Cerebral glucose and oxygen metabolism in patients with fulminant hepatic failure

Low cerebral energy metabolism in hepatic encephalopathy reflects low neuronal energy demand. Role of ammonia-induced increased GABAergic tone

Hepatic encephalopathy (HE) is a frequent and devastating but generally reversible neuropsychiatric complication secondary to chronic and acute liver failure. During HE, brain energy metabolism is markedly reduced and it remains unclear whether this is due to external or internal energy supply limitations, or secondary to depressed neuronal cellular functions - and if so, which mechanisms that are in play. The extent of deteriorated cerebral function correlates to blood ammonia levels but the metabolic link to ammonia is not clear. Early studies suggested that high levels of ammonia inhibited key tricarboxylic acid (TCA) cycle enzymes thus limiting mitochondrial energy production and oxygen consumption; however, later studies by us and others showed that this is not the case in vivo. Here, based on a series of translational studies from our group, we advocate the view that the low cerebral energy metabolism of HE is likely to be caused by neuronal metabolic depression due to an elevated GABAergic tone rather than by restricted energy availability. The increased GABAergic tone seems to be secondary to synthesis of large amounts of glutamine in astrocytes for detoxification of ammonia with the glutamine acting as a precursor for elevated neuronal synthesis of vesicular GABA.

Fifteen years of urea cycle disorders brain research: Looking back, looking forward

Urea cycle disorders (UCD) are inherited diseases resulting from deficiency in one of six enzymes or two carriers that are required to remove ammonia from the body. UCD may be associated with neurological damage encompassing a spectrum from asymptomatic/mild to severe encephalopathy, which results in most cases from Hyperammonemia (HA) and elevation of other neurotoxic intermediates of metabolism. Electroencephalography (EEG), Magnetic resonance imaging (MRI) and Proton Magnetic resonance spectroscopy (MRS) are noninvasive measures of brain function and structure that can be used during HA to guide management and provide prognostic information, in addition to being research tools to understand the pathophysiology of UCD associated brain injury. The Urea Cycle Rare disorders Consortium (UCDC) has been invested in research to understand the immediate and downstream effects of hyperammonemia (HA) on brain using electroencephalogram (EEG) and multimodal brain MRI to establish early patterns of brain injury and to track recovery and prognosis. This review highlights the evolving knowledge about the impact of UCD and HA in particular on neurological injury and recovery and use of EEG and MRI to study and evaluate prognostic factors for risk and recovery. It recognizes the work of others and discusses the UCDC's prior work and future research priorities.

Changes of Coenzyme A and Acetyl-Coenzyme A Concentrations in Rats after a Single-Dose Intraperitoneal Injection of Hepatotoxic Thioacetamide Are Not Consistent with Rapid Recovery

Small biomolecules, such as coenzyme A (CoA) and acetyl coenzyme A (acetyl-CoA), play vital roles in the regulation of cellular energy metabolism. In this paper, we evaluated the delayed effect of the potent hepatotoxin thioacetamide (TAA) on the concentrations of CoA and acetyl-CoA in plasma and in different rat tissues. Administration of TAA negatively affects liver function and leads to the development of hepatic encephalopathy (HE). In our experiments, rats were administered a single intraperitoneal injection of TAA at doses of 200, 400, or 600 mg/kg. Plasma, liver, kidney, and brain samples were collected six days after the TAA administration, a period that has been suggested to allow for restoration of liver function. The concentrations of CoA and acetyl-CoA in the group of rats exposed to different doses of TAA were compared to those observed in healthy rats. The results obtained indicate that even a single administration of TAA to rats is sufficient to alter the physiological balance of CoA and acetyl-CoA in the plasma and tissues of rats for an extended period of time. The initial concentrations of CoA and acetyl-CoA were not restored even after the completion of the liver regeneration process.

