arrest

Bedside detection and early treatment of lasting cerebral ischemia may improve outcome after out-of-hospital cardiac arrest (OHCA). This feasibility study explores the possibilities to use microdialysis (MD) for continuous monitoring of cerebral energy metabolism by analyzing the draining cerebral venous blood. Eighteen comatose patients were continuously monitored with jugular bulb and radial artery (reference) MD following resuscitation. Median time from cardiac arrest to MD was 300 min (IQR 230–390) with median monitoring time 60 h (IQR 40–81). The lactate/pyruvate ratio in cerebral venous blood was increased during the first 20 h after OHCA, and significant differences in time-averaged mean MD metabolites between jugular venous and artery measurements, were documented (p < 0.02). In patients with unfavorable outcome (72%), cerebral venous lactate and pyruvate levels remained elevated during the study period. In conclusion, the study indicates that jugular bulb microdialysis (JBM) is feasible and safe. Biochemical signs of lasting ischemia and mitochondrial dysfunction are frequent and associated with unfavorable outcome. The technique may be used in comatose OHCA patients to monitor biochemical variables reflecting ongoing brain damage and support individualized treatment early after resuscitation.

Moderately prolonged permissive hypotension results in reversible metabolic perturbation evaluated by intracerebral microdialysis - an experimental animal study

Background

Damage control resuscitation (DCR) and damage control surgery (DCS) is the main strategy in patients with uncontrollable hemorrhagic shock. One aspect of DCR is permissive hypotension. However, the duration of hypotension that can be tolerated without affecting the brain is unknown. In the present study we investigate the effect of 60 min severe hypotension on the brain’s energy metabolism and seek to verify earlier findings that venous cerebral blood can be used as a marker of global cerebral energy state.

Material and methods

Ten pigs were anaesthetized, and vital parameters recorded. Microdialysis catheters were placed in the left parietal lobe, femoral artery, and superior sagittal sinus for analysis of lactate, pyruvate, glucose, glycerol, and glutamate. Hemorrhagic shock was induced by bleeding the animal until mean arterial pressure (MAP) of 40 mmHg was achieved. After 60 min the pigs were resuscitated with autologous blood and observed for 3 h.

Results

At baseline the lactate to pyruvate ratios (LP ratio) in the hemisphere, artery, and sagittal sinus were (median (interquartile range)) 13 (8–16), 21 (18–24), and 9 (6–22), respectively. After induction of hemorrhagic shock, the LP ratio from the left hemisphere in 9 pigs increased to levels indicating a reversible perturbation of cerebral energy metabolism 19 (12–30). The same pattern was seen in LP measurements from the femoral artery 28 (20–35) and sagittal sinus 22 (19–26). At the end of the experiment hemisphere, artery and sinus LP ratios were 16 (10–23), 17 (15–25), and 17 (10–27), respectively. Although hemisphere and sinus LP ratios decreased, they did not reach baseline levels (p < 0.05). In one pig hemisphere LP ratio increased to a level indicating irreversible metabolic perturbation (LP ratio > 200).

Conclusion

During 60 min of severe hypotension intracerebral microdialysis shows signs of perturbations of cerebral energy metabolism, and these changes trend towards baseline values after resuscitation. Sagittal sinus microdialysis values followed hemisphere values but were not distinguishable from systemic arterial values. Venous (jugular bulb) microdialysis might have a place in monitoring conditions where global cerebral ischemia is a risk.

Cyclosporin A ameliorates cerebral oxidative metabolism and infarct size in the endothelin-1 rat model of transient cerebral ischaemia

