Tissue metabolism during endotoxin shock after pretreatment with monophosphoryl lipid A

Sepsis is associated with altered cerebral microcirculation and tissue hypoxia in experimental peritonitis

OBJECTIVE

Alterations in cerebral microvascular blood flow may develop during sepsis, but the consequences of these abnormalities on tissue oxygenation and metabolism are not well defined. We studied the evolution of microvascular blood flow, brain oxygen tension (PbO2), and metabolism in a clinically relevant animal model of septic shock.

DESIGN

Prospective randomized animal study.

SETTING

University hospital research laboratory.

SUBJECTS

Fifteen invasively monitored and mechanically ventilated female sheep.

INTERVENTIONS

The sheep were randomized to fecal peritonitis (n = 10) or a sham procedure (n = 5), and craniectomies were performed to enable evaluation of cerebral microvascular blood flow, PbO2, and metabolism. The microvascular network of the left frontal cortex was evaluated (at baseline, 6, 12, and 18 hr) using sidestream dark-field videomicroscopy. Using an off-line semiquantitative method, functional capillary density and the proportion of small perfused vessels were calculated. PbO2 was measured hourly by a parenchymal Clark electrode, and cerebral metabolism was assessed by the lactate/pyruvate ratio using brain microdialysis; both these systems were placed in the right frontal cortex.

MEASUREMENT AND MAIN RESULTS

In septic animals, cerebral functional capillary density (from 3.1 ± 0.5 to 1.9 ± 0.4 n/mm, p < 0.001) and proportion of small perfused vessels (from 98% ± 2% to 84% ± 7%, p = 0.004) decreased over the 18-hour study period. Concomitantly, PbO2 decreased (61 ± 5 to 41 ± 7 mm Hg, p < 0.001) and lactate/pyruvate ratio increased (23 ± 5 to 36 ± 19, p < 0.001). At 18 hours, when shock was present, animals with a mean arterial pressure less than 65 mm Hg (n = 6) had similar functional capillary density, proportion of small perfused vessels, and PbO2 values but significantly higher lactate/pyruvate ratio (46 ± 18 vs 20 ± 4, p = 0.009) compared with animals with an mean arterial pressure of 65-70 mm Hg (n = 4).

CONCLUSIONS

Impaired cerebral microcirculation during sepsis is associated with progressive impairment in PbO2 and brain metabolism. Development of severe hypotension was responsible for a further increase in anaerobic metabolism. These alterations may play an important role in the pathogenesis of brain dysfunction during sepsis.

A possible mechanism for the beneficial effect of ethanol in essential tremor

BACKGROUND

Essential tremor is one of the most common movement disorders in elderly people. The hypothesis of a disregulation of N-methyl-D-aspartate (NMDA) pathways has been suggested. It was shown experimentally that infusion of NMDA in cerebellar nuclei down-regulates glutamate release.

METHODS

We assessed the effects of intranuclear administration of harmaline on the NMDA-mediated regulation of glutamate in rats using reverse dialysis. We hypothesized that ethanol, which improves essential tremor in the clinic, antagonizes the effect of harmaline upon glutamatergic transmission. We tested the interaction of ethanol and harmaline upon glycerol (a marker of membrane turn-over), lactate, and pyruvate concentrations.

RESULTS

Harmaline increased the concentrations of glutamate and impaired the NMDA-mediated regulation of glutamate. Ethanol decreased the concentrations of glutamate during NMDA stimulation in case of pre-administration with harmaline. Concentrations of glycerol rose with harmaline. Glycerol levels markedly decreased during NMDA infusion when inhibitors of nitric oxide synthase, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate antagonists or NMDA antagonists were administered. Harmaline increased lactate/pyruvate ratios during NMDA infusion but these ratios returned to normal values in presence of ethanol.

DISCUSSION

We provide a possible mechanism for the beneficial effect of ethanol on essential tremor. The concept of glutamatergic disregulation underlying essential tremor is highlighted. Consequences for our understanding of essential tremor are discussed.

Beneficial effects of alkaline phosphatase in septic shock

OBJECTIVE

Alkaline phosphatase may decrease the harmful effects of lipopolysaccharide by detoxifying lipid A. The aim of this study was to investigate whether administration of alkaline phosphatase is beneficial in a clinically relevant septic shock model.

DESIGN

Interventional laboratory study.

SETTING

University hospital animal research laboratory.

SUBJECTS

Fourteen fasted, anesthetized, invasively monitored, mechanically ventilated, female sheep (27.6 +/- 3.9 kg).

INTERVENTIONS

Each animal received 1.5 g/kg body weight of feces intraperitoneally to induce sepsis. Ringer's lactate and a 6% hydroxyethyl starch solution were infused throughout the experiment to prevent hypovolemia. Two hours after feces injection, animals were randomized to alkaline phosphatase (60 units/kg intravenous bolus followed by a continuous infusion of 20 units/kg/hr for a total of 15 hrs) or no alkaline phosphatase (control group).

MEASUREMENTS AND MAIN RESULTS

All animals were studied until their spontaneous death or for a maximum of 30 hrs. Plasma alkaline phosphatase concentrations decreased in the control group but increased in the treatment group following alkaline phosphatase administration. In the treatment group, the Pao2/Fio2 ratio was higher (p < .05), blood interleukin-6 concentrations were lower (p < .05), and the survival time was longer (median time 23.8 vs. 17 .0 hrs, p < 0.05) than in the control group. There were no significant differences in systemic hemodynamics or diuresis.

CONCLUSIONS

In this clinically relevant septic shock model, alkaline phosphatase administration improved gas exchange, decreased interleukin-6 concentrations, and prolonged survival time.

Bench-to-bedside review: microdialysis in intensive care medicine

Microdialysis is a technique used to measure the concentrations of various compounds in the extracellular fluid of an organ or in a body fluid. It is a form of metabolic monitoring that provides real-time, continuous information on pathophysiological processes in target organs. It was introduced in the early 1970s, mainly to measure concentrations of neurotransmitters in animal experiments and clinical settings. Using commercial equipment it is now possible to conduct analyses at the bedside by collecting interstitial fluid for measurement of carbohydrate and lipid metabolites. Important research has been reported in the field of neurosurgery in recent decades, but use of metabolic monitoring in critical care medicine is not yet routine. The present review provides an overview of findings from clinical studies using microdialysis in critical care medicine, focusing on possible indications for clinical biochemical monitoring. An important message from the review is that sequential and tissue-specific metabolic monitoring, in vivo, is now available.