1H NMR-Based Metabolic Signatures in the Liver and Brain in a Rat Model of Hepatic Encephalopathy

Hepatic encephalopathy (HE) is a debilitating neuropsychiatric complication associated with acute and chronic liver failure. It is characterized by diverse symptoms with variable severity that includes cognitive and motor deficits. The aim of the study is to assess metabolic alterations in the brain and liver using nuclear magnetic resonance (NMR) spectroscopy and subsequent multivariate analyses to characterize metabolic signatures associated with HE. HE was developed by bile duct ligation (BDL) that resulted in hepatic dysfunctions and cirrhosis as shown by liver function tests. Metabolic profiles from control and BDL rats indicated increased levels of lactate, branched-chain amino acids (BCAAs), glutamate, and choline in the liver, whereas levels of glucose, phenylalanine, and pyridoxine were decreased. In brain, the levels of lactate, acetate, succinate, citrate, and malate were increased, while glucose, creatine, isoleucine, leucine, and proline levels were decreased. Furthermore, neurotransmitters such as glutamate and GABA were increased, whereas choline and myo-inositol were decreased. The alterations in neurotransmitter levels resulted in cognitive and motor defects in BDL rats. A significant correlation was found among alterations in NAA/choline, choline/creatine, and NAA/creatine with behavioral deficits. Thus, the data suggests impairment in metabolic pathways such as the tricarboxylic acid (TCA) cycle, glycolysis, and ketogenesis in the liver and brain of animals with HE. The study highlights that metabolic signatures could be potential markers to monitor HE progression and to assess therapeutic interventions.

Acute Liver Failure Induces Glial Reactivity, Oxidative Stress and Impairs Brain Energy Metabolism in Rats

Acute liver failure (ALF) implies a severe and rapid liver dysfunction that leads to impaired liver metabolism and hepatic encephalopathy (HE). Recent studies have suggested that several brain alterations such as astrocytic dysfunction and energy metabolism impairment may synergistically interact, playing a role in the development of HE. The purpose of the present study is to investigate early alterations in redox status, energy metabolism and astrocytic reactivity of rats submitted to ALF. Adult male Wistar rats were submitted either to subtotal hepatectomy (92% of liver mass) or sham operation to induce ALF. Twenty-four hours after the surgery, animals with ALF presented higher plasmatic levels of ammonia, lactate, ALT and AST and lower levels of glucose than the animals in the sham group. Animals with ALF presented several astrocytic morphological alterations indicating astrocytic reactivity. The ALF group also presented higher mitochondrial oxygen consumption, higher enzymatic activity and higher ATP levels in the brain (frontoparietal cortex). Moreover, ALF induced an increase in glutamate oxidation concomitant with a decrease in glucose and lactate oxidation. The increase in brain energy metabolism caused by astrocytic reactivity resulted in augmented levels of reactive oxygen species (ROS) and Poly [ADP-ribose] polymerase 1 (PARP1) and a decreased activity of the enzymes superoxide dismutase and glutathione peroxidase (GSH-Px). These findings suggest that in the early stages of ALF the brain presents a hypermetabolic state, oxidative stress and astrocytic reactivity, which could be in part sustained by an increase in mitochondrial oxidation of glutamate.

Pharmacologic rescue of hyperammonemia-induced toxicity in zebrafish by inhibition of ornithine aminotransferase

Hyperammonemia is the common biochemical hallmark of urea cycle disorders, activating neurotoxic pathways. If untreated, affected individuals have a high risk of irreversible brain damage and mortality. Here we show that acute hyperammonemia strongly enhances transamination-dependent formation of osmolytic glutamine and excitatory glutamate, thereby inducing neurotoxicity and death in ammoniotelic zebrafish larvae via synergistically acting overactivation of NMDA receptors and bioenergetic impairment induced by depletion of 2-oxoglutarate. Intriguingly, specific and irreversible inhibition of ornithine aminotransferase (OAT) by 5-fluoromethylornithine rescues zebrafish from lethal concentrations of ammonium acetate and corrects hyperammonemia-induced biochemical alterations. Thus, OAT inhibition is a promising and effective therapeutic approach for preventing neurotoxicity and mortality in acute hyperammonemia.