Cerebral microdialysis can be used to detect mitochondrial dysfunction, a potential target of neuroprotective treatment. Cyclosporin A (CsA) is a mitochondrial stabiliser that in a recent clinical stroke trial showed protective potential in patients with successful recanalisation. To investigate specific metabolic effects of CsA during reperfusion, and hypothesising that microdialysis values can be used as a proxy outcome measure, we assessed the temporal patterns of cerebral energy substrates related to oxidative metabolism in a model of transient focal ischaemia. Transient ischaemia was induced by intracerebral microinjection of endothelin-1 (150 pmol/15 µL) through stereotaxically implanted guide cannulas in awake, freely moving rats. This was immediately followed by an intravenous injection of CsA (NeuroSTAT; 15 mg/kg) or placebo solution during continuous microdialysis monitoring. After reperfusion, the lactate/pyruvate ratio (LPR) was significantly lower in the CsA group vs placebo (n = 17, 60.6 ± 24.3%, p = 0.013). Total and striatal infarct volumes (mm³) were reduced in the treatment group (n = 31, 61.8 ± 6.0 vs 80.6 ± 6.7, p = 0.047 and 29.9 ± 3.5 vs 41.5 ± 3.9, p = 0.033). CsA treatment thus ameliorated cerebral reperfusion metabolism and infarct size. Cerebral microdialysis may be useful in evaluating putative neuroprotectants in ischaemic stroke.

Patterns of cerebral tissue oxygen tension and cytoplasmic redox state in bacterial meningitis

BACKGROUND

Compromised cerebral energy metabolism is common in patients with bacterial meningitis. In this study, simultaneous measurements of cerebral oxygen tension and lactate/pyruvate ratio were compared to explore whether disturbed energy metabolism was usually caused by insufficient tissue oxygenation or compromised oxidative metabolism of pyruvate indicating mitochondrial dysfunction.

SUBJECT AND METHODS

Ten consecutive patients with severe streptococcus meningitis were included in this prospective cohort study. Intracranial pressure, brain tissue oxygen tension (PbtO₂ ), and energy metabolism (intracerebral microdialysis) were continuously monitored in nine patients. A cerebral lactate/pyruvate (LP) ratio <30 was considered indicating normal oxidative metabolism, LP ratio >30 simultaneously with pyruvate below lower normal level (70 µmol/L) was interpreted as biochemical indication of ischemia, and LP ratio >30 simultaneously with a normal or increased level of pyruvate was interpreted as mitochondrial dysfunction. The biochemical variables were compared with PbtO₂ simultaneously monitored within the same cerebral region.

RESULTS

In two cases, the LP ratio was normal during the whole study period and the simultaneously monitored PbtO₂ was 18 ± 6 mm Hg. In six cases, interpreted as mitochondrial dysfunction, the simultaneously monitored PbtO₂ was 20 ± 6 mm Hg and without correlation with the LP ratio. In one patient, exhibiting a pattern interpreted as ischemia, PbtO₂ decreased below 10 mm Hg and a correlation between LP and PbtO₂ was observed.

CONCLUSION

This study demonstrated that compromised cerebral energy metabolism, evidenced by increased LP ratio, was common in patients with severe bacterial meningitis while not related to insufficient tissue oxygenation.

Aspects on the Physiological and Biochemical Foundations of Neurocritical Care

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

Microdialysate concentration changes do not provide sufficient information to evaluate metabolic effects of lactate supplementation in brain-injured patients

Cerebral microdialysis is a widely used clinical tool for monitoring extracellular concentrations of selected metabolites after brain injury and to guide neurocritical care. Extracellular glucose levels and lactate/pyruvate ratios have high diagnostic value because they can detect hypoglycemia and deficits in oxidative metabolism, respectively. In addition, patterns of metabolite concentrations can distinguish between ischemia and mitochondrial dysfunction, and are helpful to choose and evaluate therapy. Increased intracranial pressure can be life-threatening after brain injury, and hypertonic solutions are commonly used for pressure reduction. Recent reports have advocated use of hypertonic sodium lactate, based on claims that it is glucose sparing and provides an oxidative fuel for injured brain. However, changes in extracellular concentrations in microdialysate are not evidence that a rise in extracellular glucose level is beneficial or that lactate is metabolized and improves neuroenergetics. The increase in glucose concentration may reflect inhibition of glycolysis, glycogenolysis, and pentose phosphate shunt pathway fluxes by lactate flooding in patients with mitochondrial dysfunction. In such cases, lactate will not be metabolizable and lactate flooding may be harmful. More rigorous approaches are required to evaluate metabolic and physiological effects of administration of hypertonic sodium lactate to brain-injured patients.