Accumulation of lactate in the rat brain during hyperammonaemia is not associated with impaired mitochondrial respiratory capacity

In acute liver failure (ALF) cerebral oedema and high intracranial pressure (ICP) are potentially deadly complications. Astrocytes cultured in ammonia have shown mitochondrial dysfunction and in rat models of liver failure, de novo lactate production in the brain has been observed and has led to a hypothesis of compromised brain metabolism during ALF. In contrast, normal lactate levels are found in cerebral microdialysate of ALF patients and the oxygen: glucose ratio of cerebral metabolic rates remains normal. To investigate this inconsistency we studied the mitochondrial function in brain tissue with respirometry in animal models of hyperammonaemia. Wistar rats with systemic inflammation induced by lipopolysaccharide or liver insufficiency induced by 90% hepatectomy were given ammonium or sodium acetate for 120 min. A cerebral cortex homogenate was studied with respirometry and substrates of the citric acid cycle, uncouplers and inhibitors of the mitochondrial complexes were successively added to investigate the mitochondrial function in detail. In a separate dose-response experiment cortex from healthy rats was incubated for 120 min in ammonium acetate in concentrations up to 80 mM prior to respirometry. Hyperammonaemia was associated with elevated ICP and increased tissue lactate concentration. No difference between groups was found in total respiratory capacity or the function of individual mitochondrial complexes. Ammonium in concentrations of 40 and 80 mM reduced the respiratory capacity in vitro. In conclusion, acute hyperammonaemia leads to elevated ICP and cerebral lactate accumulation. We found no indications of impaired oxidative metabolism in vivo but only in vitro at extreme concentrations of ammonium.

Multimodal brain monitoring in fulminant hepatic failure

Acute liver failure, also known as fulminant hepatic failure (FHF), embraces a spectrum of clinical entities characterized by acute liver injury, severe hepatocellular dysfunction, and hepatic encephalopathy. Cerebral edema and intracranial hypertension are common causes of mortality in patients with FHF. The management of patients who present acute liver failure starts with determining the cause and an initial evaluation of prognosis. Regardless of whether or not patients are listed for liver transplantation, they should still be monitored for recovery, death, or transplantation. In the past, neuromonitoring was restricted to serial clinical neurologic examination and, in some cases, intracranial pressure monitoring. Over the years, this monitoring has proven insufficient, as brain abnormalities were detected at late and irreversible stages. The need for real-time monitoring of brain functions to favor prompt treatment and avert irreversible brain injuries led to the concepts of multimodal monitoring and neurophysiological decision support. New monitoring techniques, such as brain tissue oxygen tension, continuous electroencephalogram, transcranial Doppler, and cerebral microdialysis, have been developed. These techniques enable early diagnosis of brain hemodynamic, electrical, and biochemical changes, allow brain anatomical and physiological monitoring-guided therapy, and have improved patient survival rates. The purpose of this review is to discuss the multimodality methods available for monitoring patients with FHF in the neurocritical care setting.

Cerebral effects of ammonia in liver disease: current hypotheses

Hyperammonemia is necessary for development of the cerebral complications to liver disease including hepatic encephalopathy and cerebral edema but the mechanisms are unclear. Ammonia is taken up by the brain in proportion to its arterial concentration. The flux into the brain is most likely by both diffusion of NH3 and mediated transport of NH4 (+) . Astrocytic detoxification of ammonia involves formation of glutamine at concentrations high enough to produce cellular edema, but compensatory mechanisms reduce this effect. Glutamine can be taken up by astrocytic mitochondria and initiate the mitochondrial permeability transition but the clinical relevance is uncertain. Elevated astrocytic glutamine interferes with neurotransmission. Thus, animal studies show enhanced glutamatergic neurotransmission via the NMDA receptor which may be related to the acute cerebral complications to liver failure, while impairment of the NMDA activated glutamate-NO-cGMP pathway could relate to the behavioural changes seen in hepatic encephalopathy. Elevated glutamine also increases GABA-ergic tone, an effect which is aggravated by mitochondrial production of neurosteroids; this may relate to decreased neurotransmission and precipitation of encephalopathy by GABA targeting drugs. Hyperammonemia may compromise cerebral energy metabolism as elevated cerebral lactate is generally reported. Hypoxia is unlikely since cerebral oxygen:glucose utilisation and lactate:pyruvate ratio are both normal in clinical studies. Ammonia inhibits α-ketoglutaratedehydrogenase in isolated mitochondria, but the clinical relevance is dubious due to the observed normal cerebral oxygen:glucose utilization. Recent studies suggest that ammonia stimulates glycolysis in excess of TCA cycle activity, a hypothesis that may warrant further testing, in being in accordance with the limited clinical observations.

Effects of hyperammonemia on brain energy metabolism: controversial findings in vivo and in vitro

The literature related to the effects of elevated plasma ammonia levels on brain energy metabolism is abundant, but heterogeneous in terms of the conclusions. Thus, some studies claim that ammonia has a direct, inhibitory effect on energy metabolism whereas others find no such correlation. In this review, we discuss both recent and older literature related to this controversial topic. We find that it has been consistently reported that hepatic encephalopathy and concomitant hyperammonemia lead to reduced cerebral oxygen consumption. However, this may not be directly linked to an effect of ammonia but related to the fact that hepatic encephalopathy is always associated with reduced brain activity, a condition clearly characterized by a decreased CMRO2. Whether this may be related to changes in GABAergic function remains to be elucidated.

Changes in cerebral oxidative metabolism in patients with acute liver failure

Acute liver failure patients with a persistence of hyperammonemia are at an increased risk of intracranial hypertension due to development of brain oedema. In vitro studies of brain tissue and cell cultures that indicates that exposure to ammonium inhibits enzymatic activity in the tricarboxylic acid cycle, induces substrate depletion through marked glutamate utilization for glutamine synthesis and leads to mitochondrial dysfunction. In patients with acute liver failure cerebral microdialysis studies show a linear correlation between the lactate to pyruvate ratio and the glutamine concentration, as well as to some of the adenosine triphosphate degradation products. However, clinical observations of cerebral exchange rates of oxygen, glucose, lactate and amino acids challenge the interpretation of these findings. In this review the conflicting data of cerebral metabolism during acute liver failure is discussed.

Brain edema in diseases of different etiology

Cerebral edema is a potentially life-threatening complication shared by diseases of different etiology, such as diabetic ketoacidosis, acute liver failure, high altitude exposure, dialysis disequilibrium syndrome, and salicylate intoxication. Pulmonary edema is also habitually present in these disorders, indicating that the microcirculatory disturbance causing edema is not confined to the brain. Both cerebral and pulmonary subclinical edema may be detected before it becomes clinically evident. Available evidence suggests that tissue hypoxia or intracellular acidosis is a commonality occurring in all of these disorders. Tissue ischemia induces physiological compensatory mechanisms to ensure cell oxygenation and carbon dioxide removal from tissues, including hyperventilation, elevation of red blood cell 2,3-bisphosphoglycerate content, and capillary vasodilatation. Clinical, laboratory, and necropsy findings in these diseases confirm the occurrence of low plasma carbon dioxide partial pressure, increased erythrocyte 2,3-bisphosphoglycerate concentration, and capillary vasodilatation with increased vascular permeability in all of them. Baseline tissue hypoxia or intracellular acidosis induced by the disease may further deteriorate when tissue oxygen requirement is no longer matched to oxygen delivery resulting in massive capillary vasodilatation with increased vascular permeability and plasma fluid leakage into the interstitial compartment leading to edema affecting the brain, lung, and other organs. Causative factors involved in the progression from physiological adaptation to devastating clinical edema are not well known and may include uncontrolled disease, malfunctioning adaptive responses, or unknown factors. The role of carbon monoxide and local nitric oxide production influencing tissue oxygenation is unclear.

Brain energy metabolism and mitochondrial dysfunction in acute and chronic hepatic encephalopathy

One proposed mechanism for acute and chronic hepatic encephalopathy (HE) is a disturbance in cerebral energy metabolism. It also reviews the current status of this mechanism in both acute and chronic HE, as well as in other hyperammonemic disorders. It also reviews abnormalities in glycolysis, lactate metabolism, citric acid cycle, and oxidative phosphorylation as well as associated energy impairment. Additionally, the role of mitochondrial permeability transition (mPT), a recently established factor in the pathogenesis of HE and hyperammonemia, is emphasized. Energy failure appears to be an important pathogenetic component of both acute and chronic HE and a potential target for therapy.