A technique for continuous bedside monitoring of global cerebral energy state

Background

Cerebral cytoplasmatic redox state is a sensitive indicator of cerebral oxidative metabolism and is conventionally evaluated from the extracellular lactate/pyruvate (LP) ratio. In the present experimental study of global cerebral ischemia induced by hemorrhagic shock, we investigate whether the LP ratio obtained from microdialysis of cerebral venous blood may be used as a surrogate marker of global cerebral energy state.

Methods

Six female pigs were anesthetized and vital parameters were recorded. Microdialysis catheters were placed in the left parietal lobe, the superior sagittal sinus, and the femoral artery. Hemorrhagic shock was achieved by bleeding the animals to a mean arterial pressure (MAP) of approximately 40 mmHg and kept at a MAP of about 30–40 mmHg for 90 min. The animals were resuscitated with autologous whole blood followed by 3 h of observation.

Results

The LP ratio obtained from the intracerebral and intravenous catheters immediately increased during the period of hemorrhagic shock while the LP ratio in the arterial blood remained close to normal levels. At the end of the experiment, median LP ratio (interquartile range) obtained from the intracerebral, intravenous, and intra-arterial microdialysis catheters were 846 (243–1990), 309 (103–488), and 27 (21–31), respectively. There was a significant difference in the LP ratio obtained from the intravenous location and the intra-arterial location (P < 0.001).

Conclusions

During cerebral ischemia induced by severe hemorrhagic shock, intravascular microdialysis of the draining venous blood will exhibit changes of the LP ratio revealing the deterioration of global cerebral oxidative energy metabolism. In neurocritical care, this technique might be used to give information regarding global cerebral energy metabolism in addition to the regional information obtained from intracerebral microdialysis catheters. The technique might also be used to evaluate cerebral energy state in various critical care conditions when insertion of an intracerebral microdialysis catheter may be contraindicated, e.g., resuscitation after cardiac standstill, open-heart surgery, and multi-trauma.

Bedside evaluation of cerebral energy metabolism in severe community-acquired bacterial meningitis

BACKGROUND

Mortality and morbidity have remained high in bacterial meningitis. Impairment of cerebral energy metabolism probably contributes to unfavorable outcome. Intracerebral microdialysis is routinely used to monitor cerebral energy metabolism, and recent experimental studies indicate that this technique may separate ischemia and non-ischemic mitochondrial dysfunction. The present study is a retrospective interpretation of biochemical data obtained in a series of patients with severe community-acquired meningitis.

METHODS

Cerebral energy metabolism was monitored in 15 patients with severe community-acquired meningitis utilizing intracerebral microdialysis and bedside biochemical analysis. According to previous studies, cerebral ischemia was defined as lactate/pyruvate (LP) ratio > 30 with intracerebral pyruvate level < 70 µmol L(-1). Non-ischemic mitochondrial dysfunction was defined as LP-ratio > 30 at a normal or increased interstitial concentration of pyruvate (≥ 70 μmol L(-1)). Patients with LP-ratios < 30 were classified as no mitochondrial dysfunction.

RESULTS

The biochemical pattern was in 8 patients (10 microdialysis catheters) classified as no mitochondrial dysfunction, in 5 patients classified as non-ischemic mitochondrial dysfunction, and in 2 patients (3 catheters) classified as ischemia.

CONCLUSIONS

In patients with severe community-acquired meningitis, compromised cerebral energy metabolism occurs frequently and was diagnosed in 7 out of 15 cases. A biochemical pattern of non-ischemic mitochondrial dysfunction appears to be a more common underlying condition than cerebral ischemia.

Bedside diagnosis of mitochondrial dysfunction after malignant middle cerebral artery infarction

BACKGROUND

The study explores whether the cerebral biochemical pattern in patients treated with hemicraniectomy after large middle cerebral artery infarcts reflects ongoing ischemia or non-ischemic mitochondrial dysfunction.

METHODS

The study includes 44 patients treated with decompressive hemicraniectomy (DCH) due to malignant middle cerebral artery infarctions. Chemical variables related to energy metabolism obtained by microdialysis were analyzed in the infarcted tissue and in the contralateral hemisphere from the time of DCH until 96 h after DCH.