A physiological Intensive Control Insulin-Nutrition-Glucose (ICING) model validated in critically ill patients

Intensive insulin therapy (IIT) and tight glycaemic control (TGC), particularly in intensive care unit (ICU), are the subjects of increasing and controversial debate in recent years. Model-based TGC has shown potential in delivering safe and tight glycaemic management, all the while limiting hypoglycaemia. A comprehensive, more physiologically relevant Intensive Control Insulin-Nutrition-Glucose (ICING) model is presented and validated using data from critically ill patients. Two existing glucose-insulin models are reviewed and formed the basis for the ICING model. Model limitations are discussed with respect to relevant physiology, pharmacodynamics and TGC practicality. Model identifiability issues are carefully considered for clinical settings. This article also contains significant reference to relevant physiology and clinical literature, as well as some references to the modeling efforts in this field. Identification of critical constant population parameters was performed in two stages, thus addressing model identifiability issues. Model predictive performance is the primary factor for optimizing population parameter values. The use of population values are necessary due to the limited clinical data available at the bedside in the clinical control scenario. Insulin sensitivity, S(I), the only dynamic, time-varying parameter, is identified hourly for each individual. All population parameters are justified physiologically and with respect to values reported in the clinical literature. A parameter sensitivity study confirms the validity of limiting time-varying parameters to S(I) only, as well as the choices for the population parameters. The ICING model achieves median fitting error of <1% over data from 173 patients (N=42,941 h in total) who received insulin while in the ICU and stayed for ≥ 72 h. Most importantly, the median per-patient 1-h ahead prediction error is a very low 2.80% [IQR 1.18, 6.41%]. It is significant that the 75th percentile prediction error is within the lower bound of typical glucometer measurement errors of 7-12%. These results confirm that the ICING model is suitable for developing model-based insulin therapies, and capable of delivering real-time model-based TGC with a very tight prediction error range. Finally, the detailed examination and discussion of issues surrounding model-based TGC and existing glucose-insulin models render this article a mini-review of the state of model-based TGC in critical care.

In cirrhotic patients reduced muscle strength is unrelated to muscle capacity for ATP turnover suggesting a central limitation

BACKGROUND AND AIMS

  We investigated whether in patients with liver cirrhosis reduced muscle strength is related to dysfunction of muscle mitochondria.

METHODS

  The mitochondrial respiratory capacity of the tibial anterior muscle was evaluated in seven patients and eight healthy control subjects by 31P nuclear magnetic resonance spectroscopy (31PMRS) to express ATP turnover in vivo and by respirometry of permeabilized fibres from the same muscle to express the in vitro capacity for oxygen consumption.

RESULTS

  Maximal voluntary contraction force for plantar extension was low in the patients (46% of the control value; P < 0.05), but neither the capacity for mitochondrial ATP synthesis, V(max-ATP) (0.38 ± 0.26 vs. 0.50 ± 0.07 mM s(-1) ; P = 0.13) nor the in vitro VO(2max) (0.52 ± 0.21 vs. 0.48 ± 0.21 μmol O2 (min g wet wt.)(-1) P = 0.25) were lowered correspondingly. Also, the activity of citrate synthesis and the respiratory chain complexes II and IV were similar in patients and controls. However during the contractions, the contribution to initial anaerobic ATP production from glycolysis relative to that from PCr was reduced in the patients (0.73 ± 0.22 vs. 0.99 ± 0.09; P < 0.01).

CONCLUSIONS

  These results demonstrate that the markedly lower capacity for force generation in patients with liver cirrhosis is unrelated to their capacity for muscle ATP turnover, but the attenuated initial acceleration of anaerobic glycolysis suggests that these patients could be affected by a central limitation to force generation.

Low cerebral oxygen consumption and blood flow in patients with cirrhosis and an acute episode of hepatic encephalopathy

BACKGROUND & AIMS

It is unclear whether patients with hepatic encephalopathy (HE) have disturbed brain oxygen metabolism and blood flow.

METHODS

We measured cerebral oxygen metabolism rate (CMRO(2)) by using (15)O-oxygen positron emission tomography (PET); and cerebral blood flow (CBF) by using (15)O-water PET in 6 patients with liver cirrhosis and an acute episode of overt HE, 6 cirrhotic patients without HE, and 7 healthy subjects.

RESULTS

Neither whole-brain CMRO(2) nor CBF differed significantly between cirrhotic patients without HE and healthy subjects, but were both significantly reduced in cirrhotic patients with HE (P < .01). CMRO(2) was 0.96 +/- 0.07 mumol oxygen/mL brain tissue/min (mean +/- SEM) in cirrhotic patients with HE, 1.34 +/- 0.08 in cirrhotic patients without HE, and 1.35 +/- 0.05 in healthy subjects; and CBF was 0.29 +/- 0.01 mL blood/mL brain tissue/min in patients with HE, 0.47 +/- 0.02 in patients without HE, and 0.49 +/- 0.03 in healthy subjects. CMRO(2) and CBF were correlated, and both variables correlated negatively with arterial ammonia concentration. Analysis of regional values, using individual magnetic resonance co-registrations, showed that the reductions in CMRO(2) and CBF in patients with HE were essentially generalized throughout the brain.

CONCLUSIONS

The observations imply that reduced cerebral oxygen consumption and blood flow in cirrhotic patients with an acute episode of overt HE are associated with HE and not cirrhosis as such, and that the primary event in the pathogenesis of HE could be inhibition of cerebral energy metabolism by increased blood ammonia.

Differential influence of arterial blood glucose on cerebral metabolism following severe traumatic brain injury

INTRODUCTION

Maintaining arterial blood glucose within tight limits is beneficial in critically ill patients. Upper and lower limits of detrimental blood glucose levels must be determined.

METHODS

In 69 patients with severe traumatic brain injury (TBI), cerebral metabolism was monitored by assessing changes in arterial and jugular venous blood at normocarbia (partial arterial pressure of carbon dioxide (paCO2) 4.4 to 5.6 kPa), normoxia (partial arterial pressure of oxygen (paO2) 9 to 20 kPa), stable haematocrit (27 to 36%), brain temperature 35 to 38 degrees C, and cerebral perfusion pressure (CPP) 70 to 90 mmHg. This resulted in a total of 43,896 values for glucose uptake, lactate release, oxygen extraction ratio (OER), carbon dioxide (CO2) and bicarbonate (HCO3) production, jugular venous oxygen saturation (SjvO2), oxygen-glucose index (OGI), lactate-glucose index (LGI) and lactate-oxygen index (LOI). Arterial blood glucose concentration-dependent influence was determined retrospectively by assessing changes in these parameters within pre-defined blood glucose clusters, ranging from less than 4 to more than 9 mmol/l.

RESULTS

Arterial blood glucose significantly influenced signs of cerebral metabolism reflected by increased cerebral glucose uptake, decreased cerebral lactate production, reduced oxygen consumption, negative LGI and decreased cerebral CO2/HCO3 production at arterial blood glucose levels above 6 to 7 mmol/l compared with lower arterial blood glucose concentrations. At blood glucose levels more than 8 mmol/l signs of increased anaerobic glycolysis (OGI less than 6) supervened.

CONCLUSIONS

Maintaining arterial blood glucose levels between 6 and 8 mmol/l appears superior compared with lower and higher blood glucose concentrations in terms of stabilised cerebral metabolism. It appears that arterial blood glucose values below 6 and above 8 mmol/l should be avoided. Prospective analysis is required to determine the optimal arterial blood glucose target in patients suffering from severe TBI.

Pathoetiological model of delirium: a comprehensive understanding of the neurobiology of delirium and an evidence-based approach to prevention and treatment

Delirium is the most common complication found in the general hospital setting. Yet, we know relatively little about its actual pathophysiology. This article contains a summary of what we know to date and how different proposed intrinsic and external factors may work together or by themselves to elicit the cascade of neurochemical events that leads to the development delirium. Given how devastating delirium can be, it is imperative that we better understand the causes and underlying pathophysiology. Elaborating a pathoetiology-based cohesive model to better grasp the basic mechanisms that mediate this syndrome will serve clinicians well in aspiring to find ways to correct these cascades, instituting rational treatment modalities, and developing effective preventive techniques.