RESULTS

Reperfusion of the infarcted tissue was documented in a previous report. Cerebral lactate/pyruvate ratio (L/P) and lactate were significantly elevated in the infarcted tissue compared to the non-infarcted hemisphere (p < 0.05). From 12 to 96 h after DCH the pyruvate level was significantly higher in the infarcted tissue than in the non-infarcted hemisphere (p < 0.05).

CONCLUSION

After a prolonged period of ischemia and subsequent reperfusion, cerebral tissue shows signs of protracted mitochondrial dysfunction, characterized by a marked increase in cerebral lactate level with a normal or increased cerebral pyruvate level resulting in an increased LP-ratio. This biochemical pattern contrasts to cerebral ischemia, which is characterized by a marked decrease in cerebral pyruvate. The study supports the hypothesis that it is possible to diagnose cerebral mitochondrial dysfunction and to separate it from cerebral ischemia by microdialysis and bed-side biochemical analysis.

Metabolomic profiling in perinatal asphyxia: a promising new field

Metabolomics, the latest “omic” technology, is defined as the comprehensive study of all low molecular weight biochemicals, “metabolites” present in an organism. As a systems biology approach, metabolomics has huge potential to progress our understanding of perinatal asphyxia and neonatal hypoxic-ischaemic encephalopathy, by uniquely detecting rapid biochemical pathway alterations in response to the hypoxic environment. The study of metabolomic biomarkers in the immediate neonatal period is not a trivial task and requires a number of specific considerations, unique to this disease and population. Recruiting a clearly defined cohort requires standardised multicentre recruitment with broad inclusion criteria and the participation of a range of multidisciplinary staff. Minimally invasive biospecimen collection is a priority for biomarker discovery. Umbilical cord blood presents an ideal medium as large volumes can be easily extracted and stored and the sample is not confounded by postnatal disease progression. Pristine biobanking and phenotyping are essential to ensure the validity of metabolomic findings. This paper provides an overview of the current state of the art in the field of metabolomics in perinatal asphyxia and neonatal hypoxic-ischaemic encephalopathy. We detail the considerations required to ensure high quality sampling and analysis, to support scientific progression in this important field.

The added predictive value of biphasic events in ST analysis of the fetal electrocardiogram for intrapartum fetal monitoring

OBJECTIVE

To study the predictive value of biphasic ST-events for interventions for suspected fetal distress and adverse neonatal outcome, when using ST-analysis of the fetal electrocardiogram (FECG) for intrapartum fetal monitoring.

DESIGN

Prospective cohort study.

SETTING

Three academic hospitals in Sweden.

POPULATION

Women in labor with a high-risk singleton fetus in cephalic position beyond 36 weeks of gestation.

METHODS

In women in labor who were monitored with conventional cardiotocography, ST-waveform analysis was recorded and concealed. Traces with biphasic ST-events of the FECG (index) were compared with traces without biphasic events of the FECG. The ability of biphasic events to predict interventions for suspected fetal distress and adverse outcome was assessed using univariable and multivariable logistic regression analyses.

MAIN OUTCOME MEASURES

Interventions for suspected fetal distress and adverse outcome (defined as presence of metabolic acidosis (i.e. umbilical cord pH <7.05 and base deficit in extracellular fluid >12 mmol), umbilical cord pH <7.00, 5-min Apgar score <7, admittance to neonatal intensive care unit or perinatal death).

RESULTS

Although the presence of biphasic events of the FECG was associated with more interventions for fetal distress and an increased risk of adverse outcome compared with cases with no biphasic events, the presence of significant (i.e. intervention advised according to cardiotocography interpretation) biphasic events showed no independent association with interventions for fetal distress [odds ratio (OR) 1.71, 95% confidence interval (CI) 0.65-4.50] or adverse outcome (OR 1.96, 95% CI 0.74-5.24).

CONCLUSION

The presence of significant biphasic events did not discriminate in the prediction of interventions for fetal distress or adverse outcome. Therefore, biphasic events in relation to ST-analysis monitoring during birth should be omitted if future studies confirm our findings.

Bedside diagnosis of mitochondrial dysfunction in aneurysmal subarachnoid hemorrhage

OBJECTIVES

Aneurysmal subarachnoid hemorrhage (SAH) is frequently associated with delayed neurological deterioration (DND). Several studies have shown that DND is not always related to vasospasm and ischemia. Experimental and clinical studies have recently documented that it is possible to diagnose and separate cerebral ischemia and mitochondrial dysfunction bedside. The study explores whether cerebral biochemical variables in SAH patients most frequently exhibit a pattern indicating ischemia or mitochondrial dysfunction.

METHODS

In 55 patients with severe SAH, intracerebral microdialysis was performed during neurocritical care with bedside analysis and display of glucose, pyruvate, lactate, glutamate, and glycerol. The biochemical patterns observed were compared to those previously described in animal studies of induced mitochondrial dysfunction as well as the pattern obtained in patients with recirculated cerebral infarcts.

RESULTS

In 29 patients, the biochemical pattern indicated mitochondrial dysfunction while 10 patients showed a pattern of cerebral ischemia, six of which also exhibited periods of mitochondrial dysfunction. Mitochondrial dysfunction was observed during 5162 h. An ischemic pattern was obtained during 688 h. Four of the patients (40%) with biochemical signs of ischemia died at the neurosurgical department as compared with three patients (10%) in the group of mitochondrial dysfunction.

CONCLUSIONS

The study documents that mitochondrial dysfunction is a common cause of disturbed cerebral energy metabolism in patients with SAH. Mitochondrial dysfunction may increase tissue sensitivity to secondary adverse events such as vasospasm and decreased cerebral blood flow. The separation of ischemia and mitochondrial dysfunction bedside by utilizing microdialysis offers a possibility to evaluate new therapies.

Cerebral energy metabolism during induced mitochondrial dysfunction

BACKGROUND

In patients with traumatic brain injury as well as stroke, impaired cerebral oxidative energy metabolism may be an important factor contributing to the ultimate degree of tissue damage. We hypothesize that mitochondrial dysfunction can be diagnosed bedside by comparing the simultaneous changes in brain tissue oxygen tension (PbtO(2)) and cerebral cytoplasmatic redox state. The study describes cerebral energy metabolism during mitochondrial dysfunction induced by sevoflurane in piglets.

METHODS

Ten piglets were included, seven in the experimental group (anesthetized with sevoflurane) and three in the control group (anesthetized with midazolam). PbtO(2) and cerebral levels of glucose, lactate, and pyruvate were monitored bilaterally. The biochemical variables were obtained by intracerebral microdialysis.

RESULTS

All global variables were within normal range and did not differ significantly between the groups except for blood lactate that was slightly higher in the experimental group. Mitochondrial dysfunction was observed in the group of animals initially anesthetized with sevoflurane. Cerebral glucose was significantly lower in the experimental group than in the control group whereas lactate and lactate/pyruvate ratio were significantly higher. Pyruvate and tissue oxygen tension remained within normal range in both groups. Changes of intracerebral variables indicating mitochondrial dysfunction were present already from the very start of the monitoring period.

CONCLUSION

Intracerebral microdialysis revealed mitochondrial dysfunction by marked increases in cerebral lactate and lactate/pyruvate ratio simultaneously with normal levels of pyruvate and a normal PbtO(2). This metabolic pattern is distinctively different from cerebral ischemia, which is characterized by simultaneous decreases in PbtO(2) and intracerebral pyruvate.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Cerebral energy metabolism and microdialysis in neurocritical care

It is of obvious clinical importance to monitor cerebral metabolism--in particular, cerebral energy metabolism and indicators of cellular damage-online at the bedside. The technique of cerebral microdialysis provides the opportunity for continuous monitoring of metabolic changes in the tissue before they are reflected in peripheral blood chemistry or in systemic physiological parameters. The basic idea of microdialysis is to mimic the function of a blood capillary by positioning a thin dialysis tube in the tissue and to be used to analyze the chemical composition of the interstitial fluid. The biochemical variables used during routine monitoring were chosen to cover important aspects of cerebral energy metabolism (glucose, pyruvate and lactate), to indicate excessive interstitial levels of excitatory transmitter substance (glutamate) and to give indications of degradation of cellular membranes (glycerol). Furthermore, pharmokinetic studies can be conducted using microdialysis. This article discusses technical and physiological aspects of microdialysis, and its clinical applications in brain injury.