Fuelling cerebral activity in exercising man

The metabolic response to brain activation in exercise might be expressed as the cerebral metabolic ratio (MR; uptake O2/glucose + 1/2 lactate). At rest, brain energy is provided by a balanced oxidation of glucose as MR is close to 6, but activation provokes a 'surplus' uptake of glucose relative to that of O2. Whereas MR remains stable during light exercise, it is reduced by 30% to 40% when exercise becomes demanding. The MR integrates metabolism in brain areas stimulated by sensory input from skeletal muscle, the mental effort to exercise and control of exercising limbs. The MR decreases during prolonged exhaustive exercise where blood lactate remains low, but when vigorous exercise raises blood lactate, the brain takes up lactate in an amount similar to that of glucose. This lactate taken up by the brain is oxidised as it does not accumulate within the brain and such pronounced brain uptake of substrate occurs independently of plasma hormones. The 'surplus' of glucose equivalents taken up by the activated brain may reach approximately 10 mmol, that is, an amount compatible with the global glycogen level. It is suggested that a low MR predicts shortage of energy that ultimately limits motor activation and reflects a biologic background for 'central fatigue'.

Cerebral Blood Flow in Fulminant Hepatitis

Fulminant hepatic failure (FHF) is often complicated with cerebral edema, intracranial hypertension, and coma. Cytotoxic and vasogenic factors have been recognized in the etiology of cerebral edema. One of the main causes seems to be the accumulation of glutamine in astrocytes, which is produced from ammonia and the excitatory neurotransmitter glutamate. Ammonia is detoxified within the brain in astrocytes, where it increases the osmotic pressure for water. Ammonia-induced astrocytic water accumulation seems to act as an integrative trigger for the development of intracranial hypertension. While cerebral blood flow is sometimes reduced in the first stage of FHF, as compensatory cerebral vasoconstriction to reduce mean arterial pressure, it later increases as hyperammonemia decreases cerebral arteriolar tone. Despite vasodilation in the systemic and splanchnic beds at early stages of the disease, cerebral vessel resistance may increase, so that cerebral perfusion pressure may be preserved. When cerebral vascular tone is no longer effective in the course of illness, vasodilation gradually develops and rapidly becomes poorly responsive to carbon dioxide stimulation, which signifies loss of autoregulatory tone and cerebral hyperemia develops. Prolonged excessive flow may lead to brain swelling, vasogenic edema, and intracerebral hemorrhage. Brain edema further aggravates the critically reduced cerebral perfusion and is responsible for the high mortality.

Mild hypothermia for acute liver failure: a review of mechanisms of action

Brain edema with intracranial hypertension is a major complication in patients with acute liver failure. Current therapies for this complication include a variety of pharmacologic and interventional measures, some of which are frequently associated with adverse effects or contraindications. Even though these measures usually allow the control of intracranial hypertension for a certain period of time, recurrence is common. New therapies are therefore needed. Increasing clinical and experimental evidence suggests that induction of mild hypothermia (32 degrees C-35 degrees C) may be a therapeutic alternative. Similar to traumatic brain injury or brain stroke, induction of mild hypothermia seems highly effective to reduce intracranial pressure in patients with acute liver failure. Several mechanisms by which mild hypothermia may prevent brain edema and intracranial hypertension in this condition have been disclosed and may include beneficial effects on ammonia metabolism, as well as on the disturbances of brain osmolarity, cerebrovascular hemodynamics, brain glucose metabolism, inflammation, and others. Improvement of systemic hemodynamics and amelioration of liver injury may be other benefits of the systemic induction of mild hypothermia, but the impact of potential adverse events, such as infection, should also be taken into account. At a time when mild hypothermia is increasingly used in several specialized centers, performance of a randomized controlled trial seems critical to confirm the benefits of mild hypothermia in acute liver failure and to provide adequate guidelines for its use.

Management of patients with fulminant hepatic failure and brain edema

Cerebral edema in acute liver failure is associated with a poor prognosis. Optimization of cerebral perfusion pressure and blood flow plays a key role in contemporary management of these patients. However, understanding of the pathophysiology of brain edema is required for optimal patient management. This review explains the relationships between cerebral perfusion and edema and summarizes therapies that are currently used in patients with acute liver failure to prevent and reduce intracranial pressure.